"Ed Sirett" wrote in message
news
On Fri, 31 Mar 2006 23:45:07 +0100, t wrote:

We are in the process of having central heating installed, replacing an
old
partial system. We have done this before in other properties so know
something about it. This property has 11 radiators. We are using a
Honeywell
CM37z system with radio-linked TRVs on every radiator except for the
bathroom where the steam may corrode the electics of the radiator head.

Each radiator head in this system radios back to the relay unit that
controls the boiler. If any one radiator head calls for heat the boiler
will
be active. If no radiator heads call for heat the boiler is off. There is
no
separate room thermostat as this would be entirely without a purpose.

The CORGI registered man fitting the boiler for us is saying that we must
have a bypass. I know what this is. He says it is due to 'new'
regulations.
He does not say whether these are building regulations or Corgi
regulations.
He says "safety" and that he cannot sign it off (presumably Corgi-wise)
unless it has a bypass. He is saying that in theory if every radiator
valve
failed and shut off, then the bypass radiator, which would be
uncontrolled,
would allow the passage of water for safety reasons. He also says it will
save gas. The problem to us is he is insisting it must be one radiator
that
remains uncontrolled. This is anathema to me! An uncontrolled radiator
chugging away every time the system is on for any one or more other
radiators.

Clearly this cannot save gas. I thought that a bypass was to protect the
circulating pump from pumping against nothing in the wildly unlikely
event
that all TRVs were closed or faulty. Is there another function of a
bypass?
Could we use a pipe or something automatic (I know there are various
so-called automatic bypasses) instead, rather than a radiator? As we do
not
have a radiator in a room with a system thermostat. Our system will only
fire the boiler if one or more stats calls for heat. I take this to mean
it
is fully interlocked?

Does any regulatory body, e.g. Building Regs or CORGI specify a bypass as
mandatory? If they ever do, does a system that would not call for heat in
the situation where all TRVs are -shut- require a bypass? Is it true to
say
the bypass is a safety item or is it to protect the pump? Or in some way
supposed to save energy?

What should I read? Has anyone any references? Any input would be most
appreciated

There are two things he
1) Do you need a bypass. Yes.

Open to question. It is NOT a definite yes at all.

2) Do you need one rad without a TRV. Not in this case.

Essentially you have a very extended S-plan system with each radiator
effective become a zone. Any one radiator will make for a boiler demand
(although in practice the likelihood is that several will be near calling
for heat if they are not already doing so).

Since the closing of all TRVs shuts off the boiler there IS an interlock
between the house being warm enough and the boiler demand.
So in this case the usual advice that you need a wall thermostat and a
nearby radiator without a TRV can be specifically ignored.

The bypass is almost certainly
needed to provide a path for primary water
to continue after the demand has ceased.

depends on what the boiler manufacturers say.

The boiler will need this (some
have this device internally so
no need to add an extra one
on the pipework).

Open to question.

The valve must be 'smart' not
simply a gate valve on a piece of pipe with
the handle removed. Using a radiator
(often the bathroom towel rail)
for a bypass is also no longer acceptable.

It is. The idea is to:

1. Provide a minimum flow through the boiler.
2. Pump overrun to dissipate heat from the boiler.

A bathroom rad could do this. An auto by-pass valve is not desirable. as it
raises the return temperature lowering efficiency. A flow switch, switching
the burner, with a parallel by-pass is probably the answer.

This is to comply with Part-L. CORGI
are involved with this is as much as
if the fitter is discovered to have been
self-certifying non-compliant
installations then he will put his
registration in jeopardy. CORGI have an
inspection process - he may get caught out.

A pity the CORGI man doesn't know that much. All too typical. They arm
wave a lot.