Fireplaces - heat loss or gain?

naivehomeowner

Significant other (“SO”) and I were recently lucky enough to plunk down our life savings on an exquisite circa 1800 house in the northeast. Has what looks to be a decent 20 year old cast iron oil boiler w nice burnham CI rads and similar vintage replacement windows and storm doors but no other added insulation so SO’s description as ‘nice and airy’ is accurate. There are five beautiful original fireplaces and chimneys appear compound-lined, well maintained and have chain pulled caps. Our purchase of this large, sieve-like house obviously caused oil prices to recently almost double around here and I apologize for that. SO sez “lots of cozy fires will reduce our oil bill!” I say “no, everyone knows fireplaces pull warm air out of the house and increase heating bills!” SO retorts “that’s a myth from the heating biz. For years these fireplaces were the sole heat source for the house so they add not subtract heat!” Who’s correct?

Jamie Hall

Both of you. It depends a lot on the fireplace. If you are so lucky that no one has messed with those fireplaces, they will add heat to the structure. As you note, up until a century or so ago, that's the only heat the structure had.

On the other hand, if they are "modern" or someone has "updated" them, they may indeed represent a net heat loss.

For a fireplace to provide a heat gain, it should be relatively wide in relation to its depth -- as many of them were -- and have angled sides, rather than the sides coming straight out. Take a look at this article: https://en.wikipedia.org/wiki/Rumford_fireplace : for what is sort of the ultimate in fireplace technology.

Further, if you should be so fortunate that no one has replace the chimney stack or stacks, they are massive masonry objects and are inside the building envelope -- and once they get heated up with some nice fires on nasty days, they will continue to radiate heat for hours, if not days.

Zman

Your fireplaces will add radiant heat to the house. They will also suck cold air into the house which would not normally be pulled in. Any air that leaves the house needs to be replaced by air from the outside. Your oil burner is not likely sealed combustion and does the same thing, except in a more controlled way.
The fireplaces will probably heat your house. Just not very efficiently.

What does the firewood cost you?
My guess is you will be burning at < 20% efficient, plug the numbers into the spreadsheet and see how it looks.

Larry Weingarten

Hi, Those chain pull caps are a big part of the equation. If they are closed when the fireplaces are not in use, the fireplaces won't be losing heat all of the time. I'd be tempted to do or have done an inspection with blower door and thermal camera to see where the big leaks are, so they can be identified and sealed. Also, if they are on outside walls, it's possible to bring combustion air directly to the fireplaces instead of using indoor air. Put glass doors in front of the fireplaces to keep indoor air indoors!

Yours, Larry

pecmsg

Fireplaces took the Chill off the house. Nice and warm the room it’s in, cooler too cold the further away you are. 

Hot water heat made it even and easier. 

Tinman

Those three questions that need to be answered in every heat loss calculation. 

Number of fireplaces?
Dampers?
Glass enclosures?

hot_rod

The chimney effect works when the fire is burning or not. When the fire is out you need to somehow close off those straws that are pulling you heat out.
Larry has the best advise a load calc, IR camera and blower door test

leonz

You need a blower door test and and infra red camera picture
of all the exterior walls and roof to find all the leaks.

As you are in the northeast you would be wise to consider
investing in a coal stoker insert in one one fireplace to heat
the home rather than firewood or pellets and add improvements
to the insulation and new windows when you can afford it.

For every ton of rice coal burned you would burn two tons
of wood pellets based on the heat value of hard wood pellet
fuel as compared to rice anthracite coal.

You can find heating fuel BTU comparison charts on the
internet or just visit www.coalpail.com to see one.

Jamie Hall

Blower door tests on a ca. 1800 house usually result in loud laughter... they tend to be good deal tighter than one expects them to be, assuming that someone has actually taken the time to get the windows fitting properly, but the leaks which are there are often built into the structure in ways which are difficult to fix -- particularly if the structure is post and beam, which is quite likely.

naivehomeowner

Thanks for the help! Glad we are both right in particular. The fireplaces do look to be Rumford types with angled sides and chimney w narrow opening curving back also chimneys sit within the masonry and wooden outside walls and thanks for the info on that from Jamie. Tried to attach photo. So we can enjoy some fires knowing it is a better case scenario at least w regard to the dreaded oil bills that will soon be rolling in. Thanks all

Jamie Hall

That is a classic! That will be a very positive heat source for the room -- you will be pleasantly surprised. I would suggest, if I may, keeping a fire going in it much of the time; that way it will be positive. Rumfords like that one lose surprisingly little air when not fired -- but when running, typically only the air needed for combustion.

