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# CFM calculation required

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Member Posts: 855
Have a quick glance at the code - it also allows the increase of comb air ducting size for longer runs, and check out the relief air necessity as well. As the one post indicates - interlocks and other safeguards are needed if you use a electric fan, eg air pressure switch to confirm air movement etc.

• Member Posts: 2
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CFM calculation required

I am a sales rep for a prominent boiler manufacturer and I have been asked by an engineer to provide a calculation for a fan CFM requirement for an inline fan being used to bring in combustion air. It doesn't seem like that tall of an order but I am having difficulties with it.
In Canada our code requires that 1 square inch of air be available for every 7000 Btu/hr of input, so based on 399,000 Btu/hr input, which just happens to be the boiler size that I am working with, that equates to 57 square inches. Where my difficulty begins is that this air has to be brought straight up 50' into the mechanical room. To be able to do this we have to put an inline fan in the duct work but what CFM am I gonna need so as not to create a negative or positive pressure situation? Any takers?
• Member Posts: 111
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You will need

10 cfm of air for every cfm of gas just for combustion. To that you will have to add excess air and, depending on the type of boiler, dilution air. A 400 MBH boiler will use about 400 cfh of gas, or about 7 cfm. My guess is you will need about 100 to 150 cfm of combustion air. The fan will have to move the air against the static drop presented by the 50' of duct and whatever fittings, louvres, etc. are present, but it shouldn't be much, likely less than 0.25". I would suggest you use a fan with a speed control so that you can dial in the performance once it is installed.
• Member Posts: 477
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All I can say is "good luck"! I mean that sort of sarcastically since the biggest hurdle will be getting a local Gas Inspector to accept a powered combustion air system. In 25 years in the building design business, I haven't found any inspector that would accept it. Laws of physics aside, the Gas Inspector, if he decides to accept it, will want proof that this fan is hard-wired to the boiler to insure it runs at the same time as the boiler burner, and also that it is powered by emergency power or UPS if the boiler isn't connected to a fire alarm shutdown. Tha actual CFM calculation as outlined in the other post is OK, I generally also do a check based on air/fuel ratio of 15:1 and no more than 0.03" of friction resistance on the incoming combustion air from an atmospheric opening. If you have a modulating burner, then you also have to vary the fan speed so you don't overpressure the room (and overdraft any other natural draft equipment there too). As a P.Eng, my advice is don't do it, and find a way to get the ir to the boiler as directly as possible from the outdoors.
• Member Posts: 2,322
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What Geoff said

We got involved a few years back with a system like that which another contractor had installed and couldn't get approved. Tell your customer they can't do this and propose another alternative.

My dad always said "There's ALWAYS a plan B".
• Member Posts: 213
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refer to state/local code

you really have to refer to state and local code for the required combustion air allowed. Each state, and even facilities such as the post office have more stringent requirements. The fan shall be based on that. There are no issues installing a combustion fan, we design them all the time, as long as they are sized correctly there are no problems. If oyu have a natural gas appliance your duct must come into the space up high and drop down to the floor, then you need a register at 1'-0" below criling of room and 1 register 1'-0" above the floor, once again, refer to your state code. Finally, why cant you puit a louver in the window??/ if there is a existing window put a louver in with a motorized damper to open with the boiler turning on, but provide proving switches on the blades of the md. if your 50'-0" high, cut a louver and save headacghes and money
• Member Posts: 289
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I agree

find a way,cut a hole in the wall and install a vent. If that is impossible get the local inspector out there, he is going to have final say, if you get the final say 1st you won't have to do it twice.

I'm having a hard time picturing this, 50' up to what a penthouse?, a room in the middle of a building with no outside walls?, a better description may help us help you
• Member Posts: 2
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Thanks for the help everybody

Thanks for everybodys help I have managed to piece this together and in the end the engineer will have to face the music if the inspector doesn't like what she has done. By the way to give you a better picture of what the situation looks like this is a brand new building with the mechanical room in the centre with no outside walls or openings. The combustion air is being brought up from the underground parkade 50' below the mechanical room. In the end I'd probably be better off telling the engineer to call a fan specialist but when you are continually trying to have your product specified just giving a closed door answer doesn't do me any good.
• Member Posts: 477
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Something bugging me

Steve, while I understand the need the "get the sale", I can't help thinking that the P.Eng. who should be responsible for this design is abdicating their responsibility for the design of the boiler "system". If this happened around me, and I heard about it, I'd consider calling the engineer and reminding them of their professional obligations to do the "whole design" since it's their seal on the line if something blows up later. If nothing gets resolved, the next call is to the local P.Eng Association. Sloughing off design responsibility to a supplier for something unrelated to the specific equipment is a trend that a lot of engineers in the building design industry have fallen into, and they have become "catalogue specifiers" rather than true design professionals. This is a dangerous situation in terms of risk, and you have to think about who is supposed to be doing what part of the design job at the end of the day, and maybe diplomatically remind the engineer in question that you are being asked to do something well beyond your scope of responsibility.
• Member Posts: 443
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I've...

... seen "boilerplate" language in some engineer's bid documents that basically attempted to dump the responsibility for literally everything in the lap of the contractor who was awarded the job. They were shedding responsibility for every act, code and regulation - even ones they'd botched, or missed entirely. I don't know if it would have stood up in court. I'd like to think it wouldn't.

But at the end of the chain, there's often an owner who who wants to cheap-out on everything - including the engineering.
• Member Posts: 443
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Exactly

Steve & his company should be REALLY sure that they're not the ones who'll be held responsible for this potential fiasco, and that the documentation defining just who is on the hook at the end of the day is VERY clear. This project has "trouble" written all over it.
• Member Posts: 213
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final solution

I work for a consulting firm, get a louver on the side of the building and run combustion air that way, I do not think its legal to run combustion air from a garage, hazardous/explosive fumes can be sucked in. And second THe PE is supposed to be designing this not you, put the screws to them.
• Member Posts: 2,322
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COMBUSTION AIR FROM A PARKING AREA!!!????

I don't care if it's code or not, that would be about the LAST place I would want to take C/A.

It's the Murphy's Law thing, ya know?
• Member Posts: 477
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Good call.

I didn't even comment on that one, and it's probably the biggest "no-go" item for the intended combustion air source. Parkades will be exhausted, therefore under negative pressure, and if that was used as combustion air, you could downdraft the boiler pretty quick, no matter where it was. Let alone the potential gases, etc. that are in Parkades. A Parkade will be a different Code Occupancy, and require fire separation from the rest of the building. One cannot install a fire damper in a combustion air duct, so that would mean, hypothetically of course, that a combustion air duct would have to go all the way up to the boiler room in a rated shaft (\$\$).

Bottom line- a parkade is NOT an allowable source of combustion air anyway. It might help you get the sale by sitting down with the engineer in question and run through the options and look for a more practical combustion air source.
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