Summary

Here is the International Residential Code on exterior air: R1005.1 Exterior air. Factory-built or masonry fireplaces covered in this chapter shall be equipped with an exterior air supply to assure proper fuel combustion unless the room is mechanically ventilated and controlled so that the indoor pressure is neutral or positive. Combustion air ducted from outside directly into the firebox is inadequate at best, does not improve efficiency, can lead to bad smells and blowing ashes, can cause the fireplace to smoke and, in some cases, if it can back draft, it can be a fire hazard. For the last reason, it's a controversial code issue. We recommend not installing combustion air directly into the firebox unless you are required to. If you install it, the most unobtrusive way to do it is with the Outside Air - Ash Dump Kit set in the inner hearth. Then keep the vent closed to avoid the bad smells, blowing ashes, smoky fireplace and fire hazard. Now, that doesn't absolve you of providing a balanced ventilation system. In fact the code implies as much when it exempts exterior air if you have neutral or positive indoor air pressure. Ideally you should provide as much clean, tempered outside ventilation air as you exhaust through various appliances, fans and leaks. See Balancing the Ventilation System. See also discussion in Tech 12, Exterior air code issues and smoky fireplaces

Discussion

To: Jim Buckley From: "Danky, James P - WHS" Subject: Fire in air intake We were using our Rumford for the first time this fall and a friend noticed that the back vent got so hot that it started to set the siding on fire... So we wonıt use it again until we figure this out. Our contractor had a mason do the work, his first fireplace but he had the materials you provide with the kid available, so what did we do wrong? James P. Danky* James, Outside air vents in the firebox have nothing to do with a Rumford fireplace and we do not recommend them. See our comments at http://www.rumford.com/tech7.html where we include "fire hazard" among the problems with these vents. I recommend that you replace the vent with a firebrick and solid masonry. Make sure, while you are at it, that the firebox is at least 8" thick and 4" clear of combustible material behind the firebox as required by code. I'd like to know more about this incident. Do you think the vent just got so hot it set the siding on fire or was it back-drafting? Do you have pictures? I have been trying to get rid of the misguided code requirements for combustion air and this incident may help. Hoping to hear from you soon, Jim Buckley 11/1/06 Thanks for being in touch. I spoke with our chimney/fireplace installer and he had already spoken with the veteran who helped him to do ours, his first. Of course he was in a panic about getting this right. He and that he would propose, not having been to our place just yet, that he mortar in firebrick to fill in the vent space; and that he go down to the basement where the concrete blocks support the whole and knock a hole down there so that we could draw air from the basement through the ash vent in the fireplace? I did notice the photo on the General Instructions page which has a caption, ³Note: Thatıs a ³cowbell² combustion air intake in the firebackŠ.², that should have tipped him/us off not to install it in our unit if I am reading this correctly. I am at a loss to understand why we did it this way unless it was Code, and we need to change our unit and that code I guess. So, how does that sound for a fix? Thank you. James P. Danky James, That sounds reasonable. Any chance you could send me a picture of the vent - inside and out - and tell me if you think it was back drafting or just conducting heat through to the siding? I'd still like to use the situation as an example in my effort to change the code. The air through the ash dump is probably safer and more common but, unless you have to install combustion air, I would just close off the one you have and not put any combustion air directly into the firebox. It will just lead to bad smells and blowing ashes. See http://www.rumford.com/tech7.html I think I will photoshop that "cowbell" out of that picture on the Instructions. Best, Jim Buckley _______________ 4/7/06 Dear Jim, Thanks for your thorough discussion of the modern Rumford fireplace. I am planning on building a 48" Rumford into my new well sealed home. Ventilation has been a prime consideration in the design of the house and I'll be using a heat recovery ventilation system. I live in a very cold climate and will be relying on a wood boiler as my primary heat source (with a propane backup boiler). I am using the Rumford primarily for ambience but in keeping with my conservation ethic, I want it to be efficient (thus the Rumford design). It seemed quite counter intuitive to bring make up air through the house, thus cooling the air. When I first moved into the cabin that I have lived in for the last 18 years, I opened the doors of the wood stove, picked up my book and settled in for a quiet night by the fire. The outside air temperature was about -10 F and while my feet and the front of my body were roasting, my back kept getting colder and colder until it was unbearable. I realized that the air that I was drawing in to the stove was coming out of the room and being replaced through the poorly sealed front door resulting in a wall of ice cold air moving across the room. The problem was solved by closing the woodstove doors, but it isn't an experience I will ever forget! Clearly the radiance of the open woodstove was small compared to a Rumford, but radiant heat is unidierctional, warming only one side of the receiving body, where convective heat is more even. So, I was quite surprised when I saw that you recommended not bringing makeup air directly to the fireplace. It seemed to me that while it would reduce the radiant heat input, it would reduce the heat losses at distance from the fireplace (radiant heat transfer being subject to the inverse square law, while convective heat transfer is more linear) and the net heat transfer should be equal (or less if the cold combustion air yielded a lower exhaust air temperature). Any thoughts? Thanks! Robin McClellan Potsdam, NY 4/8/06 Robin, Well, you've hit on the Achilles heel of any open radiant-heating fireplace. In very cold weather a heating appliance with a closed combustion chamber (your stove with the door closed) needs less dilution air and is therefore more efficient. It's not quite that simple, however. A radiant heater makes you more comfortable at cooler air temperatures so, even in really cold weather, your insulation works better and you pay less of a penalty for healthy ventilation. My comments at http://www.rumford.com/tech7.html about combustion air in the firebox basically add up to it doesn't work and it comes with four pretty bad side effects. My recommendation is that you find that compromise involving efficiency, ventilation and comfort that suits you and your home best. Here are some suggestions. 