Re: New Boiler (with automation inclusion)

[Jim Noble <yahoo-groups@xxxxxxxxxxxxxxxxxx>]

Robin Edwards wrote:
> The thing I can't do and would like to is to introduce a second form
of
> heating (solar?) to drive the cost down but you can't do that with a
> combi to best of my knowledge (no tank).
>

You can, I believe, but it's tricky.

One method requires a Combi that can cope with pre-heated water,
preferably with a high temperature limit as well (eg. some only cope
with "pre-heated" to 25C, which isn't much help). You'll also
want one
that can modulate down to a fairly low output for when the pre-heated
water is nearly hot enough. I don't know how successful they are at
controlling the output temperature though, but a thermostatic shower
mixer should be able to smooth out the bumps.

Another method is to use a fancy two input thermostatic mixer and take
hot water from both the solar heated source, and the combi. Actually
it's really two thermostatic mixers in series. Cold mains and solar
heated water from the cylinder go into the appropriate inputs on the
first mixer, and the output goes into the cold side of the second mixer
along with the combi boiler DHW output on the hot side. When the solar
heated water is hot enough, the output from the first mixer is hot
enough that the second mixer takes nothing from its hot side (ie the
combi). When the solar water temperature is lower and the output from
the second mixer drops down to its set point (52C or so) increasing
amounts will be taken from the hot side (heated by the combi).

Both methods have their issues. With the former you may have to mix down
the solar heated water to 50C, and the boiler will then always top it up
a bit, thus wasting gas when you theoretically don't need the boiler to
fire. With the latter, the drop across the thermostatic mixers may also
result in unnecessary use of the combi.

Jim


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