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[powerstroke]

I have the high limit on my furnace set at 167 degrees.  I notice that after it gets to that setting and the fan and damper close that if I check it 10 min or so later it will be at like 173 degrees or so.  Only around 6 or 7 degrees.  It is like clockwork.  I am thinking that the thick firebox in my Shaver is what is storing all of this heat and there is a delay in the release of this heat.  Sound about right?  I know some people were saying that "thicker fireboxes waste enegy" but wouldn't this phenomenon offset this somewhat?  It takes a LONG time for it to drain off this heat before it calls for another cycled burn.  I only have a 5 degree swing because of this too.  Any thoughts guys?

[willieG]

your fire is roaring like mad when the set temp is reached and the damper closes...there is still enough air in the fire box for your fire to continue to heat the water for a few minutes before the air is used up and the fire idles down. this is quite common. i have my settings from 150 on and 160 off and depending on the type of wood i am burning it may get near 170 before it levels out if the house is not calling for heat when the set temp is reached

[Scott7m]

Has nothing to do with heat being stored in the steel.  All furnaces spike over some, with wood mine pikes over 4-6 degrees with coal more.  You get all that wood hot and the heat has to go somewhere.

Thicker fire boxes are less efficient, we didn't realize how much less until we tested the natures comfort line.  I think the 3/8" box was like 12% less efficient than the 1/4"

[powerstroke]

What if we performed a test of this.  Say we took 2 pieces of steel, both of them 2 inch squares.  One of them is 1/8" steel, the other is 1/2 inch steel.  If we heated them both to say 1000 degrees, then drop them each into seperate cups of water that are equal, would you bet on the 1/8" cup of water being hotter after say 10 minutes?  Although it would take longer for the 1/2" piece to get to 1000 degrees, it would take longer for it to cool than the 1/8" piece.  There is absolutely no way that could be argued.  Kind of the same reason behind people putting firebrick inside fireboxes and woodstoves.  Super heavy cast iron radiators that have been used for decades are used because of the more even heating and slow dissipation from all of that heated mass.  I am in no way trying to start an argument about this but doesn't that just seem like common sense.

[BoilerHouse]

As to why your temp spikes up; as Willie G noted residual air is still supporting combustion and the residual heat mass is not being forced up the stack when the blower stops.
As far as efficiency; Thin walled fireboxes give better heat transfer which should make them more effIcient.
Thick walled Fireboxes have more thermal mass allowing the water to stay hot longer and promoting longer burn times which should also in theory promote efficiency.  The overall winner -(?)

[yoderheating]

Willie is correct that the fire will continue after the fan shuts off. I have seen my furnace spike as much as 10 degrees after shut off.
 On the argument of thin or thick fireboxes there are a few things you have to keep in mind. There are two places the heat from the fire can go, through the firebox into the water or up the chimney. Powerstroke, our theory would be correct if the heat stayed in the firebox waiting to be transferred into the water. However if the metal doesn't capture the heat immediately it goes up the chimney never to return. This is why the speed of transfer is much more important than the amount the steel is able to hold. Your theory would be correct if you burned the furnace on one or two degree cycles where the fan ran for a minute or two or if you used an natural draft furnace where less heat was produced. However, any heat that is produced that isn't transferred is going to leave rather quickly and is therefor lost. I hope this explains it, I have been know to complicate the simple sometimes.   I guess the part of the equation that many people forget is that heat is constantly being produced and needs to be captured immediately.

[Scott7m]

Like yoder is saying, it's not about how much heat you can store in the steel, you use water to store heat in and not steel.  If the heat isn't transferred to the water it simply goes up the stack. 

To not understand that this is about transferring heat into the water and not the steel confuses me.  In a perfect world we would want our fireboxes as thin as possible.  But they have to be strong enough to survive in the real world. 

Let's say you try this......  Place a pan of water on a piece 1/8" steel, take a torch and heat it, do the same with a piece of half inch, half inch will take a lot more time to boil the water, whichmen ans more acetylene/oxygen fuel was burned.  Now I know your thinking well mine will stay hot longer.  Maybe slightly but that doesnt begin to make up for what you previously gaveup.  Again this is different test cause the torch would immediately quit making heat unlike your firebox.

It's like this, there are btu's stored in the steel but very little in comparison to whats laying inide smoldering away. 

Like I said this has already been tested many times but the test I'm most familiar with was natures comforts test when there 3/8 box was 12-13% less efficient than the identically designed 1/4" box.  So i would feel safe saying that you'd lose mother 12-13% in the next 1/8" firebox material at 1/2".  Making it up to 26% less efficient on steel thickness alone. 

This is where I feel heatmaster is able to run so efficiently.  The thinner stainless transfers heat like crazy and allows us to use much smaller water capacities.  For example the 5000 sq ft model holds 130 gallons.  Most competitors would say you need 300 gallon for that. 

[rhugg]

I think you guys got it.  The goal of the OWB is to efficiently heat water.  Contrary to what I think the EPA seems to rate which is 'low emmisions' or its equivalent 'create heat'.  One can immagine that a poorly designed gassifier, while actually being more efficient from a 'cleanly burning wood' definition could actually be less efficient than a classic OWB from a water BTU per cord measurement.

The heat either goes up the flue, or into the water (to a lessor extent to the surrounding air but we'll ignore that loss).  With thicker steel or refractory more heat goes up the flue, less is transferred to the water.  Even once the damper closes the residual heat in the steel and the residual heat from the fire (it doesn't stop immediately) is transfered thru the steel to the water or to the air and up the flue.  Thinner steel is clearly better from a heat transfer standpoint. 

So now I'm thinking that a stack temperature monitor that reduced blower speed at higher temperatures might improve efficiency.  Clearly if you blow too much air thru the OWB you are putting more heat up the stack than needed.