Heater combustion air inlet size requirement.

[OH_Varmntr]

I just received my Sterling GG 120k BTU propane heater for the shop and came across NFPA specs on combustion air intake area requirements that contradict the manufacturer specs.

The manual states the NFPA requires 1 square inch of intake area for every 5,000 BTU input rating of appliances. At 120,000 BTU input rating for my heater, it would require a 24 square inch intake area. The manual also specs a 5" heater intake pipe, but that's only 19.63 square inches.

Is the heater input derated for some reason that allows the use of a smaller area intake or is it the use of a blower or ???

Not that it makes a difference to me, I'm still installing it per the manual. I'm just curious.

 

[Jackfre]

I haven't looked at the manual and don't know the heater, but your 24x24 would actually be undersized. they call for "free area" and you have to calc the body of the register you would use. Grill manuf list that in their specs, but some can bee as much a 40% restriction, so your 24x24 grows substantially.

 

[OH_Varmntr]

Per the manual...
"When buildings are so tight that normal infiltration does not meet air requirements, outside air shall be introduced per Sections 1.3.4.2 and 1.3.4.3 of ANSI Z223.1 (NFPA
54) for combustion requirements. A permanent opening or openings having a total free area of not less than one square inch per 5,000 BTU/Hr (1.5 kW) of total
input rating of all appliances within the space shall be provided."

I'm not questioning register size. I'm questioning why the manual states it needs a minimum 24 square inches of intake free area yet the 5" intake pipe it also specifies for the separated combustion air only provides a 19.63 square inch free area.

 

[finn]

They want their 5” intake pipe to be the controlling ( maximum) inlet restriction, ie, don’t have the 5” pipe sucking air out of a closet with a 4” opening.

The 5” pipe for outside combustion air breathes combustion air only, so it doesn’t matter if there is a slight negative pressure in the pipe. You don’t want a measurable negative pressure in your furnace room / closet.