> "...they seem like a better idea than air conditioning whole rooms."

in the same vein, most people's behavior around heating/cooling is comically misguided and inefficient. most people believe temperature is what we're trying to control, and that's driven by how weather reports, climate controls, and even our language ("it's hot!") emphasize temperature (which isn't nefarious, and makes sense in their separate, respective domains) rather than heat transfer, which is what we're really after.

this is most evident in cars, where folks point air vents away from themselves while running the a/c (or heat) full blast, rather than pointing the vents at themselves and running at a lower speed. the former is based on the idea that the whole car's interior temperature needs to be lowered (or raised) for effective heat transfer away from (or to) the body, which is incorrect and inefficient. pointing the vents at you (on bare skin ideally) promotes heat transfer immediately instead of waiting to create a big enough temperature differential to do the same work.

I think people absolutely understand that pointing the vent at their face will cool them faster. The problem is it’s not uniform and uncomfortable to have your left mid arm freezing while the rest of you is hot.

sure, but that's why there are multiple vents? and you can run the a/c at a lower speed to boot.

Two cold spots are not better than one. If you're claiming that the approach you describe is more efficient, then I agree with you. But for me the 'vents pointed away' approach is still subjectively more comfortable, though it does take longer.

there are controls for both speed and temperature, as well as vent direction. it's not difficult to find a comfortable combination.

I understand what you're getting at, but having 1-4 vents pointed at the front of your body is not the same as having the cabin air at a particular temperature. Neither is having air blowing on your front side the same as having air swirling about your entire body.

Basically, any fan/temp setting with vents directed at you either feels too uncomfortable or not cooling enough due to the direct impact. It's easier to fine tune ambient cooling with vents directed away from you. I wonder how these jackets work in that regard.

That is a problem that people can solve if they want to. But if you insist that you need help: I find that aiming fans across extremities, arms/hands and legs, is very pleasant and effective (a higher surface area : volume ratio?).

God this is such an engineer response. Yes every body is the same and everyone should always do the most optimal thing regardless of the actual outcome or goal.

Some people have poor blood circulation. Pointing a small fast fan at their arms will not cool their core.

Some people get dry eyes easily. Pointing a fan at their face will make them extremely uncomfortable.

Some people are extremely sensitive to temperature changes. They don’t want to be hot but pointing a 40 degree F stream of air at their chest will give them physical pain.

No, this isn’t a one size fits all problem/solution. The correct solution is actually to cool the whole car while allowing the user control over how to cool themselves by adjusting the fans/doors. It would be much better however if car manufacturers included better PID controllers as well as moisture sensors inside and outside the cabin to create better climate control. In a car that really shouldn’t be difficult, at least with the windows closed. It’s not like a house with multiple rooms that have different exposure to heating and cooling as well as sunlight.

I didn't say it one solution would suit everyone. I offered a tip.

The problem is solved by letting me point the vent away from my body to cool the ambient temperature of my car.

I don't want to have cold hands, arms, and face, while my back and my feet sweat.

Cooling my extremities seems to cool the rest of me effectively. Cooling the ambient temperature creates another problem, increasing the ambient temperature for the rest of the world; it's a big problem. However, a car is a small space; I expect the climate impact would be relatively small.

Somehow dogs can get away with cooling through their tongue but the naked apes still complain about their whole arms being cooled?

Maybe humans sweat more than dogs and so feel a much greater effect from evaporative cooling?

But having the whole cabin be at a cooler temperature allows the heat transfer to happen uniformly across your whole body, which is a lot more comfortable than having a vent pointed at yourself.

I find having a vent on my hands or face MUCH more comfortable. I’ve lived in some of the hottest places on earth and indeed on the worst days there would never be a point where you’d feel cool enough without blowing the vents directly at you. Even in more temperate climates the airflow over my face is just refreshing, and helps keep my hands completely dry for a better grip.

> I’ve lived in some of the hottest places on earth and indeed on the worst days there would never be a point where you’d feel cool enough without blowing the vents directly at you.

Surely if an air conditioner brings an indoor area to a given temperature (and humidity), it doesn't matter what the temperature is outside. So what gives? Is the air conditioner not powerful enough? Do people not want the inside to be as cold as in other locales, since they're used to the heat? The latter would make sense, but then I don't understand why you'd need to be blowing the vents directly at you.

