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High efficiency furnace with combustion make up air question

We have a 2 story approx 2000 sq ft house with an unfinished basement in Michigan that was built in 2005 with a high efficiency furnace. I sealed up most of the duct work that was leaking like a sieve, we has an ERV unit installed to bring in fresh air (definitely had some stale air issues), and have been working to close most other air leaks in the house to make the house more comfortable.

At our furnace we have two 4" outlets for combustion air - one above the furnace and one that is piped down by the ground. Even when the furnace is not running, cold MI air is not just leaking, but almost flowing out through these 4" pipes. I have not actually put a thermostat down there, but it feels much colder than the typical 55 degrees of an unheated basement.

Is there anything that we can do such as a damper, etc that would block off air from flowing in through these pipes when the unit is not running, or is this a bad idea for some reason? This area serves as a play area for two kids under 4 and I am looking at ways to reduce this drafting and make it slightly warmer down there.

Thank you!

Re: High efficiency furnace with combustion make up air question

I have a Bryant HE furnace that was installed about 18 years ago. There are two PVC 2" pipes on our unit, one brings combustion air directly into the furnace, the other takes exhaust air directly out of the house. In no case is either of the pipes open to the air inside the house. Perhaps you should have a furnace inspection to determine if your house can be set up in a similar fashion. I am not an HVAC guy, just a homeowner. I am also from Kalamazoo, so I feel your cold pain. Good luck.

Re: High efficiency furnace with combustion make up air question

Same with my furnace. No combustion or exhaust pipes are open. They go directly to the furnace.

Re: High efficiency furnace with combustion make up air question

I've never understood that whole concept of having super efficient furnaces that carry a thousand dollar premium over the "normal" furnace of only several percent less efficiency. Add to that such expensive high tech electronics that when something fails, it costs several hundred dollars to replace it! Then tightening the house up so tight that you have to add heat exchangers to bring in fresh air, or, just open the window in winter to let in fresh air. That is some efficiency!

A few years ago when I replaced the furnace, I ran the numbers. At the then existing cost of gas, it would have taken 20 years to recoup the purchase price of the high efficency unit over the lesser unit.

Now add that gas prices have been stable or falling , over the last few years, as the gas companies "frac" the hell out of America.

Re: High efficiency furnace with combustion make up air question

I'm not sure any furnaces come without the electronics anymore. I think pilot lights are pretty much a thing of the past.
In my case adding a gas fired high efficiency furnace to replace a heat pump meant not needing to add a flue up through the house to the roof. The furnace is going on 19 years and now the the company is trying to sell me a new one.

Back to the original question. I wonder if the combustion air pipes can be connected directly to the furnace. A question the manufacturer or HVAC company should be able to tell you. If not, I don't know why dampers, gravity or powered, couldn't be added at the end of the pipes if they can't be taken to the furnace. Another thought would be to enclose the furnace so the cold air doesn't get into the play room.

Ps. My mother replaced a 50 year old gas furnace a few years ago. They don't make them like that anymore. The pilot light seemed to keep the dampness out of the basement in the summer even though it was a "waste" of gas.

Re: High efficiency furnace with combustion make up air question

Contact an HVAC contractor to come and inspect your unit. It should not leak outside air into the room like this.

Re: High efficiency furnace with combustion make up air question

I spoke with the contractor that I've had out before through email and received the following message:

"....The other two pipes entering into the “CAZ” ( combustion area zone )(furnace room) are called a “hi/lo” combustion air set-up !

These are installed to assure the CAZ never goes into a “negative pressure”, which could allow the blower motor to backdraft (suck) the furnace or waterheater C/O ( carbon monoxide ) into the CAZ/furnace room/house/ductwork.

If you have a high efficiency furnace already…then only the water heater remains as a threat to be backdrafting the c/o into the caz.

If you finish your basement….of course the separated rooms will be more warmer, and the furnace room will be more confined.

However….having the furnace room more confined can create a negative pressure in the caz…even easier.

A high efficiency furnace vented to the outdoors with pvc pipes and a “power vented water heater” vented to the outdoors with a pvc pipe would allow us then to eliminate the “hi/lo” combustion air.

I personally would not remove the combustion air unless I had a furnace and water heater directly vented to the outdoors."

I guess I will have to have him back out to inspect my system, but again, would appreciate any more advice prior to his visit.


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