Efficiency boosts for a fireplace

Question

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?

Answer 1

Fireplace blower?

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

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.

Answer 2

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.

Answer 3

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.