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Comment by bluGill | Hacker News Reader

All modern furnaces I've seen take #2 - use air from outside. Despite that they can get to 99% efficient. It doesn't take much energy to heat up that cold air.

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How could it not take much energy to heat up cold air? That plus blowing air is the entirety of what goes on in a forced air ventilation system.

The efficiency rating of a gas furnace assumes the incoming air temperature is close to the desired temperature of the house- that's why it is negligible in the artificial efficiency ratings. If the incoming air is below freezing the efficiency must be different. I wish I could find a study that properly quantified this.

My experience in the US at least is that its not uncommon for the furnace air intake to draw air from inside the house (my last two homes in PNW as one example).

California resident here. Both of my last two places with gas furnaces combust unconditioned air.

The furnace my parents (who live in Ohio) installed 10+ years ago uses outside air for combustion, not conditioned inside air. As it's older, it's not got a 99 AFUE, but it's high (I think in the low 90s).

That used to be very common in the US, and there are a lot of old systems still working. However every new furnace I've seen is installed to use outside air. Using outside air needs $100 more in parts and labor and it prevents air balance issues in modern well sealed houses.

You could get high efficiency with a furnace that uses inside air, but they’re basically no longer installed.

20 years ago quite common.

The cheap homebuilders around here (midwest) are still putting standard 80% efficiency gas furnaces which draw interior air in brand new homes.

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