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.
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.
> 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).
> your eyes just slide over them and you don’t notice.
Nah.
There have been minisplits in the US for years, they are just not very common. Moreover they have been commonplace in Asia for decades now. They are noticeable. I personally think they look like ass. They are huge and you are blind if you don't notice them. There is a reason why ducted or recessed models are available internationally for luxury homes. It has nothing to do with Americans.
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.