On Wed, 08 Mar 2006 13:25:24 +0000, David Hansen wrote:

On Wed, 08 Mar 2006 11:41:40 GMT someone who may be Tony Bryer
wrote this:-

On Tue, 07 Mar 2006 20:33:24 -0400 Derek Broughton wrote :
OK, I'll accept those numbers. Anyone got a better figure on
the average cost of heating water?

The latest SAP 2005 algorithms for a 100m2 house, 110 litre factory
insulated cylinder give a required raw energy input for DHW of
3172kWh. Divide this by the boiler efficiency to get the actual
energy required.

And the efficiency of the pump and pipework. I doubt if the
conversion of gas into hot water is ever going to exceed 80%
efficiency over a year. That gives 3965kWh gas input. At 3p per kWh
that is 118.95 a year, at today's prices. A solar water heating
system should save at least 75% of this through the year, which is
£89.21 a year saved at today's prices.

If the water is heated by off-peak electricity, in theory 100%
efficient, at 4p per unit, then 3172kWh cost £126.88 at today's
prices.

I have no idea by how much the price of gas and electricity will go
up by over say 20 years, but it will go up. The price of sunshine
will remain the same though.

As for maintenance, an evacuated tube system may need a tube or two
replacing over 20 years, but that will probably be it.

I thought the 20 GBP/year was a bit conservative.

A realistic saving of around 60-90 per year depending on usage patterns
and cost of alternative methods of heating.

However your maintenance estimates are probably a bit optimistic.

Circulations pumps can and do fail. Some can go the 20 years other don't

I see no reason why this would not be broadly similar to CH pump
reliability. So let's say 50% of system will need one new pump within the
20 years.
The comparator electronics, I imagine these would be similar to the
reliability of say a an electronic time switch?

Anyway: It looks like the answer to my original question is
A few hundred quid system should pay for itself.
Differing systems and differing usage patterns and requirements make a BIG
difference in the decision. Being prepared to wait for the HW is a big
help whereas always wanting instant HW works against the solar methods.

--
Ed Sirett - Property maintainer and registered gas fitter.
The FAQ for uk.diy is at http://www.diyfaq.org.uk
Gas fitting FAQ http://www.makewrite.demon.co.uk/GasFitting.html
Sealed CH FAQ http://www.makewrite.demon.co.uk/SealedCH.html
Choosing a Boiler FAQ http://www.makewrite.demon.co.uk/BoilerChoice.html