View Full Version : Another fresh air question
07-04-2012, 11:17 AM
I have a similar situation to this one (http://hvac-talk.com/vbb/showthread.php?t=1122331). I have two Trane XB80 gas furnaces and one gas hot water heater. Last year I had a Trane fresh air exchange unit installed on the largest unit. Planning to get open cell foam and would like to make attic as tight as possible. Foam installer is concerned about combustion air for furnace and hot water heater.
House interior walls are not foam insulated. Attic is at least 2400 cubic feet. I'm wondering if I have some of the stale house air exhaust from the air exchanger diverted back into the attic instead of outside, it would provide sufficient combustion air for the furnace and hot water heater (exchanger runs both periodically 24 hrs/day and when the unit is running). I would have a net gain in air supply by bringing in fresh outside air and not exhausting all of the inside air.
I liked the idea of a dedicated air intake with a fan that runs when the unit runs, but what about combustion air for the hot water heater?
07-04-2012, 01:25 PM
I did not read the link, but if I understand correctly the air exchange would NOT be considered combustion air. If this is tied into the HVAC system it is specifically for the exchange of stale air for outside air, your combustion air is supposed to be completely separate.
Install manuals can provide insight into this as well as a knowledgeable contractor.
07-04-2012, 01:33 PM
Where are these combustion appliances located?
Regardless, the attic is considered outdoors so pumping air into it would only cause problems. The HRV must be balanced in and out-period. If you try to re-duct some of the stale exhaust back into the conditioned space, you are defeating the purpose of the unit and pressurizing the conditioned space which is not desireable.
The Combustion Appliance Zone (CAZ) should be balanced or neutral With Respect To (WRT) both outdoors and to adjoining indoor spaces. If not, it needs MakeUp Air (MUA). Passive MUA has been proven ineffective. A powered MUA source interlocked to the burner control is the best solution.
07-04-2012, 02:07 PM
Thanks for the replies. The furnaces are in a fairly large attic. The idea would be to bring the attic (and the furnaces) into the house envelope (but not condition the attic). All blown in cellulose insulation on the attic floor would be removed so anything in the attic will in theory be able to draw air from the house.
Foam guys plan to provide for passive combustion air by leaving a couple of soffits open and leaving a couple of the passive vents on the roof open. However, this will create an undesirable continuous flow of 100+ degree Texas air into the attic defeating one of the primary advantages of the foam...keeping the temp slightly above the conditioned space and creating a tighter house.
With regard to the air exchanger, I was thinking about:
* sealing off the soffits so rising hot air doesn't continuously infiltrate the attic
* diverting some portion of the cool stale air into the attic to provide "fresh" combustion air in the attic space but not necessarily directly connected to the burner controle of the furnace (I don't think my Trane 80 unit isn't designed for that?)
* based hearthman's comment above, leaving one of the passive vents on the roof open to avoid pressurizing the house (thinking I wouldn't get very much passive heat coming in from the hole in the roof).
I wouldn't have sealed the house up tight because of the open vent on the roof, but seems like it would be better than leaving soffits open. Don't want to screw anything up and don't want to create any kind of dangerous situation. I appreciate the feedback.
07-04-2012, 02:21 PM
do what the energy company says.
or get a qualified 2nd opinion from an energy rating company. it may not be possible to have combustion appliances in the sealed attic. perhaps replace them with 90+ equipment and have them draw their OWN combustion air piped in directly to the unit.
07-04-2012, 03:07 PM
Does your fresh air exchanger run 24/7? You can provide mechanical combustion air, but I don't know if it can be inside air. You will also need to make sure you are supplying enough combustion air per code.
07-04-2012, 04:07 PM
Sam, fresh air exchanger runs when the unit runs and periodically in between 24/7. The Trane XB80 install guide calls for access to passive outdoor air if there isn't enough indoor air volume for the unit to access (like there would otherwise be in an un-enclosed attic. I would think mechanical air would be as good or better than passive.
In a foamed attic there must not be any open source flame appliances such as an eighty-percent efficient furnace. We tell our customers, who are using gas heating, to only use sealed combustion 90% efficient or better furnaces.
You can make a separate equipment room area in your attic that is not foamed in. However it must have proper combustion air for the appliances that are located in it.
07-04-2012, 04:50 PM
just to be clear, the MUA he talks about is NOT what you already have!!
this is going to be an added item that, as he suggests, is tied into the demand for combustion air when the unit is heating.
07-04-2012, 05:24 PM
Thanks everyone. Great comments and much appreciated.
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