Ed Sirett wrote:

For me this is just plain nonsense.

I would agree that the concept has serious flaws in many cases.

1) I doubt very much whether any manufacture would permit the boiler to be
used in this way.
2) The boiler is not designed to take luke warm let alone hot water as
it's input.

Those two are likely to be show stoppers anyway...

3) All the while the 2ndry pump is going the boiler is in DHW mode. When
does the house get heated?

That is where the pipe stat and the timer comes in I suppose. Once the
loop is up to temperature it kills the pump and allows it to return to
heating mode. The big problem will be the heat capacity (or more the
lack of it) of the pipework - it will get cold quickly.

4) The whole system would rely on the boiler shutting down because the
primary is at or over its maximum temperature for DHW mode. This will only
happen when the DHW temperature is very hot. Ergo the heat losses on the
2ndry loop will be huge.

Where do most boilers sample the DHW output temperature. in the DHW flow
on on the return of the primary for example?

5) Whilst the burner is shutdown the fan will still be going so that the
primary will be losing heat at a rate comparable with the burner rating
i.e. tens of kW. This is going straight out the flue!
6) The water bylaws
forbid the permanent connection of the mains to a closed circuit.

Circumventable with a doublecheck valve?

If a house needs a secondary loop it also needs a grown up heating system
that will include a stored HW of some type (vented, unvented or thermal
store).

Indeed ;-)

--
Cheers,

John.

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