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Old Oct 28, 2008, 04:41 AM
56S
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Question
Fireplace screen blocks radiant heat.

Why is this? My screen is made from approx. .020" wire with about 1/8" squares of open area. Seems to me there would be some heat blocked by these wires and absorbed into the screen but that does not seem to be the case. The fireplace screen never gets very warm and it blocks quite a bit of the radiant heat from entering the room. I'm assuming the screen reflects it back into the fireplace. Reminds me of a buddy's C band wire mesh sat. dish. The mesh was the right size to reflect the signal. What gives? I'd like to find out that a slight change in mesh size would reduce the amount of heat reflected back into the fire. Sure don't want to use the fireplace without it. I live in a log home.
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Old Oct 28, 2008, 07:06 AM
St. Boondock
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Standard fireplaces aren't very efficient. They might warm the room but draw heat from the rest of the house. Rember, all the air going up the chimney must enter the house from somewhere. Even if you have vents in the fireplace itself, they often pull air and heat from other rooms. I would put a wood burning stove or insert in the fireplace. They might not be pretty but they slow the burning process down and radiate MUCH more heat into the room.
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Old Oct 28, 2008, 07:27 AM
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Heat flows in 3 ways.

Radiation - This is heat energy traveling much like light energy. The grill will block this just like it would block light from a light bulb. Some of the heat is reflected back, most is absorbed by the mesh. However once the grill has absorbed enough heat it will also start to radiate a little as well.

Conduction - This is heat flow through a material such metal or air, but without the material itself moving. For example if I apply heat to one end of a metal bar, it will 'flow' to the other end of the bar, but the molecules of the bar are not actually moving from one end to the other.

Convection - This is heat flow in a material due to movement of the material itself. Such as hot air rising in a cold room.

For your fireplace convection and conduction are not what heats the room. In fact as knifemaker pointed out, the area outside the fireplace is probably losing heat if you consider convection and conduction alone. This is because the fireplace is creating airflow up the chimney by sucking air out of the room. Also because of this airflow (convection) conduction through the air simply isn't fast enough to flow into the room. Conduction in the solid material around the fireplace does provide a little heat transfer though.

So basically the radiant heat is the only thing helping the room get warm, if you can figure out the area of the fireplace opening and then figure out the area of the screen you can get a pretty good idea of how much heat transfer you are losing.


Edit:
A 1/8" square has a surface area of 0.015625"
If the .02" wire is centered on the edge of the 1/8" square it will reduce the opening by .01" on each side leaving a .105" square opening with an area of 0.011025" so you are losing 0.0046" over the whole 1/8" square.
That is a loss of 29.4%, pretty significant.

Anyone want to double check that?
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Last edited by BushmanLA; Oct 28, 2008 at 07:36 AM.
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Old Oct 28, 2008, 07:57 AM
St. Boondock
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Quote:
Originally Posted by BushmanLA

Anyone want to double check that?
I don't want to double check it but I will agree that such a loss must be accurate. I've experienced the same thing and THOUGHT the same thing myself. It feels as if the screen is solid and has no openings.
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Old Oct 28, 2008, 09:39 AM
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Quote:
Originally Posted by N2856S
The fireplace screen never gets very warm and it blocks quite a bit of the radiant heat from entering the room.
To me, this sounds like you are running your flue wide open and most of your heat is going up the chimney. Your screen is not getting warm, because it is being cooled by the airflow going through it.
My flue has four positions closed (—), partially closed, mostly open ( / ) and full open ( | ) Once I get the fire going, I set the flue to either the partially closed or mostly open position (unless it is really windy, then full open). This works to provide both convection and radiant heat from the fireplace. We have a ceiling fan in the “fireplace room” that helps to distribute the heat and make the room more comfortable – it also helps get the heat to other parts of the house.
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Old Oct 28, 2008, 09:40 AM
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Quote:
Originally Posted by FL Knifemaker
Standard fireplaces aren't very efficient. They might warm the room but draw heat from the rest of the house. Rember, all the air going up the chimney must enter the house from somewhere. Even if you have vents in the fireplace itself, they often pull air and heat from other rooms. I would put a wood burning stove or insert in the fireplace. They might not be pretty but they slow the burning process down and radiate MUCH more heat into the room.
Often wondered why someone didn't invent a heat exchanger that fits in a chimney and supplies any recovered heat to say a radiator in the bedroom. Seems a crazy waste of heat.

