A fire in a fireplace makes people nearby feel warmer because it produces primarily radiant heat. The truth is, however, that a fireplace is a net heat loser. In the first place, a very high percentage - perhaps as much as 90% - of the heat goes straight up the chimney and out. In the second place, an open damper as the fire dies down encourages the warm interior air to rapidly disappear up th chimney. A fireback increases the radiant heat produced and may - very marginally - make you feel a bit warmer. The glass doors once closed will - assuming they're air tight and most are not - prevent warm air from going up the chimney.

The only real solution is a) a fireplace insert (fair to good) b) a free standing wood stove (much better) utilizing the present chimney. Modern EPA certified inserts and stoves are efficient and clean burning, spectacularly so when compared to an open fireplace. Installing either would be a considerable investment, but the payback period in savings in less oil burned and less wood used would likely be relatively short. Flame watching through the glass doors is just as addictive as an open fire, too.

If I can bring the ambient room temperature measured with a standard room thermometer, from 48 degrees to 74 degrees or so, with the fireplace, then it does more than make feel a "bit warmer", but I do understand you were probably referring to just the fireback there. I do understand the heat loss issue from the damper, since as I experienced, within 8 hrs most of it was lost, although maybe not all, since I don't know how cold the room would have gotten had I never used the fireplace. It seemed that had I had the glass doors at the time, there actually would have been some heat gain. I really am not considering heating with wood at this time because the cost of wood is quite high, and the work involved in cutting and splitting my own is not something I could do any more. I am just looking to use my fireplace for enjoyment without INCREASING my heating bill, and hopefully adding a small amt of heat gain if I use it judiciously only when I can keep it going long enough to heat the firebox and take advantage of the radiant heat.
It might be that no one has really measured heat gain/loss from the average fireplace under circumstances other than the usual 2 or 3 hour fire with a couple of logs, and an open fireplace, so the rest might just be guess work. I was hoping since I had heard that the glass doors do help significantly, that someone had done some measurements about how much it helps, and if the end result is still a loss.

If you're really interested in carefully measured figures for heat gain and loss, try www.hearth.com Well designed and carefully fitted glass doors probably do make a difference in that they would do the same thing as closing off the flue. If the doors can't be closed until the fire is well out, however, you will not have gained very much at all.

It does not seem at all likely that burning wood in the fireplace is increasing your oil consumption. Where you're losing is in the firewood itself. The btu content of the wood is largely wasted by going up the chimney. If you like the fire and the warmth it provides, fine. The btu loss is probably compensated for by the enjoyment you're getting. BTW, I have heated in part or solely with wood for more than 30 years. Last winter's wood consumption was 5 cords. Oil consumption was 17 gallons. A more typical winter would be 6 cords/50 gallons. Wood at $275 a cord vs. oil at $3.65 a gallon is no contest.

Thanks. If the fire was a "net heat loser" then I was afraid not just all the heat from the fire went up the chimney, but some heat from the oil, too, causing me to burn MORE oil than if I had no fire. I close the doors when fire gets low, I don't wait for it to go out completely or get cold. The instructions just warned that too much heat could crack them. With a low fire and the flu open that didn't seem to be a problem. Thanks for the opinion.

Sue if you are using fuel oil for heating the rate of return for an EPA approved insert/free standing stove is less than 2 years with a high end appliance and around 1 year for a functional appliance. What investment yields this sort of risk free return?

A fireplace has about 10% efficiency while it has a burning flame but there will be heat loss during the cooling phase of the fire. Your overall net heat gain likely is neglible.

Glass doors would help you prevent heat loss during the cooling phase but there isn't as much radiant heat.