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# Combustion makeup air for boiler room -- calculations and using mechanized dampers

Member Posts: 55
edited November 2015
Planning to insulate my basement, including foaming the sill plate, and rebuild the boiler room containing my steam boiler and water heater. The usual rule of thumb is two 6" pipes, and one of those is input (the idea being to circulate the air). That's not what I came up with from my calculations, and this is not a big house (source):

Boiler (140000 BTU), Heater (65000), Total: 205 mbh (I think that should be kbh)

Boiler room area: 11 x 7 x 6.5 = 500 cu ft
500 / 205 = 2.44
2.44 / 50 (expected) = 4.9%

Reqd: 205,000/2000 sq in = 102.5 sq in (horizontal duct)
Reqd from makeup: 102.5 * 95% = 97 sq in
i.e. 2 x 8” duct (50.2 sq in)

So a 10" pipe instead of a 6" pipe according to conventional wisdom. That sounds like crazy overkill.

Here's an alternative strategy:

I don't want to kill the energy performance of my house with the equivalent of leaving a window open. I want to put one of these combustion air dampers on each of the boiler and water heater. The boiler's exhaust pipe is 6", the water heater's is 4", so I should size input accordingly.

Appreciate any professional opinions. I would of course ask a plumber to put in the dampers.

• Member Posts: 7,356
I'm not familiar with Canadian code requirements, but do you really need two of them?
• Member Posts: 55
Those dampers are CSA approved for both Canada and US (I am in the US). They require them to be installed within 2' of the air intake for the appliance. I assume I need one for each appliance since they will rarely be running at the same time.
• Member Posts: 19,960
Conventional wisdom... frequently isn't. Your 10 inch duct is much more like it -- and only if it is straight and unobstructed.

Dampers are fine. However, in my humble opinion they MUST have an independent safety switch will positively kills power and gas flow (if it's gas) to the burner(s) -- by positively, I mean that the system fails OFF and takes an electric signal to permit firing.
Br. Jamie, osb
Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
• Member Posts: 55
Interesting that Canada has different ideas about makeup combustion air. No distinction between horizontal and vertical ducting, and the size in the inlet is smaller than required for the US. Under Canadian regulations, a single 6" duct would be okay.

From what I can gather, US regulations are based on research done in the 1940s, when energy conservation was not on anyone's radar screen.
• Member Posts: 55

Dampers are fine. However, in my humble opinion they MUST have an independent safety switch will positively kills power and gas flow (if it's gas) to the burner(s) -- by positively, I mean that the system fails OFF and takes an electric signal to permit firing.

This is what the company has to say about safety of the damper:
I developed a motorized damper for the control of combustion air for the furnace over 30 years ago and I was of the opinion that what was required was a damper that opened when the fire came on and closed when the fire stopped. The Gas Code, however, specified that this damper was to be tested to a safety standard and carry a CSA label as proof of proper operation and certification. This Standard specified that, among many other things, the damper was to be equipped with an end switch that proved the damper to be open to supply fresh air before the fire burned and also that the damper was to open by gravity should the power or electrical wiring fail (Fail-Safe-Open).
• Member Posts: 19,960
Taylor said:

Dampers are fine. However, in my humble opinion they MUST have an independent safety switch will positively kills power and gas flow (if it's gas) to the burner(s) -- by positively, I mean that the system fails OFF and takes an electric signal to permit firing.

This is what the company has to say about safety of the damper:
I developed a motorized damper for the control of combustion air for the furnace over 30 years ago and I was of the opinion that what was required was a damper that opened when the fire came on and closed when the fire stopped. The Gas Code, however, specified that this damper was to be tested to a safety standard and carry a CSA label as proof of proper operation and certification. This Standard specified that, among many other things, the damper was to be equipped with an end switch that proved the damper to be open to supply fresh air before the fire burned and also that the damper was to open by gravity should the power or electrical wiring fail (Fail-Safe-Open).
Not really a contradiction. The real question is -- what is the whole system like? Suppose, for example, that we are talking about a millivolt gas furnace with a simple thermostat, using no outside electricity for control or operation. If this is your system, what is the fail safe situation for a combustion air damper? Fail open -- which is what you have in the above quote.

Suppose instead that you have a system which requires electricity to operate -- a power burner or something of the sort. In that situation, your fail safe option for the combustion air damper is that if it doesn't open, you want to cut off the power. So you have the end switch to prove the damper open.

I don't particularly care whether the damper is open or closed on failure; what I want to be darn sure of is that there is no way the gas valve can open or the pump for oil can operate if the damper isn't fully open. Whatever floats your boat... But you have to think through the whole system, not just component by component.
Br. Jamie, osb
Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
• Member Posts: 55
As an update, in case any homeowners wander across this thread: I spoke to my AHJ, and they quoted ICC that there should be two 10" pipes in an enclosed boiler space, consistent with my calculations. They suggested insulating the boiler room, or putting in a louvered door, or just putting the pipes on the wall of the boiler room to allow exchange with the rest of the conditioned space. I will probably do the last of these.

Would not allow Canadian requirement of a single 6" pipe, which I can understand. It just seems like there is something wrong with US code.

If I bring in an outside pipe in addition to above (because of my own concern about the basement being too tight), that would not be a problem, provided I had a (certified) damper on the pipe, since I would then be bringing outside air into the conditioned space.
• Member Posts: 4,524
Why not use a" Fan in a Can" from Field Controls. That way it will be interlocked with boiler and water heater. You definitely will have a confined space and it is not make up air you need it is air for combustion, they are by the way two different things. Make up air is air required to replace air in a space which was removed by mechanical exhausting and is usually a single opening. Air for combustion is typically air from outdoors one opening 12 inches from the ceiling and one 12 inches from the floor or you can use horizontal ducts or vertical ducts.

The louvers on the door to what will now be a boiler room is a no no unless you use louvers which have a fusible link which will melt in the event of fire and close the louvers. The door also has to be a fire code door.
• Member Posts: 55
My understanding of US code is that there must be two openings less than 12" from floor and less than 12" from ceiling.

If the openings are to an adjacent room, I need to divide the btu input by 1000 to compute square inch of the opening. For 200KBtu, that means two openings 200 sq inches each. That is the equivalent of a 16" pipe. Remember, I'm required to have two of them.

If the openings are connected to outside by horizontal duct, I need to divide the btu by 2000. That gives me the equivalent of a 12" pipe (two of them).

If the openings are connected to outside by vertical duct, I can divide the btu by 4000. That gives me the equivalent of a 8" pipe (again, two of them).

If I lived in Canada, it would be sufficient to have a single 6" pipe bringing in outside air. I haven't heard of massive amounts of death by CO2 poisoning in Canada. Why is there such a discrepancy between Canada and the US?

The US code strikes me as absurd. Conventional practice (two 6" pipes) obviously ignores it, and even my own AHJ is not requiring me to follow it (they are happy with two 10" pipes). And how come the requirements for square inches take no account of the shape of the opening? A 14x8 opening has very different airflow characteristics from a 12" circular duct, even though they have the same area.
• Member Posts: 55
Thanks for the recommendation on the Fan In The Can. It looks like this may allow me to circumvent Code's ridiculous requirement, though I should run it by AHJ to see what they have to say.

It is not clear that this has a damper. If I have to install a damper with this, that's going to be a long chain of mechanical components to bring in combustion air.
• Member Posts: 1,046
An alternative to the FIC is the Tjernlund Enforcer.
http://www.tjernlund.com/combustionairintake.htm