I'm a big fan of heat-pumps, and happily heat a big old farmhouse with a ground source pump through heavy long deep cold winters in scandinavia.

However, at the small cottage I have for the first time just had the external units of two air-source pumps freeze solid due to snow blowing in in the current -16c spell. The defrost ran down inside the casing and formed a big block of ice in the bottom that has now reached up to the fan blades so the pump can't turn at all.

So I'm all out of heat pumps in the cottage just when I most need them :(

This is not enough to make me give up on heat pumps, but it makes me wary to rely solely on them.

Am planning a big new build, and will of course go air source to underfloor heating, but am seriously considering a backup combo wood burner for the panic week that comes each year.

When I got an air-water heat pump for my house, the installer also put in a small electric heater that is connected to the same loop going through my radiators. It is a backup in case my heat pump broke and needed to be serviced. I wouldn't rely solely on a heat pump either. The heater takes 3x the amount of energy to produce the same amount of heat as the heat pump, but since I'll almost never use it (didn't need to turn it on yet), that is negligible and the peace of mind knowing the backup is there is well worth it.

Most of the bigger setups I saw here in Sweden has this backup built in, not only as a backup of emergencies but mostly for cold days where the source heat (air, ground) is not enough

(parent poster) the cottage has two air-air pumps, which replaced most of the previous electric radiators. Luckily there were some electric radiators left, so they are now keeping the place just above freezing...

Similar setup, air-water system with a 700 litre heat storage tank, which has a heater rod as a back-up.

For this cold spell it's a little too late, but the solution for this to add some heat-coil/heat-wire on the bottom of the heat-pump housing that you can activate in those conditions.

The issue is that the defrost mode will defrost all the ice from the fins, but it freezes again before it can all drain through the small drain hole on the bottom of the housing. It's somewhat a known issue with air source heat pumps, sadly. I've not run into this issue myself, but on a Dutch speaking forum (tweakers) there is a long thread about this modification to "survive" the cold spells with this small modification.

You can manually attack the ice with a heating gun or hair dryer to remove it, but that's a faffy.

I think manufacturers should, and probably will, add something like this themselves in the future.

Yeap this condense re-freezing is what's got me. And I think it's also blown in snow, we've had bad drifts too.

Anyone on forums advocating for drilling bigger or more drainage holes?

Yes, I've experienced a few poeple drilling the additional drainage holes on the bottom of the outdoor unit, when they experienced similar problems not having a "nordic" unit. With the nordic unit I mean the features mentioned above - heated compressor and the heating condenser vane.

Though, if it's snow blowing directly inside then I think creating some barrier or add additional shielding of the outdoor unit is required,so that you minimize the chance of the snow DDoS-ing the unit (note: check your unit's service manual for the minimum free distances from all sides of the unit, especially the front one that is the most important to be kept enough free space).

... and your heat pump is on the ground, easily accessible.

What if you live on the Xth floor? Your gas burner is small enough to be all inside and easily serviceable when it breaks in the middle of winter (why would it even consider breaking in summer?). Where do you put the much larger heat pump and how do you reach it?

I'm building a new house and I'm based in Poland. I'm going with air heat-pump* + a simplest fireplace as a backup - not connected to central heating, no electronics. On top of that 10-20kW of solar and a place for batteries. I'll install them once they get cheaper. The house is 200m^2, so I'm hoping to pull only 200 kW/month from the grid and generate rest on my own.

Also, I've been wondering how much efficiency would I get if I could put some mirror and direct the light to the unit in the winter. It could help with defrosting as well.

*I was considering horizontal heat pump, but it's not worth it. It affects vegetation and future construction.

Battery won't help you in winter. Just a waste of money ( At least in Baltics). It will help at nights by using electricity you made at day, but is useless in winter. My solar panels at november generated 0.00kW/h, because it was under 40cm of snow. Got some double digits at December... anyways, where it snows, don't count on your panels. Even if it didn't, the energy it produces in the period when the daytime is scarce is minuscule compared to summer. Nov-Feb inclusive (~120 days) gets me about as much as I could get in 5 ideal summer days. Or a third of what I require to cover a single month electricity usage.

Of course it all depends on options government offers and in what place you live - however Poland I suspect also gets snowy. Just sell excess solar electricity and re-buy in winter. Just trying to save you some money on batteries.

Would electric heaters to clear the snow give you net positive energy?

We have an air source heat pump with a gas fired backup ("emergency" heating, it's sometimes called in the US). Don't need to use it often but it's nice to have and only added around 10-15% to the total cost.

Here in the Netherlands there's a huge incentive to move to heatpumps (subsidies and whatnot). I believe most of those (if not all) that are being installed have a defrosting mode which is automatically activated. I have seen mine defrosting in the last few days since it's finally below zero (celsius).

Yeah, it's really strange to have an heat pump without automatic defrost, as it's something that's really needed on cold and humid days

How much does it cost in the Netherlands (with incentives such as government subsidies) to install (hardware and installation) ?

Other Dutch person here... with a heat pump.

And the only answer is that it completely depends.

We spent €30k renovating our house but that included removing the entire old heating system, installing underfloor heating, extra insulation, ventilation, remove of gas pipes, new electrical switchboard, upgrade to 400V electricity and, well, the heat pump.

Just the device itself (air-water) is probably €6 or €7k excluding a €3.3k subsidy.

You can have an extra heater device installed into your external unit. I'm just about to have 3 units installed and they will fit this extra module.

Update: just had a call and it's a hack that voids warranty. There are units with built in heaters. Picking an AC is a nightmare with OCD :/

Could that have been solved by placing a tarp over the unit beforehand?

Do you have it undercover / out of the elements some how? Mine is under a concrete stair case and I've not had this problem, even when it's down to ~ 8c and very snowy.