Rumford Technical Discussion
Smoky Fireplaces

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Date: Thu, 04 Dec 1997
To: (Harry Wappelhorst)
Subject: Re: Fireplaces/Masonry/Smell


Well. I wish your fireplaces were Rumfords too. However, your problem probably has nothing to do with the fireplace design, but instead with negative indoor air pressure.

It sounds as if you're tried a lot of solutions and at least suspected pressure, but here is how I would try to approach the problem systematically:

If the fireplace draws when there is a good brisk fire in it then there is probably nothing wrong with the fireplace. If it doesn't draw well with a good fire, then look for a blocked flue, partially closed damper, restricting chimney cap or poor design like badly formed throat or too small a flue size.

Assuming the fireplace 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. The negative pressure, and resultant downdrafts, would also cause the creosote/ash smell when the fireplaces are not in use.

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 every house 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 negative pressure in the lower areas of a new house can sometimes far exceed eight pascals.

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. Usually 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. 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. But I'm getting ahead of myself.

The engineering can be complicated but it's easy to see if you have the indoor pressure under control. Use an incense burner, or something that smokes, 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 creates the negative pressure that won't allow it to draw properly. In such a case (when no down draft is observed without a fire burning) first 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 incense burner 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 palatable 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 incense burner test, or you need to keep a window open for the fireplace to draw properly, then you need more air.

Let me know what you find out.


Jim Buckley

On 12/4/97 you wrote:

I have read your web pages and thought you might have an answer to our problem.

In June 1994 we built our retirement home. It is a one story house with a 9' lower walkout level. On the first level we have a kitchen/dining area, a family room, three bedrooms, laundry room and 1 1/2 baths. About 2/3 of the lower level is finished into a family room,bath, and bedroom.

We built masonry fireplaces in the first floor family room and another in the lower level just below the on one on the first floor. Both fireplaces have independent 12" x 12" clay flues wrapped in a single concrete brick chimney with rock pavers on the outside.

The lower level has an angled flue built of brick just above the smoke shelf until it reaches the clay flues. We experienced two problems with the fireplaces.

1. When we lit fires in either fireplace smoke migrated through to the other and filled that room with smoke. Attempts to solve this problem generally failed. Chimney height was raised to a height above the tallest peak, both flues were torn down and rebuilt above the first floor, caps were added on top of the flues, extra air was added in the fireboxes, etc.

A Chimney sweep noted that the original mason had not parged the interior of the lower firebox above the firebrick and the area above the smoke shelf to the clay flues. After this was done the problem appears to be resolved.

2. In periods when there are no fires in the fireplaces, we experience a backdraft expecially in the lower level fireplace and this brings with it a creosote/ash smell that permeates the lower level and even reaches parts of the upper level. To try to solve this problem, Caps were installed on top of both chimneys. We added outside air to the lower level. We have also added an "air impeller" that forces air into the lower level. There are a number of vents on the first floor that probably compund the problem, bathroom vents, laundry dryer, and ceiling fan. We run the fan on the furnace always to balance the air/heat throughout the house.


I appreciate your counsel. Any help you can give will be appreciated. I SURE WISH I HAD HEARD OF RUMFORD FIREPLACES BEFORE I STARTED BUILDING. I thought the contractor and mason had sufficient knowledge to build a masonry fireplace, apparently not.



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