Well the topic is car A/C and indeed if the outside temperature is 45C (113F), and the air conditioner only produces a difference of 20C, and your target is 22C then you’ll fall short of that by a few degrees.

Also with car A/C (or indoor spaces which take in a lot of direct sunlight), hotter days tend to be the result of greater insolation. If direct sunlight is hitting your skin on a clear summer day it has a significant warming effect which you would want to offset with more ambient or active cooling.

It happens in houses too. People will run their heater warmer than they cool their house to in summer. My theory is that the wall temperature is causing more or less radiant heating and the thermostat gets adjusted to compensate. So if the car body is much hotter it will feel hotter even with the same air temp.

That’s because you have good blood circulation. Blood acts as your body’s coolant and transfers heat to your hands. Lots of people don’t have this so what they would get is the sensation of being very hot except their hands which will get very cold. Not a pleasant combo.

Ironically, given the context of this discussion, those people whose bodies maintain a higher differential (what you called poor circulation) are more efficient at maintaining their core temperature in other circumstances.

For example, you are out walking on cold ground. Your feet get cold while your core stays warm. This is ideal for keeping heat in your core. You may survive where those with "good" circulation would lose heat too fast.

Although this may be due to lower rate of blood circulation, there is another effect independent of circulation rate.

Heat-exchanger effects along limbs depending on the layout of your veins and arteries, which varies among people. The extreme example is penguins, whose feet are on ice yet their bodies retain heat because of heat exchange as blood circulates in their legs. It maintains a temperature differential along the legs. I'm not suggesting people have specific anatomical structure like penguins, rather I'm saying the natural variation in the placement of veins and arteries in human limbs will result in a range of heat-exchange factors among different people, which is a separate factor from circulation rate, and may indicates a thermally efficient body for some situations.

no, that's just rationalizing. you're sitting uncomfortably for 10-15 minutes waiting for that temperature to be reached[0], and even then, heat transfer is maximized by having the air flow over you. the edge case might be when there are like 4+ people in a car for which there are not enough vents, but even then, pointing the vents at people, even somewhat indirectly, is better.

heat transfer is what makes you feel cool/warm, not temperature, and air flow promotes convection (as well as conduction). it's why metal feels cooler (or warmer) than plastic at the same temperature (e.g., differing heat transfer coefficients).

[0]: note that most car trips are less than ~15 minutes, at which point, all that heating/cooling is wasted as you flush it out the door.

> note that most car trips are less than ~15 minutes, at which point, all that heating/cooling is wasted as you flush it out the door.

Talking about energy efficiency: if your car trip is <15 minutes, then ideally you shouldn't be using a car at all!

> Talking about energy efficiency: if your car trip is <15 minutes, then ideally you shouldn't be using a car at all!

That depends on where you live. It take me about 5-7 minutes to drive to the nearest store (a "Dollar General" store). That store is about 2.5 miles away, so about a 5 mile round trip. Average walking speed is about 3.2 mph, so a +2 hour trip walking it.

The next nearest store is about 10-12 miles away (a grocery store). Most of those miles are on a road with a 55mph limit but these are curvy hilly roads so probably closer to a 40-45 mph average. That's a +6 hour walk.

Riding a bicycle is akin to death wish on those 2 lane roads with no shoulder.

This sounds so weird. Like a place that has no general store in walking distance implies end of the world but then you say it has bad traffic too. And 2-lane road with no shoulder implies it's not a highway in the middle of nowhere but a place where people live. Why has nobody opened a shop closer? Seems like easy money.

He didn’t say it has bad traffic, he said it’s dangerous. I grew up in a place like this - isolated houses along a 2-lane road through the countryside with no shoulder. It’s a local road, but the speed limit is typically 55 mph because the housing density is very low. There’s not much traffic, but the traffic that is there is flying down the curvy road just trying to hang on. All of the stores are centralized in small towns and villages spaced 20 miles apart or so.

Yeah I just have no reference. Here villages spaced 30 km from each other doesn't happen. It's usually walking distance.

To get a better idea of that sort of context, look at Texas or Montana in Google Maps and zoom in on one of the low-population areas. You'll see plenty of little towns with no public transit that would be literally days away from each other if walking.

