How do you to calculate combustion air?

danitheplumber

There's a condo with 4 boilers and 4 water heaters. The spill switch keeps going out on all 4 boilers periodically. I of course suggest they get chimney cleaned and inspected. But also want to consider combustion air. The boilers are 61000 btu and water heaters are 40000 btus. The basement is 975 sq ft and 7 ft tall. So we have 6825sqft/404 MBH = 16.89 per 1000 btus which is way less then 50 sqft per 1000 btu needed. Or am I missing something here?

mattmia2

When were they installed? the code has definitely gotten stricter and more detailed about combustion air and venting.

danitheplumber

NYC 5 years ago... that calculation method i got from burnham heating helper booklet.

JohnNY

Dani, if there's any air passage to outside maybe you should recommend an intake fan and change the make-up air system from passive to active.

Jamie Hall

You're not missing anything. You don't have enough air.

realliveplumber

Its calculated in cubic feet, not square feet.

kcopp

BTU divided by 20 = ft3 needed
OR
Have ft3 x 20 = BTU support.

That room size can only support 136,500 btu

Zman

If the building is even close to "tight construction" you should be bringing in outside air as spelled out in the IMC. Using just the volume of the space is a pretty big WAG.
Try turning everything on with a combustion analyzer running and then open and close the doors and see what happens. I have seen cases where it has little effect and some where it blows your mind.

Alan (California Radiant) Forbes

The "Combustion Air" section in the 2009 Uniform Plumbing Code has three sub-sections; either using indoor air, outdoor air or a combination of the two for combustion and there are specific guidelines for each. You should get familiar with the code in your area.

As an example: If you can make outdoor air available, you can use the "Two Permanent Openings Method"; one opening within the top 12" and one within the bottom 12" of the enclosure. If you use a vertical duct, each opening shall have a free area of one square inch per 4,000 BTU; a horizontal duct has the requirement of 1 square inch per 2,000 BTU of all appliances in the enclosure.

As another example: If you are using indoor combustion air, the standard method is 50 cubic feet per 1,000 BTU. So, if you have an input of 404,000 BTU for all the equipment and 6,825 cubic feet of space, you have 59 cubic feet per 1,000 BTU.

The code in your jurisdiction may be different.

danitheplumber

So if I divide btu by cubic feet I get 59 cft/1000 btu and should be fine. If I divide cft by mbh I have 16.9 cft/1000 btu and not enough. 

There is outside air duct but just passive not active.

I see this all the time when people flip brown stone into multi family. Sometime I'll see air in can use or inducer fan type boiler but usually not and some of the newer water heater/boiler are more sensitive to make up air.