Forced air return vent issues?

[trainer]

Hi there - just stopped by to ask a question. I'm thinking of installing a new wood burning stove (Oregon, USA), alongside a gas powered whole-house forced air system. The stove will be next to (some 2-3 feet away from) the main room return vent for the forced air system. Are there any issues with this? I don't think it will create negative pressure, since there is a hot vent in the room too, but would love your thoughts and advice.
Is it possible to use the vent system and fans to circulate heat from the stove?

 

[begreen]

The return air grille must be 10 ft away from the stove according to mechanical code. Using the forced air system to circulate heat often fails due to heat losses in the ducting. However, if the supplies and returns are fully sealed and well insulated it doesn't hurt to try.

 

[Lumber-Jack]

It seems US codes allow heating duct to be run outside the insulated envelope of the house, often in the attic (seems crazy to me, but whatever), this of course would lead to massive heat lose no mater what sort of heating system you use to run heat though those ducts. If that is not the case in your home, and your ducts are inside the insulated envelope then there should be no real heat lose issues since any heating of the metal ducts will eventually radiate back into the rest of the house.
I use the duct system on my heat HVAC to circulate the air in the house when running my wood stove. The air that comes out of the vents never comes out noticeably "warm" in the bedrooms, but it does definitely moderate the temperature in those bedrooms and throughout the whole house, and consequently the bedrooms are warmer than they would be if I didn't circulate the air. My return air vent is not in the same room as my stove, but I do have 3 vents located in that room. With these vents blowing the circulated air into the stove room moderate the temperature in that room that would normally otherwise be too hot. The idea is to circulate and mix the air throughout the whole house a few times every hour, if you can do that, without losing heat to the outdoors, you can't help but moderate the temperatures in the house.
The other thing worth mentioning is that I have a circulate and fan mode on my HVAC thermostat that allows the blower to run in economical low power mode either intermittently, or constant low speed. Both are very quiet and almost inaudible.

 

[trainer]

Thanks - Lumber-Jack - US codes do allow ducts to run outside the envelope if necessary, but they still need to be sheathed in insulation for the period they are outside the envelope, so while they will loose heat, it's not as bad as you might imagine...

 

[begreen]

Mostly these ducts are insulated at R8. In the northwest's milder temps that loss is usually not too bad, but in an area with below 20F winter temps it can be substantial. The amount of heat loss will depend on the length of the run and the ambient temp. The loss is less noticeable coming from the furnace because the hot air at 130F is warm enough to lose 20 degrees (15%) and still feels warm at 110F. But when it's moving just 80F stove room air a 15% drop to 68F is very noticeable.

This is assuming a well designed and installed system. Unfortunately, there are a whole lot of poorly designed warm air systems out there and even more poorly installed systems.

 

[trainer]

Thanks BeGreen - I don't disagree. If you're running heating ducts outside your envelope in areas where 20 below are frequent, it would certainly be wise to insulate with more than R8. My ducts (in the PNW) are R12, just because I'm anal about these things, but the increase S/A that a duct has still makes it a large heat loss.

 

[begreen]

LOL, I actually meant 20F or below. When you're in the -20F range, the rules change to pure survival unless the house is well set up for cold.

Good for you on the R12 duct runs. That's excellent.

 

[madison]

Surface area as you stated and all the (leaky) connections plus potentially the heat exchanger itself which can be outside the envelope. I actually spend about one hr in the fall and spring stuffing insulation and closing off all our A/C ducts, which contributed to substantial passive heat loss in our leaky attic installed system.

One of the home owner lessons learned... next house, will not have the A/C in the attic.

 

[trainer]

LOL - I was thinking in centigrade! I think the big picture here is the tendency to view codes as the recommended amount, rather than the legal minimum. The idea that we should only install the legally mandated minimum insulation seems very odd to me...

 

[trainer]

The return air grille must be 10 ft away from the stove according to mechanical code.

