Thread: Condensing furnace - intake vent

This is a recently installed furnace. The intake vent is not piped to the exterior.

Does this furnace require the intake vent to be piped to the exterior?

It seems like some condensing type furnaces do not require the intake vent to be piped to the exterior and some do. What indicators should we be looking for to determine what type is present and what types require venting to the exterior?

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Originally Posted by Ken Amelin

This is a recently installed furnace. The intake vent is not piped to the exterior.

Does this furnace require the intake vent to be piped to the exterior?

It seems like some condensing type furnaces do not require the intake vent to be piped to the exterior and some do. What indicators should we be looking for to determine what type is present and what types require venting to the exterior?

Depends on the manufacturer but think most allow the interior intake like that. I would say about 50% of the Cat IV systems I find are just like that.

Originally Posted by Scott Patterson

Depends on the manufacturer but think most allow the interior intake like that. I would say about 50% of the Cat IV systems I find are just like that.

That's what I find too (50%-50%), but I'm really interested in when is it not correct and when is it required. What manufacturers require it and does anyone know why it is ok for some and not for others?

If it is unknown and we say nothing we could be liable for an unsafe situation.

Ken,

When you want to question these things, snap a picture of the data label and look up the manufactures installation instruction online. Sometimes you can find the instruction in a pocket right there at the furnace. One time to always question it is when a furnace is in a smaller enclosed area which might not provide adequate combustion air.

Depends on the square footage of open area that the furnace is installed in. Some manufacturers require 1 sq ft for every 100 btus, so an 80,000 btu furnace would require 800 square feet to provide combustion air.

Originally Posted by Ken Amelin

This is a recently installed furnace. The intake vent is not piped to the exterior.

Does this furnace require the intake vent to be piped to the exterior?

It seems like some condensing type furnaces do not require the intake vent to be piped to the exterior and some do. What indicators should we be looking for to determine what type is present and what types require venting to the exterior?

I always look for an alternate combustion air intake source. In colder climates, when the house is tight and closed up for several months in the winter-there must be some form of fresh air intake present.

think Cape Cod can get fairly cold?

As the others have said in different words - you need to treat it just like any other fuel burning appliance taking combustion air from the interior, starting with 'is it considered confined space space or not'.

If the equipment is in the attic of a sealed versus a ventilated attic it should get the combustion air from the outside. All the other folks have addressed the ventilated attic scenario.

Jerry, How do you calculate confined space. I have seen different methods described here. I thought you used cubic feet not square feet. Calculating Combustion Air

The only way to know is to check the installation instructions. Two things come to mind. Several manufactures require that the combustion air source and the vent be located in the same atmospheric pressure zone, which means next to each other, outside. Another is that several manufacturers state that the combustion air can not be taken from a location that is frequently dusty.

I've come across a few that required direct vent installation at high altitude or if they were vented horizontally. As mentioned, you'll need to check the manufacturer's instructions, as they are all slightly different.

Every furnace manufacturer/installation that I can remember offers the non-piped combustion as an alternate. Because the pressure switches are so sensitive in these unit you typically have to install a baffle plate at the intake to induce a small static pressure.

All in all it looks like a very clean installation with a minor exception. Nice that it's on a house keeping pad.

Originally Posted by Tom Rees

Jerry, How do you calculate confined space. I have seen different methods described here. I thought you used cubic feet not square feet. Calculating Combustion Air

Good call Tom. I have a copy of the Questar Gas good practices book in PDF format I will send you if you'd like. I think you can get a copy on line as well.

Could take a step further and recommend intake air from exterior (most require 1/4 galvanized screen to prevent clog/suffocation/pest) which then allows much safer operation as the unit can function safely up to a massive depressurization, -50 Pascals for BPI last I checked.

Originally Posted by Tom Rees

Jerry, How do you calculate confined space. I have seen different methods described here. I thought you used cubic feet not square feet. Calculating Combustion Air

You do need cubic feet, but first you must get square feet, then multiply by the height for cubic feet.

Confined space - here is an example of what is needed, what is confined space, and why being considered 'not' confined space may not matter on newer houses:
- M1702.1 Required volume. Where the volume of the space in which fuel-burning appliances are installed is greater than 50 cubic feet per 1,000 Btu/hr (4.83 L/W) of aggregate input rating in buildings of ordinary tightness, insofar as infiltration is concerned, normal infiltration shall be regarded as adequate to provide combustion air. Rooms communicating directly with the space in which the appliances are installed through openings not furnished with doors shall be considered part of the required volume.
- M1702.2 Confined space. Where the space in which the appliance is located does not meet the criterion specified in Section M1702.1, two permanent openings to adjacent spaces shall be provided so that the combined volume of all spaces meets the criterion. One opening shall be within 12 inches (305 mm) of the top and one within 12 inches (305 mm) of the bottom of the space, as illustrated in Figure M1702.2. Each opening shall have a free area equal to a minimum of 1 square inch per 1,000 Btu/h (2201 mm2/kW) input rating of all appliances installed within the space, but not less than 100 square inches (64 415 mm2).
- M1702.3 Unusually tight construction. Where the space is of adequate volume in accordance with Section M1702.1 or Section M1702.2, but is within a building sealed so tightly that infiltration air is not adequate for combustion, combustion air shall be obtained from outdoors or from spaces freely communicating with the outdoors in accordance with Section M1703.

"50 cubic feet per 1,000 Btu/hr"

Example: 75,000 Btu/hr input water heater
- long calculation: 75,000 / 1,000 = 75 x 50 = 3,750 cubic feet of space is required to *not be* considered 'confined space'
- short calculation: 75,000 / 20 = 3,750 (this just reduces the calculation down to an easier formula to remember)
- short calculation to do in your head: 75 / 2 = 37.5 then move decimal to places to the right (i.e., multiply by 100) = 3750.0
- (I use the 'divide input Btu/hr by 20 method'.)

Let's say the above water heater is in a laundry room which is 7' x 8' = 56 square feet and has a ceiling height of 8' ... 56 x 8 = 448 cubic feet of space. That laundry room is considered 'confined space' as it is less than the required 3,750 cubic feet of space.

If that laundry room is in a new house with is well insulated and is of nice and tight construction, then M1702.3 applies and "combustion air shall be obtained from outdoors or from spaces freely communicating with the outdoors".

thanks Jerry and Rod!!