Question: To use or not to use? (heating bricks for energy storage)

[MajorJinxs]

TL;DR Summary

Brick Usage in a hot water stove?

I have a Hicks hot water stove that has a Cast Iron fire box. I am looking for Clay bricks or Something like it to retain more heat to hold over night inside the fire box. But i am need help with the difference in materials as far as heat retention per brick for longer holding time. Hicks hot water stoves are a force air draft ( when heating) and natural draft when it isn't heating.

The water box at 0deg C (outside temp) has a constent temp drop of (160-100deg F) 60deg F over 8hr. Please Keep in mind that i am burning wood to heat water with a plossible max burn temperature of between.1800-2500Deg F; so durability is a factor.
The fire box size: 33x23x30
Assuming all brick sizes are 9''x4.5''x1.25'' or 9''x4.5''x2.5
Which Material would have the greatest BTU Hold time.

Brick Materials:
Clay-
Cement-
Sand Ceramic-
Ceramic-
Cast Iron?-
Ash clay Brick?-

Please some post Figures with answers to better help with this answer.

 

[gleem]

Metals have low specific heats about 0.1 BTU/deg F/lbm

materials with high water content tend to have high specific heats but are not suitable for high temperatures -e.g., wood!

Most nonmetals have specific heats that cluster in the .2 -.3 region.

Here is a list of specific heats for materials https://www.engineeringtoolbox.com/specific-heat-solids-d_154.html

 

[MajorJinxs]

I am trying to understand which material would be able to hold 1000deg heat for the longest time give or take a few hundred. I want to line the inside of the fire box so i can reduce the amount (lbs) of wood to try and save a few 100 dollars in wood cost. While i know this is a complex issue but please understand i am but a simple man and may need more explanation that just a chart.

 

[gleem]

Firebrick is probably the best material to line your firebox. It is designed to withstand high temperatures without decomposing like regular clay building bricks.

 

[Lnewqban]

Welcome, @MajorJinxs !

Most heat escape through the exhaust air and is not recovered.
The energy or heat delivered by the wood and used for heating the water is the same, bricks or no bricks.
Bricks will act like accumulators of part of that heat, at the expense of heating the water less while they accumulate the heat that will be slowly released when wood is not burning.

Please, see:
https://highperformancehvac.com/hot-water-boiler-economizer-preheater/

Is there any factory insulation between the tubes and the metal cover of the box?
Is the stove located in a cold space?
If so, are the pipes carrying hot water properly insulated?

 

[MajorJinxs]

Lnewqban:
questions:
Yes there is factory insulation but not in the fire box.
the stove is located in an area where outside temp has the greatest affect on temperature release; however it isn't outside... the building it is in has not much insulation and is only 4-6" off a cement pad.
as for the pipes they are Pec, but they dont have much in the way of insulation yet.

And what about adding Cast iron bricks?

 

[Lnewqban]

You could add any firebrick around the fire box if you have the room and it does not interfere with the heat transfer to the water tubes.

What I was trying to tell you earlier was the following:

1) You will get certain limited amount of heat or energy from the wood. Bricks will not increase that, in my humble opinion. There is no way around that. Keep and use as much of that energy as possible.

2) Preserving and transferring that heat to the water rather than to the surrounding air as much as possible will save you more money than any other thing. Internal or external dry insulation, shielding stove from cold draft, installing economizer, pipe's insulation, recirculating pump, all and each should help capturing and retaining more heat at additional cost.

3) Adding internal mass (bricks, cast iron, etc.) will not increase the amount of heat transferring from the wood into the water. It would only increase thermal inertia. Meaning that it will take longer to heat the water at first, and some heat will remain inside the hot water stove longer after the wood stops burning. Total heat transferred to the water should be the same, but in a retarded way.

 

[MajorJinxs]

would a dampner change the amount of heat loss in this system?

 

[erobz]

Do you consider the heat leaving the stove walls as waste heat?

 

[erobz]

would a dampner change the amount of heat loss in this system?

A damper is used to regulate the rate of combustion, by limiting oxygen in the combustion environment.

 

[hutchphd]

A damper is used to regulate the rate of combustion, by limiting oxygen in the combustion environment.

One of the primary reasons woodstoves (I have built three) are far more efficient than a fireplace is the ability control airflow. Ben Franklin understood this. A fireplace uses, and therefore expends up the chimney, a lot of warm air, most particularly when the fire is relatively small or waning. A sealed stove can be banked way down when it is appropriate and save all that waste.
However, woodstoves are most efficient, and produce less creosote, when burned wide open. They will also turn the room into an oven! If you can, add thermal mass external the stove but in the radiative sphere. These need not be expensive refractory materials but should be high thermal mass. My brother has an underground house in Maine whose walls are concrete backed by two feet of gravel and then insulation (extruded polystyrene) It takes days to cool down! Use your stove firebox to heat water: not enclosed brick. If you need more thermal mass add a secondary water tank and maybe a small pump.

 

[Drakkith]

I am looking for Clay bricks or Something like it to retain more heat to hold over night inside the fire box.

Why do you want to retain heat overnight?

 

[erobz]

One of the primary reasons woodstoves (I have built three) are far more efficient than a fireplace is the ability control airflow. Ben Franklin understood this. A fireplace uses, and therefore expends up the chimney, a lot of warm air, most particularly when the fire is relatively small or waning. A sealed stove can be banked way down when it is appropriate and save all that waste.
However, woodstoves are most efficient, and produce less creosote, when burned wide open. They will also turn the room into an oven! If you can, add thermal mass external the stove but in the radiative sphere. These need not be expensive refractory materials but should be high thermal mass. My brother has an underground house in Maine whose walls are concrete backed by two feet of gravel and then insulation (extruded polystyrene) It takes days to cool down! Use your stove firebox to heat water: not enclosed brick. If you need more thermal mass add a secondary water tank and maybe a small pump.

