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Insert, or Woodstove Standing in the Fireplace?
Q: I am hoping you can help me. I am looking for a wood burner for a masonry fireplace. My wife likes inserts, but I prefer a freestanding stove
inside the fireplace, thinking that the existing firebrick will absorb heat and radiate it back to my room. I also think that an insert sure must trap a
lot of the heat behind the surround panels.
Any recommendations on what would be best choice would be appreciated. I am not trying to heat the whole house, just the living room, although if
the adjoining rooms get some of the heat that would be great.
You're right about the masonry absorbing heat from free-standing stoves, but when it comes to reclaiming that heat, there's another factor that
comes into play. Take a spoon out of a cup of hot coffee or tea. Now, touch an ice cube to the very top for a couple of seconds. You'll find that the
bottom of the hot spoon rapidly turns cold, as the heat transfers away at the top through a process known as thermal conduction. Masonry is also an
excellent conductor of heat, and masonry chimneys transfer heat away in similar fashion due to the fact that the top of the chimney is exposed to
cold outdoor temperatures. Chimneys that are built on outside walls (and exposed to cold outdoor temperatures for their entire length) transfer heat
away at an even higher rate.
Heat conducts more readily into a colder environment than it does into a warmer one. The result is, although a small amount of the heat absorbed
by the masonry structure might radiate into the room-temperature living space, the great majority will be transfered away into the colder outdoors.
This is a common complaint whenever a free-standing stove is installed inside a fireplace, and is the very phenomenon that gave birth to the modern
Fireplace inserts are specifically designed to minimize heat loss to the masonry, using another heat transfer technique called air convection. Inserts
have an outer box that goes all the way around the firebox bottom, top and sides, with an airspace between that is open only to the room in front.
With this design, radiant energy from the fire is reflected back toward the firebox by the outer box BEFORE it can be absorbed and conducted
away by the masonry structure. This causes the air between the two boxes to superheat, rise and and flow out the top opening, carrying the heat into
the room and drawing replacement air into the airspace at the bottom to be heated in turn. Incorporating a blower that forces more air through the
airspace increases the flow of heated air into the living space, and further decreases heat loss to the masonry.
There's another advantage to this constant flow of heated air into the living space; as this air circulates from room to room, it carries the heat from
the insert with it, enabling the insert to heat a larger portion of the house.
10/19/09: Hang on, Gentle Reader; a little clarification might be in order.
Q: Just want to clarify your Q&A on inserts vs. hearth stoves. I understood you to write that inserts, due to their design, are more efficient at
producing heat or not losing heat, than hearth stoves or as the email described free standing stoves. Correct? I just want to buy an insert or hearth
stove to put into an existing fireplace that we use for wood fires now but lose alot of heat. Thx and thx for the Web site and Q&As. Very helpful.
Check above, and you'll see that Jeff was asking about putting a free standing stove inside the fireplace. Typically, hearth stoves stand outside the
fireplace on the hearth (hence the name), and have rear shields to minimize heat loss to the masonry structure behind. A shielded hearth stove
installed on the hearth is generally considered to be a bit more efficient (less heat loss to the masonry) than an insert. Remove the rear heat shield
and back the hearth stove up so it is inside the fireplace, and the insert regains its advantage.
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