Thread: Fresh Air Fiasco

I got this question from someone yesterday. Any ideas from the hoary heads out there?

"The fireplaces were spec’d as a Heatilator unit, but the Lennox brand was installed instead. The one we are concerned about is in the master bed room and has an outside fresh air supply that is installed in the adjacent exterior chimney wall and about the same height as the fire box. The fireplace (it is not one with a blower) has cold outside air that flows out from the side and bottom vents of the metal firebox during the cold winter days and increases when the HVAC is running. I assume when summer heat comes, this will draw in warm/humid outside air thus working against the air conditioning system. There was also a moderate smoky smell in the hall, master bath area, and the bedroom when the fireplace was being used. I could not detect any smoke coming out of the firebox. I did not check the attic space (which is conditioned) above this area.

The builder and the Lennox rep tells us this is normal, but we have friends that have a similar unit and when their HVAC unit is on there is no air that comes from the side and lower vents as ours does. I am told there should not be fresh air freely, or forced by the HVAC, that should come from these vents and we are worried about safety first and energy efficiency next."

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Thats an interesting one. The first obvious response is, can I see the installation manual for either/both units.
If the fireplace is set-up for outside combustion air intake then there should be an operable damper to open/close for when the unit is not in use. Without an operable damper I'd consider it a bad/incomplete install.
Is there a return register near the fireplace. Maybe it's creating suction and pulling air out of the firebox when the hvac is on.
Interesting to me is the idea that the outside air duct goes through the adjacent chimney. Does the furnace possibly vent into that chimney? A cross connection in the chimney wouldn't surprise me.
Is the outside air duct properly sized, attached, what type of hat on the outside, does then orientation of the hat (say in a gangway) contribute to air being pushed in excessively?
I would probably start by taking some airflow readings at the front of the firebox with my meter with units ON/OFF and see what things read. This is also one of those times I'd pull my seesnake out and go probing.
Hope that helps. Markus

Originally Posted by Markus Keller

Thats an interesting one. The first obvious response is, can I see the installation manual for either/both units.
If the fireplace is set-up for outside combustion air intake then there should be an operable damper to open/close for when the unit is not in use. Without an operable damper I'd consider it a bad/incomplete install.

I would think that having a damper in the combustion air would NOT be allowed.

If there was a damper in the combustion air, the fireplace could be operated with the damper closed, meaning no combustion air, meaning the house supplies all combustion air, meaning someone may die.

Or maybe that is just me thinking logically?

I absolutely agree with you on the dangers Jerry. People will do the dumbest things (myself included). I just don't like the idea of an open ended air intake without any control over it, unless the run has a loop in it. I've stood in front of a few fireplaces and could feel the draft coming in. If it were my install I would consider a powered louver damper, low voltage wired to the unit to open and close. I'm not a big fireplace fan either way.

I must wonder where "Hot Foot" Bob Harper stands on this. I assumed from his postings that this was his area of expertise . . .

Aaron, sometimes I hold back responding because I want to see the thought processes of everyone. If I provide answers immediately to every issue it tends to discourage people from doing their own research for answers and that is where the real learning comes from--not from some loud mouth like me providing answers. Also, I've been really busy and have not had the time to post.;-)

My knee jerk reaction to this thread was to question if an open hearth fireplace is appropriate for a bedroom application in the first place. The gas code clearly prohibits taking combustion air from the bedroom but for some reason open hearth fireplaces do not address prohibited locations in the IRC or NFPA 211. So, while there may not be a direct code prohibition, the practical concern remains.

If they are smelling smoke in other areas of the house, they most assuredly are getting some level of carbon monoxide as well. This is a problem anywhere in the house be especially in the bedroom! That's why open hearth fireplaces should be banned from bedrooms. Only sealed combustion direct vent or electric appliances should be allowed in sleeping rooms IMHO.

The fireplace must be installed per the UL 127 listing. That includes the combustion air kit. The entire kit must be listed with this fireplace and installed to the listing. Now, the IRC prohibits combustion air intakes from being located higher than the inlet to the combustion chamber, however, if a mfr. has a detail in their listed instructions showing how to locate this intake much higher than the Fp but installing a trap feature where it attaches, then the listing takes precedence and is allowed. Heatilator has done this (at my suggestion) and it is universally accepted so far as I know. If you have an intake higher than the fire chamber and the intake is located in a negative pressure zone such as from wind, you can backdraft heat, smoke and gases out the air intake and cause a fire. The trap defeats the backdraft while minimizing cold air infiltration at standby. Now, all factory built fireplaces I'm aware of have a 'damper' for the combustion air, which is a simple flap cover right where the air enters the shell of the fireplace. Some are friction fit and some are magnetic. When not in use, they tend to be points of cold air infiltration as long as the house is depressurized. Even closed, most leak some air.

