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Heating and cooling: Maximum temperature of wood stove bodies... Does 230C seem quite low?

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    • CommentAuthorgreenfinger
    • CommentTimeDec 31st 2020 edited
     
    Seconds up - Round 2 of the wood burning stove questions from me...

    This time I'm curious as to what temperature the body of your stoves get to (anyone use a thermometer on the stove itself?). And/Or - if you've read the manual that came with it - what the manufacturer species as the maximum? The reason I ask is because we've narrowed our search down to the Woodwarm Fireview 5kW Slender. I was reading through the manual online and it says the maximum is 230C. That seems quite low to me. A quick glance at some of the stove thermometers out there and I see they show an optimum burn range. One has this at 150-300C, another at 200-350C. The Woodwarm made thermometer shows up to 300C as optimum!

    For the record, the manual I was reading refers to the whole range of Fireviews, which has models up to 20kW. So the 230C max. applies to them all. And the manual goes on to say "...avoid continuous running at the maximum..."

    Does anyone have any thoughts as to whether a maximum 230C body temperature is low and whether that will affect performance? I appreciate a lot goes into the design of wood stoves and there's a lot more to them than the statistics. And I've also read many glowing reports on Woodwarm; often mentioned in the same breath as Clearview. Nevertheless, this surprised me.

    Ta
    • CommentAuthorphiledge
    • CommentTimeDec 31st 2020
     
    I think the temp range could well be fire specific and position specific. Clearveiws reccomendation for our 750 specifies the temp of the door airwash duct rather than the 'body'. The recommended temp range for the duct above the doors is around 210-260 degC with a maximum of around 320.
  3.  
    Thanks @philedge.

    With regards the Fireview, it's the same manual for all versions and there's no mention of position.

    A 320C maximum is quite different to a 230C maximum. So for the same given nominal kW rating, I can't figure how they can both give the same room heat when the actual temperature of the stove body is so different.
    • CommentAuthorphiledge
    • CommentTimeDec 31st 2020
     
    You probably need to ask the manufacturer whereabouts on the stove the running temp should be measured. Our temp is measured on the outside of the airwash duct and with air running through it I assume that would be considerably cooler than without the airflow?? Id be certain that the top of the stove will be much hotter than the airwash duct. Equally the sides and back are way cooler as we have a wrap around boiler in the fire that soaks up the heat. I assume a stove lined with firebrick will be cooler on the sides and back than it would on the front or top??
  5.  
    Good thinking. I plan to call them when they re-open to discuss it all generally, but will definitely bring that up too. Thanks for the thought.
    • CommentAuthortychwarel
    • CommentTimeDec 31st 2020
     
    I have a thermometer on my clearveiw 400 it is bolted to the flue at the point it leaves the stove so probably gives a reliable indication of combustion chamber temperature. I run it at 400 f approx 205 C and after 10 years of operating it, that seems to be the sweet spot between excessive wood consumption and it petering out.
  7.  
    That's very helpful @tychwarel - thank you
    • CommentAuthorrevor
    • CommentTimeJan 2nd 2021
     
    I have measured my Heta stove with IR thermometer Internal temperature from burning log 350 sides of stove 170/180 top 180/190 exit to flu 160 500 mm up flue 100 all centigrade
  9.  
    Thanks @revor - that's really helpful and very kind of you to share those figures. What kW is your stove?
    • CommentAuthorrevor
    • CommentTimeJan 3rd 2021
     
    5Kw don't think the size will affect temperature guess moisture content of logs will be a more important factor as heat is lost in the process of drying out the log if it is wet, i.e. 16% +
  11.  
    Hi all

    Just thought I'd revisit this as I can't stop mulling it over to try to make sense of it!  A couple of things...

    I noticed yesterday that the Woodwarm Foxfire (4kW) has a maximum stove body temperature of 350C.  120C more than the Fireview.  They both have a similar internal layout, are both steel and both have 30mm mica based fire boards.  So that's got me scratching my head! 

    Since learning more about all this, I can now see how the glass plays a part in the heat output, and how the internal heat is relevant to many things (burning of secondary gases, flue temps, emissions, etc).  But it still seems logical to me that the temperature of the stove body will have a direct bearing on the amount of heat that can radiate out into the room.  And as such, I can't understand how a stove that's only a bit bigger physically can quadruple the kW output.  Unless it's logarithmic...   ???

    Americans tend to like to do things on a grander scale (no offence to any Americans reading, and I mean it as a sweeping statement).  Having read a fair few threads on the net discussing stove temps, some of the more cavalier wood burning folk discussing running their stoves to a body temp of over 1,000F.  I've even seen 1,200F mentioned (that's 650C in our money)!  Whilst that sounds like a serious recipe for disaster and I have no aspirations to burn my house down, it did on the other hand make 230C maximum seem a touch puny. 

    Cheers   
    • CommentAuthorEd Davies
    • CommentTimeJan 20th 2021
     
    Posted By: greenfingerAnd as such, I can't understand how a stove that's only a bit bigger physically can quadruple the kW output.  Unless it's logarithmic...
    It's not logarithmic but it is proportional to T⁴. For lower temperatures like radiators that's not so important as that temperature is absolute (e.g., in kelvins) so it only increases output slightly. However, once you're getting up to these sort of temperatures it does begin to matter.

    E.g., all other things being equal a 350°C stove will have a radiative output proportional to (350+273.15)**4 ~= 151e9 whereas a 230°C will have an output proportional to (230+273.15)**4 ~= 64.1e9 so the hotter stove will produce something like 2.36 times as much output.

    Not ~1.5 times as much as some might very naively expect by dividing 350 by 230 and not 1.24 times as somebody slightly less naive might think by dividing the absolute temperatures.

    Of course, in reality other things aren't equal and stoves don't produce all of their output by radiation as a not insignificant proportion goes by conduction/convection which has its own non-linearities which aren't so simple to deal with. Still, a slightly bigger stove producing 4 times the output doesn't seem out of the question to me.

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