You hit the nail on the head. I live in rural southern Missouri in the Ozark Mountains.

Likely restrictive zoning. In much of North America, commercial and mixed-use buildings are not permitted in areas zoned for single-family residences.

> It take me about 5-7 minutes to drive to the nearest store (a "Dollar General" store)

Assuming you live in an urban area, this seem like absolute madness to me. Does this not represent a huge opportunity for someone to start a local grocery store business?

I have never lived further than 15 minutes walk to the local grocery store (and I considered that a pretty inconvenient location). Where I live now I have a 2 local shops ~1 minute walk away, with a small high street with a small supermarket ~10 minutes walk away. And I would say this is pretty typical across the entirety of urban/suburban UK.

US post-war neighborhoods built after the advent of cars, were mostly designed for them. It's not madness, as much as it is the inevitable result of consumer demands for what was at the time, new housing.

> Does this not represent a huge opportunity for someone to start a local grocery store business?

Those stores do mostly exist where they are viable. The limiting factors are:

1. many of those neighborhoods don't have commercial space, and are filled with people who live there specifically because they do not want to live by commercial property, and support zoning regulations to keep grocery stores from being built besides their property.

2. because many people do have cars, neighborhood grocery stores are sometimes at a commercial disadvantage, because many of their potential customers can easily drive to stores with greater selection.

I do like living in more lively neighborhoods, and I picked one where I do have two small grocery stores within 15 minutes walking distance. The thing is, they're both ethnic grocery stores with limited selections. I try to support them when I can but to get everything that I want to buy in one shopping trip, it's much quicker and easier to drive out of my neighborhood to a much larger store.

In the US, that frequently involves crossing 6 to 8 lane roads with 40mph+ speed limits, and 200ft setbacks to allow for huge parking lots. Or an actual highway where only motorized vehicles are allowed.

if only we could collectively realize this and reconfigure our urban spaces according to that consideration! for instance, adding many, many more street trees everywhere, replacing street parking with protected bike lanes, and converting a lane on major streets to dedicated bus lanes with synchronized/sensored lights.

but yes, for those majority short trips, the wind in your hair on a (electric) bike/scooter is likely just as cooling, and the trip likely no longer.

I wonder how improved traffic could be if we somehow had managed that people driving alone would be driving in a vehicle that is suited for exactly 1 person. (although now I'm having pictures in my head of the insane traffic in India's cities... :))

Maybe electric vehicles could be an incentive to downsize vehicles?

for sure. on top of energy taxes, we could assess escalating maintenance fees on cars by size and weight, to encourage right-sized vehicles, especially in metro areas where it really matters.

There are several studies on the relationship between vehicle weight and road wear and tear. 1 fully loaded semi does more damage than nearly 10,000 cars. The reality is if costs were assigned appropriately large aspects of life would become unworkable.

> that's just rationalizing

That's not called for.

> you're sitting uncomfortably for 10-15 minutes waiting for that temperature to be reached

It doesn't take that long, and you can compromise by hitting yourself with some of the full-power cooling until the cabin reaches a comfortable temperature.

> heat transfer is what makes you feel cool/warm, not temperature, and air flow promotes convection (as well as conduction).

Heat transfer yes, but everywhere on the body. You can't just look at the total.

You are arguing too technically here. How people feel and what they like is very subjective. If people don't like air blowing unnaturally at them, no amount of arguing will change that.

you're implying that there is something incorrigible about such feelings, and that it's useless to point out other, objective factors that can impact decision-making (and habits). if that were true, we'd be a way more predictable, and boring, species than we are.

The purpose of air conditioning in comfort -- so arguing that people can make themselves slightly less comfortable by doing something different is moot. It's even more energy efficient to not run the AC at all so why argue about vent position?

I don’t think they are implying that. If someone’s preferences are based on metric A, and another method provides better results with regards to metric B, even if metrics A and B are correlated, this needn’t be a reason for the person to prefer the proposed alternate method, because the current method may still provide results with regards to metric A.

Telling someone that tomato is a more vibrant red than chicken, will not compel them to prefer the taste of a tomato to the taste of chicken. (In this example the “correlation” bit doesn’t apply, but, whatever.)