Still remember those big roaring coal fires though from when I was a kid, toasted front, ............ frozen back.
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Old Oct 28, 2008, 09:43 AM
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Quote:
Originally Posted by N2856S
Why is this? My screen is made from approx. .020" wire with about 1/8" squares of open area. Seems to me there would be some heat blocked by these wires and absorbed into the screen but that does not seem to be the case. The fireplace screen never gets very warm and it blocks quite a bit of the radiant heat from entering the room.
your screen may not be blocking thermal radiation because it's possible that not
much such radiation reaches it in the first place. It maybe interesting if you
experiment with the shape of the fire...
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Old Oct 28, 2008, 11:20 AM
Not THAT Ira
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Coupeville, Wa
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You wood (pun) be waaaaaay better off with a fireplace insert that kind of converts it into a wood stove.
A fireplace will actually make most of a home colder by forcibly drawing cold air in through every crack to feed the fires draw. This is the reason for those high back chairs with wing sides, so people could sit in front of the fire to catch radiant heat without the draft at their back freezing their butt off.
Most modern wood stoves and inserts allow air flow to the fire to be finely controlled at a much lower level and many actually draw outside air to feed the fire through ducting or the under home crawl space (I own one of these).
We have been using wood only heat for years and get by with about 3-6 chords per winter. With a fireplace it would be more like 12-15 chords!
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Old Oct 28, 2008, 11:40 AM
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Quote:
Originally Posted by eflightray
Often wondered why someone didn't invent a heat exchanger that fits in a chimney and supplies any recovered heat to say a radiator in the bedroom. Seems a crazy waste of heat.

.

You need that heat to carry out the gases that you don't want condensing on the inside of the chimney and building up into ....either a need to call the chimney sweep monthly or a big chimney fire down the road



Jim
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Old Oct 28, 2008, 11:51 AM
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With a good air-tight insert, like a Lopi http://www.lopistoves.com/, You'd be able to heat your entire house with a whole lot less wood.
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Old Oct 28, 2008, 12:05 PM
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Quote:
Originally Posted by me11owman
You need that heat to carry out the gases that you don't want condensing on the inside of the chimney and building up into ....either a need to call the chimney sweep monthly or a big chimney fire down the road



Jim
a long time ago I helped my father build a U-shaped array of 5 tubes that
held the fire and sucked air from the front of the fireplace (at a level
below the level of the fire) which was then heated and returned to the room
shooting out the tubes diagonally upwards under the mantle. It was ugly but
it worked...it interfered with our ability to grill steaks in the living room
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Old Oct 28, 2008, 04:43 PM
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Quote:
Originally Posted by eflightray
Often wondered why someone didn't invent a heat exchanger that fits in a chimney and supplies any recovered heat to say a radiator in the bedroom. Seems a crazy waste of heat.

Still remember those big roaring coal fires though from when I was a kid, toasted front, ............ frozen back.


Eliza Furnace in PA......


Quote:
Eliza Furnace.
The rear and side of the furnace stack showing bridge connection to the top of the furnace stack and the hot blast heat exchanger on the top of the furnace stack with the down blast tube on the side of the furnace.
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Old Oct 28, 2008, 06:28 PM
56S
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I do have an adjustable flue that will prevent most of the air used for combustion from being drawn from the room. As well as a fresh air vent at the bottom of the fireplace insert that doubles as a clean out door. I agree that a woodstove would do much better but in our house form does not always follow function here. I'm married. The stones from floor to ceiling are quite nice and do hold alot of heat. The insert has a built in heatilator arangement that takes cold air from the lower sides, pushes it through about 6 tubes in the top of the insert and blows out hot and I mean hot air out openings in the stones. I have measured air temps over 400F coming out those holes in the stones! But, that screen does block alot more heat than I think it should. I'm thinking of placing something dark a few feet away and blocking the front 1/2 way with the screen. Take measurements, let cool and repeat with the screen moved to the other side. Oh, well. It sure was nice to come home to a warm wife, a hot meal and a energy wasting fireplace.
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Old Oct 28, 2008, 06:32 PM
St. Boondock
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Quote:
Originally Posted by eflightray
Often wondered why someone didn't invent a heat exchanger that fits in a chimney and supplies any recovered heat to say a radiator in the bedroom. Seems a crazy waste of heat.

Still remember those big roaring coal fires though from when I was a kid, toasted front, ............ frozen back.
You can buy a woodburning furnace that works just like an oil or propane one. They also heat water. No matter how you look at them, if a fireplace is beautiful, it's most likely SUCKING the heat out of your home
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Old Oct 28, 2008, 07:01 PM
56S
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As compared to my propane furnace and clothes drier the fireplace doesn't do bad. With the damper set and the cold air coming in through the bottom from outside it will quickly run us out of the house if we keep a fire in it. When the OAT is around freezing 3-4 hours in the AM is all it is used. We do like the colder bedroom though. I will build the fire before bed and light 'er off in the morning when I get up in the AM. I have looked in to the outside wood furnaces but figure the ROA would take me 10 or more years. I have plenty of wood here and with the Emerald Ash Borer headed this way I'll have more than I need. A also have free wood from a pallet shop if I don't feel Paul Bunyonish. Last year I figured we used about $400-500 in propane and about 3 cords of wood. BTW, I've tried using the reason of scrap balsa for kindling to get a new airplane kit! She just smiles and buys the kit for me.
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