Wood burning stove efficiency.

[Bodidly]

I am trying to calculate how much energy we use to heat the house. I have weighed the amount of wood we use and I know how dry it is but how efficient is the stove? The SAP calculations presume 65% efficiency. The stove specification claims something like 73% but this is probably when it is working hard but does anybody have any figures on efficiency when stoves are shut down.

Beau

[StBarnabas]

Beau
sadly it is very difficult. Wood typically has about 3-4kWh/kg of stored energy, so some uncertainty there. You would need to know your building well to be able to work anything useful out.

[Bodidly]

Yes  I think you right.
I could try letting the fire out  and running the heat pump to keep the house at the same temperature as now, then log the energy use as I have fitted a meter to it and presume a COP of 4. Do you think this might give me some idea

Beau

[dhaslam]

That should give a reasonable answer.       Because I have a buffer tank of known  size I can measure the output to water fairly easily but the  output to the  room is a bit harder  to work out.   The only way is to use an electric heater of known  output and measure the  increase in temperature in the room.    There is quite a difference in stoves, the present one outputs 3.5 times as much  heat  to water compared to the previous one with roughly the same amount of wood used.   

[titan]

I am trying to calculate how much energy we use to heat the house. I have weighed the amount of wood we use and I know how dry it is but how efficient is the stove? The SAP calculations presume 65% efficiency. The stove specification claims something like 73% but this is probably when it is working hard but does anybody have any figures on efficiency when stoves are shut down.

Beau

It may be easier to calculate your building heat loss then measure how much wood it takes to maintain the heat loss ie to keep a stable inside temperature. If this was done at various  delta T you should get a linear  graph. This should also give an estimate of the stove efficiency. since you know both the input and output.

[Bodidly]

Hi titan

This might work if I had a conventional insulation but I have a multifoil type and there is a lot of disagreement over there effectiveness. To give you an idea about the differences my predicted energy use through the SAP report suggests our total energy use of 126kWh/m2/year but my initial figures suggest an actual energy use of 61kWh/m2/ per year this is based on a 65% efficiency of the stove. This is what I am trying to get to grips with.



I am going to do an experiment.

For the next few days I will weigh all the wood we burn and take temperature readings all over the house and also make a note of the external temperature. Then I will do they same with the GSHP running and the stove off. Are there any other parameters that I should consider?

Beau

[titan]

I am building a low energy house and have followed the mulifoil saga over the years ( I don't have any) but if you do your own heat calcs it is simple enough to have an interactive speadsheet where you can see the effect of the different factors. As the fabric losses decrease the air losses are more significant.I think a lot of the benefit with multifoils when fitted correctly is that they reduce air leakage.  Doing your own trials are going to be more relevant than any theoretical calculation.The main practical issue will be just matching the heat loss  but I suppose logging the internal/external temperatures frequently as you measure your heating output should give you some usable figures. 

[Baz]

You will need to try and run at the same temperature in the test or the thermal mass may make a difference. It might be worth calculating the mass too, and cross checking by monitoring temperature change against heat pump input if you have fine enough grained measurements to establish its relevance. The weather forecast change in conditions is not conducive to an accurate assessment this week.

[Bodidly]

You will need to try and run at the same temperature in the test or the thermal mass may make a difference. It might be worth calculating the mass too, and cross checking by monitoring temperature change against heat pump input if you have fine enough grained measurements to establish its relevance. The weather forecast change in conditions is not conducive to an accurate assessment this week.

Thanks Baz

I was wondering about the weather this week. I don't quite understand what you mean about "temperature change against heat pump input" do you mean me to measure the input and output on the ground loop?

Beau

[Baz]

What I meant was that if say you find you raise the temp by 2 degrees while putting in X amount from the pump, then cruise at Y input for steady state maybe y-x is the heat needed to raise the mass by 2 degrees.

[Bodidly]

I was planning to leave the GSHP on for 12-24 hours before starting recording temps just to let things balance out. The wood burner is not plumbed into the UFH so current UFH fluid is at about 19c and to keep the house at steady 19c-21c the GSHP will have to raise this to something like 28c . Is this what you mean, Sorry I'm being a bit thick.

[dhaslam]

If the stove isn't connected to  the same outlets as the  heat pump then you cannot make a valid comparison.   

[Bodidly]

If the stove isn't connected to  the same outlets as the  heat pump then you cannot make a valid comparison.   

If I take temperature readings from all over the house and averaged them out would that not be a fair comparison?
This morning I took readings, and everywhere except the balcony was within 3c.

Lowest 18c
Highest 23.5c (balcony near fire)
Average 20.4c

[Bodidly]

I have been taking temperature readings for the last 4 days with the wood burner only. The readings have been taken from multiple places in the house 3 times a day to balance things out.

Average internal temp               21.01c
Average temp lift from outside    12.27c
Wood consumed                       20.575Kg per day

Despite concern over the change in the weather we have surprisingly steady temperatures, warm throughout. 
The wood is all hardwood with a moisture content of 20%-22%

I have the GSHP running now. When I have got the temperatures balanced I will start taking readings again.

Beau

[StBarnabas]

If we assume the wood has 4kWh/kg and a stove efficiency of 70% this gives about 4.7/24 =196W/degree. This is very good and much better than StBC, indeed I need to check my calculations I get something like 500W/K structural and about 250W/K air-change (same units K -Kelvin). How many m^2 is your house again? Of course you will need to fairly confident of your COP for the heat pump.