It has been suggested that combustion air drawn from the attic space can be a problem. Although this space has ridge vents and gable vents warm air from the house could migrate upward and introduce moisure into that area with the potential for mold etc. Although there are no obvious problems I am considering a straight horizontal run from the furnace room to the exterior of 6″ duct with a bird screen. Is 6″ adequate with 120K BTU between the furnace and HWH? Can a damper be installed to prevent cold air intrusion when the furnace is not calling for air? Thanks for the help.
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i ran a 4" inslated pipe to a area near my oil furnace as per installers instructions
it terminates in a 5 gal bucket about 6 " from the bottom , also has a screen on the inlet to prevent critters from entering
I'd suggest a chat with your building inspector. Mine told me that if I had a 6" exhaust vent going out, I'd need two 6" makeup air vents.
Excerpted from the 2003 International Residential Code (one and two family dwellings):
M1701.3 Volume dampers prohibited
M1701.5 Opening Area.
M1703 All air from outdoors
M1703.2.1 Size of openings.
M1703.3 Attic combustion air
M1703.5 Opening requirements.
*my wording* There is no mention in the 2003 IRC about any of the attic air issues you cite.
Lots of pictures and useful info if you can get your hands on a copy of the 2003 IRC.
Volume dampers ... vs. shut off dampers? Even if so, you could probably request an exception under alternative materials and methods. This is standard fair in the commercial industry.
Now we are getting into gray areas where the person needing answers to the questions needs to go to the building inspector and ask them.
I know of residential combustion air intakes with dampers that are always shut off... and some with fiberglass stuck in the end since the occupancy permit was granted... some with the end stuck in a bucket... lots of variations, many not to code, but good enough for the inspections at the time.
My only intent was to provide a correct answer and the resources to back it up.
Just like anything else, your mileage may vary.
right. Didn't think you were doing different. Jus providing another view. In Washington State at one point, the energy code specifically required 'shut off dampers' (not a quote) in the combustion air. I even had commercial inspectors scratching their head a bit on this on one or two occasions ... even though it was common practice in most commercial applications (had been for decades).
Which is also why I recommended to the OP to have a little chat w/ the BO. Allowing unconditioned air continuously into the mechanical room .... especially when there is water present is a problem ... IMO.
Also, if the undampered air goes to the mechanical room, it is now considered unconditioned space and the walls/ceilings must be insulated per the energy code (which can exacerbate the situation). Simple intentions can have larger ramifications.
I think I'll "debate" this with you. :)
"Allowing unconditioned air continuously into the mechanical room .... especially when there is water present is a problem ... IMO."
Our code requires combustion air for anything over (IIRC) 50,000 btuh. We use a trap for the vent that is made by taking the vent to the floor and then taking it back up about 3 feet. The trap allows two things.
Cold air will enter the pipe, fall to the floor, but does not want to rise back up. Hot air rises, cold air falls. If the pressure in the house is the same as outside, no air will enter the house. If the house is negative, air will enter.
Raising the outlet off of the floor also allows the cold air to mix with the room air and become tempered.
Probably 95% of the installations here are done that way. We can get temps of -40, but freezing water lines are not a problem. It's also an easy way to tell if the house has a problem. If no exhaust sources are operating, and there is air pouring out of the trap, the house is leaking badly.
Air HAS to come in the house somewhere, the mechanical room is a good choice. It is not "uncontrolled", the physics of the house control it.
BTW, taking combustion air from the attic is a poor choice. We don't do it any more. It is a real possibility that there is enough draft out of the attic that you could downdraft any appliance. Seen it happen.
Discussion is good ...
Most codes I've seen REQUIRE that combustion air be dumped into the room w/in 12" of the floor AND 12" of the ceiling (both). Getting to those points doesn't really matter, I don't think. Your 'trap' would have to turn back down again (I suppose that is OK, if not even better).
Not having the trap also allows the cold air to be tempered by the warmer air in the space ... really makes no difference. The air is still unconditioned. Both ways allow the air to mix w/ the air in the room and get tempered (although preheating in the duct may help a little).
If this combustion air is low in the house and it is cold outside, the natural neutral pressure plane of the house will undoubtedly be above this opening and therefore there will ALWAYS be a flow of unconditioned cold air into the mech. room. Also when you use the exhaust fans in the house, as you indicated, it will always flow in then as well. If the house is pretty tight, this will be smaller (the amount of air coming in), but it will still come in ... and my guess is rarely go out.
Whether you get freezing depends on other particulars. If the mech room is insulated to the levels of the exterior walls (which energy codes may require since having the combustion air to the outside (in the Pacific Northwest) has made, by definition, that mechanical room an unconditioned space. So if you have -40 degree air and possibly little source of heat (i.e. insulated hot water piping, water heater, etc.), you could drop the temp of that space very quickly.
Also ... if the physics of the air leakage of the house are uncontrolled ... i.e. left to the whim of the wind and weather, then it is also uncontrolled air coming into the mechanical room. If you really don't control when it comes in, it is uncontrolled. Even if the mech room is tight, the flue itself will ALWAYS draw a negative on the mech. room (even with a induced or power burner).
Letting the air spill directly onto the floor can result in freezing of water lines along the floor depending on air flows. Making it fall from at least 3 feet allows for mixing with the room air. Dumping it into a bucket has the same effect as a trap.
I think that a relief air vent at the ceiling is a fallback to the days of boiler rooms where the room temps could get high enough to cause problems with any electrical components or wiring.
We still require a relief vent in a commercial boiler room, but not a furnace room.
