Please could someone check my calculations...

[acresswell]

I'd really appreciate it if someone could check my calculations and assumptions...  I've set up the attached spreadsheet (which I've converted to open document format) to help with sizing a log gasification boiler top cope in the depths of winter.

The structure of the building is now fixed, so the only variables are the internal and external temperatures, occupancy and water use. The outputs I'm interested in are how many times each day I'd have to load the boiler in particularly poor weather (-5degrees C average temp).  I'm trying to decide between the 25kW boiler and the 40kW boiler.  Either of them has plenty of output overall, but the 40kW boiler has a bigger loading chamber, so would need loading less often.

The boiler itself will be outside the main thermal envelope (in a space that is separately insulated to keep it frost-free).
The thermal store will be inside the main envelope (which is why I haven't included heat losses from there in the main calculation... any heat lost from the thermal store will just reduce the demand on the UFH)
We're fitting a MVHR system - stated efficiency is over 90% but I never believe manufacturers figures, which is why I've reduced it to 70%.
We're fitting solar thermal panels, but this "midwinter" scenario assumes that the solar thermal isn't making any noticeable contribution.
If the average temperature dropped below -5 degrees C, we could allow the internal temperature to drop a little, light the woodburner in the evenings, or resort to using an electric heater.  I've ignored the contribution of our metabolic heat to allow for weeks when we are at work all day, but the dogs need to stay warm.
The house will have a fairly high thermal mass, so varying the temps sigificantly at different times of day is unlikely to make a big difference, though we'd certainly try it if necessary.

Have I missed anything obvious (or important but not obvious?)

Thanks for your help

[rondurrans]

acresswell is the floor area only 107 m2?

[acresswell]

Thank you... you're right.   Big gold star to rondurrans!

The ground floor is 137m². Typing error, I'm afraid   .
The total floor area is double the ground floor.  New version of spreadsheet attached. I've double-checked the other sizes and they're correct.

[KLD]

Hm, only looking at the bottom line, I'm surprised by the total amount of heat you seem to require. This is for a new-build house, isn't it? Just for comparison, we live in an 1850th Cotswold stone cottage, some added improvements, but no major changes to the old centre of the cottage. Our daily heating rate is about 3kWh per deg C. So, for 20°C in- and -5°C outside, that's 75kWh a day.

Klaus

[acresswell]

Klaus,

Does your 3kW per deg C include water heating?

Thanks

[rondurrans]

I think there is a problem with the calc in cell N11 - you will not require that amount of kWh for 365 days year; my 205m2 1960's house uses approx. 120 kWh per square meter for heating and DHW annually.

Try:

http://www.idhee.org.uk/calculator.html

http://www.retscreen.net/ang/home.php

Laughton, C. (2006) Home Heating with Wood. Aberystwyth: Cambrian
‘Rule of thumb’ volume to be heated divided by 34 (Laughton, 2006)
Peak boiler size ‘rule of thumb’ volume divided by 32 (Laughton, 2006)
 

[KLD]

Klaus,

Does your 3kW per deg C include water heating?

Thanks

Yes, that is all inclusive, daily consumption figures measured for several years. See attached plot. For lower temp differences you'll see more scattering in the data, since both DHW and solar input for DHW make a higher contribution. There's hardly any solar input in winter, so don't worry about that effect here.

But please do note the units: this is 3kWh per day per degree Celsius.

Klaus