OWB design. More water? Less water?

[leon]

I'm no expert nor do I own an OWB but I think some are missing the point of having a large amount of storage. Once the large mass of water is up to temperature, you don't have to heat it from ambient temperature back to 165-180. All you are doing is making up the difference from what btu's you have extracted to heat with. If you have say 1000 gallons of water in a well insulated tank and it is at 170 degrees and you draw it down to 160 degrees you only have to heat it back up to 170. More storage will mean that you can draw off that storage for a longer period of time. If everything works as it should you shouldn't need to have a continuous burn but rather a burn when needed to raise the storage temp back up.
just my .02
dave
Again I say "google Garn".
taken from the garn website: When the GARN WHS unit drops below the operating temperature of your system, start a fire and heat the water again.

Dave, your right on saying:

"If you have say 1000 gallons of water in a well insulated tank and it is at 170 degrees and you draw it down to 160 degrees you only have to heat it back up to 170. More storage will mean that you can draw off that storage for a longer period of time. If everything works as it should you shouldn't need to have a continuous burn but rather a burn when needed to raise the storage temp back up."
I always fire my stove to 195 degrees and then let it go out. Here in NC it's not as cold as it is up north. I fire it hard then let it go out at night then in the morning it's down to maybe 160 then I put some good seasoned wood in, throw alittle diesel fuel, light it up and turn on the fan and make it burn as hard as I can back up to 190. By the time it recovers to 190 there is nothing but good hot coals, no smoke.You be surprise how long a box of hot coals will keep the water up there. When you have more water storage you can play around with the firing.

Thank you dave_dj1 and NR wooduser for further proofing what I have stated, wherein working
with more water/thermal mass/firebrick allows for more heat transfer and easier firing to maintain
the water temprature desired as you are only heating it enought to bring it back up to the desired
temperature.

 

[AIM]

Well I won't deny that you can draw off a 1000 gallons longer than you can draw off of 250 gallons. That's just logic.

How much wood do you think it will take to recover 10° in that 1000 versus 250.

I have no idea but I would guess 4 times as much. Do you agree?

 

[John D]

Of course it would take 4x the wood....but you would be adding 4x the btus too 1000 gallons to bring it up 10 degrees as you would to bring 250 up 10 degrees ..so nothing lost...the 250 gallon owb will have to fire 3 more x before the 1000gallon owb needs to open the draft/ fan again....

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[John D]

The biggest benefit of a larger water storage is the ability to burn very large unsplit rounds and maintain water temps longer while any slow burning wood is in the firebox ...even during times of peak demand. My water capacity is 230 gallons and at times it drops 5-10 degrees behind the low set point especially after periods of long idle time or when loaded with the biggest rounds I can get in there ( which burn slow and the most efficiently).If I had double the storage I could probably burn huge rounds all the time ...without having to split wood so it could burn faster ...this would save me time and money. I split most of my wood just so it drys and lights faster ...to keep owb temps up in times of high demand ...

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[NRwooduser]

Well I won't deny that you can draw off a 1000 gallons longer than you can draw off of 250 gallons. That's just logic.

How much wood do you think it will take to recover 10° in that 1000 versus 250.

I have no idea but I would guess 4 times as much. Do you agree?

I don't know about the 250, but I have 800 gallons, there's some things you have to look at when firing the stove. Like is the heat running hard in the house, is wife, son, running the dw at the point they're trying to run the well dry. If you want to raise the temp of 800 gallons with very little wood at high demands then I would put some dry pine about maybe 7 medium split pcs and fire it up. I can stand there and watch the temp gauge start climbing. But now with oak it will take longer. I don't know how true this is but I had a taylor rep tell me that they put a guage in the firebox while firing and the temp inside the firebox was close to 2000 degs. Now I don't know what wood they was using but that's what he said. He made a comment that if the firebox wasn't surrounded with water it would warp the firebox.

 

[mtfallsmikey]

That's the bad part about system design, how do/would you know what the drawdown capacity is before you drop your water temp. by, let's say 10 deg. or so? With electric/fossil fuels that is a constant, but with wood too many variables.

 

[J.W Younger]

I can't figure out why anyone would put firebrick in an OWB.

 

[John D]

That's the bad part about system design, how do/would you know what the drawdown capacity is before you drop your water temp. by, let's say 10 deg. or so? With electric/fossil fuels that is a constant, but with wood too many variables.

Your last statement is the biggest reason to error on the side of too much water capacity ....there is no set recovery rate for a wood boiler ...sometimes its not very fast and sometimes it is....with oil, natural gas, electric and propane heat we can calculate the recovery btus and rate ..therefore be able to select the proper size boiler, and nozzle to get the btus per hour to keep up with the highest demand the system will see ...in my house (2family) there can be very high demand at peak hours and none during the day...I can't tell my tenants when to wash clothes dishes or take showers ...if were both using simultaneous it can be a problem for the Shaver to keep up IF I am loaded with large rounds and slow burning wood....thank god I didn't listen to Shaver and get the model 165 OWB...they recommended ...I dont think I couldve got by with it....I wish I'd gotten the biggest one now...(340)...the water storage makes a big difference in dealing with sudden high demands.

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[mtfallsmikey]

Took that into consideration when designing my system. Actual heat loss for the house was 46,800K BTU. I installed a hydrocoil rated at 56,000 BTU @180 deg. water, even if my water temps drop to 150, 160, can still heat the house well. A heat loss figure in my area represents the heat loss @ 0 deg. outside temp, heat loss is a lot less at 40 or 50 deg. outside temp.

 

[AIM]

I can't figure out why anyone would put firebrick in an OWB.

I agree with this.

 

[mtfallsmikey]

I agree with this.

