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.


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This situation and the comments that follow are good reminders of how we sometimes get caught up in the cost concerns of a customer who may not fully appreciate the hazards of the situation combined with(possibly?) an effort by Patrick to do the basement to perfection in a sincere effort to give a high quality performance and be able to completely check off the basement as a region completed . We then are not doing what auditors must do first, last, and always-stick with the science and the hard truth of the unqualified benefit of a sealed combustion unit. Let the homeowners take the risks & reap the rewards.

Going with a sealed combustion unit in both the basement & attic, to me is a no-brainer. Sorry, Mrs. McGillicuddy. I sympathize with your budget woes-we all have them, but safety trumps budget. There are always band-aids that can be applied, but they are just that, and besides, you don't need my permission to do something I advise against. 


I doubt it would be much, but it may be enough of a counterpoint to ease the customer into the 2nd unit to say stick with the old ductwork in the basement for now, and apply those costs to the attic furnace. Afterall, indications are that the house is pretty tight-don't know if the basement is in the envelope(?). Considering the attic is, then one could assume it is.

Field controls makes a "fan in a can" that can tie into the furnace, such that the "hole" is only there when the furnace is operating. You could also explore a transfer grille to the conditioned space of the home, sized per code...

Certainly money can be spent on solutions that will end up using more energy, but isn't good advice about coming up with solutions that actually help pay for themselves with energy SAVINGS?

Why, when the homes load has clearly been significantly reduced, is new duct work required if it will not be satisfying any new spaces?  There may be really good reason in this case, but I've found that usually fixing the house reduces or removes the need and benefit of duct replacement. 

How much consideration/exploration was given to "1 system".  An hour, 10 minutes, none?  

I get it, a nice sale has been made before a comprehensive understanding of the situation was fully grasped, and now extraordinary efforts will go into saving the sale.  The homeowner is purchasing a product, not a solution.  They are a potential customer, not a paying client.  

If faced with spending $5k on another hi zoot system, do you think Mrs. McGillicuddy might be more open to the expense of running supply and return to the attic if it were to make the second system unnecessary?  Might she be willing to pay for design efforts instead of expecting this engineering for free? 

This seems another great example of how putting all compensation on the sale causes shortcuts to design because there is no appreciation for the cost of design or commitment to it from the customer.  

We all need to be paid.  This delivery model is so broken.  We need to help the consumer understand the value of paying for design or solutions will be "what can I easily sell so I can get paid for this time".

Very good points Ted. This is worth its own discussion (and maybe more than just one).

Most closed cell foam manufacturers have improved their foam so it does not require an ignition barrioer where the code calls for ignition barriers.  .  see this link 

so, IF that foam meets AC-377 appendix X and the installer can prove it with an ICC ESR the foam is not improperly installed. 

the building code allows the installation of foam with an IGNITION Barrier in 'attic spaces accessible only for the servicing of mechanical devices. ' then things get mushy in the real world.  I have seen situations where the attic only held a furnace, but it was large, had a floor, and was easily acessible, where code enforcement required a THERMAL Barrier because that attic could be used for storage. 

Let the arguing begin. 

Also, if that furnace predates the insulation, it would be worth asking if they have tried not using it.  the foam is a massive change to the building envelope.  It is quite reaonable to think they don't need the second furnace now.  I would first suggest they shut it off for a few months and see what happens.  if they are not satisfied with the results, then put in a rennai or a sealed combustion furnace, or a ductless spilt.  if the house is any reaonable size, the foam was installed to an R-21 or higher, and it is sealed to the second floor ceiling all along the perimeter of the attic,  then it should not require a furnace on the second floor . 


"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"

I agree with Sean on this one. If you are not calculating the heat load of the two zones, you are missing an important part of design considerations.
What kind of square footage per zone are we talking about? What is the R-value and area of the thermal boundaries? How much glazing and in which orientation do you have per zone? What size ducts and what is the required airflow per room? These are all questions to be asked first, and then you can start to consider if you can run a single furnace and maybe even use zoning dampers in the duct system. Of course this will also be contingent upon whether or not you can connect the lower duct system to the upper system. In many situations converting to a single furnace may have cost prohibitions due to the duct re-design, but then again if you can get the system dialed in, the homeowner could potentially see great savings, and a comfort increase.
"You could also explore a transfer grille to the conditioned space of the home"

I think this may be the easiest, and most affordable solution. If the furnace is starved for air, then give it some more with a bypass to the conditioned space below it. Since the pressure boundary is on the roof deck this should not increase building leakage, and it will provide more air volume for combustion. But of course if that furnace has been experiencing carbon buildup, and improper firing, there is a possibility that the unit will not last very long, and they will be looking at replacing it in the near future.

If you solve the venting issue, and satisfy the homeowner, then they will probably call you back when the furnace dies anyway, but of course most of us out there want to do it right the first time.

