Water heater: Makeup combustion air source
I’m energy retrofitting a house built in 1991 which had bad ice dams this winter. The blown-in cellulose is being removed so we can completely air-seal the attic with a 2 inch layer of closed cell foam followed by R60 of blown-in fiberglass. I’m working to eliminate as many heat sources to the attics as possible. My gas fired water heater draws combustion air from the attic and is a heat source to the attic.
There is a utility room between the garage and the house which houses the forced air furnace and 75 gallon water heater, both natural gas fired. The Bryant furnace has closed combustion and draws outdoor air through a sealed concentric vent, but the water heater draws combustion air from the utility room. Makeup combustion air for the water heater is via a floor vent to the crawl space and a ceiling vent to the unheated attic above. The ceiling vent is a 6 inch rigid, vertical circular air duct that opens to the attic and extends about 4 feet up from the utility room ceiling. The utility room is insulated and, in winter, had a typical winter low temperature of 50F overnight and a daytime high of 70F.
This combustion air makeup duct dumps heat to the attic and contributes to the ice dam problem we had this winter (we had about 20 inches of snow on the roof).
I need to figure out the best solution for providing makeup combustion air to the water heater.
Questions:
1. Is the heat dumped to the attic from the ceiling makeup air duct enough to worry about with regard to ice dams? Should I just leave the ceiling combustion air inlet as-is and not worry about it? You can feel heat from the utility room rising into the attic in the winter at the duct opening in the attic. This is an excellent natural convection system with cold air coming in the floor, the heaters in the utility room warming the air, and the warmed air convecting to the attic.
2. Should I extend the combustion air inlet through the roof to the outdoors? This would be a direct source of 5 degree F air to the room in the winter.
3. Should I seal off the ceiling air inlet and rely on the crawl space makeup air? I know this does not comply with code (I don’t understand why code requires BOTH a ceiling source and a floor source).
4. Should I change out the water heater to a new power-vented model with a closed combustion system like the furnace? This is really expensive – about $1,500 for the water heater alone.
5. Change to an electric water heater?
6. Other alternatives?
7. Does it make sense to insulate the air makeup duct and the exhaust duct passing through the attic to reduce their heat contribution to the attic?
The insulation crew is removing the existing cellulose today, so I need a strategy for the the water heater makeup air ASAP. Thanks in advance for any help and advice!
GBA Detail Library
A collection of one thousand construction details organized by climate and house part
Replies
Steve,
Codes can vary from jurisdiction to jurisdiction, and I am not a code expert on fuel gas codes. That said, here's what I know: "The International Fuel Gas Code requires one of the following options for equipment installed in a mechanical room: either (a) a ceiling duct to provide outside combustion air with a minimum of one square inch of free area for each 4,000 Btu/h input of gas-burning appliances in the room, or (b) a wall duct to provide outside combustion air with a minimum of one square inch of free area for each 2,000 Btu/h input of gas-burning appliances in the room, or (c) a fan that provides 1 cfm of outdoor air for each 2,400 BTU/h of gas-burning appliances in the room."
This summary of the code differs from your understanding. Note that even if you have a ceiling grille in your mechanical room to provide outdoor makeup air, the ceiling grille doesn't have to be connected to a vertical duct (as far as I know) -- it could be connected to a horizontal duct.
You should talk to your local code official to determine local requirements. You may be able to reroute your combustion makeup air to a single duct -- one that is unconnected to the attic.
Of course, switching to a sealed-combustion gas water heater or an electric water heater would also solve your problem.
How big is your utility room? I don't see how having a make up air vent going into the attic is any help and the floor vent should suffice. You obviously know what is happening, hot air is rising up into the attic rather than being sucked down. Perhaps if you extend the vent, coming from the attic into the ceiling of the utility room, down to the floor level as it should be, you may eliminate the stack effect. The option of running the MU air vent in the attic to the outside is also a possibility, but as you mention, now you are introducing cold outside air. Where or how does your gas HW exhaust vent out?
Thanks, Tom. The WH exhaust is an uninsulated round rigid duct through the ceiling, passes through the attic, and exits the roof to a cap.
