A contractor that I hired messed up quite a bit and I am looking for the easiest way to resolve my situation before winter comes (Is it safe to use Category 1 80,000BTU furnace with 2 in PVC exhaust?)

So far, it seems like the path of least resistance is to get a high efficiency unit and re-install 2inch PVC pipe for exhaust. All high efficiency furnaces require an outside air intake, but google is finding information that suggests otherwise.

HVAC system is sitting in about 24x28.25x7 garage that has minimal airflow. Old system was 75,000 BTU with 92% efficiency, and was keeping up OK for the most part. An upgrade to heating power would be welcome. (100,000BTU if possible).

Can someone explain how not having an outside air intake will affect safety/longevity/performance of a high efficiency system?

  • 3
    What does the manufacturer of this unit say about outside air intake and exhaust? You must install it according to their specifications or you risk damaging the unit or causing a dangerous situations for occupants in the home.
    – jwh20
    Sep 9, 2021 at 9:34
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    What you had : (75kBTU x 0.92) = 69kBTU effective heating power. What you "upgraded" to : (80kBTU x 0.8) = 64kBTU. So your crappy new inefficient furnace is even weaker than what you had before. Yes, if it's not too late, definitely go for higher efficiency. If that much heating power can't keep your house warm it's either a mansion or it's insulated like a cardboard box. I'm in Canada with freezing winters and a big house and 80kBTU-95% has plenty of headroom. Fix your insulation!
    – J...
    Sep 9, 2021 at 17:08
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    Heat pumps: "If I go with electrical heater in my area, I would need to spend about $300-$500 more per season. I am researching pros/cons of electrical heaters right now to see if there are any benefits besides safety". Research MUCH harder. That's not how heat pumps work. I know what you mean by electrical heating, that's not what heat pumps are. Technology briefer 1: what is a heat pump. Briefer 2: an even bigger win. Same guy: gas furnaces. Sep 9, 2021 at 18:30
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    Update: Not sure if one of the agencies that I reported him to reached out to him, or if one of his 'colleagues' was an actual certified HVAC technician, but contractor completely changed his tune and went from "I already did all the work. Contract is done. I cannot return unit. If you want new one you have to pay me for new unit and full re-installation costs" to "Just pay me the difference for high efficiency furnace and one that you have, I will replace it for free.". Thanks everyone for giving me courage and information I needed. Y'all might have just saved my life. Will post updates.
    – Dimi
    Sep 9, 2021 at 19:52
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    @J... Reached out to a couple of business in the area and got a fully licensed, experienced HVAC technician to come out. Saying that he was 'extremely concerned' for my safety and safety of contractor's previous clients when he heard about PVC exhaust pipe is an understatement. He explain that 'contractor' probably was confused because old furnace that he was replacing was janitrol gun075-3, a category IV furnace WITHOUT an air intake. He probably believed that it was 80% system, and it was completely fine to just replace it without doing anything else. House is ~3000 sqft with 7-12ft ceilings
    – Dimi
    Sep 9, 2021 at 20:25

3 Answers 3


Problem #1: It can kill you

If you have a modern, competently built house, you have a very tight house that does not like to leak air from outside. That is for heating/cooling efficiency. Air is a thing, it's not just magic. If you push air out of a well-sealed house, it will draw a vacuum on the house, just like charging an air compressor tank, but in reverse. The furnace is not designed to do that, so weird things will happen, like carbon monoxide staying in the home and killing you.

Problem #2: It's inefficient.

It is the same problem as portable A/C's, really. Both machines have 2 air streams: a) the house air that is being treated (warmed/cooled)..... and b) the process air the machine need to run on. (A/C makes this air hot; furnace burns it). So we have 4 air ins and outs:

  • House air input (raw air from the house, already near correct temp, so reuse it)
  • house air output (treated air into the house)
  • Process air input (???????)
  • Process air exhaust (CANNOT discharge inside the house!!!)

But out of general laziness and cheapness, portable A/C's and 80% gas furnaces only have exhaust pipes for process air output. They steal their process air *input" from inside the house.

This tries to "draw a vacuum" on the house, e.g. Problem #1. But even if your house is old and leaky and the stolen "process air" can easily be replaced by air leakage from outside... there's still another problem. The outside air is exactly what you DON'T want. When you're heating, you don't want ice cold, dry outside air that will have to be heated AND humidified - and this makes drafts! When you're air conditioning, you don't want to draw in hot muggy outside air.

Because those things defeat the purpose. Making the system work even harder at worse efficiency still.

This is why these 1-pipe appliances are stupid.

"I don't like electric heat"

Who can blame you? Electric resistive heat (i.e. just having a bunch of toaster coils to make heat) is the most inefficient thing on earth. It's laughable to even discuss it, unless you're in a place with such a glut of winter power that they give you favorable electric rates (North Carolina, Ontario).

But you say you don't like electric heat? Let's talk about humidification. Humidity is made of the same stuff as heat. Let's suppose the air is dry and you want to add 1 pound of water to the air to make it more humid. Thanks to the latent heat of vaporization, turning 1 pound of tap water from liquid to vapor takes 1200 BTU of heat energy. It's heat. It's the same stuff.

