Simulate the fireplace during combustion safety testing or not???

Hello all,

I have noticed that BPI Standards for combustion safety do not require the simulation of a fireplace using 300 cfm during worst case depressurization but Resnet does. Is this correct or am I missing something here? Where did the 300 cfm value come from? Is it truly accurate and can it be applied to all homes no matter what the total ft2 is. It seems to me every time I simulate the fireplace at 300 cfm most natural draft appliances will fail spillage. Thiis is causing action to install fan assists or spillage alarms that can be extremely costly to the homeowners. I have been monitoring the depressurization and it seems that when the fireplace is simulated it changes the CAZ by about -4 pa.

Is everyone simulating the fireplace at 300 cfm? Thoughts???

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Why are natural draft appliances begin installed in conditioned space to begin with? On existent homes you don't have a lot of choice, but the fireplace will almost always cause a backdrop in a reasonably tight home unless a window is cracked or the fireplace has it's own intake air. The only ND appliances we see around here are water heaters and very old furnaces, everything else has inducer fans. Moving the water heater to unconditioned space or replacing it with a tankless could be a solution, but both are expensive.. I'm not sure how much of a problem backdrafting is going to cause if there is 300CFM of air being exchanged. How much combustion byproducts can buildup with that high of an air exchange rate?

Thanks for your comments Bob.

In the DC region, due to limited space, more often than not the combustion appliances are within the thermal envelope. In most cases the natural draft furnace or boiler is old enough to be replaced and I am recommending doing so with seal combustion- condensing unit. On the other hand, I encounter brand new natural draft water heaters that the homeowner would not want to replace. Sealed combustion tankless is definitely the way to go, but not going to happen since the unit was just replaced. Most often the install did not include adding a fan assist. These appliances are passing at worst case without the simulation, the spillage is only occurring as a direct result of the 300 extra cfm.  My issues are that I cannot find anywhere in the BPI instructions for setting up worst case using 300 cfm simulated for the fireplace. I keep asking myself is this the right thing to be doing or not. What to do??? Any thoughts?

Even if the appliances aren't sealed combustion, furnaces and tankless water heaters are at least induced draft. Induced draft solves the backdraft issue, although there is still some negative pressure generated when they run. As for fireplaces 300CFM is a kind estimate, most fireplaces draw much more than that if they are burning wood.

Boilers we don't see in Oklahoma, so I can't say about them. I've heard they last practically forever though..

I guess my main question is that BPI standards do not state to simulate the fireplace in its procedures to set up worst case. If I were to follow procedures to an exact I would not simulate the fireplace potentially not depressurizing the CAZ to its worst case. If I do simulate I am causing the HVAC equipment to fail. Who has it right BPI or Resnet? I would hate to keep adding problems to a home’s HVAC system if I did not need to or just the opposite potentially missing something that could create hazardous conditions for the homeowner. Common sense says just simulate and cover all grounds but in the end it is causing a scare too because you are telling the homeowner there system is possibly poisoning them. Regardless we are recommending CO monitors on all levels but just wanted an overall consensus. Thanks ahead of time for all comments and inputs.

I am relatively new to all of this, and have not encountered this scenario yet, but it seems to me that creating a worse case scenario-establishing winter time conditions with doors & windows closed, and running the active appliances that would create a backdraft (exhaust fans) would mean that the fireplace flue is closed(it is more a window than an exhaust fan) and so it would not contribute to depressurization(assuming the flue seals relatively well). Creating the 300 cfm for the fireplace would be akin to opening a window. No?

I'd be interested in hearing from one of the pros in the NE or Upper MW where this may be a common concern.

Hi Bruce,
I can get you started, but there is much to consider when a house has a natural draft fireplace. Obviously, when it is not in use and the damper is closed there is nothing to simulate, assuming the damper seals well. But when it is running, not only does it jeopardize the natural draft appliances, but powered vents run the risk of backdrafting the fireplace. The later is not part of his question, but it is a serious concern as the fire dies down and produces a lot of CO with little draft.

