Masonry fireplace heat exchanger

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Masonryfireplace88

New Member
Jan 1, 2019
4
Springfield, IL
I recently had a cheap DIY fireplace insert created with the assistance of a local muffler shop (total cost = $47 including the fan). They bent a 2.5" diameter ten foot exhaust tube to wind into the back of my fireplace and then back out. I bought an 4" inline duct fan rated at 100 cfm and an adapter to connect them. I know the fan isn't putting out 100 cfm, maybe 50-60% because of the difference in cross sectional area. Regardless, I'm incredibly disappointed at the seemingly minimal effect on heating the room and the temperature of the air coming out of the tube.

Has anyone else created a DIY fireplace heat exchanger that noticeably increased the room temperature? During the day I close the door to the back bedrooms so I'm trying to heat around 800-1000 sqft. My current setup at a temperature of 20-30 degrees outside can maintain the temperature inside once the gas furnace has got the house to a comfortable temperature so it is helpful in keeping the furnace shut off all day, but once the temperature starts dipping in the teens I start going backwards again. It doesn't seem like there is a huge difference between heat output with or without the exchanger.

One of the main things that baffles me is how the fire burns somewhere between 1200-1500 degrees and the environment inside the fireplace where the pipe winds is around 1000 degrees and yet the air coming out of the tube is warm, but not nearly as hot as I was expecting. Something else that surprised me was if I increased the airflow through the tube (reverse hose hook up on shopvac and blew through), the temperature coming out was the same despite seemingly 3-5x more airflow. This suggests to me the the amount of heat output from the pipe is limited by the airflow, at least at the current flow rates. The pipe on the output side, even outside the fireplace is so hot it will burn hand if touched. My main question is why isn't the air coming out hotter. How does the air coming out of a pipe that passes through 1000 environment not burn my hand when I put where the air immediately comes out?

One thought I have is to have several smaller pipes, maybe two inches ran through in the same way and connect to a fan with 200 or more cfm. This would allow more air to be in closer contact with the metal and increase the flow.

I get my wood for free, so yes I know this isn't the most efficient way to heat my house, but even 15% efficiency at zero cost beats 85% efficiency at cost. Also, I'm only going to live here for 3-4 years so it isn't worth putting an insert and chimney liner it since it would never pay for itself, best case scenario i might break even (might as well just crank up the gas thermostat).
 

pen

There are some who call me...mod.
Staff member
Aug 2, 2007
7,965
N.E. Penna
As you are seeing, there is simply only so much efficiency to be had out of a fireplace, with so much air going up the chimney.

Also, as hot as the pipes might be, they just don't transfer heat to air that efficiently. .

Got any pics of the setup?

As a heads up, keep an eye on this thing. If a hole were to burn through, hot ashes would be pushed out the tubes and into the room. Safety first.

pen
 

bholler

Chimney sweep
Staff member
Jan 14, 2014
28,473
central pa
I recently had a cheap DIY fireplace insert created with the assistance of a local muffler shop (total cost = $47 including the fan). They bent a 2.5" diameter ten foot exhaust tube to wind into the back of my fireplace and then back out. I bought an 4" inline duct fan rated at 100 cfm and an adapter to connect them. I know the fan isn't putting out 100 cfm, maybe 50-60% because of the difference in cross sectional area. Regardless, I'm incredibly disappointed at the seemingly minimal effect on heating the room and the temperature of the air coming out of the tube.

Has anyone else created a DIY fireplace heat exchanger that noticeably increased the room temperature? During the day I close the door to the back bedrooms so I'm trying to heat around 800-1000 sqft. My current setup at a temperature of 20-30 degrees outside can maintain the temperature inside once the gas furnace has got the house to a comfortable temperature so it is helpful in keeping the furnace shut off all day, but once the temperature starts dipping in the teens I start going backwards again. It doesn't seem like there is a huge difference between heat output with or without the exchanger.

One of the main things that baffles me is how the fire burns somewhere between 1200-1500 degrees and the environment inside the fireplace where the pipe winds is around 1000 degrees and yet the air coming out of the tube is warm, but not nearly as hot as I was expecting. Something else that surprised me was if I increased the airflow through the tube (reverse hose hook up on shopvac and blew through), the temperature coming out was the same despite seemingly 3-5x more airflow. This suggests to me the the amount of heat output from the pipe is limited by the airflow, at least at the current flow rates. The pipe on the output side, even outside the fireplace is so hot it will burn hand if touched. My main question is why isn't the air coming out hotter. How does the air coming out of a pipe that passes through 1000 environment not burn my hand when I put where the air immediately comes out?

One thought I have is to have several smaller pipes, maybe two inches ran through in the same way and connect to a fan with 200 or more cfm. This would allow more air to be in closer contact with the metal and increase the flow.

I get my wood for free, so yes I know this isn't the most efficient way to heat my house, but even 15% efficiency at zero cost beats 85% efficiency at cost. Also, I'm only going to live here for 3-4 years so it isn't worth putting an insert and chimney liner it since it would never pay for itself, best case scenario i might break even (might as well just crank up the gas thermostat).
The problem is if you do manage to extract enough heat to heat your house you will no longer have enough heat for the fireplace to work correctly.
 
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Masonryfireplace88

New Member
Jan 1, 2019
4
Springfield, IL
As you are seeing, there is simply only so much efficiency to be had out of a fireplace, with so much air going up the chimney.

Also, as hot as the pipes might be, they just don't transfer heat to air that efficiently. .

Got any pics of the setup?