When you're heating your house, replacing this humidity takes more heat. When you're air conditioning, removing this humidity by condensing it takes more cooling.

So you say "no problem, I'll correct the humidity shortage with this humidifier". An electric humidifier, right? Well, that humidifier is also spending 1200 BTU per pound of water, and you're doing that with inefficient electricity. Which is the thing not to do.

So sucking process air from outside is much more costly than you would initially think.

Get the second pipe.

PSA: Heat pumps are vastly cooler than you can imagine.

I saw comments that were like "Heat pump" and "electric heat no way". It's not like that.

Heat pumps are actual, literal magic. They do the thermodynamically impossible: create more heat than they use. Way more. (Well, they're not creating it, they are stealing it thanks to our old frenemy Latent Heat of Vaporization.)

Anyway, here are some technology briefers on the subject:

Early heat pumps had a problem working below freezing weather - and that was a problem in Texas during their cold snap; all the electric emergency heat overloaded the grid.

However first, extended range heat pumps are now available that work at much colder temperatures, and second, ground-sourcing moots the issue.

  • While heat pumps are indeed cool, there's a lot of factors you have to consider to see whether installing one makes sense for you: ratio of gas/electricity price, climate, compatibility of the heating system. Say, here in Europe a MJ in gas costs about a third of a MJ in electricity, so your heat pump needs to have a coefficient of performance of at least 3 to be worth it. That's easy to achieve with a low-temperature system (floor heating, I guess forced air as well), fairly hard to achieve with hot-water radiators.
    – TooTea
    Sep 10, 2021 at 9:56
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    Are you able to get a commitment from your gas vendor that gas prices will not change through the depreciation lifetime of the furnace? Where does Europe get its gas again? Sep 10, 2021 at 21:01

Today's heat pumps blow gas out of the water for you, especially given how awful your new furnace is

Given your location, the current rates your utility provider (KUB) charges of $1.0189/therm for all therms of gas over the first 30 (and higher for the first 30 therms) and $0.09186/kWh for residential electricity (or lower for offpeak), 23°F for your location's 99% winter design temperature, a quite generous operating furnace efficiency of 90% for your existing furnace, and the procedure for finding the economic balance point that Dave Butler gives in his last comments on this EnergyVanguard post, we can compute the economic crossover COP for a heat pump in your situation. Going through the computation assuming all regulatory and service charges are fixed, your gas heat cost per-therm comes out to $1.1321, while your electric heat cost per-therm for resistance heat (COP of 1) is $2.6915. Dividing the electric heat cost per-therm by the gas heat cost per-therm gives us our economic crossover COP, which is 2.38 under the rather pessimistic assumptions given for your case.

As it turns out, this is not a terribly hard number for a modern heat pump to beat. In fact, given that you'd need a 5-ton/60kBTU heat pump to match your existing system, current generation mini-splits can nearly match this number at 17°F, with the MXZ-8C60NA2 turning in a COP of 2.2 at that temperature while providing 65kBTU of heat. Furthermore, I seriously doubt your current furnace will ever make 90% running efficiency and the first 30 therms of gas are more expensive than the rate we used for this calculation, so the actual economic crossover COP for your situation is likely to be lower than the number computed here, making the heat pump even more attractive. Atop this, if you weatherize well enough to get the load down to 48-50kBTU or less, you can downsize the heat pump, which opens up many more options, some which will have better performance yet! (Do ask your local housing authority about weatherization financing programs while you're at it -- many places have grants or low-cost loans available for these sorts of envelope upgrades.)


Oxygen is burned during heating process. If air circulating in closed volume, after time oxygen percentage is down. It causes lower efficiency of gas heater.

  • In a well sealed structure , the furnace will not work. It must have fresh air ( oxygen) at atmospheric pressure . The exhaust/flue gas, leaving the furnace must be replaced. Sep 9, 2021 at 14:45
  • Is there an easy way to measure efficiency/performance/airflow of a system? Like a CO sensor for exhaust or oxygen sensor for a room? I think we will try to install a high efficiency system without an outside air intake and only have a single PVC exhaust pipe. I would like to see if there are 'enough holes in walls' for this system, and maybe hire a different contractor to do air intake if it is necessary.
    – Dimi
    Sep 9, 2021 at 17:08
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    @Dimi Furnaces don't need the outside air intake, but it does dramatically improve the air quality in the house and the overall ability of the house to retain heat. Without it you're using room air to combust and are venting it outside. That means that combustion air is replaced by cold outside air which has to be sucked into the house. The intake pipe lets you properly air seal the house, get rid of the cold makeup air duct, and install a proper ventilator like an HRV so that your fresh air can come in with much less heat loss.
    – J...
    Sep 9, 2021 at 17:23
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    @user253751 SAY WHAT? You have to have an outside exhaust. Every fuel burning furnace must have an exhaust or you will kill everyone in the house. That's not even a question! The only discussable matter is whether it has an intake. Sep 9, 2021 at 18:23

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