"John Rumm" wrote in message
...
Ed Sirett wrote:

For me this is just plain nonsense.

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

What serious flaws might these be?

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...

Any evidence of this? Or just making it up again.

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.

Not it heavily lagged.

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?

DHW outlet

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?

A rank DIYer and confused professional agreeing with each other. My, oh,
my!

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 ;-)

A rank DIYer and confused professional agreeing with each other. My, oh,
my! The professional should stick to Vaillants.