Rumford Technical Discussion

Rumford Technical Discussion
Smoky Fireplace Checklist
12/30/07

Email: buckley@rumford.com to ask technical questions or contribute to the discussion.

Buckley Rumford fireplaces, like other fireplaces or woodburning appliances, should draw well. None of the products of combustion should escape through the fireplace opening into the house. The good news is that very few Buckley Rumford fireplaces smoke and so far we have been able to correct them all. To draw correctly a Buckley Rumford fireplace must

be built according to our Instructions with the right sized throat and flue, etc., and
be free of restrictions such as a partially closed damper or restricting cap or improper spark arrester, and
be vented into a chimney that draws when it's cold, which usually means that the chimney is taller than the house, and/or
be within a house or building that has an adequate and balanced ventilation system so that the fireplace doesn't have to fight more than 8 Pascals negative pressure. Neutral or positive pressure is even better. Outdoor fireplaces are an exception. They tend to be temperamental in a breeze and many of them have short chimneys but clearly negative indoor air pressure is not an issue outdoors. See "Smoky Outdoor Fireplaces".
Besides the above requirements, tall chimneys and chimneys built within the exterior walls of the house draw better than short chimneys or chimneys built on the outside of the house. For mechanically ventilated spaces see "Calculations and assumptions behind exterior air requirements for fireplaces" below. And it might help to read Balancing the Ventilation System linked to our Instructions.
These are the rules. Pretty simple, eh? They are in theory, but it's not always so easy in practice to analyze a problem fireplace and decide what to do to fix it. Here is what to do based on some seventeenth century science (mostly smoke and mirrors) and some common sense based on experience.

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Smoky Fireplace Sleuthing

1) First, think about how and when the fireplace smokes. If the fireplace works fine with a good fire burning in it, but smokes (or you get bad smells in the house) after the fire dies down, there's probably nothing wrong with the fireplace and the problem is excessive indoor negative air pressure. Skip to item #4. If the fireplace smokes all the time, or especially when a brisk hot fire is burning in it, keep reading.
2) Look for restrictions. Make sure the damper is fully open and not jammed or in cockeyed. Take the damper blade out if necessary. Make sure there are no other restrictions. Tops and caps should have a combined opening (and spark arrester screens should have a surface area) four times the cross-sectional area of the flue. Take off any tops or screens if in doubt. Look up or down the flue to make sure it's clear. Push a brush or rag through the flue if in doubt. Even spider webs in flues that haven't been used for a long time can be restrictive.
3) Check the fireplace construction against our Instructions to make sure it was built correctly. Check especially to make sure the throat opening is the right size. The fireback should be straight and the throat above and behind the top of the fireplace opening should be rounded and smooth with no ledges or drop-offs that could cause turbulence. The throat should fit well at the top of the firebox, especially at the back and the damper frame should not restrict the throat opening. Look for smooth streamlined surfaces with no projections that would be restrictive or cause turbulence. Make sure the flue is the specified size checking the Instructions and the flue size tables and the article on flue sized if in doubt.
4) If the fireplace is built correctly and there are no restrictions and especially if it works OK with a good fire but is temperamental to start, or smokes after the fire dies down, then the problem is usually negative indoor air pressure or too short of a chimney. The negative pressure, or short chimney, and resultant downdrafts, might also cause a creosote/ash smell when the fireplace is not in use.
Neutral pressure level: The warm air in the house tends to rise and find a way out somewhere high in the house. In other words the whole house acts like a chimney. In most houses there is a neutral pressure level about half way between the ground floor and the roof. Everything above that level is positively pressurized and air will leak out of any open windows or holes, while below the neutral pressure level the house is negatively pressurized and air will leak into the house through any openings. In old leaky houses the negative pressure rarely exceeds about eight Pascals, which is about the difference in pressure in ten feet of altitude - not much. Furnaces and fireplaces usually do all right pulling against a negative pressure of up to eight Pascals.
Modern houses, however, tend to be more tightly built, wrapped, sealed and caulked. They also tend to be full of powerful kitchen and bathroom exhaust fans. So the neutral pressure level may be higher or maybe the whole house is negative and the negative pressure in the lower areas of a new house can sometimes far exceed eight Pascals.
Chimney too short: Extending the chimney is intuitive, logical and often one of the first attempted "fixes" for a smoky fireplace. It's not a guarantee but building the chimney taller than the house always helps. When the house is tightly closed up you can create a positive updraft with mechanical ventilation - even if the chimney is not as tall as the house. Conversely, in a tight house, an imbalance in the ventilation system can cause a downdraft, for example when the kitchen fan is turned on, even if the chimney is taller than the house. But, with the windows open, especially upstairs, about the only way to get draft in the chimney is to build the chimney taller than the house so it draws better than the house.
Wind: Wind can cause turbulence at the top of a chimney or blow down the chimney and cause a fireplace to smoke, especially if there are tall roofs or trees nearby. A far more common way the wind can cause a fireplace to smoke is by de pressurizing the side of the house on which the air intake is located. The wind causes the windward side of the house to be pressurized and the leeward side to be depressurized - by as much as 30 Pascals. It tilts the neutral pressure level so that one side of the house can be all positive while the other side all negative. When opening windows and doors to let in air or locating makeup air intakes be aware of this effect of the wind and make sure air is actually coming in the supposed intake.
Makeup air: For a fireplace to draw well, especially as the fire dies down and little heat is being produced to overcome negative pressure, you may have to let in some makeup air near the fireplace or somewhere low in the house. The six square inch combustion air kits designed to be built into the firebox are not big enough. And they can lead to other problems like creosote or ash smells and enough turbulence to cause the fireplace to smoke. See "Combustion Air". If you have one of these make-up air systems ducted into the firebox, close it. Better to open a window or add makeup air to a cold air return in the heating system or install an air-to-air heat exchanger in the mechanical, utility or laundry room. How much air does it need? See "Calculations and assumptions behind exterior air requirements for fireplaces" below. But I'm getting ahead of myself.
Energy concerns: Many people object to opening a window or punching a hole in their house to let in cold air, especially if they have just spent a lot of time and money to see that their home is tight, well insulated and energy efficient. We recommend balance and control of the ventilation system - not necessarily increasing the amount of ventilation. Some houses are full of fans competing with each other to blow bad air out of the house with no provision for make-up fresh air to come in. Make-up air has to come in. It will come in through leaks or backwards through the kitchen and bath vents or down the chimney. Sometimes it's a good thing the fireplace smokes because when the real problem is discovered we find out the furnace and hot water heater vents aren't drawing and are spilling carbon monoxide into the house. A home with a balanced ventilation system with approximately neutral indoor air pressure is safer, more comfortable, and more efficient.
Testing with smoke: The engineering can be complicated but it's easy to see if you have the indoor pressure under control. Use a smoke pencil or some incense or a candle, and hold it up in the throat of the fireplace to see if the smoke goes up the chimney. If it does, great - no problem. But if the smoke blows down and out into the room, indicating there is a down draft in the chimney, that means the room is too negatively pressurized and the easiest way for makeup air to replace the air being lost up high somewhere in the house is down the chimney.

