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I have a brick and masonry chimney that is built on the outside edge of our 1920s house. Originally, the house was primarily heated by an oil furnace, and the fumes were vented through a second flue that is now decomissioned. The other flue connects to a fireplace in the living room, and still my family likes to sit around and burn a big fire and enjoy the flames. However, it does absolutely nothing to heat the house, it actually cools things down. I feel like I've reached a stalemate trying to improve how much heat we might actually get from this thing.

  • According to city code, an insert or freestanding stove cannot legally be placed in there because no standard 8" pipe, even when ovalized, can fit through the very narrow double-bricked ledge leading into the first flue. http://www.seattle.gov/documents/Departments/SDCI/Codes/SeattleResidentialCode/2015SRCChapter10.pdf

  • Even if a stove were vented directly into the smoke chamber to draft up the flue, the hot air doesn't draft through the house at all, it merely rises up and out of the flue. This is particularly bad because the firebox is basically on the exterior of the house. My memories of "grandpa's little wood stove" heating the whole house (with no fan) was because the stove was centrally located with a stove pipe venting through the roof.

  • For this reason, I'd be averse to trying something like a fireplace back which, while it might get really hot, would simply sit in the dark, cold chimney chamber and slowly conduct heat upwards, out the flue. I don't believe the claims that fireplace backs "reflect heat" into the house as stated here: https://www.owenschimneysystems.com/benefits-using-fireplace-fireback/

  • The negative pressure of hot air rising out of the flue, and from burning is merely drawing cold air in from the windows all around the house.

I think I were an engineer, I would design a stove or insert that supplies cold air from outdoors via a separate line to burn the fire, vent it out of the chimney in a completely sealed system, and then use a separate electric fan to blow the hot air around the stove into the room.

My question is: am I completely out of options for moderately efficient wood burning heat?

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  • When I was a kid, my father, with a bit of help, converted an old water heater into a wood burning stove (added legs, fire brick on the bottom, a baffle of some sort & a chimney vent). After about 6 guys helped deliver & slide it into the fireplace, some quantity of pipe was shoved up the flue and the rest blocked with fireproof insulation. A nice fire would get the living room to >80°F and convection would help warm the rest of the house. The stove did stick out of the fireplace ~24" or so. Can't help you with the Seattle city code about flue pipe, though.
    – FreeMan
    Jan 13, 2022 at 18:48
  • @FreeMan I guess I hadn't considered that, if the smoke chamber is batted down with fiberglass insulation that the heat will necessarily move out into the living room.
    – AdamO
    Jan 13, 2022 at 19:11
  • I think that the key takeaway is that our stove was long and stuck out into the room (though not beyond the hearth). This allowed it to radiate heat into the living room instead of straight up the chimney. It was, as noted, custom built, and I don't think too many pre-fab fireplace insert stoves are going to stick out into the room very much. People don't want that because it puts hot metal where people could touch it. I was ~10 when this was installed and we lived there for about 8 years afterwards. Nobody managed to burn themselves on the hot stove in that time period.
    – FreeMan
    Jan 13, 2022 at 19:15
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    @FreeMan kept telling my son “hot - don’t touch”… he touched it once and lesson learnt. Now has his own woodstove and loves it.
    – Solar Mike
    Jan 13, 2022 at 19:49
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    Have you checked with a local fireplace installer? It may be possible to chip out some of the ledge that blocks access for the pipe, or even bore straight through the back and run a new pipe parallel to the existing chimney.
    – Mark
    Jan 13, 2022 at 20:32

3 Answers 3

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Fireplace blower?

https://www.regency-fire.com/en/Blog/fireplace-blowers-explained

blower

How Does a Fireplace Blower Work? A fireplace blower pulls air in from the room and forces it through a channel on the fireplace's exterior. As the air travels around the hot fireplace, it heats up, and by the time it is released into the room, the air is much warmer than the rest of the room. Because of the warm air's convective properties, the now warmer air will rise in the room, settling near the ceiling and causing the cooler air to be pushed down towards the fireplace.

Some friends of my folks had a fireplace blower made of motorcycle mufflers. These extended from the room above the fireplace, into the fireplace down the back and then out the bottom. Fans in the top pushed air thru these pipes and it got hot from the fire. The air coming out was hot. It more than made up for heat losses up the chimney. When the room got too hot they turned off the blower fan.

Regular fireplaces heat by radiation only because air does not come out the fireplace into the room (it would be full of smoke!). Fireplaces cool by convection and loss of warm air up the chimney. The commonality with the various fireplace blowers is to heat the room by convection by moving clean air in a closed circuit into and back out of the fireplace.

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  • ...and provide a fresh air intake. And doors, so the combustion air has to come from the fresh air intake, not the room.
    – Ecnerwal
    Jan 14, 2022 at 15:33
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Like you, I love a wood-burning fireplace but they aren't heat efficient. The obvious problems:

  • Combustion requires oxygen and
  • heat rises

A number of years ago we built a highly energy-efficient home with two wood burning fireplaces. The only way we could get them half-way efficient was by installing glass doors, an outside air supply vent that fed directly into the firebox and fan-driven heatolators that drew in cool air from the floor, circulated it over the firebox and blew the warmed air out into the room through vents.

I'm guessing that you're probably not going to want, or be able, to do that kind of retrofit. However, since you're on an exterior wall it might be worthwhile to consider running a fresh air supply from the exterior into the firebox and installing glass doors.

This would resolve the biggest energy drain which is drawing the heat from your home up the chimney. You will obviously need to check local building and fire codes to make sure you are in compliance.

Given the constraints you've outlined I don't know of another reliable option for you.

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    A major efficiency loss in a cold climate is the humidified warm air goes up the chimney for both combustion and draft. Cold , dry air replaces it. Jan 13, 2022 at 20:58
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    An inefficiency in a warmer climate like East Tx is that often people turn on the AC because the fireplace warms too much. Jan 13, 2022 at 21:01
  • @blacksmith37 - all true!
    – HoneyDo
    Jan 13, 2022 at 22:03
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We experienced the same problem of cooling the house down with a open fireplace. Indeed the fire needs a lot of fresh air and if it cannot get that from an air vent it attracts the air from elswhere thus cooling down the house. The first thing you should do is 1. Get an air vent close to the fireplace. You will get used to regulating it in such a way that you won't get cool air flows through your house. Second thing you should do is 2. to get a cast iron plate (fireback) and put it behind your fire. Your belief that a cast iron plate doesn't radiate heat into the room is quite different from mine. The hot air goes into the chimney, but the radiation doesn't. You will feel the radiation immediately if you light the fire. Radiation and hot air aren't the same. Hot air indeed goes up into the chimney, but radiation doesn't. Yet the radiation of a cast iron plate only works at short distance, so if you sit in the vicinity of the fireplace. That's what your family enjoys when sitting around the fireplace. It is efficient only close by as you could lower the central heating then. So forget the idea that a fireplace will heat your house, but it can heat your family around the fireplace (and allow you to lower the central heating). The way you make the fire can also influence the heat from the fireplace. A big fire isn't necessarily resulting in heat production as it also attracts more cold air. Measures to increase the heat efficiency of your open fireplace may be using dry fire wood, make a constant and quietly burning, fire (instead of a big fire) and watch the influence of a fireplace screen blocking the radiation. Read also this blog on getting more heat from your fireplace.

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