Minimum distace of return form furnace - InspectionNews

Michael Thomas · #1 (**permalink**) 01-09-2008, 06:45 AM

For years I've been referencing the "10 foot" rule for the separation of returns from furnaces, noting the importance of properly sealed filter openings and the like - but now discover that I really can't cite the sources for such requirement or their methods of application (for example, to what point should the return distance measurement be made - the draft hood?)

Does anyone have a link to a really through discussion of these issue, including the relevant code cites or manufacturer's recommendations?

- Thanks

Jerry Peck · #2 (**permalink**) 01-09-2008, 06:56 AM

Michael,

Answered at your other post asking essentially the same question.

See that other post and the answer.

Michael Thomas · #3 (**permalink**) 01-09-2008, 06:57 AM

I should have been more patient - while I was posting this Jerry P answered some of this this is another, older, thread where I had tracked my questions on to the end yesterday.

Thanks!

Next, I assume there must be a separate section of of the mechanical code which addresses the "unintentional" creation of "returns" at locations such as air-filter access openings under the rubric of "duct sealing"?

Jerry Peck · #4 (**permalink**) 01-09-2008, 07:01 AM

Quote:

Originally Posted by Michael Thomas

the "unintentional" creation of "returns" at locations such as air-filter access openings

Michael,

Not quite following you there.

If the return is not sealed at the access opening and that is within 10 feet, then the return is also within 10 feet right?

But, yes, *ALL* ducts and plenums must be sealed, under the Mechanical Code.

Michael Thomas · #5 (**permalink**) 01-09-2008, 07:03 AM

I copied Jerry's response here, so it will be easier for someone to find by searching the thread's header.

From the 2006 IRC. (bold and underlining are mine)

- M1602.2 Prohibited sources. Outdoor and return air for a forced-air heating or cooling system shall not be taken from the following locations:
- - 1. Closer than 10 feet (3048 mm) to an appliance vent outlet, a vent opening from a plumbing drainage system or the discharge outlet of an exhaust fan, unless the outlet is 3 feet (914 mm) above the outside air inlet.
- - 2. Where flammable vapors are present; or where located less than 10 feet (3048 mm) above the surface of any abutting public way or driveway; or where located at grade level by a sidewalk, street, alley or driveway.
- - 3. A room or space, the volume of which is less than 25 percent of the entire volume served by such system. Where connected by a permanent opening having an area sized in accordance with ACCA Manual D, adjoining rooms or spaces shall be considered as a single room or space for the purpose of determining the volume of such rooms or spaces.
- - - Exception:The minimum volume requirement shall not apply where the amount of return air taken from a room or space is less than or equal to the amount of supply air delivered to such room or space.
- - 4. A closet, bathroom, toilet room, kitchen, garage, mechanical room, furnace room or other dwelling unit.
- - 5. A room or space containing a fuel-burning appliance where such room or space serves as the sole source of return air.
- - - Exceptions:
- - - - 1. The fuel-burning appliance is a direct-vent appliance or an appliance not requiring a vent in accordance with Section M1801.1 or Chapter 24.
- - - - 2. The room or space complies with the following requirements:
- - - - - 2.1. The return air shall be taken from a room or space having a volume exceeding 1 cubic foot for each 10 Btu/h (9.6 L/W) of combined input rating of all fuel-burning appliances therein.
- - - - - 2.2. The volume of supply air discharged back into the same space shall be approximately equal to the volume of return air taken from the space.
- - - - - 2.3. Return-air inlets shall not be located within 10 feet (3048 mm) of any appliance firebox or draft hood in the same room or space.
- - - - 3. Rooms or spaces containing solid-fuel burning appliances, provided that return-air inlets are located not less than 10 feet (3048 mm) from the firebox of such appliances.

Note that those listed under 5. (underlined) *are not* applicable *unless* "room or space serves as the sole source of return air."

I believe that some of these may often be mis-used by applying them to locations where the return air inlet *is not the sole source* of return air.

Michael Thomas · #6 (**permalink**) 01-09-2008, 07:41 AM

OK, so bear with me as I try to parse the application of this to the most typical inspection in my area: 2 story single family home with a full basement connected to the first floor by a stairway with a door at the top.

"Outdoor and return air for a forced-air heating or cooling system shall not be taken from... 3. A room or space, the volume of which is less than 25 percent of the entire volume served by such system. Where connected by a permanent opening having an area sized in accordance with ACCA Manual D, adjoining rooms or spaces shall be considered as a single room or space for the purpose of determining the volume of such rooms or spaces."

