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Green Building Bible, Fourth Edition
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    • CommentAuthorjwd
    • CommentTimeSep 15th 2017
     
    Hi.

    Can anyone suggest a simple way to test if my heat meter is giving accurate readings?

    I've got a 40kw log gasification boiler with a thermal store. It has a heat meter installed because of a backup oil boiler. My meter readings are much lower than expected. The meter didn't work properly at first as the return probe was incorrectly installed but since then it's still giving lower readings than I had hoped - about 15% of what I expected from the heating requirement calculation for the deemed rate.
    The house is tenented and the tenents tell me they have been using wood almost constantly and have hardly used the oil boiler at all.

    The boiler burn chamber has a capacity of 185 litres and is located in an enclosed boiler room and the is in the adjacent room, inside the insulated envelope. the heat meter probes are fitted close to the thermal store on the flow and return. The water in the tank has inhibitor in it. The wood fuel is dry and mainly norway spruce.

    Any advice would be very welcome.

    Cheers

    Jwd
    • CommentAuthorjwd
    • CommentTimeOct 22nd 2017
     
    Bump- Just putting in an a comment as no responses to first post.

    I was thinking it might be worth trying to work out how much heat energy would be produced by burning a full load of norway spruce and seeing if that is reflected in the increased heat meter reading. The only problem is I have no idea how to go about calculating such a figure.
    Cheers

    J
    •  
      CommentAuthorSteamyTea
    • CommentTimeOct 22nd 2017
     
    Timber has around 4.1 kWh/kg. (http://www.fao.org/3/a-i4441e.pdf)
    Your boiler will have an efficiency of somewhere between 30 and 80%, depending on the length of burn and the relative temperature differences.

    I would think that if you could start with a cold system, read the meter, burn 5 kg of timber, take meter readings every ten minutes while it burns, then take meter readings while it cools (if you can be bothered).
    The meter should show something like half the energy content of the fuel, so around 10 kWh.
    • CommentAuthorjwd
    • CommentTimeOct 22nd 2017
     
    Thanks SteamyTea.
    I was thinking it might be better to start with a cold thermal store( ours is quite large compared to the boiler size and can easily take two burns to get up to temperature from cold esp when the heating is on). Having burnt the boiler once, perhaps not a full load, I could reload and burn again taking measurements on the second burn. This would minimise the loss of heat in getting the water jacket around the boiler up to temp. I could easily weigh the wood and take moisture level measurements. I hadn't thought about using the weight of wood rather than volume which now seems obvious.

    Cheers
    J
    •  
      CommentAuthorSteamyTea
    • CommentTimeOct 23rd 2017
     
    If your store can take a couple of decent burns then that is the better way.
    You could make your own meter with a 10 quid raspberry pi, a temp sensor and a relay to sense when the pump is on.
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