On 09 Oct 2004 14:40:32 GMT, Ian Stirling
wrote:


Let's be optimistic, and say that the heat pump takes 1Kwh to pump
7Kwh of heat. (about the theoretical maximum efficiency for pumping
from 0-40C.)
But, electricity is some 3-4 times dearer than gas.
So, unless you are comparing it with electric heating it's not looking
so rosy.

Hi,

With a typical GSHP it would be 3:1 or 5:1 for a very good one.

It's (eventually, assuming it lasts for ever) 7 times cheaper than
electricity.
But, it's only about 2 times cheaper than gas.

What makes it worthwhile for a building with a lot of thermal mass is
that you can heat overnight on economy 7 and the building will keep
warm in the day.

Also the thick walls will average out the outside temp to around 5°C
for NI winters, averaging out the heat load which helps heat pump
capacity and efficiency.

AFAICS the economy 7 rate in NI is 3.44p and the gas rate is 2.1p, so
with a 4:1 heat pump running overnight it comes out at 3.44/5 = 0.688,
3x cheaper than gas.

Payback can happen faster if energy prices rise (gas and electricity
will tend to rise together unless we go nuclear), but it will never
outperform adding insulation unless electricity is free.

Electricity will lag behind a bit, so the saving will increase
slightly as gas prices rise.

I'll take a guess that the walls of the church are 75cm stone on average.
That's a R value of around .6, or a U value of 1.6W/m^2/K.
To halve the thermal loss you only need about 10-20mm extra insulation.

True, if they dry line the walls that would make a big difference as
heat is released through the floor so can only escape through the
walls, windows and roof.

cheers,
Pete.