One method optimizes for energy efficiency and the other for comfort.

This material is a passive method that I think rather soon recoups any extra energy expended during manufacturing.

You really think people point vents away from themselves because they don't understand how heat transfer works, rather than because they dislike having the air blowing directly on them?

Not many people know it, but blasting detainees with air conditioning for prolonged periods is one of the Enhanced Interrogation techniques we use on detainees in Guantanamo Bay

Ever since I got lasik my eyes get dry and uncomfortable if I have dry air moving over my face for too long. This is what happens if the vent is pointed anywhere around my upper body sadly.

I could not agree more: People don't wear insulation (e.g., sweaters) in winter, the heat escapes from their body, and then they heat the entire house/office in order to warm themselves up again. You already have the heat! It's getting away!

Summer has similar situations. Point a small fan at yourself and you can save a lot on cooling the entire unit. A fan is blowing a gentle breeze across my hands right now (tip: aim it across the keyboard; it cools the laptop and you, and it doesn't dehydrate me). As long as the ambient temperature is below about 85, I'm fine. Think of it: 80 degrees, in the shade, with a breeze. That's pretty nice.

If the vehicle's air conditioner compressor only runs at one stage, does it even matter? Are there any automotive air compressors with multiple stages/speeds?

I suspect they use something like a TXV (thermostatic expansion valve) that is found on newer household AC units.

This acts as sort of a regulator for the system, which ultimately ends up reducing the load on the compressor as the evaporator is colder.

So while the compressor runs at one stage in the sense of it being directly driven by the engine, the resistance to spin the compressor varies based on the load of the AC system.

that's valid, but even then, there's also the efficiency of whisking away heat from your body to consider, which is the more direct/relevant efficiency concern here.

Your pithy analysis seems to entirely discount having passengers in the back seat, or multiple rows of seating like minivans.

I think you're giving them too little credit. I'd rather have even and comfortable warmth through-out the cabin, than having to sit in front of a hot draft the entire ride, despite what might be more effective. That is a matter of comfort, and not physics.

You can point them at you if you only have front seat passengers - if that is comfortable for you. If you have folks in the back seat or want to keep your groceries coolish, whole cabin cooling is much better.

I think what is evident for all parents is the importance of cooling down the entire car, not just the driver or front passenger but all kids in the back, sometimes with a lot of stuff in the way of the vents

I use a 3-step approach: point the vents at me while I'm very warm, then away from me when I start to get comfortable. If I actually get cold, I then turn the blower speed down.

Personally I don’t enjoy the sensation of the air blowing against my skin. It’s distracting and usually moves my hair around when I’m trying to focus on the road.

People sit in the back seat of cars too.

I have seen these and pondered if it is rude to wear them. While it is actively cooling your body, you are aggressively blowing your heat to those around you. In a crowded subway train I would not want to be next to someone wearing this. However, perhaps I overestimate their effects.

They appear to just circulate air around the body, which makes sweat evaporate cooling the body. If they were actual air conditioners they would be indeed moving heat from the body into the surrounding air.

Edit: to be clear I am not suggesting heat/energy disappears. In this case they are cooling the air and the body using evaporation. The system appears to suck in hot air on one side and ejects cooler humid air on the other. More or less human swamp coolers.

https://en.m.wikipedia.org/wiki/Evaporative_cooler#Physical_...

It's the same dilemma though. Now you are raising the humidity around you, making other peoples' sweat less effective at cooling them.

Although lowering humidity can be more easily done with just ventilation.

Not really, because the body self-regulates temperature by pushing out as much sweat as it needs to reach target temperature. A tool that facilitates evaporation allows the body to get by with less sweating. The amount evaporated will be the same, but the carrier won't be drenched. (PS: roughly the same, additional heat transfered from air to dry skin will need to be shed by sweating, but I doubt that this would make much difference compared to all the watts resulting from metabolistic base load, even in absence of physical activity)

Mandatory slightly related anecdote: on a summer drive through Morocco back when car air conditioners where a thing exclusive to America and the one percent I never felt sweaty as long as the car was moving.

Yeah I rarely ever feel sweat on my body when I run. It’s when I stop running that I get drenched and need to put a headband on.

