Open combustion furnace in an attic that has been sealed at the end walls and roof.

I have a client that had the attic of their home insulated a couple years back with spray foam. The attic contains an open combustion furnace.

First off, the spray foam was never protected with an ignition barrier.

Secondly, the furnace has been starving for combustion air, and as a result they have witnessed incomplete combustion (represented as a higher than recommended CO level in the spent gasses).

They want to have an air source added to the attic so that the furnace is not starved for combustion air. They are thinking about having a vent added in the roof.

Do you folks know of any products and\or practices for this situation. I have a general sense of what to do, but I need the input of the pros on this one.

Thanks in advance.


Views: 3589

Reply to This

Replies to This Discussion

Hi Patric,

My first concern would be code compliance.  If an ignition barrier is required, it must be "in-place" before I would get involved.  As for combustion air, some units have kits for providing outside air and they should probably be installed by their HVAC service person.  As for a vent, this is a case where the sealed attic should also have included a visit from the HVAC contractor to specify how and how much.  Depending upon your state, this may not be a job for the energy auditor.  My state, I cannot touch.



I share your first concern. I mentioned this to the HOs yesterday.

I wanted to get some info before consulting an HVAC partner of mine. The HOs are looking for estimated costs of installing an ignition barrier and providing outside combustion air to the furnace.

The spray foam company they hired really screwed up on this one.

In VA we are operating under the 2009 Residential Building Code (and 2009 Energy Conservation Code). 

Thanks Bud.

I have run into the foam guys telling the HO that "they usually don't care" if the ignition barrier is there so convince the HO to go without it.  Or they wink and say, "you are going to install that yourself, right".

Anything to get the job, get paid, and get out of there.


Ain't that the darn truth!

Providing combustion air for a furnace can be tricky. The NFPA 54 calls for a minimum 50cuft/1000btu/hr for combustion air. However, how the combustion air is brought into the combustion appliance zone determines what size venting and how it should be placed. I would probably say that talking with a qualified professional would be the best bet. Different types of furnaces may need different solutions. Good luck!

Thank you Doug, that is very helpful!

They want to have an air source added to the attic so that the furnace is not starved for combustion air. They are thinking about having a vent added in the roof.

So, are these people really inclined to go to extraordinary measures, cutting holes in what sounds like an excellent insulation/air sealing project, all to keep a crappy non-condensing low efficiency furnace rather then simply update equipment (heat pump or sealed combustion)?  

Or hasn't the discussion occurred yet?  

If it has, and that's the way they lean, I think demonizing the spray job due to intumenscent barrier is deflecting.  BTW, some foams meet fire code without a barrier (I believe Demilec has a number of them). 



But you do agree that foam in an attic with an open combustion furnace (without an ignition barrier) is a problem, right?

The case is this: The furnace, while crappy, is less than 10 years old. Do you really want to junk a less-than-10-year-old furnace and incur a few thousand bucks for a new, high-efficiency unit?

On top of that, we are replacing their old furnace in the basement with a 95 AFUE bad-ass, and re-doing the ductwork. I hope you can understand why they do not want to spend another few thousand, and have to say good bye to a relatively new furnace in the attic.

"Problem" is pretty vague.  Sounds like there are lots of problems with this home.  I'm not sure unprotected foam in unoccupied space meets the cut given there may be some serious budget constraints preventing more important work from being considered.  I think addressing the hazardous combustion appliance comes at the very top of the list. 

Doing really stupid things to save a 70% depreciated, crappy inefficient furnace seems pretty myopic (anyone think providing uncontrolled ventilation at the top of the stack for combustion air is smart?).  That's spending money to have a higher energy bill.  Sorry, that simply doesn't make sense to me.  (But I have a total cost bias, and many homeowners are heavily upfront biased and need a fair amount of educating to even begin to understand what I'm talking about.  You know the audience here, it may be time poorly spent for you if your homeowners don't have the capacity.)