I just consulted with an installer in my area, and he is not aware of this, saying he just got a stove permitted that was sitting directly on top of a return vent. Could you point me to the source of this code? Thanks!

 

[mellow]

Could you point me to the source of this code? Thanks!

International Residential Code
(IRC 2006)

- Mechanical and gravity outdoor
air intake openings shall be located a
minimum of 10 feet from any hazardous or
noxious contaminant (e.g., vents, chimneys,
plumbing vents, streets, alleys, parking lots
and loading docks, except bathroom or
kitchen vents), or 2 feet below the source.

International Mechanical
Code (IMC-2009)a

- OA intake openings shall be
located a minimum of 10 feet
from lot lines or contaminant
sources, or 3 feet below
sources.

ASHRAE Standard 62.1-2010

- OA intakes located at prescribed
distances from known contaminant
sources.

Source: http://www.epa.gov/iaq/pdfs/building_codes_and_iaq.pdf

 

[trainer]

Thanks - it looks like those are referring to outdoor air intakes - to be clear mine is a cold air return to a forced air heating system, not an air intake.

 

[begreen]

That is the wrong section of code. I quoted it a while back but you'll have to find it with a search. No time now

 

[begreen]

OK, I found it: https://www.hearth.com/talk/threads...s-jotul-600-firelight.6474/page-3#post-156963

 

[trainer]

OK - I think I understand the confusion - you're referring to generic international codes - I'm asking whether such a code exists in Oregon. As far as I can tell it doesn't - you can't have an intake closer than 10 feet from a vent, but there doesn't seem to be a rule about fireboxes and internal return vents.

 

[begreen]

State of WA and OR go by international code as do most states I believe. Stop by on Monday and have a chat with the inspecting authority.

http://ecodes.biz/ecodes_support/free_resources/Oregon/10_Mechanical/10_PDF/Chapter 9_Specific Appliances_Fireplaces and Solid Fuel-Burning Equipment.pdf

 

[blwncrewchief]

When it comes to a question on codes you always want to consult with your AHJ or inspecting authority. Codes are really "guidelines" to go by. Every area can be different, local codes can add to or override other codes, and the inspecting authority can override codes one way or the other. That is why they are referred to as "Authority Having Jurisdiction".

The code sighted:

http://ecodes.biz/ecodes_support/fr...replaces and Solid Fuel-Burning Equipment.pdf

reefers to "A room or space containing a fuel-burning appliance where such room or space serves as the SOLE source of return air." According to how that is written it should only apply to a forced air system with a "central" return or a system where the return air is drawn from a single room only, and that room is where the stove is located. If you have return air vents in more than one room that code should not apply. Now, I have seen that code written a few different ways and in several different versions. That is where it can get sticky. Some versions refer to "any return vent" and some refer to "a return that is the sole source of return air for the system".

 

[trainer]

It's certainly rather confusingly written - 918.6-7 reads [Prohibited sources] A room or space containing a fuel-burning appliance where such room or space serves as the sole source of return air.
Then there are a series of exceptions to that, including 7.2, this shall not apply where the room or space complies with the following requirements: ... 7.2.3 Return air inlets shall not be located within 10 feet etc.
It looks like the requirement for return air inlets not to be less than 10 feet from the stove is part of a series of exceptions, where if that is the case then the previous items do not apply.
As blwncrewchief suggests, the only way to sort this out is to talk to an inspector. Worst case it won't be that hard to move the return vent.

I presume all this is about concerns re negative pressure and pulling fumes into the forced air system?

Thanks for your help!

 

[begreen]

Agreed, the language they use is awkward. The way I read it is that the exception is ok if the return is not less than 10 ft away from the solid fuel appliance. The exception being a return in the same room as the wood stove.

 

[trainer]

Well, I talked to the city permit office, the installer and the stove sales folks, and none of them are familiar with this, all say they see lots of installations with return vents next to stoves without issues. I'm guessing this isn't used where I live. Thanks for your help!