(I live in a dry climate and generally don’t sweat much anyways, YMMV)

A few years ago I stopped sweating, almost completely. Trying to cool off is nearly impossible now.

I have an ice vest if I really want to go outside for then a few minutes.

Mirror clothing sounds like a dream.

Only in poorly ventilated enclosed spaces. But the sweat usually gets evaporated anyway only slower.

And when they are used outside, the added humidity is negligible compared to volume of surrounding air.

Evaporated sweat only cools the body if it leaves the area where the body is. So presumably they would be radiating warm air away from them.

Not necessarily, they might just be radiating humid air. It's the evaporation itself that leads to the cooling.

But the humid air makes it harder for people around you to cool evaporatively (eg. sweating)

Well yes, you must (irreversibly) increase entropy somehow. The air itself won't be warmer though.

This is why we have wet and dry bulb temperatures.

Your intuition about temperature is focused on the dry-bulb temperature that a simple probe gives you.

A wet-bulb is literally a probe with a wet cloth or tissue around it.

As the humidity in the air increases the wet-bulb temperature approaches the dry bulb temperature.

This is absolutely “warming”. The latent heat of the air increases. The sensible heat remains the same.

Without the jacket the person will emit the same heat. The jacket fans do generate a trivial amount of heat, but it’s likely less than a desktop fan.

ventilation is a lot cheaper than AC though

Hey as long as I feel cool and look cool I don’t care what it does.

Isn't that just the same thing, but using a natural process (sweating) rather than phase change in a refrigerant?

Evaporative cooling also moves heat from the body into the surrounding air, just encapsulated in water droplets.

So heat goes poof?

I wonder if that would be true at steady-state (e.g. when the train is effectively empty), but for a train at rush hour, whether much of the capacity of the AC system is already being used to remove heat generated from the humans, and if that means the temperature difference to the outside is actually mostly unimportant.

That is to say, the person next to you is generating and dissipating a fixed rate of heating onto you (equal to their basal metabolic rate consumption), so all that the jacket can have changed is their steady-state perception of the local temperature.

They are usually worn by workers with long sleeve jacket uniforms. They need to cool down during summer when it gets really hot and they need to wear their uniforms. Ordinary people do wear them and remove the sleeves and they are not that loud. You wont even notice them unless you see the fan. The DIY jackets with fan are the loud one, I've seen a couple them.

While it does not actively cool your body, it transfers the heat to any surface that absorbs MIR. So in a crowded space the wearer will indeed dump the heat on other people.

I've only seen these in use by construction workers and parcel delivery/mail people

It looks like it's more of an air "cooler" than an air conditioner. It probably works somewhat depending on the climate, but air conditioning actively cools the air (it produces colder air than the exterior air temperature) and reduces humidity.

This page explains (not very clearly) how it works: https://kuchoufuku.com/?mode=f1

> The wind that runs around the clothes by the fans takes heat away from the heat of vaporization.

Yeah so as I expected. It would work well in a lot of climates today, but it's basically just a very well directed fan. If the wet-bulb temperature is high, it's not going to help.

(In Japanese)

> they seem like a better idea than air conditioning whole rooms

Is that really true? It's more efficient to cool larger rooms, because their surface area to the outside world is lower relative to the volume of the room.

Keeping entire factories or warehouses cool uses surprisingly little energy. Once they reach the desired temperature they stay there with only minor additional cooling.

House cooling doesn't really need A/C most of the time. Most countries in Europe and North America (exceptions are mostly Mediterranean countries) aren't even using blinds that prevent sunrays from reaching the glass of the windows. People are living in greenhouses, not houses, and the ridiculous part is that they are not even in there most of the time during the day, so the light is unused and the heat accumulates so much that you get home to an unbearable 30C/80F temperature.

If the heat was kept out of the house properly during the day, you could just keep the house cool enough by opening the windows during the night, without any A/C at all.

Most people do use blinds to keep the sun out but they are on the inside of the glass. Blinds on the inside of the glass are sufficient since we are only talking about blocking/reflecting infrared heat. In the US at least, modern windows in the last 20 years can have an inferred reflective coating meaning you don't need blinds at all.

