How to Optimize Heat Output from a Gas Fireplace?
January 6, 2014 1:45 PM   Subscribe

We have a 24 inch by 36 inch vented gas log fireplace. See image. I'm pretty sure most of the heat goes up and out the flue. Are there any good modifications for optimizing heat output? Should we consider switching to a ventless system? Add some glass doors to the exiting setup?
posted by steinwald to Home & Garden (11 answers total) 1 user marked this as a favorite
We had a ventless gas fireplace in our previous house and it put out some serious heat. There were "Hi" and "Lo" settings and "Lo" was plenty hot. Owing to the placement of the fireplace, a lot of the heat went upstairs as well. Compared to the standard gas-starting fireplace we had in the prior house, the ventless system was a dream. Every BTU stayed right inside the house. No flue to clean or worry about. It was awesome.

Our current house has a comically small firebox with a regular flue and we're strongly considering converting it to a wood-burning stove setup.
posted by jquinby at 1:52 PM on January 6, 2014

We've had some luck with firebacks in our woodburning fireplaces. I wonder if you can use one in a gas fireplace? (Edit: Apparently you can)
posted by cecic at 2:19 PM on January 6, 2014

A ventless fireplace will basically put 100% of its heat output into your house, so yes, switching to ventless will help dramatically. A traditional "B-vent" system like you have, where air is drawn in from the room and then expelled up the chimney, is something like 30-40% efficient and really for ornamental use rather than heating.

To improve the efficiency of your fireplace you'd need to change it to a "direct vent" (or ventless) setup, and that may not be possible. It's been my experience that you basically can't modify manufactured fireplace products without voiding their warranties and UL certification, so basically nobody will do it, even if it's technically possible to do. (And this was with wood-stove related stuff; I suspect it's even stricter with gas.)

If your power goes out and you were really without heat, sure by all means fire up the gas log. But I wouldn't use it for auxiliary heating on a regular basis.
posted by Kadin2048 at 2:41 PM on January 6, 2014

Do not switch to a ventless gas fireplace. The combustion products (water and CO2) not to mention the combustion byproducts (nasty stuff) end up in your living space.

Your current setup is probably not adding net heat to your house. When the damper is open, lots of air travels up the flue. That air is nice warm air from inside your house, which is replaced by cold air from outside coming in through cracks in your house. You lose a lot of heat in that air and only gain a little bit from the burning gas. That kind of fireplace is best regarded as decorative (it looks nice when burning, but it doesn't heat up the house).

Also note that there is some soot-staining above the fireplace which indicates that you have a draft problem (at least sometimes, the exhaust is not travelling up the flue as it should).

Glass doors added to your existing setup would help a little (by reducing the amount of air travelling up the flue). You would want to make sure it was installed with sufficient ventilation to provide combustion air.

It would be a much better idea to replace the whole thing with a modern unit. Bear in mind that gas fireplaces are never going to be as efficient as a good furnace or boiler, so it is better to use those for heat if available.
posted by ssg at 2:46 PM on January 6, 2014 [1 favorite]

I am not terribly knowledgeable of fireplace technology, but I have a background in fluid dynamics and thermodynamics.

Basically, what's happening with your fireplace is that natural gas is being sprayed in to the fireplace and ignited with ambient air from the room, and then the hot exhaust air is lifting through the flue and outside. In an industrial setting (like a power plant), those exhaust gases pass over tubes carrying water which draw heat from the exhaust and power whatever's downstream in the system. Your exhaust is heating up the chimney, but bricks are insulating so there's not a whole lot of heat transfer.

So you're getting kind of a double-whammy here - (warm) air from the inside of your house is being used to burn gas and expelled out the chimney without a whole lot of heat transfer back into the room. Plus, all that room air that was heated by your furnace is being replaced by colder air from your basement, gaps in the window, whatever.

To keep the room hot from a fireplace like that, you'd probably want to do a number of (impractical) modifications. First, seal the large opening to the room with a good heat sink and add a second way to draw air into the fireplace from outside, probably by adding a small intake in the back of the fireplace. You could also recirculate some of the exhaust back to the fire - there's a lot of excess air being drawn through without participating in combustion, so this would accomplish a number of beneficial things including reducing the amount of air being drawn in from the room, preheating your combustion products for a higher flame temperature, and reduce pollutants.

As I said, this is all pretty impractical. However, one thing you could try is to place a heavy cast-iron grate in front of the fire. This will act as your heat sink as well as restrict the airflow somewhat into the fireplace. The iron will draw heat from the fire, and since it is sitting in the room instead of in the fireplace it will exchange heat with the room air more efficiently.

Also note that there is some soot-staining

Natural gas doesn't produce soot - this was probably a conversion from a wood-burning fireplace.
posted by backseatpilot at 2:58 PM on January 6, 2014

Yes - the current setup is most definitely a gas log sitting in an old wood burning fireplace.
posted by steinwald at 3:03 PM on January 6, 2014

Glad to hear that you don't have a major combustion problem.

Natural gas doesn't produce soot - this was probably a conversion from a wood-burning fireplace.

Natural gas, when combusting poorly, can most certainly produce soot.
posted by ssg at 3:14 PM on January 6, 2014

I'd recommend looking into a gas fireplace insert. They have a double flue system where the combustion air is drawn down the chimney so you are using no house air. I heated my whole house with one as long as it wasn't too cold out.
posted by beagle at 4:14 PM on January 6, 2014

I think you have to decide what you want - heat (ie efficiency), or atmospherics (the fireplace effect).

You can a have a bit of both worlds, and there are better options which still give the 'burning logs' effect.

One option would be to replace the gas logs with a proper (ie high efficiency) gas heater located elsewhere in the room/house, and just use the fireplace as originally intended (hopefully your fireplace has a damper which can be closed to prevent heat loss when the fire is not going).
posted by GeeEmm at 5:00 PM on January 6, 2014

If you want heat, go the whole hog and get an insert, so you have venting and efficiency. More expensive up front, but a better investment in the long run.
posted by holgate at 7:20 PM on January 6, 2014

Do not switch to a ventless gas fireplace. The combustion products (water and CO2) not to mention the combustion byproducts (nasty stuff) end up in your living space.

This is not a major problem. Modern ventless gas heaters have low-O2 cutoffs and unless you are using them in a very tightly-sealed space (which any house with a wood-burning fireplace almost certainly isn't, or you'd have backdraft problems) they're fine.

Ventless gas and even kerosene heaters ('paraffin heaters') are quite popular in other parts of the world and used extensively for space heating. The risk and horror stories you hear of them come chiefly from people operating old ones without low-O2 safety cutoffs in tightly-sealed spaces like trailer homes. That can lead to CO poisoning. But all modern natural gas and the propane/kero heaters designed for indoor use (not workshop / jobsite / utility heaters, which you should never use in a living space) should have the cutoff.
posted by Kadin2048 at 8:51 AM on January 7, 2014

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