AskCodeMan

Hey Jerry,
For my own info; I was unable to find anything that stipulates the installation methods of the attic furnace. I was only able to find code that states the unit should be supported according to the manufactures requirements.
CMC states the units shall maintain 6" clearance from the deck(wood).
BTW, Fuel line needs support.
What say you?

Hi Marc,

First, that needs to meet the clearances from combustible material, hence the CMC stating it needs 6 inches from wood.

The manufacturer will also give clearance to combustible material, which includes those wood supports.

The gas line not only needs support, but it also needs a sediment trap too.

And if there is an evaporator coil there, that needs a pan under it which is large enough to catch any condensate running back into that gas furnace (typically means under the furnace too).

Thanks Jerry.
As far as terms go, does combustion air have the same meaning as make up air?

Hi Marc,

Marc M wrote:As far as terms go, does combustion air have the same meaning as make up air?

Yes ... and ... no.

The code addresses combustion air as make air for combustion, i.e., the air burned for combustion must be made up for.

The code also addresses make air for exhaust air, which, similarly to being combusted and sucked out of the area, it is being sucked out of the area by exhaust fans, and that negative pressure (not only correct over 'being sucked out' but sound better too) must be made up for with fresh air, otherwise unwanted air will be drawn in through and all all openings, cracks, crevices, failed seals, failed sealants, etc., from the outdoors to the indoors trying to equalize the pressure back to neutral pressure.

Drawing outside air in where not intended causes energy losses, indoor air contamination with outdoor air contamination (which is frequently actually less contaminated than indoor air is), etc., however, along with all that air comes moisture, and moisture being drawn into a structure is not a good thing.

And that is just the tip of the ice berg as to the negative effects of negative pressure indoors.

Thus, what you need to be alert for is what the reference it too: if fuel burning appliances, the reference to make up air is likely meant as a reference to combustion air.