The big issue is humidity. Attic fans work great in low humidity places but by their nature they pull in exterior air. If it's 70F but very humid, then you will not have a great time opening windows even at night. The scenarios in which you don't need AC also work really well with AC; the efficiency (COPR) of a heat pump depends on the temperature difference that you pump heat across. If the exterior temperature is lower at night then the heat pump is going to have great efficiency anyway. Also AC has to condense water (cooler air holds less water), letting in (humid) air at night only to run AC during the day is not as efficient as looks.

I big part of people saving money on cooling when using an attic fan or opening windows is they are probably also just letting the interior reach a higher temperature.

To keep this somewhat on topic, in is more efficient for heat pumps to cool a room than to cool individual people/things inside that room typically because when you are cooling the room you are moving the heat to the outside of an insulated area.

> Most people do use blinds to keep the sun out but they are on the inside of the glass.

That's exactly my point, in Southern Europe blinds are on the outside of the glass, not inside. Reflective coating and insulating double glass is good but not the same as they're not 100% efficient (with outside blinds, they absorb the heat and insulating glass is very good at keeping conduction at bay).

I can come home in the evening and barely notice the difference in temperature from when I left (25C-26C in the morning, 26C-27.5C in the evening, so about 1.5C difference); that's even with 33-36C highs outside. By 5PM it's possible to open blinds because I don't get direct sunlight anymore (my house faces South and East and has other buildings towards west), and I open the windows from 10PM till dawn at 5-6AM.

Blinds on the outside... How does that work? How do the blinds not get destroyed by the wind?

They are rigid, typically of some metal.

Air conditioning removes heat and moisture from indoor air, which has a big impact on your breathing comfort. This is very different to the cool suits racing drivers and others wear, who also have cooled breathing air piped into their helmets.

Also, a ridiculous review of these by AvE (a channel I recommend to anyone who’s mechanically inclined): https://youtu.be/ySw0IHIwQ3s

Wow, I find his manner so disagreeable.

He affects an overly exaggerated Canukistan patois that I find endlessly amusing, but I can see why it would grate on others.

Some of the other "vidjaeos" are quite interesting when he is tearing down power tools to see how well they are constructed, and which ones are really worth the money.

Agreed. Quite a bit of casual racism, among other things.

If you're just going by that video, I can see why you might think so.

But he has a lot of respect for Japanese products in general, because they are well engineered and manufactured. And a lot of the stuff for sale these days in Norte America is very cheaply made, and he really doesn't like that.

> If you're just going by that video, I can see why you might think so.

That video isn't unique in that way. I do enjoy his channel, but he's not afraid to let his racist/sexist flag fly.

Now you're making me question how much I should tolerate that sort of thing. Where is the line between "he's just joking around, he's not really serious" vs. "deep down he really believes this". Am I just too complacent? I'll have to think about that.

People who don't believe those kinds of thing deep down tend not to make jokes like that.

Same! If you figure it it, please let me know.

The only device that gets that small is a Peltier cooler, and those have far less efficiency than a proper Air Conditioner / Refrigerant based pump.

You can experiment with Peltiers yourself: they're widely available on Digikey and are extremely small and effective at "air conditioning" like tasks (moving heat to the hot side, while cooling the cold side).

But due to their lack of efficiency, they can never properly compete against a large scale central air conditioner: be it a proper dehumidifier, refrigerator, or actual AC / HVAC unit.

-------

A 100W Peltier might be able to move 50W (meaning the hot-side is 150W of heat, while the cold side is -50W of cooling). Perfect efficiency (50W heat hot side / -50W of cooling cold side) is impossible, much like perpetual motion machines.

In contrast, a proper air conditioner might be closer to 100W of power delivering 300W of cooling (Emit 400W of heat on the hot side, and provide -300W cooling on the cold side).

Furthermore, the temperature difference from a Peltier is small, maybe 10C or 20C (and the bigger the delta, the less efficient a Peltier gets). A proper compressor / refrigerator can handle much larger temperature differences at much higher efficiency.

-------

So the key for Peltier is to find low temperature differences (maybe keep it to 10C to minimize the efficiency loss), at low-wattages (so that the inefficiency won't generate too much heat). Maybe 10W to provide -5W of cooling (cold side) and 15W of heating (hot side) which won't raise the temperature of the room too much, and still provide some degree of comfort to the people wearing the jacket.

