When does an 80% furnace need to have fresh air?

[electrajim]

How do you establish when an 80% furnace needs to have fresh air suppied (ducted in from the outside) for combustion?

Jim

[BaldLoonie]

All furnaces need air for combustion. Code says the area around the furnace (and WH) is sufficient if there is 50 cu ft of space for each 1000 BTU of input. So typical 100,000 BTU furnace needs 5,000 cu ft basement or room. If it doesn't have, then properly sized openings to a larger area are needed.

[hvaclover]

All furnaces need air for combustion. Code says the area around the furnace (and WH) is sufficient if there is 50 cu ft of space for each 1000 BTU of input. So typical 100,000 BTU furnace needs 5,000 cu ft basement or room. If it doesn't have, then properly sized openings to a larger area are needed.

Some folks make the mistake that if the basement (or where ever the furnace and WH are located)does have the stated volume its, ok to build a room with a door around them.

I run into this quite a bit around my area. When I do an installation in an open basement I explain that if they build a room to isolate the furnace with a door that they intend to keep closed they must provide a means of combustion since they are cutting down the necessary area they can draw fresh air from.

In this case there must be ventilation provided, 2sq in per 100k in put.

Registers or louvered grills at floor and ceiling level with the free area stated can be used.

Alternately if the furnace an WH are behind a tight wall and the cubic footage area of the home does not lend it self to the 50 cubic foot rule, than
a round duct from out side terminating a foot above the floor will suffice.
The air opening from outside looks like a dryer vent with a screen to keep critters out. Check with local authorities for the size of round duct you need.
I have seen it vary from place to place. Around here a four or five inch round duct would suffice.