Simple question – Is it ok to get make up air for a FHA gas furnace located in a closet from a vented attic space? I’m in the great white north (Duluth,MN) and the house is getting tighter as I seal things up. Also what’s the best way to keep the cold air from spilling into the house?
Ted
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I don't know the answer to the source question, but if you are really trying to save money and energy, there are air-to-air heat exchangers that pull some of the heat out of something (the exhaust maybe?) and move the heat to the incoming fresh air.
Hopefully, someone far more knowledgeable than me will explain better.
Here are some ways to get combustion air.
http://www.codecheck.com/pg21_22mechanical.html#combustionair
http://www.buildingscience.com has design advice in their technical section on bringing makeup air into the return air plenum.
This is the way our house (Rochester, MN) was built 30 years ago. A duct the same size as the flu, coming down from the attic and ending about a foot off the floor, next to the furnace.
(Will be replacing that setup with a new sealed-combustion furnace this month.)
happy?
Does cold air pour out of the supply pipe? Warm air go up?
If the BI will let you get away with it, you can make a heat trap. Run the makeup air duct down into an empty 55 gallon drum or equivalent about 2/3 of the way to the bottom (that's how I've seen it done and it seems to work).
"A job well done is its own reward. Now would you prefer to make the final payment by cash, check or Master Card?"
You not only need the "heat trap" to Trap" air, but to prevent the air inlet from accidently becoming the flue - pretty rare, but it has happened.
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Sojourners: Christians for Justice and Peace
Good to know. So you mean the makeup air duct will "chimney" casuing negative pressure in the actual flue? Makes sense if I'm getting this right.
"A job well done is its own reward. Now would you prefer to make the final payment by cash, check or Master Card?"
>>Good to know. So you mean the makeup air duct will "chimney" casuing negative pressure in the actual flue? Makes sense if I'm getting this right.Yeah, the guru on this stuff, Jim Davis of National Comfort institute tells a story oif a commercial application which was doing exactly that: the furnace (boiler?) flue was acting as the combustion air supply and the combustion air supply was acting as the flue, and the combustion gases were actually being pulled out of the front of the combustion chamber.A maximum case of backdrafting, if you will!He says it was the warmest floor he's ever walked across!I believ it is extremely rare, though
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Sojourners: Christians for Justice and Peace
Not as rare as you would think, especially if the combustion air is coming from the attic and the attic has an exhaust fan. Best to bring combustion air in direct from outside.
this was my question in the first place - The web sites refered to in the begining of the thread show attic air. The furnace I have has a power draft blower thing. Shouldn't be a reverse problem. I just want to use cold air instead of the precious heated air for the combustion without replacing my furnace.
It is a gas furnace, you don't have a power draft, you have an induced draft. Difference is first is positive pressure in the chimney, second is negative. If you have an attic fan, it may cause problems. Bring the combustion air in from outside.
Edited 3/7/2006 11:37 pm ET by rich1
>>"Difference is first is positive pressure in the chimney, second is negative."
Never thought about it that way but the chimney comes down to "does it suck or blow?" When referring to induced versus forced, of course . . . Induced versus forced venting that is -- what were you all thinking.
"A job well done is its own reward. Now would you prefer to make the final payment by cash, check or Master Card?"
:)
sorry, I meant induced draft. I will get it from outside the attic, that makes sense. Thanks for the advice. Lots of work to do on this old house.
Not enough to notice, really. When the furnace is running you can feel a draft from there, but not when things are off.Keep in mind that your furnace flue (in a "conventional" setup) is a hole several inches in diameter all the way through the roof, and the draft/losses through that aren't really enormous. (Though certainly closing that hole with a modern furnace is worthwhile.)
If ignorance is bliss why aren't more people
happy?
My house has the FHA furnace and Gas DHW heater built in a closet space in the center of my home. They used an opening in the ceiling of the utility closet for the combustion air for both. It seems like the simplest way to do this.
If I was building a new place I would do it different.....
Just my 2 cents ....