Looks like the basement has to be considered separately from "adjoining areas or spaces" (the first and second floors)" as the door is not a "permanent opening". The attic is not conditioned space, and thus not served by the system. In my area the typical basement ceilings in older homes are around 6 '6", and the typical ceiling elsewhere is around 8', so the basement volume is around 28% of the volume served, and the 10' rule does not apply under (3) above.

"- 3... Exception:The minimum volume requirement shall not apply where the amount of return air taken from a room or space is less than or equal to the amount of supply air delivered to such room or space."

If I'm reading this correctly, you could have a return cut into a duct adjacent to a draft hood as long as you had supply registers in the basement providing a greater amount of air at the far ends of the basement, even if the basement is less than 25% of the total volume, which lead us to:

"from... 4. A closet, bathroom, toilet room, kitchen, garage, mechanical room, furnace room or other dwelling unit. "

The entire basement serves as the "furnace room", but... is this intended to apply in such situations, or rather to something like an enclosed utility closet within a basement or other "unconfined space"?

"From ... A room or space containing a fuel-burning appliance where such room or space serves as the sole source of return air."

Typically, there will be return air ducted from rooms above, so this would not seem to apply either...

So, assuming that the basement is 25% or more of total volume, what prohibits someone from putting a big honking return grill in the return plenum at the furnace?

Or even assuming it's smaller, what prohibits someone from putting a smaller return opening there if the supply to the basement is greater than the return volume at the grill?

Am I missing something here?

Jerry Peck · #7 (**permalink**) 01-09-2008, 08:23 AM

Hope you don't mind me quoting you separately - but it is the only to logically answer this.

Quote:

Originally Posted by Michael Thomas

"Outdoor and return air for a forced-air heating or cooling system shall not be taken from... 3. A room or space, the volume of which is less than 25 percent of the entire volume served by such system. Where connected by a permanent opening having an area sized in accordance with ACCA Manual D, adjoining rooms or spaces shall be considered as a single room or space for the purpose of determining the volume of such rooms or spaces."

Looks like the basement has to be considered separately from "adjoining areas or spaces" (the first and second floors)" as the door is not a "permanent opening".

The attic is not conditioned space, and thus not served by the system. In my area the typical basement ceilings in older homes are around 6 '6", and the typical ceiling elsewhere is around 8', so the basement volume is around 28% of the volume served, and the 10' rule does not apply under (3) above.

That puts the basement at over 25% based on your calculations, thus 3. does not apply, but that does not remove 1. from the equation. EACH of those conditions is taken separately as stated in each condition.

Quote:

"- 3... Exception:The minimum volume requirement shall not apply where the amount of return air taken from a room or space is less than or equal to the amount of supply air delivered to such room or space."

If I'm reading this correctly, you could have a return cut into a duct adjacent to a draft hood as long as you had supply registers in the basement providing a greater amount of air at the far ends of the basement, even if the basement is less than 25% of the total volume, which lead us to:

Nope.

First, you are trying to apply different requirements and intermingle them with each other. Apply each separately.

Second, are you even allowed to draw return air from the basement? Is the basement insulated and part of the thermal envelope? If not, drawing return air from the basement would be drawing unconditioned air into the system, which would require additional heating or cooling (and probably dehumidification) to get the 'return air' to be compatible with the 'environmental air' in the conditioned space. Also, is it even 'return air' if no 'supply air' is provided? Would it not be just 'additional air', such as 'outside air', in which case, where would the 'return air' be located?

Are your basements insulated and part of the thermal envelope of the house?

Are your basements 'conditioned'? Or, are they like the attic, just 'separated from' the conditioned space?

I remember (vaguely) the basement of the house we had in New York (40 miles south of Buffalo) and I recall the basement being much colder than the living space above - not freezing cold, just much colder, like it was not heated, but the earth insulated some of the cold out and the heated area above provided some heat through the floor into the basement. But I was young then (we moved to Florida when I was 10) and my memory might be off.

Quote:

"from... 4. A closet, bathroom, toilet room, kitchen, garage, mechanical room, furnace room or other dwelling unit. "

The entire basement serves as the "furnace room", but... is this intended to apply in such situations, or rather to something like an enclosed utility closet within a basement or other "unconfined space"?

First, is the basement part of the conditioned space (see questions above)?

If yes, then I would treat the basement like a mechanical room, furnace room, or even a garage (similar things may be stored in a basement, depending on the basement and access to outside).

If no, then should you be taking 'return' air from the basement anyway?

Quote:

"From ... A room or space containing a fuel-burning appliance where such room or space serves as the sole source of return air."

