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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If you have 2 AC systems in a 3000 sf house with a foamed in roof you have too much AC and you are headed for a cold/wet summer experience. The AC system reacts to temp. The need is dehumidification. It will probably get cold before it gets dry. Really it sounds more and more likely to me that a ductless split is your best solution. Get rid of the furnace and second AC system. With no duct lossses and the efficiency of the het pump your client will see drastic drops in energy costs, improvements in comfort, and a much better health & safety situation.
Inspect the foam install to be certain the foam connects the roof to the second floor exterior wall top plates. If it doesn't we are all assuming a level of performance that is severely compromised. In that case, the split may not work.
If the foam is relatively constant in thickness and it makes the roof/wall connection, it will allow the split to work fine. At first calculation, the ductless split will seem way too small, but that is good. it will have longer run times, which means better dehumidification. The downstairs furnace is probably already oversized, so the small heat pump in a split will be a good auxiliary solution.
What kind f water heater o they have? Natural draft? Does it work @ worst case?

We know they need heat, the thing has been running.  

1000 sf may mean quite a few rooms.  One mini will likely not be satisfactory.  

Maybe a ducted minisplit in place of the POS that's coming out.  Again, as an option rather than a recommendation, multiple systems makes managing one space complicated, and you want that risk on their shoulders.  

I have a client  in MD with 4000 sf, pretty well insulated but still fairly leaky 1960's split, cooling with a 2 ton Greenspeed (he couldn't believe the dehumidification this summer - keeps stat at 76 because lower than that his wife get's cold! His bill is about 1/2 that of his neighbors, who don't have energy sucking sun-room additions.).  

So I'm with Pat, you can't use 2 split systems on that house without being grossly over-sized unless they're both 2 ton modulating units.  As these units typically run 12-15k each, I doubt they're going to go there.  

Are you recommending some type of fresh air?  They may need 2 strategies if they have 2 hvac systems. 

At this point I am taking in all that you folks have been saying.

The previous builder I worked with really gave me the impression that these 3,000+ square foot homes needed two separate systems to maintain comfort. This is why I was not originally pushing hard for them to move to a single heat\AC system. I figured that their HVAC guys put in two separate systems because they felt the HOs needed it. I am under the impression that the systems were installed before they foamed the walls and the roof.

I am intrigued by the option of a ducted mini-split.

With the advent of PROPERLY SIZED, staged or modulating equipment, which operates  continuously or near continuously when the home is under load, the need to club the house to death with equipment has greatly diminished. *** 

When you seal off what were huge 150f reverse stack induced attic leaks into the second floor, then add enough insulation that tossing eggs against the ceiling doesn't result in sunny side down cooking, and top it all off with sealed ducts, preventing ice box basements, we are in a completely new paradigm.  

But change, and even staying current, is tough for some.  So much of the "rules of thumb" you will be exposed to are 30 year old "avoid complaint phone call" oriented, not current best practices oriented. 

Always recommend ERV's.  Don't arm twist, but since it's permanently documented in your proposal it'll make you look like you were really thinking about their health.  And when the kids get asthma, you won't feel the least bit responsible. 

(*** Nice thing about this is where you'd sell 2 crappy systems with "acceptable" comfort results, huge energy bills, and high maintenance cost you can now sell 1 AMAZING system that'll provide mind blowing total comfort management, completely silently, and result in incredibly low energy bills.  I have bills, houses, and clients that will attest to this.)  

Can't argue with that. Thanks for all the advice on this matter Ted.

I will inspect for the roof\wall connection of the foam.

Natural gas, atmospherically-vented DHW. It passed worst case when I tested it in March 2012.

How would you suggest that I approach the situation of getting rid of the AC for the 2nd story? They have had no complaints thus far.

Ask if they ever noticed it running after the foam was installed. Houses are forgiving sometimes. Folks like sleeping quarters cooler than living spaces. It is possible the stats are set differently and once the foam went in one system has been doing all the AC work without their knowledge. I don't know how you would prove that if they cannot tell you though.
Remember the 'floor' of the attic has a thickness. If there is a floor deck that has not been disturbed then the foam likely stops on that deck and does not extend down through the floor cavity to the lower level ceiling. This type of flaw will diminish performance catastrophically .

Great point Pat. I will let you all know what I find when I do my next analysis.

Simple solution: Do a load calc, install a  low btu sealed combustion condensing 2 stage variable speed furnace.  Based on the load calc, determine wether or not you are going to hook up or allow the second stage of heat to come on or not.  The other option is to add a hot water coil and air handler instead of a combustion product.  Most importantly, do a load calc to help you decide

Thanks Joe.

What are your thoughts (as mentioned in previous threads) about getting rid of the furnace for the 2nd story all together, and just having local AC on the 2nd?

The walls have been foamed as well as the roof and end walls of the attic.



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