1) You might consider a smaller Rumford. A 30" Rumford would radiate far more heat than the open door of your stove but require about a third as much dilution air that a 48" Rumford needs. 2) You have a heat recovery ventilation system. Does it bring in as much fresh air as it exhausts or does it have a net exhaust? Can it be adjusted to bring in more fresh air than it exhausts to help make up for other appliances and fans and the fireplace that exhaust air? 3) You have to bring in as much air as you leak or pump out. If you don't plan for it, it will leak in anyway. Comfort depends largely on where you bring in the fresh air. Bringing it in somewhere remote or where it can be tempered, filtered and humidified such as in a cold air return or behind the refrigerator would be the most comfortable. Introducing fresh air near the fireplace such as an open window next to the fireplace so the cold air will flow from the window to the fireplace without you being in the draft would be next most comfortable. Opening a window behind you so that you are in the draft would be the least comfortable. All would be equally efficient - or inefficient. 4) Radiation is, as you say, directional. Maybe we could invent a rotating spit-like easy chair. But in practice, after a degree of equilibrium has been established, the wall behind you gets warmer and the whole room is more evenly comfortable. All this adds up to taking a whole house view of your heating and ventilation system. Obviously you need some ventilation for good health even if it does make your heating system less efficient. Instead of pumping even more stale air out of your house and creating a situation where all your appliances are competing with each other, consider pumping fresh air in and balancing your ventilation system. In a time before mechanical ventilation, Rumford was pleased than his fireplaces could be used to ventilate as well as heat. He suggested cutting a vent into the flue near the ceiling to clear stale air out of the room and argued that radiant heat was compatible with good ventilation. Of course he lived in Bavaria and England - not Siberia. Warm regards, Jim Buckley >12/18/97 >I am in the process of planning a house to be built. It includes a >fireplace, most certainly a Rumford design. > >I would like to use outside air for combustion in the fireplace to >minimize the amount of heat lost from the rest of the house when the >fireplace is in use. What is the best way to do this, especially >considering the rumford design, that would eliminate any turbulence it >might create by being close to the fireplace opening or maybe even in >the fireplace itself. > >I personally know little, if anything, about fireplace design and would >appreciate any guidance you can give, so that I know how to communicate >my desires to the contractor that will build the fireplace. > >Thanks, Tom tom wright Tom, Your Rumford fireplace will be an efficient radiant heater. Like the sun, it will heat people and surfaces - not the air. It'll make you feel comfortable at cooler air temperatures. That's why they use commercial radiant heaters in UPS garages, aircraft hangars and outdoor restaurants where it's not very practical to heat the air. Most people these days rely on a furnace to heat the house and, if they have a Rumford, they use it to make a cozy and comfortable place to settle for the evening without having to turn up the thermostat. Rumfords are pretty efficient heaters. If the thermostat for your furnace is in the room with the fireplace, however, you may have a calibration problem. The furnace thinks the house is warm and will not come on when you have a fire, so the parts of the house that the fireplace doesn't "see" and heat radiantly, get cooler. That doesn't mean the fireplace is inefficient. If you had an extra very efficient gas space heater or pellet stove in the same room with the thermostat, the rest of the house would also cool off. The fix is to plan for zone control heating, put the thermostat in a bedroom or crank it up higher to make up for the calibration problem. The addition of outside combustion air has no effect on efficiency. It's called the first law of thermodynamics and is related to the conservation of energy and is commonly known as "there ain't no free lunch". To provide enough combustion air for the fireplace to work you have two choices: Duct outside air directly into the firebox which cools off the firebox with the result that you get less radiant heat out of the fireplace. Or you can let air in through an open window or other vent remote from the fireplace which allows the fireplace to burn hotter and radiate more heat - exactly the same amount of extra heat that it takes to heat up the cold outside air coming in the house remotely. The real question then becomes where to introduce outside air for ventilation and combustion and to make up for all the air being pumped out by various fans so that it won't cause draftiness, like an open window might, or bad smells and enough turbulence to make the fireplace smoke, like air vents in the firebox might. I prefer adding makeup air to the cold air return of the air handling HVAC equipment, or adding a vent into a mechanical or laundry room. Simply opening a window a crack somewhere near the fireplace isn't a bad solution either. Remember it's a radiant heater and doesn't heat the air, so the efficiency penalty for good ventilation isn't bad. Best, Jim Buckley

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