Yeah, figuring out how to optimize without some kind of diagram is tricky. It sounds like at least part of this system (heat exchanger for the water) is outdoors. I would want to insulate that box, so long as doing so didn't cause the water in the system to boil? Is the stove itself used as ambient heat for a room? If it's not, I think insulating the exterior of the stove would be a good idea. It should lower the fuel consumption to maintain a given operating temperature.

 

[jrmichler]

the building it is in has not much insulation

This sounds like an older outdoor wood boiler that controls heat by controlling combustion air. If it is a typical older design that smokes heavily when the draft is shut off, then it is throwing away as much as half of the wood. The thrown away wood shows up as smoke and creosote. I once saw one of those boilers that had a full two inches of creosote on the inside of the firebox.

I recently saw one of the new EPA approved outdoor wood boilers, where the manufacturer claimed it used 60% less wood to get the same heat. I'm doubtful about the 60% number, but would believe 50% less wood. It is a downdraft design, and was not generating visible smoke when I saw it burning. They claimed that it would hold the fire overnight, then start up in the morning by starting the draft.

Are the water pipes to the house insulated? If not, there is a product specifically designed for this application. I don't know any trade names, but a photo of a piece of the water lines to my workshop is below:

Two lengths of 3/4" ID tube inside a 4" diameter plastic sleeve.

If there is noticeable heat from the boiler inside its building, then that building should be insulated. It's too late now, but that slab should have at least 4" foam underneath it.

@hutchphd, you beat me. I only built one woodstove. It burned clean, with no visible smoke and no charcoal in the ash. Although I have added firebricks to two other woodstoves in mostly futile attempts to make them burn cleaner with less charcoal in the ash.

I did the design work for a friend who wanted to store heat from his indoor wood boiler overnight. The calculations called for 1100 gallons of water storage. It worked as designed. He could build one fire in the evening and have full control of the heat in his house until the next evening at outside temperatures down to zero deg F. At -20 deg F outside, he built a small fire in the morning. He had a four zone hydronic system.

 

[Mister T]

as for the pipes they are Pec, but they dont have much in the way of insulation yet.

Without knowing the specifics, I would say that insulating the pex lines will give you the bigger bang for your buck.

As has already been stated, adding thermal mass (your fire bricks) will not alter the total amount of heat available, but insulating the pex lines will waste less of that heat energy.

 

[MajorJinxs]

would a dampner change the amount of heat loss in this system?
the System is a firebox inside, cast iron pipes that hold water in the middle, and Insulation out side that will the a pretty metal shell. The Flue is a straight so there is probally 60(natural draft)-80(forced air)% heat loss... i am just trying to reduce the amount of wood burn per burn by either holding heat in the box or ... Any other ideas i am open to.

 

[MajorJinxs]

This sounds like an older outdoor wood boiler that controls heat by controlling combustion air. If it is a typical older design that smokes heavily when the draft is shut off, then it is throwing away as much as half of the wood. The thrown away wood shows up as smoke and creosote. I once saw one of those boilers that had a full two inches of creosote on the inside of the firebox.

I recently saw one of the new EPA approved outdoor wood boilers, where the manufacturer claimed it used 60% less wood to get the same heat. I'm doubtful about the 60% number, but would believe 50% less wood. It is a downdraft design, and was not generating visible smoke when I saw it burning. They claimed that it would hold the fire overnight, then start up in the morning by starting the draft.

Are the water pipes to the house insulated? If not, there is a product specifically designed for this application. I don't know any trade names, but a photo of a piece of the water lines to my workshop is below:
View attachment 319466
Two lengths of 3/4" ID tube inside a 4" diameter plastic sleeve.

If there is noticeable heat from the boiler inside its building, then that building should be insulated. It's too late now, but that slab should have at least 4" foam underneath it.

@hutchphd, you beat me. I only built one woodstove. It burned clean, with no visible smoke and no charcoal in the ash. Although I have added firebricks to two other woodstoves in mostly futile attempts to make them burn cleaner with less charcoal in the ash.

I did the design work for a friend who wanted to store heat from his indoor wood boiler overnight. The calculations called for 1100 gallons of water storage. It worked as designed. He could build one fire in the evening and have full control of the heat in his house until the next evening at outside temperatures down to zero deg F. At -20 deg F outside, he built a small fire in the morning. He had a four zone hydronic system.

yeah but look at my last post. it is an water stove not boilier... would love a boiler or as you say a hydronic system.

 

[MajorJinxs]

would a dampner change the amount of heat loss in this system?

 

[erobz]

would a dampner change the amount of heat loss in this system?

A damper is basically a throttle. You could throttle down, so it doesn't consume as much fuel. It will also be a "colder" fire...unless it's well insulated.

 

[MajorJinxs]

thanks for the thoughts

 

[Lnewqban]

would a dampner change the amount of heat loss in this system?

A damper controls the flow of intake combustion air and exhaust gases.
Less of those means less burned wood per hour; therefore, less heat available to heat the water in that hour.

Try to keep all the generated heat from the precious wood (while heating the water and after is hot).
Expose as much cold water to that heat as possible, the colder the better for heat transfer (make up cold water and economizers get along well).
Keep tubes as clean as possible, in and out.

Happy New Year!