Edited 9/8/2008 1:45 am ET by rich1
I think the high/low requirement is to accomodate gas fired appliances and the fact that they may switch from natural gas to propane ... avoid gas collection in the event of a 'problem' by making sure that the combustion air comes both high and low. It is NOT a requirement of relief venting, I'm fairly certain ... they are combustion air intakes, not hot air reliefs. Seem like that is what I recall. Although if you have a propane device below grade, you have to install special controls to eliviate a problem in the event the LPG 'spills' into the room.
I agree, you'd get some better mixing, but you can't always guarantee results and you will always run into the weak installation that is unique enough to take advantage of the situation.
Again ... installing the damper helps take care of problems and help ensure that cold air spillage isn't as much of a problem.
Edited 9/8/2008 2:03 am ET by Clewless1
the vent you describe was called a Saskatoon loop by a engineer i used here & looks like a P trap , must be a canuck thingy
We had combustion air drawn from out attic for 30 years, and never had a problem with it. Never seemed to be a need for a damper, based on what I could feel coming out of the duct when the furnace wasn't firing.
Yeah. Talk w/ the BO and/or follow mfg instructions. Combustion air openings historically are REQUIRED to terminate both w/in 12" of the ceiling AND 12" of the floor. You don't necessarily need two ducts, but you need to provide the two openings. Let's say you bring the duct in high ... you provide an outlet near the ceiling and then also take it to the floor and provide another. If you are on an outside wall ... one high and one low through the wall would work.
A damper ... absolutely!! Also talk w/ the BO, but don't let him be casual. Commercial applications have been using dampered combustion air for decades. Usually, the damper is interlocked with the combustion device so that the device will not fire until the damper is 'proved open' ... this means when the device calls to fire, it triggers a damper motor and when that damper motor opens enough to close a switch, then the device can fire/ignite.
I know in the Pacific Northwest at one point there were lots of discussions and such about this issue. I don't recall what kinds of solutions were acceptable, though. It is not unusual for you to breach this issue w/ the BO and have him look at you like you are a little off. He will have a tendency to allready know this or he will balk at your suggestion. This is a safety (and energy) issue, so you should do it right to ensure good operation and energy efficiency.
In your case w/ hot water heater and furnace, you will have some girations to deal with to interlock both w/ the same damper. A piece of cake for a controls guy, I suspect.
You need to review the 2003 IRC for single family dwellings.
In your case w/ hot water heater and furnace, you will have some girations to deal with to interlock both w/ the same damper. A piece of cake for a controls guy, I suspect.
So, how would you integrate the pilot light with the damper control system?
You could get fancy off of a power vent water heater, breaking the circiut that fires the inducer motor... then you might have warranty and UL listing issues.
Same basic thing for tankless gas fired units, same potential issues unless the manufacturer has provided a control tap.
Edited 9/6/2008 10:36 am ET by danski0224
So ... what does the 2003 IRC say? Share it ... I don't have one in front of me. Read us the salient points so we have a starting point, here.
Not knowing the specifics, a stat calling for heat can send a signal to the damper motor and once the damper is proven open, it closes the circuit to the gas valve (you simply substitute the make switch for a thermostat; it doesn't matter what energizes or sends a signal to the furnace). I think that side is easy. Maybe the same for the water heater; interrupt the stat signal to go to the damper. It's integrating the two to work properly that creates some complexity (at least in my mind).
While your concern for violating U-L listing is much appreciated and may be valid depending on the approach taken, my sense is that some extra control wiring done properly would not be an issue (although I've been known to be wrong).
I'm thinking the pilot light is a non issue. A pilot has little or nothing to do with this.
This is done all the time on commercial boilers ... add the damper make switch and integrate w/ the burner controls. It's fairly simple components and technology. I'd be surprised if the code didn't require it for dampers in residential applications where you have a furnace in a 'tight room'.
Edited 9/6/2008 12:22 pm ET by Clewless1
http://forums.taunton.com/tp-breaktime/messages?msg=109397.4
In addition, all that fancy integrated control stuff has a very limited market in residential work. Very few will pay for it.
Edited 9/6/2008 1:19 pm ET by danski0224
Wow...thanks for all the good info. I had envisioned a "freewheeling" type damper like you might find on a woodstove draft control. I see they are available in 6" pipe. Likely not up to code?
Your "freewheeling" damper is probably a part for a specific application- not combustion air. I am not familiar with the term or the product.
There is a company that makes barometric relief dampers for intake purposes- Skuttle. One could also adapt barometric relief zoning dampers to the application.
However, the 2003 IRC is quite specific in pointing out that dampers are not allowed (residential applications).
Municipalities might have codes that supercede the base IRC coding.
If the combustion air intakes are installed properly, you will not have any problems unless there is a big hole in the side of your house open to the prevailing wind.
That is why damper interlocks are done on commercial installations- some of the louver assemblies are quite large.
When the furnace is operating, you will feel a draft at the air intake- which is better than sucking cold air in from around windows, doors, and exterior wall electric outlets.
What kind of FURNACE ROOM are you talking about.
Is the furnace in the attic?
If the area where the furance is mount has at least 59 cu ft of volume per 1000 BTU rating of the furnace then combustion air can be drawn from that space.
.
A-holes. Hey every group has to have one. And I have been elected to be the one. I should make that my tagline.
Maybe best to talk w/ your BO about the damper, first. Then to a knowlegeable HVAC guy ... the BO may even refer you to someone he knows that has worked w/ this before. The concept is solid. It's done routinely ... w/out rocket science. It is not common in residential applications, but it is not unheard of, either.