Very good points Tom. And like the rest of you, I would like to do it right the first time.

I wonder if trying this and that is similar to band-aids. My sense, judging on all your responses, is that I should push hard for a sealed combustion furnace in the attic.

A bypass is something I considered. I also considered the "vent in a can" approach. I considered tying in the 1st story ductwork to the second. It all comes down to the fact that the HOs like having two separate systems, do not want a bunch of disturbance to the house, and are going to spend the money if it truly makes sense. 

I think I am left with one option: Two new sealed combustion furnaces. Do not mess with ductwork (yet). 

Assuming the HOs are on-board, do you folks have any suggestions for small, high-efficiency furnaces? My area is boilers (up to this point). 

Thank you all for all the advice and help thus far!

 It all comes down to the fact that the HOs like having two separate systems, do not want a bunch of disturbance to the house, and are going to spend the money if it truly makes sense. 

I think I am left with one option: Two new sealed combustion furnaces. Do not mess with ductwork (yet). 


These people are at a major crossroad here.  You are both consultant and contractor, which is huge conflict of interest.  How can you minimize that conflict and the risk to your reputation that goes with it?  Encourage and own what you feel is the best long term solution.  

Recommending one "from the hip" option that follows what is their really outdated/misguided preference for two systems seems unnecessary and dangerous.  Time to take off the salesman's hat.  There will be future unanticipated consequences to this decision.  I've seen these consequences and would never want ownership of them.  You aren't getting paid enough for that.  

I think you'd best serve the client and protect yourself by offering 2-3 paths, discussing them, and letting them choose.    

  1. If they want two systems, sell them two of the high zoot systems they've already signed up for.  (I imagine it's Modulating Communicating Dual Fuel?).  If they aren't going for one system, installing one good system and one crappy one is a bad decision along the same line as the band aid solutions.  
  2. Spend some money on design.  Come up with options for one high zoot system that can manage the whole house, which probably means some invasive changes but they'll get past it.  Reassure them that it will provide the best long term solution.
  3. Start educating about communicating zoning to cure ignorance and allay fears.  (Curing ignorance and allaying fears is one of the biggest parts of our job.)  Let them know they can have even better control with one system than they have with two.  

Remember, with one system you can effectively mix all the air in the house.  When it comes to managing humidity and IAQ and temperature there are tremendous advantages of having one system.  Long term cost, as you are seeing, is another big reason to go to one system.  Sizing is another.  The list is really endless. 

Don't encourage band aids.  Then you own them and they reflect on your creativity and character.  Just because you don't recommend or encourage band aids doesn't mean you can't discuss or sell them.  

I'm not sure my communication skills are up to conveying the subtlety here.  They are already inclined toward the two unit solution, no need for you to take any ownership of it.  The position you find yourself in down the road when they start realizing they chose the solution you discouraged is much stronger than if you encourage that choice.  When they realize they made a bad decision by taking shortcuts you want THEM to own all responsibility for it rather than blame you for it. 



How big is this house? Where is the house? 1500 HDD or 7000 HDD? Does the client need/want AC? What is the going rate for the furnace replacement? What would a ductless split cost? How much foam is there?

You need more detail than we have so far, but I can tell you in my 6800HDD climate, I have insulated roofs over very substantial attics and clients have finished them into 2 bedrooms and a bathroom without adding furnaces or ducts/radiators and they work. One guy put a third floor apartment on a 30x60 over/under duplex and installed a ductless split in the third floor as the heat and AC because he couldn't get ducts up there. That works. I would at least get an HVAC guy to do a load study on the house as a whole, then ask the owner about AC, then talk about options & solutions.

3,000 square feet conditioned space (1,400 on 1st floor, 1,000 on 2nd floor, and 600 in basement).

Northern VA = 4,200 HDD.

Client currently has two AC condenser units (one for 2nd story and the other for rest of the house). 

Going rate for furnace replacement is high in our area, approx. $5,000.

Ductless split is out of the picture for HOs.

6" foam in between rafters. 3.5" foam in the exterior walls of original section of house, and 5.5" FG in walls of addition.

Since the house has been super insulated with foam the heating load for upstairs will be minimal in most areas. They may not need ANY heat upstairs. I've seen builders use 5KW of strip heat with good results for upstairs installs in Oklahoma. The heating load is so little buying a 2nd furnace simply doesn't justify the cost.

Bob, you and others have been speaking to this possible need for no heat upstairs. I too have thought about this, but HOs are convinced that they need heat for upstairs, and they like the idea of having one if the other hits the bed. 

The HVAC and other contractors in this area (for the most part) just feed bad info to folks and they begin to take it as the bible over the years.

Let me shoot off an e-mail to HOs with all that you and others have mentioned and then I'll follow back up here.


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