How to add a boiler extension to a wood stove?

  • Active since 1995, Hearth.com is THE place on the internet for free information and advice about wood stoves, pellet stoves and other energy saving equipment.

    We strive to provide opinions, articles, discussions and history related to Hearth Products and in a more general sense, energy issues.

    We promote the EFFICIENT, RESPONSIBLE, CLEAN and SAFE use of all fuels, whether renewable or fossil.

monteville

Feeling the Heat
Nov 23, 2019
346
Dallas
If you have an existing wood stove, but want to extend the warmth to other parts of the house, with airducts it's easy, simply add a fan to blow air from near the wood stove into the air duct, but assume your house don't have air ducts. so it has to be via steam or hot water.

  • What boiler pipes or heat exchanger will you use to extract heat from the hot flue?
  • Is it OK to simply hook into existing steam heating pipes of the house?
  • How will you control the water flow, such that flue exit temperature is above 300°F to avoid creosote condensation?
 

bholler

Chimney sweep
Staff member
Jan 14, 2014
28,452
central pa
simply add a fan to blow air from near the wood stove into the air duct, but assume your house
Nope against code because it can and has created negative pressure near the stove causing leakage of co.

What boiler pipes or heat exchanger will you use to extract heat from the hot flue?
If run correctly there shouldn't be extra heat in the flue to be extracted.

Is it OK to simply hook into existing steam heating pipes of the house?
You could but generally a woodstove with a hot water coil won't produce enough heat to work properly.

How will you control the water flow, such that flue exit temperature is above 300°F to avoid creosote condensation?
That gets very complicated with bypass valves pressure relief valves control boards etc. This is why we keep telling you hydronic is the expensive way to go.
 
  • Like
Reactions: monteville

monteville

Feeling the Heat
Nov 23, 2019
346
Dallas
Nope against code because it can and has created negative pressure near the stove causing leakage of co.
So... instead we install two identical fans, the other one in the return register to suck in cold air from the air handler, which will balance out the negative pressure.

Maybe it doesn't have to be so complex, just max the air handler without the AC/furnace on could also circulate air among the whole house.
 
Last edited:

begreen

Mooderator
Staff member
Nov 18, 2005
93,171
South Puget Sound, WA
So... instead we install two identical fans, the other one in the return register to suck in cold air from the air handler, which will balance out the negative pressure.
A simpler and more effective way is to use the duct to pull colder (denser) air from the remote room and blow it into the stove room. Negative pressure in the colder area will pull the warmer air from the stove room into the colder area. If the duct travels through an unconditioned space it should be insulated.
Furnace air handlers work sometimes if the ductwork is fully insulated (supply and return) and travels through a conditioned space. This is more common with heatpump systems that operate at lower duct temps. Most hot air systems have lossy ductwork that is designed for moving 120-130º air. If the ductwork is bare and running through an unheated basement, the heat losses negate the circulation benefit.
 

Rickb

Minister of Fire
Oct 24, 2012
1,197
St.Louis
I use a ducted fan to blow air from my cold room into the stove room. In my case its from our main floor family room into the basement family room.

You would be surprised how much that helps move the air around. Plus its very very quiet.
 

monteville

Feeling the Heat
Nov 23, 2019
346
Dallas
That gets very complicated with bypass valves pressure relief valves control boards etc. This is why we keep telling you hydronic is the expensive way to go.
The control parts are not really expensive compared to installation costs. Solenoid valves are, maybe, $25 a piece? They can be driven by 12V-36V solid state relay($5) with input signal from an $15 Arduino.

Hydronic is more complicated than other medium, but it's easy for average people to get right, and if it's not right, there are obvious signs -- losing water, pipe rupture, noisy steam discharge etc before catastrophe happens.

and there are mature, easy to execute ways to test a hydronic system, like pressurizing it with hot water a few times above the operating PSI to see if it leaks under extreme condition.

You can put the high temperature/combustible/potentially CO generating parts far away from the house with hydronic, but not with air ducts.
For example,
  • a packaged heat pump unit running on R290 (propane as refrigerant) sending hot or chilled water into house. R290 is cheap, efficient and green, drop in replacement of R22 (even the oil is compatible) so you can reuse proven designs, but a leak of R290 inside house is dangerous.
  • a chimney fire in outdoor wood boiler destroy only the chimney, rather than the house.
 

bholler

Chimney sweep
Staff member
Jan 14, 2014
28,452
central pa
The control parts are not really expensive compared to installation costs. Solenoid valves are, maybe, $25 a piece? They can be driven by 12V-36V solid state relay($5) with input signal from an $15 Arduino.

Hydronic is more complicated than other medium, but it's easy for average people to get right, and if it's not right, there are obvious signs -- losing water, pipe rupture, noisy steam discharge etc before catastrophe happens.

and there are mature, easy to execute ways to test a hydronic system, like pressurizing it with hot water a few times above the operating PSI to see if it leaks under extreme condition.