Typically, there will be return air ducted from rooms above, so this would not seem to apply either...

But are there "returns" from the basement? Are there "supplies" to the basement?

Quote:

So, assuming that the basement is 25% or more of total volume, what prohibits someone from putting a big honking return grill in the return plenum at the furnace?

Or even assuming it's smaller, what prohibits someone form putting a smaller return opening there if the supply to the basement is greater than the return volume at the grill?

See my above questions and answers (more questions than answers, though).

Bob Harper · #8 (**permalink**) 01-09-2008, 08:32 AM

Remember, codes are a minimum std. If this is installed in a confined space but with the return 10ft 1 inch away from the vent collar or draft hood, you still have grounds to advise them to:
a) address the confined space with makeup air per the code OR BETTER
b) move the return even farther away such as outside the confined space.

Appliances can't read code books. Even with the code required MUA, the return could overpower the vent. Fans are very powerful---atmospheric vents are very, very weak.

This is a situation that warrants a more comprehensive system evaluation including a Worst Case Depressurization Test, duct balancing & sealing and combustion analysis.

Bob

Michael Thomas · #9 (**permalink**) 01-09-2008, 11:08 AM

Quote:

Originally Posted by Jerry Peck

Hope you don't mind me quoting you separately - but it is the only to logically answer this.
.... you are trying to apply different requirements and intermingle them with each other. Apply each separately.

No problem, I'm not touchy about such things.

- 1. Closer than 10 feet (3048 mm) to an appliance vent outlet, a vent opening from a plumbing drainage system or the discharge outlet of an exhaust fan, unless the outlet is 3 feet (914 mm) above the outside air inlet.
In the case of a natural draft gas appliance vented to the exterior, what is the "appliance vent outlet" at the interior with violates the 10' requirement in (1) above, the draft hood?

James Duffin · #10 (**permalink**) 01-09-2008, 11:41 AM

How does all this apply to a furnace with the return at the furnace...like when the furnace is in a closet and the return in the closet door?

John Arnold · #11 (**permalink**) 01-09-2008, 11:49 AM

Quote:

Originally Posted by Michael Thomas

In the case of a natural draft gas appliance vented to the exterior, what is the "appliance vent outlet" at the interior with violates the 10' requirement in (1) above, the draft hood?

Yes. The 10 foot rule applies to the burner area and the draft hood. Same with a water heater - the furnace return cannot be within 10 feet of the water heater burner or draft hood.

Jerry Peck · #12 (**permalink**) 01-09-2008, 11:58 AM

Quote:

Originally Posted by Michael Thomas

what is the "appliance vent outlet" ... the draft hood?

Yes. Where, or to what, you would connect the vent to.

Jerry Peck · #13 (**permalink**) 01-09-2008, 12:05 PM

Quote:

Originally Posted by James Duffin

How does all this apply to a furnace with the return at the furnace...like when the furnace is in a closet and the return in the closet door?

James,

First, an "assumption" ... The return opening in the closet door 'seals to' the return duct when the door is closed.

Now (assuming the door is weather stripped and has a weather stripped threshold) the return is 'outside the closet' and there is no path for the air to get from the return inlet in the closet door to the furnace in less than a 10 foot path.

Of course, when we make the above assumption, we need to recognize that shortly after things are installed the 'seals to' seal will begin wearing and may leak at the door / return air duct joint, and, that the weather stripping around the door will begin to wear and may leak around the door ... in which case it would likely create a problem which would need to be addressed. And the best way to address it is to duct the return someplace else where one is not relying on 'seals at the door' to seal the return air from the furnace - however, after construction, it is not always practical to re-route ducts.

James Duffin · #14 (**permalink**) 01-09-2008, 12:33 PM

I was thinking of all of the Coleman gas and oil furnaces I have seen in manufactured homes with the return in the front bottom panel of the blower compartment of the furnace. How did the manufacture get by with that configuration I wonder?

Jim Luttrall · #15 (**permalink**) 01-09-2008, 12:37 PM

Those are not installed in a confined space (typically) and mobil homes fall under a different code. Some also have a almost hidden air inlet through the floor.

Michael Thomas · #16 (**permalink**) 01-09-2008, 01:40 PM

Quote:

Originally Posted by John Arnold

Yes. The 10 foot rule applies to the burner area and the draft hood. Same with a water heater - the furnace return cannot be within 10 feet of the water heater burner or draft hood.

OK, that makes sense, and would answer my question (that's why I was asking earlier if the distance should be measured from the draft hood).

Now, all I need is is code or commentary cite for that...