Replies

1. GBA Editor
   Martin Holladay | Feb 20, 2013 01:18pm | #1

   Ron,
   Your post raises a lot of questions.

   The first issue is the size of your AC systems -- one 5-ton and one 2.5-ton unit sounds like a lot of cooling capacity for a 2,600-s.f. house. My guess is that the HVAC contractor never did an accurate load calculation.

   Second, I'm not sure what you mean by EVR. Perhaps you mean "ERV" (energy-recovery ventilator)?

   Third, I'm not sure what is causing your fireplace to backdraft. One common cause of this symptom is turning on an exhaust appliance (for example, a range hood fan). It's also possible that your house is under negative pressure due to an unbalanced forced-air system (due, for example, to a leak in one of the supply ducts in your attic).

   Opening the front door shouldn't cause your fireplace to backdraft.

2. Expert Member
   Dana Dorsett | Feb 20, 2013 02:09pm | #2

   +1 on the HVAC oversizing issue. On a house that tight and well insulated you'd need huge expanse of west-facing glass to need even 5 tons of cooling for a 2600' house. Most houses that size could run with 2.5-3 tons or less for the whole house. How you get to 7.5 tons cooling (2.9 tons per 1000' of conditioned space) is completely beyond me! If the air handlers and ducts are in the attic above the attic insulation (bad practice, but common in many areas) that could add a ton or a large fraction thereof, maybe even two if the the ducts aren't insulated.

   No matter how tight or leaky your house, mechanical ventilation is always a good idea, and in TX using ERV is preferable to most other methods, due to the high summertime dew points. With ERV the ventilation will cut the latent cooling load of the ventilation air in half compared to HRV or supply-only/exhaust only schemes. While ERV systems will put holes in the side of your house, they're in no way going to big enough to act as makeup air for a fireplace, and even if they were, it might not be enough to stop the backdrafting issue, depending on the exact causes of the backdrafting. ERV would certainly clear your house of smoke-odor more quickly though, but stopping the backdrafting entirely is the goal.

   Debugging the backdrafting fireplace issue starts with first verifying that the stack is clear of bird's nests, dead squirrels, etc. (I'm not kidding- last year I had to remove the well-dessicated and cresote encrusted skeletal remains of a squirrel just above a woodstove stack's damper to correct a drafting issue at a relative's house.) Then, check the air-sealing all HVAC ducts and register-boots, (maybe even duct-blaster testing, if you don't have good access). Also verify that the vent stack for the fireplace is insulated, and extends at least the code-min above the ridge line of the house. Also start looking for the air leaks in the pressure envelope of the house, particularly at the attic floor plane. Leaks into a ventilated attic depressurizing the house, unless the fireplace stack is well above the ridge-vents the house could have a higher stack effect draw than the fireplace stack under some wind conditions. You may be able to fix the issue by raising the top of the chimney 5' above the ridge line & any plumbing vent stacks, etc) to fix the problem but it's far better to just kill-off the house's stack effect by air-sealing.

   If the fireplace and stack is on an exterior wall (sounds like it is, if it's adjacent to the front door),and the stack is not well insulated, it's draw (particularly when cold-starting, or at low fire) will be much lower than if it was running up the center of the house. I've seen instances of poor draw on similar fireplace units where the chimney chase was not insulated, and the stack piping had R4 at best, resulting in smoky starts and backdraft susceptibility, particularly on colder days.

3. 
   davidmeiland | Feb 20, 2013 03:19pm | #3

   I wouldn't rush out and install an ERV as a way to stop the fireplace from backdrafting, especially given that the problem happens when you OPEN the front door. If it were happening when the door was closed, that would be more easily understandable, and it might be an indication that the house is too tight, and that the fireplace lacks enough combustion air... but even in that case an ERV is not intended to be the source of combustion air. You could spend a bunch of money and not solve the immediate problem that way.

   I would start with the fireplace rep. Ask them to visit the house and/or talk to them on the phone. They will need details about the house which hopefully you can provide accurately if they don't make a site visit. They should make sure that the whole thing is installed and working correctly. It's worth considering glass doors on the fireplace if you do not have them, assuming the unit has adequate combustion air into the firebox.

   I would also consider having someone come out and perform a blower door test and an energy inspection. That person can help you determine if the house is "too tight" and perhaps if there are other issues. A good technician may be able to inspect the chimney system, perform some air pressure diagnostics on the house, and figure out the problem. You are looking for an energy auditor, a building performance contractor, possibly a HVAC contractor.

4. Expert Member
   Dana Dorsett | Feb 20, 2013 04:41pm | #4

   David: I thought most (all?) of the FMI fireplaces already came with glass doors and many or most come with outside combustion air provisions. I presumed the "...Freshe Air Lever..." referred to in the original was the draft control for the outside combustion air(?). (They're not exactly woodstoves, but some models come pretty close.)

   http://www.fmiproducts.com/prod_list.cgi?cat=0.1

   (Ron- which model?)

   I'm guessing there's a drafting issue related to either a flue obstruction or insufficient flue height as the most likely causes. But tightening up the house won't hurt, and will only help, if it's a model that takes it's combustion air from the outdoors.

   If the front door swings out rather than in it can create a short depressurizing blip which arguably could cause some smoke spillage, but you'd have to make some HUGE holes in your house to make that issue go away. Making the front glass more air tight would take care of it though (which may take a consultation with the manufacturer/distributor.)

5. 
   davidmeiland | Feb 20, 2013 08:54pm | #5

   Dana, I did a quick google on FMI and the first few images I saw were open fireplaces, no doors. You couldn't do that in this jurisdiction, and of course I'm not sure what the OP really has--we need clarification. I've done a few inspections here with fireplace issues and people sometimes, um, install some temporary doors on the fireplace for the, um, final inspection, and then remove them.

6. Expert Member
   Dana Dorsett | Feb 21, 2013 03:38pm | #6

   I get it, people will do the strangest things to defeat a design. But there's no point to having the outdoor air supply & damper if you don't install the glass, eh? (Sort of a "The windows are free, glass costs extra" kind of thing, mayhaps? )

   I'm not a big fan of open hearth fireplaces of any type for their high pollution/low efficiency aspects. An engineered glass-front unit with separate outdoor combustion air and flue damper controls can probably hit 30% efficiency as a heating appliance but not 50%, but still way better than the 7-10% efficiency of an open hearth. An EPA rated wood burning insert will usually be more than twice as efficient as any glass front fireplace, with 1/10th the soot emissions.

   The FMI PureFire units come with a catalytic cannister to clean it up, but is still considerably dirtier than most non-catalytic EPA rated woodstoves with secondary-burner/air injection.

   But whatever this fireplace is, it is. He should be able to debug it well enough that the sooty particles end up outside, rather than backdrafting into the house whenever the door is open.