But its clear to me that "air conditioning" is one of those "bigger is better" deals. You'd rather more centralize the hot-side vs cold-side (ex: make sure the hot side is outside, rather than having many hot-sides inside fighting each other, raising the temperature of the room you're working in)

There are also cooling vests packed with ice or other substances e.g. PureTemp from Glacier Tek [1], although I wonder how good are these compared to the traditional ice.

[1]: https://glaciertek.com/

Funny seeing all of the comments span multiple years asking to buy or distribute this product.

Is that shirt effective when it’s 40 degrees outside? I have noticed that the fan stops working for me after certain temperature.

I can't remember the exact situation, but it's better to keep a gigantic house at temperature than to let it get far out of temperature only to be recooled.

With something like this you'd need near constant cooling. Although still you are only cooling a small volume, so it could be better.

That sounds like a convenient thing to believe, similar to family telling me to just leave the heater on all day on a base level heat to supposedly save energy because it doesn't have to do a lot of heating when I come home from work. But, obviously, a lower temperature differential between inside and outside is going to lose less heat/cold, so unless the heater/AC is more efficient at lower power levels (they're not) it cannot possibly make sense to leave the heater/AC on when you're not making use of it to save energy. Sure it'll have to work harder when you get home, but that's better than it trying to maintain that temperature all the time. I guess people want to believe this though because you'll have a higher thermal comfort if you don't come home to a hot/cold house.

I tried to find the source for this claim too in the past. The nearest I can find is that it implies the HVAC system is sized correctly: if the system is optimally sized, it should need to run most of the time to stay at the desired temperature. If it changes the temperature too fast, then it may end up with wild temperature swings that overshoot the desired comfort level and waste energy that way. In certain situations, with good insulation and building design, this waste can apparently exceed the savings from a few percent lower degrees for part of the day. If I understand correctly, this is a bigger problem for heat pumps that take a long time to spool up/down so simple bang-bang hysteresis control has poor performance in this scenario.

Edit: as another poster mentioned, for heat pumps (like AC, but frequently not furnaces), it is also much more efficient to run them when it is cool outside (the morning), and then maintain that temperature through the day as a cold reservoir, than to try to expel the heat when it is hot outside. Which is probably a better explanation for the effect.

I find that a bit hard to believe.

The rate of heat flow across an insulator is proportional to the temperature difference across the insulator, if I recall correctly.

Suppose you want the inside of you house to be at temperature Tin when occupied and the temperature outside is Tout.

The rate of heat flow across the boundary between inside and outside will be k|Tin-Tout|.

Imagine two identical houses.

In house #1 we keep it at a constant Tin. In house #2 we keep it at Tin while occupied but when empty we let it move toward Tout, turning on heating or cooling when it is going to be occupied again to get it back to Tin when the people return.

For each house, if we plot |Tin-Tout| over time, the total energy used over a given interval will be proportional to the area under that |Tin-Tout| curve.

For house #1, that curve is simply a horizontal straight line.

For house #2, that curve matches the curve of #1 while the house is occupied, but during the unoccupied times falls toward zero until it is time to get it back to Tin in anticipation of people returning home. It then rises back up to Tin.

The area under house #2's curve will be less than the area under house #1's curve.

Thus, in terms of energy required to produce the desired temperature curve, house #2 uses less than house #1.

There are inefficiencies in producing and applying that energy, though, and so getting a given amount of energy applied to heating or cooling the house from your heating or cooling system will take more energy, and that extra energy might depend on Tin or Tout, so it is possible that this might be a big enough effect to counter the savings from letting the unoccupied temperature move toward Tout.

Isn't it harder to heat 32 degree air 1 degree than heat 70 degree air?

This was my 400 level thermo 2 class, we calculated everything, inefficiency, convection, conduction.

> Isn't it harder to heat 32 degree air 1 degree than heat 70 degree air?

From a pure heat capacity viewpoint, heating cooler air is very slightly easier (on the order of a few thousandths at most for realistic temperatures) [0]. Don't think it'll be noticeable for the average heating system, though.

> we calculated everything, inefficiency, convection, conduction.