Ya, I want max heat transfer myself...

 

[mtfallsmikey]

Heating efficiency=Heat transfer efficiency X Combustion efficiency.



To much heat transfer efficiency can lower combustion efficiency due to a cold firebox. Building an efficient boiler involves more then cutting and welding metal.

True, to a degree (badabing)....more so w/fossil fuels than wood, combustion temps. w/wood are all over the map.

 

[leon]

OWB design

I can't figure out why anyone would put firebrick in an OWB.

The reasons you want fire brick in any furnace or boiler are:

1. you are creating a thermal mas that absorbs heat energy
2. you have a thermal mass that sheds heat energy when the fire is colder as heat always travels to cold
3. the fire brick maintains high combustion temperatures by shedding heat back into the fire box which
reduces smoke which is lost heat energy; allowing you to burn a small intense fire at all times.
4. the fire brick aid in the reduction of hot spots on boiler and furnace combustion chamber and the chance of a perforated firebox.
5. the hotter the burn the less smoke and the fewer upset neighbors you have.

 

[leon]

OWB boiler design

Heating efficiency=Heat transfer efficiency X Combustion efficiency.



To much heat transfer efficiency can lower combustion efficiency due to a cold firebox. Building an efficient boiler involves more then cutting and welding metal.

If you have and maintain hot fire and have a scavenger like single pass set of four fire tubes in a water jacket
exiting the combustion chamber to take exhaust heat out of the combustion as it exits the boiler
flue how is that inefficient?



If one was to obtain a used insulated tank car that was in good condition and bought it before it headed to the scrapper
the railroad car tank is a huge asset where the tank is fully insulated and can be a huge source of thermal
mass and useable energy alllowing one to have a hot intense fire with zero smoke and massive heat transference
into the water which is in the railroad car tank allowing you to burn a very hot fire only afew times per week or less
as you would have the thermal mass of this huge salvaged insulated tank cawr to feed to the home heat exchanger or directly
to the homes hydronic heating system or steam boiler feed water reservoir.

 

[J.W Younger]

I don't believe the firebrick is gonna do much for lowering wood use. When the water temp calls for heat i want it het and the forced air shut down as fast as posible. That 6" hole in the top will send a lot of heat out during a cycle, no need to put anything between it and what your trying to heat.

 

[leon]

I don't believe the firebrick is gonna do much for lowering wood use.

When the water temp calls for heat I want it heat and the forced air shut down
as fast as posible. That 6" hole in the top will send a lot of heat out during a cycle,
no need to put anything between it and what your trying to heat.

Again you are missing the point, you lose huge amount of potential energy as smoke.

Look at any russian Masonry stove or Finnish masonry stove design and you will understand
why it works.




Again it is a THERMAL MASS. If you fill the fire box half full of standard firebrick, not the crap half bricks(they crack)
you will see it works.

 

[J.W Younger]

The reasons you want fire brick in any furnace or boiler are:

1. you are creating a thermal mas that absorbs heat energy
2. you have a thermal mass that sheds heat energy when the fire is colder as heat always travels to cold
3. the fire brick maintains high combustion temperatures by shedding heat back into the fire box which
reduces smoke which is lost heat energy; allowing you to burn a small intense fire at all times.
4. the fire brick aid in the reduction of hot spots on boiler and furnace combustion chamber and the chance of a perforated firebox.
5. the hotter the burn the less smoke and the fewer upset neighbors you have.

Less smoke and fewer upset neighbors is a different subject. My goal is heat in the house and putting a barrier between the fire and the water thats moving that heat inside won't show me anything i'm intrested in seeing. After heating with this unit for 22 seasons i have a pretty good idea what works and what don't.

 

[J.W Younger]

Less smoke also means more heat for a given volume of wood consumed.

We are going to see fewer and fewer 'smoke shacks', and that is a good thing for everyone.

How cute and clean and hot and stuff of a little fire it is means nothing to me. How long a burn time and total wood consumed each burn cycle does. My carbon footprint is likely smaller than average but you go ahead and lose enuff sleep over it for both of us.

 

[AIM]

I understand wanting a hotter cleaner burning fire but not at the expense of impeding heat transfer to my water, where heating my water is my ultimate goal.

If you have a 1000° fire on the backside of a bare steel plate. What would the temp be on the opposite side of that steel?

Now put the same 1000° fire on the backside of a firebrick lined steel plate. What would the temp be on the opposite side of this? I gotta believe it would be much lower.

Now your gonna say "Well that fire is going to burn hotter than 1000° due to the firebrick."
Ok but how much hotter does it need to burn to get equal temp transfer through the steel plate to my water? Does the wood have the ability to get this much hotter to equal it?

Do you follow me here?

I think firebrick is a fabulous idea for wood stoves and such but a bad idea for boilers. I've been wrong before though.

 

[wndwlkr]

I understand wanting a hotter cleaner burning fire but not at the expense of impeding heat transfer to my water, where heating my water is my ultimate goal.

If you have a 1000° fire on the backside of a bare steel plate. What would the temp be on the opposite side of that steel?

Now put the same 1000° fire on the backside of a firebrick lined steel plate. What would the temp be on the opposite side of this? I gotta believe it would be much lower.

Now your gonna say "Well that fire is going to burn hotter than 1000° due to the firebrick."
Ok but how much hotter does it need to burn to get equal temp transfer through the steel plate to my water? Does the wood have the ability to get this much hotter to equal it?

Do you follow me here?

I think firebrick is a fabulous idea for wood stoves and such but a bad idea for boilers. I've been wrong before though.

[/I]
I put firebrick in mine & it works good. I build a fire every other day for three hours with 15 sticks of wood and heat 1350 gallons .View attachment 245014 :msp_rolleyes: