Not so rare. Look at the Eco-Homtec site as they have a boiler dedicated for the task. It really improves combi performance having the inlet raised to
25C and above. I prefer a thermal store to have the solar heat dumped into, and a heating or system boiler heat the store. Low pressure and faster DHW rates, if you need high rates of course, not everyone does.
It was a combi - a great system and not overly complicated to fit either.
I have the system you say you prefer above, evacuated tubes dumping heat via a twin-coil cylinder (sealed for solar, open for boiler), the other coil heated from my trusty old Glowworm BBU! Gets too hot in the summer though (80c +), thinking about adding a rad and a zone valve to dump the heat when necessary, shame to waste it though.
Angus
It has an unvented cylinder, which required an annual service, and full of pressurised water.
BBU? Yuk.
Excessive heat could be problematical on an unvented cylinder. What you could do is increase the thermal store size by adding another cylinder instead of rads. Have the return from the main cylinder run through the additional cylinders coil and either have a shunt pump to pump from one to the other, or use gravity. Do you have a conventional cylinder or thermal store which instantly heats cold mains water? A thermal store can easily store water at 95C.
You could do much the same at less cost by having a do-nothing store in the combi feed. The water would gradually come up to the house ambient (of course this heat is being provided by the combi) so go in to the combi at 15-20 instead of the winter 5 or whatever.
It would have to be tall thin cylinders with no insulation, and depends on frequency if use. They take a time to get to ambient if cooled quickly to
5C will quite a time rise.You could use the heat stored in radiators and use then radiators as a store of heat. they would cool rapidly and when switched to CH re-heat rapidly too. They will give 20-25C in summer and maybe even cool the house.
The heat in a full heating system can be used to pre-heat cold mains pressure hot water. This can be done. I have seen this done using a plate heat exchanger, pump, flowswitch and two check valves. The system should not have thermostatic rad valves, or few of them. On a combi system:
- a by-pass pipe between the flow and return at the boiler
- on this pipe a re-heat plate heat exchanger is fitted
- The pump is fitted between the plate and the return by-pass pipe tee to the return.
- A check valve between the pump and the plate
- A check valve on the boiler flow before the tee to the by-pass pipe.
- A flow switch on the cold mains water before the pre-heated plate heat exchanger.
The check valve on the flow pipe ensure no flow back into the boiler, although the internal 3-way valve should do this. The check valve on the by-pass pipe ensure no short circuit in normal CH operation. The cold mains water runs through the pre-heat plate heat exchanger. This pre-heated main water then runs into the combi as normal. When calling for DHW the combi diverts to DHW only to heat the incoming cold water. The flow of mains water is detected by the flow switch and switches on the by-pass pump. This pumps water from the rads into the pre-heat plate heat exchanger. This will raise the mains water substantially and the combi tops up.
You can fill a bath up in a few minutes doing it this way. The rads cool down a lot. This doesn't matter as when the system switches over to CH, the boiler re-heats the rads ASAP, with loss in room temp so small it is not noticeable to the occupants.
The combi flow rate in summer, when the CH is off is better than an average flowrate combi as the water in the rads will be around 20 -25C when the CH is off. This stored 20C plus heat is used to pre-heat the cold mains water, which is around 10-12C. Depending on the efficiency of the plate heat exchanger and power of the boiler, the flow rate may be very good, even in summer. Cooling the rads also helps to cool the house in summer too.
A simple and cheap way to vastly improve the output of a combi.
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