That furnace was likely grossly oversized BEFORE the foam, what does that make it now?  I wonder if it's cycling on limit - which means the Heat Exchanger could be on the verge or failure.  Savings from replacement could be 40-60%.  

we are replacing their old furnace in the basement with a 95 AFUE bad-ass, and re-doing the ductwork. 

WAIT!  2 systems?!  This is the exactly why two systems is a colossally bad design.  The enclosure is forever burdened with cost of maintaining and replacing two mechanical systems.   I don't know the house, but if you are doing furnace work AND duct work, replacing 2 furnaces with one would be top on my design list.  

Taking the longer view will move them away from reactive design towards proactive design, and should make solutions more rewarding for everyone. 

If they take the step of ventilating the attic they can say good-bye to a high efficiency home envelope!  They may also be looking at issues like condensation in the attic as a result of this change.  I doubt that the manufacturer will have a "kit" to retrofit an 80% naturally aspirating furnace to be a sealed combustion unit as someone mentioned.  You can't introduce ventilation into a sealed attic without compromising the building envelope.  You very likley can't alter the existing furnace to allow it operate safely in that air sealed environment. 

So, realistically, you only have one option.  Buy a sealed combustion condensing furnace and sue the foam insulation guy for the cost.  He violated the code, the manufacturer specifications and could have killed someone.  Let's not get so wrapped up in the energy issues that we forget that doing things like starving a furnace for combustion air has killed people.  Protecting human life must be our first priority.  He needs to know that what he did was potentially fatal to that family. 

Thanks Doug.

Unless any others have any objections, I will be going to the HOs and telling them they will just have to use this as a (costly) opportunity to go high-efficiency.

Now, the question is whether we try to make a single heat pump (with zoning) work? Is this risky (i.e. potential for compromised comfort on a significant scale)? I remember my old boss making such a situation work with new construction, but in an existing house I wonder if zoning is going to involve a whole lot of tearing down gyp board, etc.

Do a Manual J with all the improvements being made. Then do the rest of the alphabet S T & D and you will know exactly what is needed, where, etc... I bet if they do it properly you will find out one unit might be enough

As an FYI, they make retrofit zoning pieces that simply slide into the boots or they can be let into the ducting work


Latest Activity

Don Fugler replied to Don Fugler's discussion joining ducts in the group HVAC
"David, If the IMC mandates metal dryer duct and prohibits metal screws, they must be accepting…"
54 minutes ago
David Butler replied to Don Fugler's discussion joining ducts in the group HVAC
"I never checked that before. Checking now... Section M1502.4.2 of the International Residential…"
1 hour ago
Joseph Frasier is now a member of Home Energy Pros Forum
1 hour ago
Don Fugler replied to Don Fugler's discussion joining ducts in the group HVAC
"Brendan, Thanks for that response. Because of your warning, I just went down to the basement and…"
5 hours ago
Brendan Reid replied to Don Fugler's discussion joining ducts in the group HVAC
"Hi Don, Summary: DRYER VENT: screws bad idea, possibly prohibited by code. METAL DUCT: must be…"
5 hours ago
Profile IconBrendan Reid and David Butler joined Allison A. Bailes III's group


HVAC design, Manuals J, S, T, & D, Duct leakage, Air flow, ENERGY STAR new home requirements,…See More
5 hours ago
Profile IconSean Wiens and David Butler joined Chris Laumer-Giddens's group

Energy Efficient Design

Energy efficiency starts at the drafting table...How architects and designers can help buildings…See More
13 hours ago
Profile IconSean Wiens and David Butler joined Mark Richardson's group

Renewable Energy

This group will focus on the issues associated with incorporating renewable energy sources like…See More
13 hours ago


  • Add Photos
  • View All

© 2017   Created by Home Performance Coalition (HPC)   Powered by

Badges  |  Report an Issue  |  Terms of Service