You might want to consider getting a glass screen for it, more for peace of mind than anything else. Some are made (though a little hard to find) which have dampers and can be closed on an active fire.

Another thought -- be sure to use dried hard wood. Green or wet wood, hard or soft, will not burn happily -- and softwood (such as pine) will put a lot of creosote in the chimney, which you don't want. And on that -- have the chimney swept professionally at least once a year. It's cheap insurance...

Solid_Fuel_Man

If you want to burn wood efficiently, consider a wood boiler. You will enjoy having just one fire and heating up all that wonderful cast iron in each room. I've heated a large house with 100% wood for many decades. 

Depending on what type of boiler you get your efficiency will be anywhere from 50% to 85%. 

You must....I repeat MUST have the chimney(s) inspected, cleaned, and burn properly seasoned wood. That means it has been cut, split, and stacked for at least 12 months prior to you burning it. 

hot_rod

Mainly radiant heat from a fireplace which is line of sight. They feel great when you are directly in front of them.
Maybe have a brick mason, experienced with fireplaces check them out. Looks like some mortar patches have been made and the brick in the bottom back are ??

A CO detector is a must. In the event of any back draft you don't want the byproducts of combustion in your home.

Lance

This has been a concern of Ben Franklins as he was a forward thinker and doer for efficiency. He designed the Franklin stove to help conserve wood supplies and provide a more productive economical multipurpose fire. Control the fire, heat the home and cook.

Voyager

A fireplace will not be a net heat loss while in operation unless you let the fire get extremely low. The big issue with fireplaces is the presence or absence of a good damper. The best dampers are those on top of the chimney as they keep most of the cold air out of the chimney to prevent heat loss and lessen creosote formation in the top of the chimney. Dampers on the top of the firebox are second best and I would rank glass doors on the firebox as third best.

The problem with dampers down low as they still allow cold air to get into the chimney and cool it much quicker than otherwise and this not only costs in efficiency, but that cold chimney will also condense out creosote when the fire is first started causing the need for more frequent cleanings and greater risk of chimney fires.

I have a masonry heater in my house that has a chimney top damper and glass doors and it is very efficient and creates zero creosote. I vacuum the fine dust out of the bottom of the chimney once a year and that is all that is required. I had the chimney cleaned once after about 15 years of use and the chimney sweep said there was basically nothing to clean.

So, if you have dampers that seal reasonably well, and you remember to close them once the fire is out, your heat loss will be minimized. You do have to wait until the fire is out though to avoid CO issues and you definitely want a couple good CO detectors in any event. And if you have no dampers or you have only firebox dampers, you may want to consider installing chimney top dampers. They are a pain to install in an existing chimney, but do work the best.

Magnus

Some experience with big old chimneys and a question. I have a 1964 house and reclaimed brick central chimney with 4 flues. Three are fireplaces (two large, one small) and one was the exhaust for the oil burner. The oil flue was retired and sealed several years ago when I switched to propane. Not too many fires in the following years before I retired and the bricks soaked up a lot of moisture, bleeding into the attic. Seems the oil burner was keeping the bricks dry. When I installed a large wood burning insert in the large basement fireplace last season, regular use has solved the spongey brick problem. My advice then is, use it or lose it.

My question: why dont we use outside makeup air piped into the hearth, to overcome the drafting problem with too tightly sealed homes? I think this can be done with woodstoves when necessary but I've never seen it used for a masonary hearth.

Voyager

My masonry heater does use an outside air duct that comes into the firebox through the door frame to help cool it and through a vent under the door frame to feed the fire as well. I don’t think anyone saw the need “back in the day” as homes were so leaky that make-up air was simply not a problem nor was energy efficiency as great a concern then.

OldPro69

When I had a fireplace installed in my home, I installed a fresh air duct from the attic to inside the firebox. I installed a glass door over the front and closed the dampers in the glass door setup. When I first light a fire, I close the glass doors to start the draft pulling thru the duct and then open the glass doors for the radiant effect. The fireplace has an insert with a blower to supply heated air to the space. This setup is very efficient.

hot_rod

yes, somehow somewhere you need enough air coming into the space to support combustion, if not you could consume all the O2 in the room. Not healthy!
perhaps Bob Harper know the math to calculate that combustion air?

