Open combustion furnace in sealed attic
GBA Editor
| Posted in Building Code Questions on
I have an older 1950s house and got a bid to do icynene on the roof deck after sealing off the attic entirely. I do not have a high efficiency closed combustion furnace. I have been told it is an open combustion furnace. Do I have to upgrade my furnace if I want to retrofit my attic with spray foam on the roof deck?
GBA Detail Library
A collection of one thousand construction details organized by climate and house part
Search and download construction details
Replies
Yes.
Kasey,
Your current set-up is nuts anyway, so you have to do something. Installing a furnace in an unconditioned attic is very wasteful. You're now losing a tremendous amount of heat; you probably also have a lot of ductwork up there.
The right thing to do is to enclose the attic — install insulation along the roof plane — and bring the furnace into the conditioned area of your home. To do that, you need to buy a new sealed-combustion furnace with a ducted source of combustion air.
I live in a hot, humid area of Texas (near Houston) so cooling is more of a priority. I have been told by a spray foam installer that all I need to do is retrofit a fresh air intake onto my current furnace. One HVAC guy said the same while a couple of others said I had to replace my furnace with a sealed unit. I was given a rough quote of $4,200 for a new furnace. If I have to replace my furnace (which is only 6 years old), then it does not make financial sense for me to go the spray foam route considering it would take many years to break even. Any other thoughts on whether replacement of furnace is mandatory or if there is a retrofit option? Seeing the post from New Orleans about a retrofit spray foam application on the roof deck causes me to want to just go with cellulose on the attic floor in a vented attic.
Kasey,
You can't turn an atmospheric combustion furnace into a sealed combustion, direct-vent furnace. But you can put it in a sealed space as long as a sufficient amount of combustion air is provided to that space. Check with the building inspector on code requirements.
Aside: It must be all those ten-gallon hats in Texas that makes builders think they can put HVAC in the attic. All hat, no cattle. Too much space, too few brains.
Since it looks like I'm going to have to use traditional insulation on the attic floor, which is best: cellulose, blown fiberglass, or cotton batts?
Kasey - you could build a room around your furnace keeping it outside of your thermal envelope - not ideal but will save you from replacing a unit that is only 6 years old. You will still get the benefit of having your ductwork in a conditioned space just won't be getting the benefit of having the actual unit in the space - add a radiant barrier on the rafters in your vented, unconditioned HVAC room and it will keep it cooler. A decent and safe compromise.
Is this the only unit for your house? May want to also look into reducing your fan speed or some other adjustment for the latent load - one problem with using spray foam in a retrofit is the HVAC system short cycling and not getting the humidity out of the air.
Kasey,
Cellulose. And start putting a few pennies a week in a big jar labeled "furnace fund."
Why is cellulose recommended over something like Johns Manville Climate Pro? I'm concerned about the dust and longevity of cellulose vs blown fiberglass in a hot humild Texas attic. Thanks.
Kasey,
If you are concerned about cellulose, then go with the blown-in-place fiberglass. It's your house.
I prefer cellulose because (when settled) it's denser than blown fiberglass, and better able to resist air flow and convection currents.
Regardless of which insulation you choose, be sure to do an impeccable job of air sealing the attic floor before installing any insulation. If you go with blown-in-place fiberglass, have the contractor blow in 6 or 8 inches more than you are now thinking of installing. That will help (somewhat) make-up for the real-world performance problems with fiberglass.
When an attic is sealed, it becomes part of the conditioned space and separated from the outdoors. If the inside of the house becomes depressurized by ventilation fans such as bath exhaust fans, kitchen venthoods, or by the stack effect such as a burning fireplace, it can draw exhaust fumes from the furnace into your house. The byproducts of the burnig gas includes carbonmooxide- an oderless and deadly gas. SO DON"T RISK it. A sealed combustion furnace(high effi) is a must for a sealed attic. An ordinary furnace will kill you when your sleeping.
George
Kasey,
You could have an energy audtior come to your house and do some testing to determine how likely it will be that the house becomes "too tight" for your furnace. He can also do a full blown combustion safety test afterwards.
But lets back up a minute. There is a difference between "non-sealed" and natural draft. If your furnace is that new, it may already be an induced-draft unit, which has a fan to power combustion air out of the house. It is much safer than a natural draft unit because if it can't move enough exhaust out, it will shut off. This type of furnace is less likely to be unsafe.
If your non-sealed unit is natural draft, no combustion air fan, just a chimney, then it may already be unsafe. You will have a short length of chimney, and thus low draft pressure. In any case, an auditor can answer these questions in under an hour.
I suspect your furnace is draft induced and will be fine as-is, but you need to check it if you don't want to spend $4200. And I don't blame you.
