On Dec 19, 9:32*am, "Dave Liquorice"
wrote:
On Mon, 19 Dec 2011 01:01:44 -0800 (PST), harry wrote:
Typical bodge made by plumbers.
What is needed is an auto bypass valve.

Which is another bodge on the bodge that didn't fit a room stat
to
prevent the boiler short cycling once the house is up to
temperature.

I thought the auto bypass valves were more to allow a sink of heat
for
the boiler when the motorised valve closed, and would still be
needed
even if one rad had no TRV, as there would be no circulation path
for
the pump overrun.

No, the bypass valve is not a bodge.

To work properly, the boiler needs a constant volume of water going
through it.

Only more modern low water content boilers. Our floor standing lump
of cast iron with 12 gallons of water in it doesn't care a two hoots
about being turned off at full chat.

You only need an auto bypass valve if the boiler needs pump over run
and there is no path for that flow due to a motorised valve under
control of a room stat or no rad without a TRV. Even with a motorised
valve if you have a HW cylinder and three port valve the HW cylinder
will provide the heat dump for any required pump over run.

A blanket "fit a an auto bypass valve" is a bodge in that some
systems do not require one, either because the boiler doesn't need
one or because there is a inherent heat dump available.

The radiators/remainder of heating system needs a constant pressure.

Why?

--
Cheers
Dave.- Hide quoted text -

- Show quoted text -

As the rad valves close off, the pressure rises. this tends to defeat
the action of (especially thermostatic) valves and can lead to noises
in the sytem.
Also the balancing of the system is unaffected by valves opening/
closing.
The bypass valve relieves this pressure.

It also allows a constant flow of water through the boiler. Reduced
flows can lead to hot spots in the boiler, sludge settlement and
reduced efficiency. There is an optimum flow for each boiler design.

All of the above is the main reason why all this stuff us pre-
installed inside the boiler cabinets these days.