JustinTheCarpenter

With regard to this idea-

My question: why dont we use outside makeup air piped into the hearth, to overcome the drafting problem with too tightly sealed homes? I think this can be done with woodstoves when necessary but I've never seen it used for a masonary hearth.

What I was told over the years by masons who knew a bit about fireplaces was that the make up air needs to match the flue size to be really effective. It's hard to integrate into a house. Houses like yours are air sieves allowing a steady flow from thousands of little sources, & likely a few obvious large ones. Your fireplaces will draw air from outside from every single opening in the envelope subtly chilling anything & anywhere not withing range of the fireplaces radiance or subsequent stored heat in the masonry.
The advice about a blower door & infrared survey will show you the condition of the house & how leaky it is.
The last thing is the very important question of "warm". I'd be willing to bet warm & toasty when the house was built is a temperature about 10-15 degrees cooler than you SO's idea of what warm & toasty is.

Larry Weingarten

Hi, To the topic of combustion air inlet vs flue size, my wood burning stove has a six inch flue and a four inch air inlet, so the inlet is roughly half the airflow of the flue. It works fine, with no creosote build-up. I do let room air in while starting the fire, but once it's hot, I can close it down without getting a smoky fire. So, the question is, how much combustion air is really needed? If it's less than flue size, running the intake will be less problematic, which is a good thing. Or, maybe I just don't build roaring fires!

Yours, Larry

ethicalpaul

> A fireplace will not be a net heat loss while in operation unless you let the fire get extremely low

Which means it will be a net heat loss every time you operate it. I would put a wood burner or modern insert into every fireplace you actually want to use.

Roger

Great posts on this! Someone very experienced in building efficiency once told me that conventional fireplaces are good if you want your front hot and your back cold (from drafts feeding the fire).
I've always thought that it was interesting that the wing back chair was designed to add comfort in fireplace rooms. Simple, clever, and effective design combining the wing back and the Rumford (thanks @Jamie Hall ) from the pre-steam Dead Men!

hot_rod

Remember also any kitchen or bath exhaust fans may have an effect on the operation of a fireplace.

This may be more info then you wanted, here is some code language.

https://basc.pnnl.gov/resource-guides/fireplaces-and-wood-stoves-have-proper-ventilation#edit-group-compliance

Voyager

My masonry heater has an 8” circular intake duct and an 8”x8” tile flue so intake is roughly 78% of the area of the exhaust.

Voyager

ethicalpaul said:

> A fireplace will not be a net heat loss while in operation unless you let the fire get extremely low

Which means it will be a net heat loss every time you operate it. I would put a wood burner or modern insert into every fireplace you actually want to use.

It all depends on who is using the fireplace. Most people I know will have a roaring fire in the evening on a cold night and then let it burn out and close the damper before going to bed. Used like that, a fireplace is great. Now, if you burn a really slow fire all day long or let the fire go out at night and go to bed leaving the damper open all night, then not so good.

No question that a good wood stove is better efficiency-wise, but not nearly as nice to sit in front of and watch. I installed a masonry heater as it is nearly as nice to watch as is a fireplace, yet it is even more efficient than a wood stove given the thermal mass. However, they are expensive and not easy to retrofit.

Jamie Hall

It really varies dramatically with the fireplace. Some fireplaces are, indeed, heat sinks. Others -- such as the main one in Cedric's home, which happily eats cord wood (4 foot lengths) alive in a firebox only a foot and a half deep, put out incredible amounts of heat. One does need to remember to close the damper when one isn't using it...

Solid_Fuel_Man

The truth of the matter about open burning is that you really cant combust all the fuel properly...ie smoke. This is wasted fuel, and there is the inconvenience that heat and smoke rise, so pretty much all the of the air heated by the fire must go up the flue. We all have discussed that air is pulled in through the structure. 

So a fireplace is good at radiant heat directly at you, and the radiant heat that the brick absorbs and releases later into the building envelope. This can be somewhat dubious. 

To burn wood efficiently there has to be sufficient heat in the fire to combust all the wood gasses and that is not remotely possible in any type of masonry heater short of a rocket stove (russian fireplace). 

I'm very passionate about this subject as you may be able to tell. Much research was done here in Maine back in the late 70s and early 80s about efficient wood combustion in a practical appliance. 