Steve,
What do you mean by "WH exhaust"? Are you talking about the flue of the gas-fired water heater? If so, that shouldn't be single-walled pipe. It should be B-vent (double-walled vent pipe for gas-burning appliances) or better.
Yes, the water heater (WH) exhaust flue. It sure looks like single wall, not double wall B-vent. I haven't looked in the attic, but I expect it is the same. Photos attached.
Steve,
I'm not a venting expert, but in many jurisdictions that would be illegal. You shouldn't penetrate a ceiling or a roof with single-wall pipe that is venting flue gases.
Tom, the utility room is 8'-7" x 3'-5" with 10 ft ceiling. It is within the physical house envelope, but I think it is not within the house air / weather barrier envelope. The utility room door enters to the garage and it is not weather sealed (3/8 air gap at the bottom to the threshold). I presume that the four walls of the utility room are insulated, but I haven't verified. I just posted photos of the WH exhaust vent.
Sounds like you should be all set with just the vent in the floor and the gap under the door to the garage, especially since it's only the HW heater that needs the combustion air which only cycles occasionally. As I mentioned earlier, extending the vent going into the attic down to the utility room floor, with an elbow basically resting on the floor may prevent hot air leaving, if you feel you still need it. ( You could reduce it to 4" so it won't take up too much space)This is normally how a MU air vent is installed, closer to the combustion, just like laundry closets with gas dryers. If you think about it, the open vent in the ceiling is pulling air through the floor vent and door gap and out of the room rather than the exhaust flue doing the drawing......... which is why it has to be lower.
>"...we can completely air-seal the attic with a 2 inch layer of closed cell foam followed by R60 of blown-in fiberglass."
Why 2" of closed cell foam?
Why NOT 2" of closed cell foam? Let's see...
It doesn't take a continuous 2" layer of foam to air seal an attic, and closed cell foam is the most expensive (environmentally and financially) way to get there.
If for simplicity's sake you want to do it with a continuous layer of foam:
3" of half-pound density open cell foam...
... air seals as well or better than 2" of closed cell...
...uses less polymer than just 1" of closed cell at about the price of just 1" of closed cell, and the R-value of 2" of closed cell...
...and uses water rather than HFC245fa (an extremely powerful greenhouse gas) as the blowing agent.
A sizeable vent hole into the attic from the ceiling from a room that never drops below 50F in winter is always going to be a potential ice dam problem, no matter how well you air-seal the rest of the attic floor. Side venting a power-vented (not necessarily sealed combustion or concentric) water heater out some path other than the attic and sealing up that vent is a reliable solution.
Is the crawl space vented to the outdoors?
Without knowing your water use or utility rates it's hard to advise on whether an electric water heater is a viable solution.
Dana...I can't reply once a thread is about 5 levels deep. What a silly restriction! I also can't use "@" tags to address a reply to you. So I'm responding to you on this earlier comment of yours.
On April 30, you wrote "The average heat pump water heater is north of 300% efficiency, provided it's in a big enough space, and is usually cheaper to operate than natural gas. Recovery times in heat pump mode are a bit slow, but an 80 gallon heat pump water heater is overkill for most homes. Why were you thinking a 75 gallon gas was the way to go?"
The house came with a 75 gallon WH. We've always had 50 gallon raising our a family of three kids, but this 75 gallon unit in the new house is really nice. Sure it's overkill, but when we have visitors, it'll be nice to not run out of hot water.
I'm thinking again of electric WH rather than gas. Zero noise. I can close off the top and bottom fresh air vents. I can remove the existing gas WH exhaust vent stack from the roof. Recovery may be a bit slower than gas, but that shouldn't be an issue with a 75 gallon tank. I also may have to add an electrical sub panel because we have a full 20o A service main panel.
If you have 5 cent electricity it may not matter, but if you have 15 cent electricity there will be "payback" well within the 10 warranty period of a heat pump water heater.
The new Rheem heat pump water heaters are quieter than many refrigerators, and quieter than your furnace. They come in 50, 65 and 80 gallon sizes. At box stores they run about $1300 for the 50, $17.50 for the 65, $2K for the 80.
