Commercial building - relocating thermal/air barrier

I have a confused 8,000 square foot commercial office space. It has a dropped ceiling suspended from wooden truss system. It has a vented attic with pitched shingle roof. We are in climate Zone 5 at 7,000ft elevation with around 7,000 HDD.Around $9,000 in gas per year.


There is no sheathing top or bottom of trusses. The insulation has been stapled in floating style and we all know how that goes. Pretty much no air barrier and insulation dropping all over the place. There are multiple furnaces(I think 4 or 5) all installed on top of trusses with grilles incorporated into the dropped ceiling. Ideally we would like to go with an unvented roof assembly with foam on the underside of the deck. My concern is all the non-sealed combustion appliances and corresponding combustion air requirements. We feel that not only is there no space to move the furnaces, it most likely would not be cost effective. We considered various insulation strategies on the existing attic floor, but seems to patchwork to me. Two options left were foam the underside of roof and either A) buy all condensing furnaces, or B) create a box around each furnace with combustion air running to the outside.


I would like comments on Option B, as I do not think new condensing units are going to fit in the budget.



Flagstaff, AZ

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You mean the furnaces are installed under the trusses,not above them right? My first thought is those are commercial vent-less units & thank goodness you are in Flagstaff.

I am not sure I would be worried about combustion air running to the units, but rather changing air out for the occupants with an HRV which would provide needed combustion air. Now if they are vented units, you have other issues  

The furnaces are installed above the bottom chord of the trusses - so in the attic space. These are normal 80% non-condensing, forced air, vented furnaces.


I didn't think about using conditioned volume for combustion air requirements. We will have to run the calcs and see if there is enough volume. If needed we can have pass thru vents from the living space to conditioned attic and introducing fresh air with an HRV.


We're you thinking of code issues for vented furnaces? That is what I was concerned about with having non-sealed combustion ap.0pliances all in conditioned space once the thermal barrier is moved to the roof deck and venting removed.


It seems to me, barring any specific code issues, you can pull combustion air from conditioned space if the volume is sufficient. It just makes me nervous, but barring replacing all the hvac, it may be the only solution.


Any other ideas or comments?


I think option B makes great sense. The air injector fans shouldn't run more than a few hundred dollars each. Much cheaper upfront cost than replacing furnaces.

However, you should definitely map out a payback on the furnace upgrade option. With $9k in heating costs (will be less, of course, after the insulation retrofit), the payback may seem appealing to the building owner. It may also be possible to completely drop one of the systems, and replace it with a zone, avoiding one of the system upgrades all together. In general, I think option B is certainly a reasonable approach.

Isaac - Can you tell me what you mean by "air injection fans". I was assuming we would connect the furnace "boxes" to the outdoor for combustion air with simple vertical hi/lo ducts sized to code for the btu of each furnace, just as you would on an inside mechanical closet.

Field Controls has a "fan in a can" product that can be wired to the furnace. A pressure switch must be proved before the furnace will fire. Could be used in lieu of hi/low if only one envelope penetration was desired. This also works well for furnaces in crawlspaces that require "sealing" but don't have the budget for a furnace upgrade.

There's a statement in the code that allows combustion air grilles from the conditioned space of a building (larger sqin/btu ratio than to outside, if I recall correctly). It may be worth investigating whether or not you could deliver this combustion air from the conditioned space. Obviously not ideal to exhaust conditioned air. But, would be more affordable upfront, as you wouldn't need to box-in around each unit. This may be an issue related to thermal barrier coatings on foam. But, you'll be facing that anyhow if there is only a dropped-tile ceiling separating the attic from the office space.

I like it. My only question is, is there enough volume to meet the requirements for combustion air in the building by volume alone(either in the attic space, or by connecting the work space and the attic space with pass thru vents sized to meet code). If there is.. do we need to even consider the box around each furnace, or the "fan in a can"?
50 cubic feet of air per 1000 btu's of input is the code requirement I'm familiar with. I'd imagine you'd have plenty if the office space was connected to the attic. If your code official let's this fly, you should not need the boxes or fans.
If your attic is considered a "confined space" (less than 50 cubic feet per 1000 btu's of input), then, according to section 702, your openings to "inside" should be sized based on 1 square inch per 1000 btu's of fuel input. You should confirm that your code is the same, of course.

What's the budget?  


If you drop the bill to $2000, will they let you have the $7000?  


I'd guess that'll cover cost of spray foam the space and allow for 2 hybrid heat systems.  Can't image you couldn't manage that space with two 100mbtu/5 ton hybrids.  I did this with a similar space.  Building owner is now unhappy because he spends more on energy for his 2200 sf house.


I wouldn't start doing analysis without some participation from the client.  They need to give you some $ for design and analysis work.  


Also, you have the PAIN - $9000 a year.  You need MONEY, what they are willing to allocate to this project.  Then you need DECISION, commitment to move forward with the work if it solves pain for less than or equal to money.   




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