Note: Once in a while we have discovered a very tight house that doesn't leak in or out so there is no down draft in the chimney, but when a fire is burning the fireplace itself creates the negative pressure that won't allow it to draw properly. You may be able to simulate the air loss caused by the fire by turning on the kitchen fan to see if that creates as downdraft. Then add ventilation air until the chimney drafts up even with the kitchen fan on and you will know how much air is needed for the fireplace. If there is no kitchen fan, open a window or door and build a fire in the fireplace. Then slowly close the door or window and go through all the pressure sleuthing smoke tests.

Find a way to neutralize the down draft. Open a window or door low in the house. Turn off a fan or the furnace. Close the skylight or upstairs window. Do these things one at a time and give the air enough time (a couple of minutes) to turn around and reverse itself. See what it takes to control the pressure so that there is no down draft in the chimney even before you light a fire in the fireplace. The specific things you try depend on your house but always think of reducing the air escaping high in the house and increasing the air coming into the house on the lower floors. The cold air return in the room with the fireplace may need balancing, it may make a difference which window you open or close, especially in a breeze. But you want to let more air in low in the house so when you open a window, check with the smoke pencil to see if air is actually flowing into the house.
Once you've closed off as many leaks as you can high in the house and found out where and how much makeup air you have to let in low in the house, then you can think of a permanent solution like makeup air into the cold air return or an air-to-air heat exchanger that might be more comfortable and more efficient than opening a window. You may have tried some of these solutions already, but if there is still a down draft, as shown with the smoke pencil test, or you need to keep a window open for the fireplace to draw properly, then you need more air.
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Calculations and assumptions behind exterior air requirements for fireplaces:
The 1979 ASHRAE Handbook reports on empirical studies which show that modern masonry fireplaces require a minimum average face velocity or 0.2 feet/second in order to exhaust all the products of combustion.
According to this calculation, a fireplace with an opening of 6 square feet would need a minimum of only 72 cfm so in our code proposal we exempted fireplaces smaller than 6 square feet in opening area from any exterior make-up air requirement.
This same fireplace, with a 12"x12" flue at one cfm per square inch of cross-sectional flue area would, from TABLE 2116.12b in the IRC, require 91 cfm. A four foot wide fireplace with a 16"x20" flue would require a minimum of 214 cfm makeup air.
While these fireplaces might actually require more air with a brisk fire, the greatest potential for spillage is when the fire is dying down and the draft and required makeup air volumes are reduced, so we think sizing the minimum makeup air requirement to the flue size (one cfm per square inch of flue area) and based on the ASHRAE formula is about right and easy to regulate without having to go beyond the IRC code.

Pressure Controlled Make-up Air Systems up to 1000 CFM