That compares to about $700 for a glass lined residential 80 gallon electric water heater, $1K for a limited warranty or $1250 for a "lifetime warranty" stainless 4.5kw residential 80 gallon electric.
Residential electric rates effective 4/1/19:
First 600 kWh 9.116¢
Over 600 kWh 10.179¢
Residential natural gas rates effective 1/1/19:
Flat rate, $0.50087 per therm (100,000 BTU)
Dana, thanks for the thoughts on OC vs CC foam. I decided on CC because of the structural integrity in the attic. It can prevent an occasional errant foot from going through ceiling sheetrock. It also acts as a vapor barrier, so water and moisture penetration are less likely. The foam itself is unharmed by water damage.
Yes, the crawl space is vented to the outdoors. I forgot to mention I'm in North Idaho in Climate Zone 5. The crawl space vents have a handle to pull them shut in the winter. There are four crawl space vents (about 6 x 12) on the south side and three on the east side (also 6 x 12).
You wrote "A sizeable vent hole into the attic from the ceiling from a room that never drops below 50F in winter is always going to be a potential ice dam problem, no matter how well you air-seal the rest of the attic floor." -- that is exactly what I was thinking. That's a lot of heat flowing straight up to the attic through that big hole. Most of the time the water heater is off and the 50 degree air is flowing straight up to the attic. I went into that attic on a cold winter day and it was quite warm.
>”Dana, thanks for the thoughts on OC vs CC foam. I decided on CC because of the structural integrity in the attic. It can prevent an occasional errant foot from going through ceiling sheetrock. ”
I wouldn’t count on that. Ccspf does help with the structure, but mostly laterally. I wouldn’t trust it, especially an only 2” thick layer, to support the weight of a person. Maybe if you had very closely spaced trusses or something like that it would be enough, but I still wouldn’t use ccspf just to protect the drywall from a trip and fall in the attic.
Bill
What you currently have looks to me like an atmospherically vented, standard B vent (single wall) gas water heater.
Consider simply replacing the atmospherically vented gas water heater with a direct vent gas water heater if your WH is within 6' of the outside.
The direct vent will vent and bring in makeup air through an outside wall no more than 6' away. It is a closed system and will work in a power outage. It works via a coaxial pipe (pipe in pipe) that takes in fresh air in around the outer pipe, and vents out the combustion from the inner pipe.
Then you can get rid of all other holes/vents/ air leaks, etc.
Thanks. I just talked to our building inspector and he mentioned that as well. The utility room opens to the garage. Does that count as "within 6' of the outside"? The utility room is far away from any true outside wall. The closest true outside wall is probably 22' away across the garage.
Steve
Look for a direct power vented tank (one with two inlet and exhaust pipe like the box store GSW G650T58N-PDV-ES2). Depending on the size of the vent pipe and the number of bends, these can run 40' without issues.
Thanks, Akos. Yes, I was looking at a 75 gallon powered-vent, closed combustion unit like you suggested. Here's a good page on the GSW Power Direct Vent water heaters.
http://www.gsw-wh.com/assets/documents/current/spec-sheets/GSW%20Power%20Direct%20Vent%20English%20GPD435N_0316rev1.pdf
We are in the northwest and it might make sense to go electric. They are 100% efficient, but $/Btu is higher for electricity. I talked to our building inspector today and he said the overall operating cost of the electric hot water heater is about the same as natural gas fired (with lower efficiency).
Why would you talk to the building inspector instead of the plumbing inspector? The plg inspector, or any licensed plumber, should be able to show you or tell you what the codes are with regards to when there is a need for MU air and how MU air venting should be installed.
>"We are in the northwest and it might make sense to go electric. They are 100% efficient, but $/Btu is higher for electricity."
The average heat pump water heater is north of 300% efficiency, provided it's in a big enough space, and is usually cheaper to operate than natural gas.
Recovery times in heat pump mode are a bit slow, but an 80 gallon heat pump water heater is overkill for most homes. Why were you thinking a 75 gallon gas was the way to go?
@Tom May...our little town subcontracts all planning inspections to a State of Idaho Building Inspector. I think he covers review and approval of all aspects of construction. I don't think we have a separate plumbing inspector. He's the person the planning & permits office at City Hall referred me to.