How do they affect the air quality indoors? Don't modern (less than fifty years old) gas boilers have balanced flues so that they take in air from the outside and exhaust the combustion products outside too?

Surely properly installed gas boilers always did that even without balanced flues too.

My experience (sample of one) is that they don't. They're supposed to, but VOCs are definitely higher all winter running heat than all summer running air conditioning. Natural gas combustion is at best _mostly_ exhausted.

It could also be that outdoor AQ is worse around you in the winter because everyone is running their boilers, and then that outdoor air ends up inside. I can't test it because I have a heat pump, but I would be curious to know what happens to your VOCs if you turn off your boiler at a time when your neighbors are still running theirs.

My meter here in boulder, CO reads effectively 0 VOC outdoors all year round.

> My experience (sample of one) is that they don't.

There is something very very wrong with your equipment (or you're measuring something else not having anything to do with the heating equipment - more likely). A high efficiency furnace has a sealed combustion chamber, the entire thing runs in a circuit vented to the outside - combustion air comes from the outside and it is vented to the outside. If it is not airtight sealed from the indoors, it is broken. Even a mid-efficiency furnace with a non-sealed system will vent 100% of the flue gas outside.

> Natural gas combustion is at best _mostly_ exhausted.

Bullshit for any modern equipment (ie installed in the last 50 years).

At least where I live in the US if you want a ventless natural gas heater you can just go buy one. It's not something I'd recommend.

Mid-efficiency furnaces are still commonplace in the US, and they don't have sealed combustion systems - they get combustion air from the indoor space, but they vent 100% of combustion products out. High-efficiency furnaces in the US made in the past 30 years or so use sealed combustion with intake and outtake to the outside. (North American high efficiency furnaces older than about 30 years were not necessarily sealed).

But yes, the GP is full of shit. Maybe they have some other non-induced vented gas burning appliance (ie a hot water heater or gas hob), or their measurement equipment is faulty.

> But yes, the GP is full of shit.

Or you live insulated from shitty alliances and low income life.

A friend's husband was killed by a faulty boiler, carbon monoxide poisoning. In 2015 I lived in a house where boiler ignition didn't work and you had to reach inside with a lighter. That unit was definately not sealed.

UK only required consenser boilers since 2005, the ones before that used to air from inside your house.

Condenser boilers and balanced flue are separate concepts. The house I bought in 1978 had balanced flue gas heaters but they were not condensing. Condensing boilers are an efficiency measure. Balanced flue boilers have been available in the UK since the mid sixties.

All I can do is share personal experience of living in rental properties in UK, and if these boilers were really sealed, my acquaintance wouldn't be dead.

Whether most boilers are prehistoric, or it's just years of neglect, I don't know.