[Greenbuilding] Fireplace questions

[Norbert Senf]

At 10:52 AM 11/6/2006 -0600, Ian Remmler wrote:


>I know most fireplaces just suck out warmth,
>but I would go with a Rumford fireplace, which is supposed to be
>pretty efficient.  I'm in the Austin, Texas area, so I don't
>need to heat the house with it; I just like sitting by a nice
>fire!  So as long as there is no net energy loss, I'm fine.

Hi Ian:

All open fireplaces are limited in efficiency by the amount of excess
air that they have to move. They use 10 times or more air than a fireplace
with a glass door.

The efficiency of a large open fireplace is pretty much a function of
outside air temperature, which has to get drawn into the house and heated
to room temperature. With an outside air temp of -40F, pretty much any open
fireplace will run in the minus efficiency range. With an outside temperature
above freezing, a small open fireplace with a reasonably sized flue is
actually not that bad.



>1. I've read that bringing in outside combustion air is the only
>way to go, but rumford.com indicates that due to the Rumford's
>efficiency, it's unnecessary, and that your ventilation system
>should take care of bringing in makeup air.  Who's right?

There is no such thing as outside combustion air for an open fireplace.
It is more correctly termed makeup air. Combustion air goes directly into
the firebox. An open fireplace doesn't actually 
have a separate firebox, strictly
speaking, until you put a door on it and separate 
it from the house. Even though
the codes call for bringing the air in near the 
fireplace opening, the fireplace
doesn't actually care -- the air could be brought in on the other side of the
room (through a cracked window, for example).

Whether or not you need makeup air doesn't have much to do with energy
efficiency. It depends on how tight your house is. If your fireplace is large
enough, and your house is tight enough, the fireplace can draw enough air
(around 500 cfm for a 4'x4' Rumford) to depressurize your house,
causing it (and other combustion appliances) to spill into the house. Again,
cracking a window is a surefire cure. Actually, 
how much you need to crack the window
is a pretty good measure of how tight your house is.


>2. Assuming rumford.com is right on question 1, would it be
>better to put the fireplace in the interior, or would an
>exterior wall be OK assuming everything is well sealed,

Exterior wall fireplaces are a really, really bad idea, for numerous reasons.

Best ...... Norbert Senf


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Norbert Senf---------- mheat(at)heatkit.com
Masonry Stove Builders
25 Brouse Rd.
RR 5, Shawville------- www.heatkit.com
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