Both the gas hot water heater and boiler are in separate closets on the 2nd floor. Both closets have a short metal vent in the ceiling (assuming to let warm air out (to the attic)) and combustion air in from the attic.
Are these necessary? What are the draw backs and general comments.
It doesn’t sit right with me, but I can’t put my finger on it.
That is the way we draw combustion air from attic here in this state all the time almost on every one The short pipe is suppose to be into the closet at least 12 inches and the longer pipe is suppose to be 12 inches from the floor creates a natural circulation.
Took a 3 day co/combustion training course a couple years back. The room is relying on passive combustion air. The combustion air pipe is warm air will be trying flow up and out of the attic. There should be at least two openings. One high and one low. Even if the pipe extends from the ceiling to the floor it is still a high opening and will heat and pull air from the room.
Let me relate. A site with a boiler system in a small 6’ x 6’ x 8’ building. That’s it basically a boiler room building. It had a louvered door which had been covered over with a piece of sheet metal. It also had an opening through the roof for combustion air and a vent for a natural draft boiler. After extended run times the co levels in the undiluted vent would start to climb. Once the sheet metal was removed from the louvered door co levels would remain normal during long run times. The venting action of the boiler and the high combustion air opening was now allowing fresh air to be pulled into the boiler room.
The IMC may allow a single passive combustion air opening but if it results in bad combustion testing it would need to be corrected.
If you check the committees that set the rules for combustion appliances you usually will find no one with in depth building science training, especially with building pressures and depressurization, combustion gas spillage/backdrafting. In the past, the “rules” worked because houses/buildings were so loose (and US buildings are generally much looser than Canadian due to overall climate and cheap fuel prices) that even when you may have something that didn’t follow the rules of nature (physics), it appeared to work OK due to good luck, not good design and practice
(I give many examples of this when I do training and gave one on a thread a few months ago…the one about smoke emanating from the combustion air inlet located in the rim joist of the main floor and serving a “zero clearance wood heating airtight fireplace” on the main floor…during strong wind gusts, the chimney supplied the fire with air, the fire burnt downwards and smoke exited the uninsulated metal combustion supply air duct installed beside the wood floor joists…the homeowner had a video of this occurrence!!)
As part of my energy analyst’s job with the gov’t here, I was one of 4 energy advisors answering calls from the general public on energy queries. (they now have 20-22 year old kids giving this advice today??? They bid it out privately for a lot less $$$$ than hiring experts)
One late summer day, I took a call from a gent with a question about a 6" diameter air inlet hood and duct from his rim joist area to supply combustion air for his oil furnace room. After determining he had an older, looser home, I had him go to his furnace room and check for air movement in or out of the combustion air supply duct. By chance, the inlet hood of the supply air system was in the lee of the winds that day (therefore negative pressure was acting on the hood) and air was being sucked out of the combustion room…so much for the theory!!
The winds were not very strong that day but were from the direction of prevailing winter winds and with the much stronger winter winds that would act on that hood, the effect would be much stronger with more air being sucked out of the room during the heating season!!
Another story from my gov’t days:
Mid March: call from a lady about an oil exhaust gas smell in her daughter’s second story room (new full 2 story house with full basement) after the oil furnace heating cycle was done. The furnace room is in the basement 2 floors down in the same south facing corner of the house with a living room on the main floor. There was no odour detected in the living room.
After determining the particular venting system that was installed (a fan powered sidewall venter through the rim joist), I asked her if this had been happening since the first week of January. (We had an unusual 10 cold week steady very cold period where the stack/chimney effect would be stronger inside the house plus this area of the walls/roof/attic is in the lee of the prevailing north/northwest winds). Very surprised she asked “How did you know that??”
She said she’d had the heating service company (ESSO- EXXON branch plant) out every week since January and they hadn’t been able to solve the problem and someone had passed my name along. I explained what I thought the problem was, so she asked if I could go out to her place. My supervisor phoned ESSO to check out the story and said go out as soon as I could!!
Got there same afternoon (by chance, house was close to being on my way home). Found that (1) the sidewall venter had been modified by adding a damper so that any UL/ULC/CSA certifications would now be void; (2) the heating duct serving the girl’s bedroom was taken directly off the hot air plenum (bad practice to begin with) and travelled essentially vertically direct to the room; (3) ducting/plenums not sealed at joints, (4) furnace was non-airtight design from the era of furnaces that vented into chimneys with small negative draft acting continually.
What was happening late in the heating cycle was: (1) after the burner was shut down by the anticipator/thermostat, there was a 2 -3 minute post purge cycle designed to remove all exhaust gases from the fire chamber/heat exchanger; (2) after the air handler fan shut down, the ducts were still warm and would encourage/enhance the stack effect…it would be strongest in the vertical duct directly to the girl’s room, having being taken off the plenum with a top take-off boot oriented vertically right above the still hot heat exchanger/fire chamber.
The negative pressure in the plenum/duct system was pulling air with remnants of fire chamber odour out of some very fine openings at the oil burner insertion opening up into the hot air plenum above the fire chamber. (they had already checked the furnace for combustion gas leakage during operation but found none). Normally, this type of furnace on a warm, naturally drafted chimney would have a continual negative pressure on the exhuast gas removal system that would overcome any competing negative pressure from the air handler/fan side of the system or the furnace room.
My recommendation was that since you can’t overcome the forces of nature, even by invaildating certifications on tested equipment, install a Class L (approved for oil) interior stainless steel chimney to provide the vent system needed to provide a constant negative draft. They didn’t do that but installed a better insulated Class A chimney on the exterior*. I assume the problem was obviously solved as I did not hear back from the lady but I have heard of others complaining about similar “after-the-cycle” odours in the mechanical room on other sidewall powered venters with these 10-15 year old furnaces.
*Why do they install chimneys on the outside of houses???
Because they don’t know any better.!!!
A chimney flue has to be warm to hot to provide an adequate to good draft that will help the appliance operate properly and…SAFELY. But let’s put them outdoors where it gets down to -20 to -30 or more (both F and C) in some areas of North America. Try and work properly now…you SOC!! (son of a chimney) SAD!! SAD!! SAD!!
In Canada, WETT would like to see outdoor chimneys banned but it makes…too much common sense… and we all know what Will Rogers said: “Common sense isn’t all that common, is it?” By the way, WETT certified chimney sweeps with much lower levels of formal education, know way more about house and chimney pressures than most architects do!!!