I'd be curious to how those calculations affect things. If the heating element is the same temperature, I'd think heating cooler air would be faster because heat conduction is faster across a larger temperature differential and you'd get "faster" convective mixing for air with bigger temperature differences. Have to admit I'm not sure what inefficiencies might come into play, and I haven't taken classes at that level so I wouldn't be surprised if I missed something.

[0]: https://www.engineeringtoolbox.com/air-specific-heat-capacit...

I could not find the problem I was referencing. I'm either wrong, or the problem wasn't in the book and was in the professors notes. I'm leaning on the later, but unless I reply with something else, assume I'm wrong.

Sorry to hear that you couldn't find the problem. Was hoping that I'd get to learn something new

Much of the energy drain when heating/cooling a home is the fan to blow the air. These days, keeping the fan running is quite efficient. Turning it off, then putting in a large draw to get it moving again is expensive.

So at least part of the efficiency comes from not restarting the fan repeatedly.

My furnace has an 80,000 BTU burner (equivalent to 23,000 watts), and a 1/2 horsepower blower, which would use around 600W.

So the blower uses less than 3% of the energy used by the burner. Some of the waste heat from the blower motor probably helps heat the home, so in reality it's probably less than that.

While startup current is significant (for a very short time), starting the fan up in the morning and again in the evening after work doesn't add that much to the total power drain, or even cycling it on/off a few times an hour to maintain temperature. Starting wattage on that motor is around 1800W, but only lasts for a few seconds.

This is wrong on several levels. The energy expended by a fan is proportional to the mass flow. The energy required to accelerate the rotor is negligible compared to the ongoing energy required to keep it turning and doing work. Also, I'm pretty sure that the energy use of the fan is itself negligible compared to the heat pump.

Hm - everything people are saying makes sense. I was repeating what I've been told by literally every HVAC guy I've had work on multiple houses over the last 10+ years. If the professionals are all wrong, I suppose it could just be one of those tropes that persists in an industry?

If the fan is off for at least a minute, the energy to start it should be much smaller than the energy to keep it going. If that's not true, then fix the fan design.

What fan are you talking about ?

This seems like it would heavily depend on how well insulated the house is, and for how long it will be unoccupied.

Based on my experience with most houses’ insulation, especially bigger ones, I cannot imagine it being economical to cool it for 8+ hours when no one is there (for work and school), rather than letting it warm and then starting the AC 30min before people start coming back home.

The short answer is that you should keep the house relatively cool, but maybe above your comfort level while you're not there, and well insulated. The details of when it's best to run the AC for maximum efficiency pales in comparison.

The AC will be more efficient in the morning, for one, than during the heat of the day. And if you let the entire house get truly hot inside, it will take a lot longer than 30 minutes to bring the temperature down (it's not just air that has to get cool, it's all the solid objects as well). And when it's fairly hot outside, it may not actually be possible to get it cooled down if you wait until the heat of the day. E.g. my AC will keep my house at 70F when the temperature outside is over 110F, but if I let it drift up to 80F inside and then try to bring it down to 70F in the mid-afternoon, it won't get any lower than maybe 75F until the ambient temperature outside drops into the 90s.

The same effect works in reverse though, maintaining temperature through the heat of the day is wasteful if the temperature drops before you get home. If you’re dealing with significant solar gain then running a fan before turning on the AC becomes even more efficient.

Only if it's already cooler outside than inside the house. On a day where it's only 90 out during the day, it's not dropping to 75 until 8-9pm at the earliest.

Also can't use a fan on the west coast in the later parts of the summer when the air is full of wildfire smoke.

An example of significant solar gain would be a car sitting outside in sunlight. A surprising amount of modern architecture would work just fine as a greenhouse. Assuming a 9 hour workday it’s going to get well above ambient before you get home.

I could not find the example problem I was referencing in my textbook, consider me either incorrect or ambiguous until I do. I'm going to look through my notes next. I believe it was a problem from my professor.

I remember this problem being absurd, like bigger than a mansion.

Gosh I wish I could find it. I've referenced this problem a few times, I need to find it. I might have the book, if I can find it I'll update this post.

These jackets are powered for hours from a rechargeable battery about as big as a fist. I don't know of any house-sized AC unit that could run from such a battery for more than a few seconds. So that doesn't make intuitive sense.

they have something like this but full body - for those people who wear the full body mascot outfits eg games/ Disney etc..