As a heads up, keep an eye on this thing. If a hole were to burn through, hot ashes would be pushed out the tubes and into the room. Safety first.

pen
 

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Masonryfireplace88

New Member
Jan 1, 2019
4
Springfield, IL
As you are seeing, there is simply only so much efficiency to be had out of a fireplace, with so much air going up the chimney.

Also, as hot as the pipes might be, they just don't transfer heat to air that efficiently. .

Got any pics of the setup?

As a heads up, keep an eye on this thing. If a hole were to burn through, hot ashes would be pushed out the tubes and into the room. Safety first.

pen

I was a little skeptical of using exhaust tubing but was surprised at how thick it was. Definitely will be keeping eye on the condition, especially the bend areas since they are now weaker and more stretched than the rest, I don't expect it to last more than a few years.
 

pen

There are some who call me...mod.
Staff member
Aug 2, 2007
7,965
N.E. Penna
consider the surface area of the pipe section that is getting heated, versus what a wood stove insert has air moving over.

As bholler mentioned, the next problem is, you harvest enough energy from those coals, and you are diminishing what makes those logs burn well in this environment. Wood in an open fireplace has air coming in all over, trying to steal heat needed to make a clean burn, and then sends it right up the chimney.

Fireplaces are beautiful, but inefficient heaters by nature. I'm afraid you are fighting an uphill battle.

pen
 
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Masonryfireplace88

New Member
Jan 1, 2019
4
Springfield, IL
consider the surface area of the pipe section that is getting heated, versus what a wood stove insert has air moving over.

As bholler mentioned, the next problem is, you harvest enough energy from those coals, and you are diminishing what makes those logs burn well in this environment. Wood in an open fireplace has air coming in all over, trying to steal heat needed to make a clean burn, and then sends it right up the chimney.

Fireplaces are beautiful, but inefficient heaters by nature. I'm afraid you are fighting an uphill battle.

pen

Totally get what you're saying. The goal was to add just a little more heat to the room. I have seen some masonry fireplaces with build in vents within the bricks which is what i was trying to simulate but there is probably a higher contact/air ratio.
 

webby3650

Master of Fire
Sep 2, 2008
11,121
Indiana
Totally get what you're saying. The goal was to add just a little more heat to the room. I have seen some masonry fireplaces with build in vents within the bricks which is what i was trying to simulate but there is probably a higher contact/air ratio.
Those fireplaces with the vents on the side did almost nothing. Most people report barely warm air moving through. Most of the time, just cold air infiltration.
 

webby3650

Master of Fire
Sep 2, 2008
11,121
Indiana
The heat exchange grates from the past that your are imitating had about 10 2” tubes that went from the hearth, under the fire and acted as the grate, then as a fireback, then up and over the fire. A constant arch that’s in the fire. Even these make marginal heat output.
 

Ludlow

Minister of Fire
Jun 4, 2018
1,437
PA
We had one of these type of things when I was a kid. Worked marginally well for the room the fireplace was in, didnt improve the negative overall efficiency of the fireplace. When it burned up my Dad threw it in the trash. Once you got a good bed of coals on it the heat coming out the vent was nuclear hot. Trouble being it immediately made a u-turn and went up the chimney. Welcome to the forum future insert owner! (You will get there...lol)
ters-door-not-included-20-fireplace-grate-heater-heat-exchanger-zero-clearance-blower-heatilator.jpg
 

maple1

Minister of Fire
Sep 15, 2011
10,932
Nova Scotia
One of the main things that baffles me is how the fire burns somewhere between 1200-1500 degrees and the environment inside the fireplace where the pipe winds is around 1000 degrees

What are you basing those numbers one? Pretty sure it is nowhere near that hot in that fireplace.

How does the air coming out of a pipe that passes through 1000 environment not burn my hand when I put where the air immediately comes out?

More heat is being exchanged to the air from the room that is coming into the fireplace and then going right up the chimney.

I suspect you might be better off overall if you stopped burning the fireplace, and closed the opening up tight. The large amount of air going up the chimney is sucking all kinds of heat up with it. You might feel some heat from it, right in front of it, but the farther reaches of the house are being made colder at the same time. We have a fireplace, gets used for ambiance only.
 
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Das Jugghead

Burning Hunk
Jan 2, 2019
160
Indiana
I recently had a cheap DIY fireplace insert created with the assistance of a local muffler shop (total cost = $47 including the fan). They bent a 2.5" diameter ten foot exhaust tube to wind into the back of my fireplace and then back out. I bought an 4" inline duct fan rated at 100 cfm and an adapter to connect them. I know the fan isn't putting out 100 cfm, maybe 50-60% because of the difference in cross sectional area. Regardless, I'm incredibly disappointed at the seemingly minimal effect on heating the room and the temperature of the air coming out of the tube.


2.5 inch tube has a cross-sectional area of 4.71 square inches

100 CFM fan running at 50% theoretically pushes 86,400 CIM (cubic inches per minute)

86,400 CIM (fan)/4.71 square inches (cross section of tube) yields 18,343 lineal inches per minute or 1,528 lineal feet per minute.

Assuming frictional losses of 70% due to boundary layer and bends (an arbitrary number I pulled out of thin air) the air moving through the tube is still moving at 458 feet per minute or 5 MPH.

In order for air to pick up the heat from the walls of the pipe (not withstanding the Delta T1 - Delta T2 of the rate of heat transfer through the pipe walls) it has to move much much slower. Having fins or tabs in the pipe to cause turbulence or tumbling of the air greatly aids in warming the air as well. Slowing down the air would help raise the temperature by virtue of contact time. Think about it - would you run through a burning house or stroll through it?

Even so, no matter how much improvement you make to your system to raise outlet air temperatures, as many others have pointed out you are fighting a losing battle trying to heat with a fireplace.
 
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