Wood burning stove efficiency.

[Bodidly]

The internal floor area is classed as 163m2 but it actually is 120m2 because our sitting room is double hight.

The volume is 367m3 according to the SAP report.

I am afraid I do not understand the maths involved in your description. Any chance of an idiots guide to what you are telling me.

I would love to be able to do heat-loss calculations, are there any straight forward books on this or does it take years of education?

Thanks
Beau

[dhaslam]

The  more complicated element  of heat loss to work out is the  ventilation one.   If you change the air in the house once per hour  that would be 367 cubic metres of air that has to be heated up to the inside temperature.  This takes about 1/3 rd of one watt per degree for each cubic metre.      If the increase is say 15 degrees  you would need  367 X 15 /3  =1835 watts and over 24 hours that would be  44 kWh.   If you are only using  20.6 kilos of wood, equivilant to 57.7 kWh at best (4 kWh per kilo of wood at 70%)  it would only leave  about 14 kWh for heat loss through the walls etc.     Of course there would be  passive heat as well from  sunlight,  electrical appliances  and occupation   but it  is  quite a low figure. 

In any case it should  give a  good comparison for the heat pump.  The two factors that will vary from day to day apart from outside temperature is the amount of sun and the wind speed, both have quite an influence on heat requirement.   

[Bodidly]

"sunlight" what's that? sounds nice though

[Baz]

367 m3 of air per hour 
How about closing the front door to start with. 

[Bodidly]

The barn maybe quite airtight because I used a basement tanking system on the walls and the roof was carefully taped on the outside of the PUR insulation.

 "Of course there would be  passive heat as well from  sunlight,  electrical appliances  and occupation   but it  is  quite a low figure."

It was sunny for one morning and it was windy on the same day, we used 1kg less wood that day.
The barn runs north-south so the solar gain that morning was noticeable.



Thanks for your help on this.

Beau

[StBarnabas]

The internal floor area is classed as 163m2 but it actually is 120m2 because our sitting room is double hight.

The volume is 367m3 according to the SAP report.

I am afraid I do not understand the maths involved in your description. Any chance of an idiots guide to what you are telling me.

I would love to be able to do heat-loss calculations, are there any straight forward books on this or does it take years of education?

Thanks
Beau

Hi Beau,

thanks. Yes it is therefore a lot smaller than StBC, which is around 400m^2 depending on how it is measured. Though I'm sure it is plenty big enough a lot of modern houses are less than 100m^2.

Possibly easier to understand the calculation slightly differently.

Over a 24 hour period you are burning  20.575kg. This is at a rate of 20.575/24 kg/h = 0.8573 kg/h

If we assume your wood has 4kWh/kg you are burning at a rate of 4kWh/kg x 0.8573 kg/h=3.43kW

I assume your stove is  70% efficient so the actual heat going into the house is 0.7*3.43= 2.4kW

You state that you are mantaining a 12.27°C temperature difference so the heating loss per degree centigrade is 2.4/12.27=0.196kW/°C=196W/°C

Hope that makes more sense

Sean

[Bodidly]

Thanks Sean

That makes sense to me.

When I have some figures from the GSHP I will post them up.

I am presuming a COP of 4 as the temperature lift from from the ground loop to the UFH is 27c does this sound right?

Beau

[Bodidly]

I am starting to become concerned about the heat pump. After a quick call to the suppliers the suggested COP is 4.53  for 35c temperature lift. On it's current energy use the heat pump is well below a COP 4.53 or the wood burner is 110% efficient

[optic]

does the cop include the water pumps etc that have to run aswell ??

[StBarnabas]

I am starting to become concerned about the heat pump. After a quick call to the suppliers the suggested COP is 4.53  for 35c temperature lift. On it's current energy use the heat pump is well below a COP 4.53 or the wood burner is 110% efficient

Actually getting a COP of 4.53 is very good. I hope my GSHP has a COP of 4.0 but I'm not sure. Air change is a big issue and passive solar gains. Not easy I'm afraid

Sean

[Bodidly]

This morning the heat pump is between cycles. I have just measured the power used at standby this includes the UFH circulation pump. Total 112watts

These are the numbers I have from the first 2 days of heat pump use.

Average total used per day               20.5kW

Average used for hot water per day    2.26kW

Standby and pump per day               2.668kW

Presumed energy used for heating     15.56kW

15.56kW at a COP of 4.53 would be 70.48 kW of heat. This seems improbable as the stove was using 20.575kg of wood per day.At 100% efficiency this would be 82.3kW

[dhaslam]

It isn't so  bad  just to get  that close to the same answer.  There are so many different factors involved.    If the stove is 65% efficient it could mean the COP from the heat pump is  about one  less than the manufacturers figure but it unlikely to work out quite so accurately as that.   One possible reason for  extra heat requirement from the heat pump is  the extra heat absorbed into the floor that may take quite a long time to settle to a constant figure and also there would be more radiant heat transmitted from the floor into the house structure.     

In any case you have confirmation that your woodstove is working well.  Normally that would the  more suspect device.   

[Bodidly]

I have just had an interesting chat with the GSHP suppliers and it appears the power used by the two internal pumps are not included in the COP figure they gave me. The supposed energy use is 1.3kW in and 5.9kW out, but I have measured the power in at 1.666kW presuming the output figure given is right then real world COP is 3.54 at a 35c temperature lift.

[titan]

I have just had an interesting chat with the GSHP suppliers and it appears the power used by the two internal pumps are not included in the COP figure they gave me. The supposed energy use is 1.3kW in and 5.9kW out, but I have measured the power in at 1.666kW presuming the output figure given is right then real world COP is 3.54 at a 35c temperature lift.

How old is your pump the new ISO standard has been around for years and includes the circ pumps. If you go here www.kensaengineering.com/downloads.asp   and download the 8th from the bottom item in the factsheet list " COP variations with different temperatures" you should be able to get a good idea of your cop from the ground temperature.

[Bodidly]

Hi titan

Yes the COP fact sheet fits with what I thought we should get but reality seems different. Our source is 0c but our lift is only 27c-30c so I would expect at least a COP of 4.5 but as I said earlier this would imply our wood burner is 110% efficient. I will continue for a few more days in case what dhaslam said about the floor taking a long time to properly heat up is causing the disparity.

The heat pump was commissioned in 2006
I have found my original installation manual, the figures given for COP are 1.4kW in and 5.7kW out that's a COP of 4.3 "In accordance with EN 255" it says in the manual.

Thank you everyone for helping with this
Beau