I’m asking for factual information and not just opinions so I hope this question stays in GQ. I want to put glass doors on our fireplace to keep us from losing so much heat up the chimney when we’re having a fire. My belief is that most of the heat from a fireplace comes from radiation, and glass doors won’t make much negative impact on the heat we derive. But they will limit the amount of air that goes out of our house, saving a good bit on heating costs. When I went to a fireplace store to price glass doors, the guy said to open the doors when you’re having a fire. The glass can break with the heat, etc. But that would totally obviate the need for having glass doors, in my understanding. What’s the SD on this? Do the doors stay closed during a fire, keeping the heating costs down, or are they just decorative items that should be open during a fire (in which case, I wouldn’t buy them). Thanks, dopers. xo, C.
Well, my guess would be that you’d lose a significant amount of heat by not having the doors open. If it were as easy as closing the doors to increase the total efficiency of the system, someone would have started used Pyrex for fireplace doors.
You close the glass doors on the fireplace when there’s no fire burning. This is to prevent heating loss through air going up the chimney and to keep the ash in the fireplace. You open the glass doors when there’s a fire in the fireplace because (as you’ve already been told) then glass can break in the heat and because the fire needs air to burn. This air comes from the room, not from outside.
All fireplace doors have vents in them to permit some air to come in to feed the fire. I’ve seen many fireplaces with closed doors, so I’m skeptical of the claims that the doors will crack. I presume the glass is treated - pyrex, borosilicate, or something. So I have conflicting information.
When we had glass doors on our wood burning fireplace, we kept them shut.
My daughter’s house has a gas fireplace with glass doors, and the instructions say to keep them OPEN when burning the gas logs.
~VOW
We keep ours shut. It doesn’t really seal the fireplace which is made from rough stone, but it cuts down on the amount of air going up the chimney. The glass in fireplace doors is intended for high heat, but not necessarily as hot as can be encountered with the doors shut. You probably want to leave them at least partially open with a very hot fire.
I’ve noticed that fires tend to burn hotter and faster with the doors shut - the air comes in though vents at the bottom of the doors, and fans the coals directly.
When I have a fire, I slightly open a window near the fireplace. This provides “make up” air, and keeps the rest of the house warmer.
I have no data on fireplaces. They almost do not exist here. I will share wood stove data and possibly some of it will be relevant.
Glass door wood stoves are common. I have never seen one break. Even when the flames are linking at with with ferocity. keeping the doors closed and vent damped down makes the fire burn much much longer with improved efficiency. A well sealed wood stove damped down can burn a seasoned log for as long as 24 hours. Not at full heat but you won’t come back to a cold stove there will still be coals in it. Keeping the doors open will burn the wood much faster and allow for little to no radiant heat. It will just go up the stack. So the idea is really heat up the box and keep the heat inside the house. I don’t really know how fireplaces work but if I had one I would try to operate it in the same manner. Possibly to disasters results.
My honest answer: you will have to experiment with your particular set up.
The glass won’t break. It’s made for a fireplace. It does make a big difference to airflow, so the particular doors, any venting present, where the airflow travels (from under the logs or into the top of the flame), … will affect what happens.
In my case, if I leave the doors open, the fire gets good oxygen, the heat is good, but there is a lot of flow up the chimney. If I close them partway, the fire is very strong as targeted air rushes into the base of the logs from under the doors, so the fuel burns fast, the roar is a bit loud for ambience, the fire has reduced perceived heat, but the flow through the chimney is reduced. If I close them all the way, the fire burns long and slowly due to the reduced airflow. The perceived heat is minimal.
I thought the idea was to combust outside air and not send warm room air up the chimney.
you want to use glass doors to not loose heat up the chimney when you aren’t having a fire. with the damper closed you still loose much heat through it from warm room air against it.
best situation is a fireplace insert with outside combustion air, glass doors closed when burning and room heat provided by air circulation through tubes or channels.
if you didn’t get an insert then look for doors that can operate closed. if you could install a combustion air pipe all the better, then you could get airtight doors.
Closed when the fire is small or out, open when the fire is large.
The glass does block a fair amount of the radiant energy.
You might looking into a fireplace insert, either wood or wood pellet for much increased efficiency.
We added glass doors to our fireplace last year, along with a grate heater. We were told that because we were keeping the doors shut to use the grate heater, we needed special heavy-duty glass in the doors. Otherwise, there would be a risk of glass breakage from the temperatures involved.
The grate heater was expensive, but totally worth it in my opinion. We keep the doors closed, and the fan blows heat out into the room without sucking all the room air up out the flue.
You only want to let enough air into the fireplace to sustain the fire. A totally open fireplace sucks in much more air than that. Thats already warmed air going up the chimney.
We have a fireplace insert. When the sealed glass door is closed there are two air vents to let air in for the fire to burn. Those vents in total about the size of a 3 by 5 inch index card. Obviously an open fireplace sucks in much more air than that.
Can you close the vents, like a Franklin stove?
Yes, you can go from “way” open to totally closed. To keep it burning all night with a few coals left to get things going again the morning we leave it open so that there is about the area of 6 postage stamps providing air flow. Now, you do have to keep it full open when starting a fire or getting it all warmed up and loaded with coals before you close down for the night.
And let me tell you, these “sealed” fire place inserts can pump out a bunch of warm air.
But, I guess my point is, it takes much less air to “run” a wood fire than an open fireplace provides. And more than that is just warm air sucked up the flume.
Cool!
while it might seem useful to keep coals over night for some heat or to start a fire in the morning, it can be not the best thing. a smoldering fire is cool and doesn’t keep the chimney hot and fully combust the wood, this can leave creosote in the chimney and later cause a chimney fire.
it is good to burn it at a safe hot temperature that doesn’t overheat the house/room. flames show the fire is hot, too tall flames might be wasted fuel.
So do those advertised logs actually do any good burning off the creosote? Or is that a scam?
My parents have lived in their house for 16 years. They use their fireplace probably about 10 times a year (not for heat, just for ambiance when there’s company over). It’s a dual sided fireplace with glass doors on both sides. They almost always have them closed and have never had them break, or really even get warm. Or course, the fireplace is pretty decently sized. Even when the fire is roaring, the distance from the flames to the glass is probably still 18-24 inches. So far as I can tell, the previous owners never had any issues either.
Also, the glass is pretty thin, I don’t think it would take much to break it. But like I said, it barely even gets warm.