Where the 300cfm comes from I don't know, but that is probably reasonable for a fireplace running full blast. Even though one should not overwork a fireplace, we have far too many issues with first time "fireplace users" not understanding the human adjustments that are often required to avoid problems. The old "open a window" suggestion is one of the patches that is used to get by fixing the real problem.

That real problem is understanding how each fireplace works in each home, inside vs outside, tall vs short, the wood being burned, and how tight the house is, are some of the concerns.

As for simulating the 300cfm, as Johnny stated, few will pass the CAZ test and that should be the red light for the rest of us. Add in the backdraft issues for a traditional old fireplace and the conclusion I have drawn is to NOT recommend any work that would result in a reduction of the air leakage, until the FP is upgraded to a sealed combustion style unit. Drastic, but it is the solution that is needed. Trying to tighten to some unknown level that will avoid all of the problems is a target that has not been established.


This is a good place to begin and lots of great insite. Thanks for the info. Any other thoughts?

The 300 cfm number is in the right ballpark. CMHC (in Canada) tested many fireplaces, both masonry and metal, twenty years ago. We saw some that would do 1000 cfm (big masonry with roaring fire) and some manufactured units that were in the order of 50-100 cfm with the door closed. The same fireplace will have much different chimney flows depending upon the chimney temperature. No one number is accurate, but 300 cfm is a reasonable first approximation.

A big fireplace, like a big kitchen fan, will backdraft almost any chimney. The usage of fireplaces varies a great deal. Some I have seen burning several times a week in the heating season, but there are many that are not used from one year to the next. It is difficult to justify an expensive fix for a house where the fireplace is rarely used. 

There is no right answer on whether a fireplace should be tested as one of the exhaust devices. It depends upon the policies of the program doing the testing. Those that lean toward the conservative, risk-avoidance side can mandate fireplace simulation. Those that are trying to avoid high expense to homeowners can look at alternative solutions: installation of alarms (smoke and CO), education of homeowners, fireplace labelling, education, etc. By the way, in the other testing that CMHC did, they never found a wood-burning appliance that was spillage-resistant. Tighter doors just delayed the spillage. Dedicated air intakes did not help. Wood-burning, tight houses, and depressurization do not cohabit well. Any "solution" is a compromise. On the other hand, banning wood-burning is too draconian and is not realistic. Maybe the split should be between conscious and unconscious homeowners. If they do not want to think about it, tell them to switch to a sealed gas unit. If they are prepared to open windows while burning, and respect their alarms, wood-burning can be made to work at very low risk.


This is one of the big problems with RESNET and BPI not being on the same page. If we go with true depressurization, then we have to consider the fireplaces. Bud hits things right on the head here as there are so many variables. Imagine houses with 3 fireplaces. One thing I have suggested to people with fireplaces who do not use them and don't want to seal them up is to install balloons. The issue is like whole house fans..I've yet to test a house and turn one on that does not make everything spill. 

I think Bud did a great job on nailing the issue on the head & yes to the chimney balloons as most dampers don't do so well.

As for the question, who has it right - that is simple as neither of them do as they both have their heads... (feel free to choose your own metaphor)

The answers are pretty simple, you have a ventilation standard for occupancy & then you have an additional ones for said equipment which should NEVER involve indoor air. This is not only common sense but addressed in the code books and also from manufacturers. Granted we have older houses that were not built to the most current codes or standards, but once you start working on them you need to bring them up to said standards.

While it maybe impractical to replace the equipment, or move the equipment outside of the thermal envelope / conditioned space, there are plenty of ways for making sure that the CAZ is getting it's own supply of air that is required for the equipment & if that means that you take an energy saving penalty there, well so be it as dead or sick customers do not make good referalls... Rant Off

lol..."sick customers do not make good referalls"...



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