If you want to blow fiberglass in your attic in Houston Texas, then my advice would be to not bother. Your biggest heat gain is infra-red radiation from the roof deck to the ceiling below. Fiberglass doesn't block infrared (along with it's other non-performance issues), so thus don't bother. You literally would be better of throwing dollar bills in your attic. But that's a side issue.
I'd test your furnace and foam the roof. It's easy and cheap to get air to it if needed.
And no one mentioned that if your furnace is natural draft, you can add a draft inducer for like $500.
To everyone else who commented. Just want to point out that by telling him to "absolutely" replace the furnace he went from a conditioned foam attic to fiberglass with gross leaky ductwork running through the attic. It was that quick - you lost him. In your quest to be green you killed the patient. Good job.
There's lots of safe middle ground on this furnace issue, lets try to focus on the goal. Do no harm, and then save energy.
-Rob
Carbonmooxide? Is that the gas that comes out of the back end or the front end of a moo cow?
And why do people assume an original poster still wants advice six months after the initial question? Jeezum Crow, let sleeping cows stand.
Thanks for the advice, but I sprayed cellulose in my attic back in May. It has made the house feel cooler during the summer. My energy bill was around $100 per month all summer for approximately 1,600 sq foot house. My average price per kWh was 11.6 cents. I used 792 kWh from 7/28/10-8/26/10. The average high temperature in Houston last month was 97.6 degrees. I live right outside of Houston. Spray foam was too expensive. Maybe if I ever build my own house, I will consider it. In my situation, there were too many unknowns. I receive conflicting advice in regards to my furnace so I decided to not risk carbon monoxide poisoning.
Kasey,
How do those electrical bills compare to last year? And how is your house cooled?
I was not living in the house last summer so I do not know what the cooling costs were before the cellulose installation. It has a central a/c for cooling with metal ducting wrapped in fiberglass insulation. I kind of regret not replacing the ducting as well since it is probably not possible now. My pool pump and a refrigerator in the pool house which are on a separate meter use as much energy if not more than my main house.
Wow. I'm in a similar predicament, but luckily I'm actually in the HVAC field. I'm also in Texas, fwiw.
Most unit down here are in the attic. We don't have full basements down here, most of the foundations are slab or pier and beam. That means if the unit isn't in the attic, it's taking up closet space. There's a few homes down here that put the unit in the garage, which presents it's own set of problems, but all of the duct work is in the attic anyway.
We run our furnaces maybe 2 months out of the year total. Our winter season is still about 3 or 4 month long, but we don't usually run the furnace every day, and even then it's not running as often during the day.
It doesn't make a lot of sense down here to spend the money on a 90% furnace (especially if the present one is only a couple years old--furnaces can last of 20 years or more down here with decent maintenance).
If someone was going to seal the attic, then it would make much more sense to build a "closet" around the furnace that gives adequate ventilation to the outside. 90% of the attic would get the insulation benefits of the spray foam, and then there's other options to help with the new furnace "closet" in the attic.
What's maybe more to the point is that doing foam for an attic probably isn't the most cost effective solution anyway. "Drill and fill" foam for the walls of the home can be great depending on what kind of insulation the walls already have. For an attic that's just used for storage, or even left vacant, it doesn't need to be sealed.
A blast of radiant barrier paint, combined with a fresh layer of insulation in the attic will work wonders, for a fraction of the cost of a new furnace and/or foam spray--at least down here in Texas.
My AC is on it's way out (14 years old) and my furnace is also acting up (control board issue) so I'll be installing a new AC and furnace. I'm only doing a new furnace to get a variable speed blower. Otherwise I'd just fix the old one. I'm definitely only doing an 80% furnace (Trane XT80) because all of the expense and hassles that are involved with retrofitting a house for a 90% furnace far outweigh the benefits.
Maybe after this project is out of the way, I'll get to work on cooling the attic down. Unfortunately, the AC/furnace combo will be eating up just about all of my home improvement budget this year.
"My pool pump and a refrigerator in the pool house which are on a separate meter use as much energy if not more than my main house."
And here dear readers we get a glimpse of the 'elephant in the room' of energy consumption discussions in this culture.
More on pool pump electricity use: Can Swimming Pools Be Green?
Absolutely. If they are neither heated nor filtered nor chlorinated (or otherwise poisoned), then they quickly become green with algae, and you know they are a healthy environment for living things.
It's funny that, while we'll swim in a lake or pond or ocean full of growing, wriggling, dying things that excrete all sorts of bodily wastes in the water, we insist on poisoning our artificial "ponds" and consider that "healthy". Of course, we do the same thing with municipal water supplies. Go figure.