Balancing the Ventilation System

Rumford Fireplaces
Balancing the Ventilation System

11/13/14
In any house 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.

Chimney height and location matter. If the chimney is not as tall as the house or is on an outside wall so the air in the chimney is cold, the house may "draw" better than the chimney. When possible, locate chimneys inside the exterior walls of the house and build them taller than the highest part of the house.

Modern houses tend to be 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 vents in the firebox 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.
Any open fireplace will need at least one cubic foot per minute of make-up air per cubic inch of flue area just to keep it from smoking as the fire dies out. It may take two or three times that much air when a fire is burning briskly. See "Calculations and assumptions behind exterior air requirements for fireplaces".
The engineering can be complicated but it's easy to see if you have the indoor pressure under control. Use a stick of incense, 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 chimney isn't tall enough or 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 (if the fireplace smokes 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. Sometimes you can simulate the effect of the fireplace by turning on the kitchen fan and see if that induces a downdraft in the chimney.

Find a way to neutralize the down draft. Open a window or door low in the house. Turn off fans and the furnace. Close the skylight and upstairs windows. 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 smoke 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.
Bottom line is, under normal conditions - bathroom fan, furnace, clothes dryer on or various windows open - the air must go up the chimney when there is no fire in the fireplace and the chimney is cold.
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