You can put the high temperature/combustible/potentially CO generating parts far away from the house with hydronic, but not with air ducts.
For example,
  • a packaged heat pump unit running on R290 (propane as refrigerant) sending hot or chilled water into house. R290 is cheap, efficient and green, drop in replacement of R22 (even the oil is compatible) so you can reuse proven designs, but a leak of R290 inside house is dangerous.
  • a chimney fire in outdoor wood boiler destroy only the chimney, rather than the house.
Have you ever designed and built a hydronic system?
 

monteville

Feeling the Heat
Nov 23, 2019
346
Dallas
Have you ever designed and built a hydronic system?
never, I only repaired steam pipes in a steam heated house.
The operating principle looks pretty straightforward,
for NG and oil boiler: burner shuts down when steam pressure reaches an upper threshold, and fires up when steam pressure drops below a lower threshold.
I suppose if you have a variable burner you can also do PID control instead of on-off.
 

bholler

Chimney sweep
Staff member
Jan 14, 2014
28,452
central pa
never, I only repaired steam pipes in a steam heated house.
The operating principle looks pretty straightforward,
for NG and oil boiler: burner shuts down when steam pressure reaches an upper threshold, and fires up when steam pressure drops below a lower threshold.
I suppose if you have a variable burner you can also do PID control instead of on-off.
You do realize steam and hydronic are completely different systems right?
 

monteville

Feeling the Heat
Nov 23, 2019
346
Dallas
You do realize steam and hydronic are completely different systems right?
"hydronic" simply means using water as heat carrying medium, in most of the technical documents I read, hydronic includes both steam and hot water.
Hot water system use a pump to circulate hot water, hot water has higher energy density than steam so the pipes and radiators can be made smaller or rated for lower pressure. Hot water system use water temperature instead of pressure for feedback control.
 

bholler

Chimney sweep
Staff member
Jan 14, 2014
28,452
central pa
"hydronic" simply means using water as heat carrying medium, in most of the technical documents I read, hydronic includes both steam and hot water.
Hot water system use a pump to circulate hot water, hot water has higher energy density than steam so the pipes and radiators can be made smaller or rated for lower pressure. Hot water system use water temperature instead of pressure for feedback control.
Steam systems are also almost always single pipe systems with much much larger pipes.
 

begreen

Mooderator
Staff member
Nov 18, 2005
93,171
South Puget Sound, WA
This conversation is better carried on in the boiler room.
 
never, I only repaired steam pipes in a steam heated house.
The operating principle looks pretty straightforward,
for NG and oil boiler: burner shuts down when steam pressure reaches an upper threshold, and fires up when steam pressure drops below a lower threshold.
I suppose if you have a variable burner you can also do PID control instead of on-off.
You know how easy it is to turn a hydronic system accidentally into a steam system and cause serious damage or injury right??
 
  • Like
Reactions: bholler

bholler

Chimney sweep
Staff member
Jan 14, 2014
28,452
central pa
You know how easy it is to turn a hydronic system accidentally into a steam system and cause serious damage or injury right??
But it's easy anyone can do it. Lol what could go wrong?
 

monteville

Feeling the Heat
Nov 23, 2019
346
Dallas
You know how easy it is to turn a hydronic system accidentally into a steam system and cause serious damage or injury right??
Thermostat and T&P valve prevent that, but it's an ugly solution because once you lose water you need to replenish it from a backup/expansion tank.

The challenges:
1. How far can we tune down a wood stove (and be able to start it back up)
2. In case the stove cannot be tuned "down" enough to stop hot water from reaching boiling temperature, how to stop heat transfer from stove to the hydronic system.
 
Last edited:

bholler

Chimney sweep
Staff member
Jan 14, 2014
28,452
central pa
Thermostat and T&P valve prevent that, but it's an ugly solution because once you lose water you need to replenish it from a backup/expansion tank.

The challenges:
1. How far can we tune down a wood stove (and be able to start it back up)
2. In case the stove cannot be tuned "down" enough to stop hot water from reaching boiling temperature, how to stop heat transfer from stove to the hydronic system.
If done correctly yes they will.
 

brenndatomu

Minister of Fire
Aug 21, 2013
7,128
NE Ohio
Same issue, if a house has no duct, how will you transfer heat to another room without hydronic?
A small fan sitting on the floor running on low at the other end of the house blowing cool air back toward the stove is a long time well known way...not saying that the whole house will be 70*, but in many homes the temps can be kept in acceptable ranges after a lil experimentation.
 

monteville

Feeling the Heat
Nov 23, 2019
346
Dallas
A small fan sitting on the floor running on low at the other end of the house blowing cool air back toward the stove is a long time well known way...not saying that the whole house will be 70*, but in many homes the temps can be kept in acceptable ranges after a lil experimentation.
The same way all of us do it that are heating with woodstoves. Air circulation
This way requires loss of privacy and noise proof among multiple rooms (because we need to open up holes for fans on walls or doors).

Not every family can use this configuration, for example, two people working remotely from two rooms -- they should not be able to hear each other talking with colleagues.
 

bholler

Chimney sweep
Staff member
Jan 14, 2014
28,452
central pa
This way requires loss of privacy and noise proof among multiple rooms (because we need to open up holes for fans on walls or doors).

Not every family can use this configuration, for example, two people working remotely from two rooms -- they should not be able to hear each other talking with colleagues.
Well it has worked fine for many many people for well over 100 years now. You want to cut costs that's how you do it. Sometimes cheap heat requires some compromises. Btw our bedroom door is shut every night with no holes etc and it's a little cool but fine
 
  • Like
Reactions: salecker