A modern EPA woodstove or gasification boiler are able to get the most complete combustion and then extract as much heat as practical from the flue gasses. There are many nice wood stoves which have huge glass doors which stay clean for a nice wood fire glow. 

I grew up in a roaring 20s bungalow, with a shallow rumford fireplace, and it heated the room it was in while the rest of the house grew colder. But my parents used it frequently for the ambiance. 

Marc_18

1920’s Colonial in Eastern Massachusetts - There’s a living room fireplace that I use occasionally not for heat, but only for the ambience of a wood-burning fire.

There’s an ash dump chute going down to the basement in the rear of the fireplace. I installed ceramic (high heat) glass doors that seal the face of the fireplace. I removed the chute cover and placed a heavy metal plate with corner legs over the chute opening. The plate is slightly larger than the chute opening itself. The corner legs hold the metal plate up off the fireplace floor positioning it just below the fireplace grate.

The gap between the plate and fireplace floor creates an entry space for combustion air that can be pulled in from the (unheated) basement rather than the heated living space. The plate serves as a shelf for the ash and embers from the fire above. A basement window is kept partially open.

I can now have a roaring fire going behind the large glass doors while pulling in negligible heated air from the living room. The doors do subdue the ‘snap, crackle, and pop’ of the fire, but offer good protection against flying embers.

The glass door assembly includes a sliding door at the bottom allowing the entry of some heated space air into the fire place if desired. I found that this is useful for better control of combustion as the fire dies down.

BTW, I do not close the fire place damper until the next morning when any remaining coals are cold.

hot_rod

Marc_18 said:

1920’s Colonial in Eastern Massachusetts - There’s a living room fireplace that I use occasionally not for heat, but only for the ambience of a wood-burning fire. There’s an ash dump chute going down to the basement in the rear of the fireplace. I installed ceramic (high heat) glass doors that seal the face of the fireplace. I removed the chute cover and placed a heavy metal plate with corner legs over the chute opening. The plate is slightly larger than the chute opening itself. The corner legs hold the metal plate up off the fireplace floor positioning it just below the fireplace grate. The gap between the plate and fireplace floor creates an entry space for combustion air that can be pulled in from the (unheated) basement rather than the heated living space. The plate serves as a shelf for the ash and embers from the fire above. A basement window is kept partially open. I can now have a roaring fire going behind the large glass doors while pulling in negligible heated air from the living room. The doors do subdue the ‘snap, crackle, and pop’ of the fire, but offer good protection against flying embers. The glass door assembly includes a sliding door at the bottom allowing the entry of some heated space air into the fire place if desired. I found that this is useful for better control of combustion as the fire dies down. BTW, I do not close the fire place damper until the next morning when any remaining coals are cold.

Interesting modifications😊

Voyager

This is what a masonry heater looks like at full tilt. First time we’ve fired it this season.

This is about the best way you can burn wood for both efficiency and cleanliness. You only see visible smoke from my chimney for the first few minutes as the fire heats up. After about 5 minutes, there is no visible smoke from the chimney.

Solid_Fuel_Man

@Voyager is that a factory built unit with a cultured stone surround built around it? Beautiful! All masonry flue or class A?

Someday, I'd like to replace this with a beautiful unit like that! 

Voyager

Solid_Fuel_Man said:

@Voyager is that a factory built unit with a cultured stone surround built around it? Beautiful! All masonry flue or class A?

Someday, I'd like to replace this with a beautiful unit like that!

It is a factory built unit from Tempcast in Canada. https://tempcast.com/

No cultured stone. The masonry heater and chimney are surrounded by 30 tons of real stone (an entire tractor-trailer load) averaging 6” thick. That was the recommendation from Tempcast to ensure adequate thermal mass. The chimney is clay tile lined for both flues. The Tempcast uses the right side flue and my Harman pellet stove in the basement uses the left side flue.

At my prior house, we had a wood stove in the basement and an exterior concrete block chimney. The flue gases from the wood stove were not sufficient to keep the chimney warm on colder winter nights and I got a lot of creosote and water condensing. The creosote would run down the sides of the chimney and the water would run into the house through the thimble. It was a mess. I decided I would never use an exterior chimney again.

When I built my log house, I designed it with the chimney dead center in the house so that the only cold part is the 3’ or so above the roof. This combined with the masonry heater and pellet stove, both very clean burning, and the damper on the very top of the chimney has worked better than I expected. No condensation or creosote or all.