As for the performance of these air kits, understand that while required now by code, they do NOT provide adequate combustion air but, as noted above, could backdraft. A study in Canada proved an air flow of about 10-12 CFM at a 5 Pascal negative pressure indoors WRT outdoors and a short unrestricted duct. Your typical fireplace inhales about 400-600 CFM so this is like peeing on a forest fire. The fireplace uses house air the same as a Cat. I water heater, boiler or furnace. Yes, if the Combustion Appliance Zone is depressurized, it can backdraft and be hazardous. Fix the house and the appliances will work in general. FYI, I have seen more than one instance where these kits contributed to the smoking issue rather than helping. The code requires you install them but it doesn't require you to use them. Fix the house.

If you are getting cold air infiltration through the fireplace, fix the house and it will stop. Same for odors. Now, understand in summer the natural draft can reverse making it very difficult to stop this process. Leaky ducts are a major contributor. Again, the solution is to fix the house and its mechanical systems. The fireplace is just a symptom of a larger problem.

Markus, don't worry, the fireplace in question doesn't have a 'fan' either!

HTH,
Bob

Originally Posted by Bob Harper

Aaron, sometimes I hold back responding because I want to see the thought processes of everyone. If I provide answers immediately to every issue it tends to discourage people from doing their own research for answers and that is where the real learning comes from--not from some loud mouth like me providing answers. Also, I've been really busy and have not had the time to post.;-)

My knee jerk reaction to this thread was to question if an open hearth fireplace is appropriate for a bedroom application in the first place. The gas code clearly prohibits taking combustion air from the bedroom but for some reason open hearth fireplaces do not address prohibited locations in the IRC or NFPA 211. So, while there may not be a direct code prohibition, the practical concern remains.

If they are smelling smoke in other areas of the house, they most assuredly are getting some level of carbon monoxide as well. This is a problem anywhere in the house be especially in the bedroom! That's why open hearth fireplaces should be banned from bedrooms. Only sealed combustion direct vent or electric appliances should be allowed in sleeping rooms IMHO.

The fireplace must be installed per the UL 127 listing. That includes the combustion air kit. The entire kit must be listed with this fireplace and installed to the listing. Now, the IRC prohibits combustion air intakes from being located higher than the inlet to the combustion chamber, however, if a mfr. has a detail in their listed instructions showing how to locate this intake much higher than the Fp but installing a trap feature where it attaches, then the listing takes precedence and is allowed. Heatilator has done this (at my suggestion) and it is universally accepted so far as I know. If you have an intake higher than the fire chamber and the intake is located in a negative pressure zone such as from wind, you can backdraft heat, smoke and gases out the air intake and cause a fire. The trap defeats the backdraft while minimizing cold air infiltration at standby. Now, all factory built fireplaces I'm aware of have a 'damper' for the combustion air, which is a simple flap cover right where the air enters the shell of the fireplace. Some are friction fit and some are magnetic. When not in use, they tend to be points of cold air infiltration as long as the house is depressurized. Even closed, most leak some air.

As for the performance of these air kits, understand that while required now by code, they do NOT provide adequate combustion air but, as noted above, could backdraft. A study in Canada proved an air flow of about 10-12 CFM at a 5 Pascal negative pressure indoors WRT outdoors and a short unrestricted duct. Your typical fireplace inhales about 400-600 CFM so this is like peeing on a forest fire. The fireplace uses house air the same as a Cat. I water heater, boiler or furnace. Yes, if the Combustion Appliance Zone is depressurized, it can backdraft and be hazardous. Fix the house and the appliances will work in general. FYI, I have seen more than one instance where these kits contributed to the smoking issue rather than helping. The code requires you install them but it doesn't require you to use them. Fix the house.

If you are getting cold air infiltration through the fireplace, fix the house and it will stop. Same for odors. Now, understand in summer the natural draft can reverse making it very difficult to stop this process. Leaky ducts are a major contributor. Again, the solution is to fix the house and its mechanical systems. The fireplace is just a symptom of a larger problem.

Markus, don't worry, the fireplace in question doesn't have a 'fan' either!

HTH,
Bob

Bob: Great information. Thanks!