As a further comment Gabrial relating to the pre-heating of freezing water that passes into a plate heat exchanger of a Combi boiler.

Ignoring for now what gas boiler manufactures tell you, all Combi Boilers can except pre-heated water as long as you first understand that from the first frost on the ground until May, the incoming water is freezing, meaning 4c in Scotland and 6c in the rest of England, even down here in Dorset.

The performance of a Combi begins to improve on the domestic hot water side around May when the sun is over head for 4-5 hrs a day which heats up the ground where the water pipes are buried as it heats up the swimming pool and sea.

The incoming water temperature in summer months rises to around 15c, that’s cold but not freezing, so in effect the water has been preheated indirectly from the sun.

The old Combi boilers had a Diaphragm which can take a maximum temperature of 23c where the new Combi is installed with a Mechanical Valve, that can take any temperature.

Its a proven fact that most older Combi boilers are rubbish during winter months when you need high flow and good temperatures over the winter when it comes to the delivery of hot water to where its needed, but perform very well once you get to summer and a warmer cold water.

So supplying solar pre-heated water through what we designed as a Combi Fortic thermal store cylinder to keep it simple with a 20 tube heat pipe collector we designed as freeze free to heat that fortic direct, not through a coil and everything else sold with closed loop systems, the system is affordable at £2,500 installed.

If the Combi is due to be changed, my advice would be, not to replace it, instead go back to a traditonal boiler with a thermal store heated by solar that can provide hot water and space heating

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