How much heat is stored in my iron stove?

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bokehman

bokehman

Feeling the Heat
Nov 25, 2007
445
Spain
The other day someone mentioned that it takes a lot of energy to get an iron stove up to operating temperature and this got me thinking how much heat is stored in my stove. The stove weighs 230 pounds. When the stove is really roaring the flue collar gets to around 600ºF while the lower parts can still be touched (maybe 150ºF). Overall, I guess the average temperature of the whole stove is in the region of 400ºF, an increase of maybe 350ºF compared to when there is no fire. Specific heat capacity of iron is 0.1076 BTU/lb/ºF so for my stove 230 lb * 350ºF * 0.1076 = 8661 BTU. Wood at 20% water content is 6800BTU/lb so we are looking at the heat from 1lb 4oz of wood being stored in the iron once operating temperature has been reached.

Does it surprise you how little it is?
 
Not really surprised- but it is an interesting thought on the subject. Metals have a low heat capacity. Heat water- it's almost 10 times the heat capacity. Ceramic is about twice that of cast iron. Those soapstone stoves seem to do a good job holding heat- I'd guess it's got a better heat capacity than ceramic.
 
Larry Lopez said:
http://www.allmeasures.com/Formulae/static/formulae/specific_heat_capacity_300K/17.htm

show specific heat of iron to be 449 J/(kg.K)

Assuming 212 F rise, 300 pounds
it comes to 6800 btu.
Click to expand...
6843 BTU's. But I don't see your point. Why 212ºF? What does that relate to? Also, I used BTU/lb/ºF because this is a US based forum, but 449 J/(kg.K) is exactly the same as 0.1076 BTU/lb/ºF.
 
Whatever...the whole purpose of a stove is to serve as a space heater...to transfer the heat generated by combustion out to the living space, through radiation and convection. Typical metal (steel/cast/combo) woodstoves aren't intended to be heat sinks or thermal storage masses. Material characteristics vary to a small degree in this regard among the available stove types. Now, if you want to start talking about a masonry heater, that's a different animal. In any case, whatever heat is absorbed by your stove materials during start-up is going to eventually be released during cool-down. Rick
 
fossil said:
Whatever...the whole purpose of a stove is to serve as a space heater...to transfer the heat generated by combustion out to the living space, through radiation and convection. Typical metal (steel/cast/combo) woodstoves aren't intended to be heat sinks or thermal storage masses. Material characteristics vary to a small degree in this regard among the available stove types. Now, if you want to start talking about a masonry heater, that's a different animal. In any case, whatever heat is absorbed by your stove materials during start-up is going to eventually be released during cool-down. Rick
Click to expand...

Yeah, but the OPs analysis is interesting and useful. Consider, for example, the Pacific Energy T6 versus Summit. T6 is essentially a Summit clad in cast iron. For someone trying to decide between these two appliances, using Bokemon's analysis, a rational buyer might conclude as follows: A good reason to buy the T6 is because I like the look of it. A bad reason is for the energy bank feature of the cast iron cladding, which would be minimal.
 
fossil said:
whatever heat is absorbed by your stove materials during start-up is going to eventually be released during cool-down. Rick
Click to expand...
I realise that but as it cools it radiates less and less. What I am really getting at is how much wood I need to burn from a cold start to get the stove to operating temp, where it starts to radiate a reasonable amount of heat.
 
A function of the mass and thermal properties of the material, as I'm quite sure you grasp firmly. Rick
 
Yes, it surprises me.

Intuitively I would have thought a cast iron stove of 200+ pounds would hold several times more heat than that.
 
The other variable involved (more correctly I should say among the other variables involved), is how quickly the temperature inside the stove rises, or the rate at which heat is generated inside the stove at start-up. The faster this happens, the sooner the stove begins to radiate. Build a fire that will light, kindle, and build rapidly, and give it plenty of air until it's well established, and you'll minimize both the time required to begin to feel the stove's warmth, and creosote build-up in the connector pipe & chimney. Rick
 
this is why it is good strategy to build a masonry hearth around a strove... lets the stove stay nimble and get up to temperature more quickly.. let the masonry store the heat and even out the heating cycle.... win win
 
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