Can someone please explain the meaning and function of different bits
of a central heating system:
-Is there a difference between a by-pass and an automatic by-pass
valve?
-Do all new boilers turn off if the return is the same as the water
going out (which I presume is what would happen with a by-pass)?
-If not, is there such a thing as a flow switch to turn the system off
in conjunction with the by-pass?
--
Hugo Nebula
'What you have to ask yourself is, "if no-one on the internet wants
a piece of this, just how far from the pack have you strayed?"'

On Wed, 24 Nov 2004 21:59:30 +0000, Hugo Nebula abuse@localhost
wrote:

Can someone please explain the meaning and function of different bits
of a central heating system:
-Is there a difference between a by-pass and an automatic by-pass
valve?
-Do all new boilers turn off if the return is the same as the water
going out (which I presume is what would happen with a by-pass)?
-If not, is there such a thing as a flow switch to turn the system off
in conjunction with the by-pass?

A bypass is used with some types of boiler to cover the case when both
(or the active)heating and HW thermostats are satisfied and the boiler
is in full output.

Typically there is an over-run arrangement on the boiler internal
thermostat so that it continues to power the pump when this happens.
The idea is that water continues to be circulated around the system
until the water cools enough to stop it actually boiling in the heat
exchanger because the heat was not dispersed.

If the motorised valve(s) are closed at this point, there isn't a path
for the water, so hence a bypass is used. This goes after the pump
but before the motorised valve and in the simple case is a lockshield
valve slightly open.

An automatic bypass is pressure sensitive and so doesn't open until
the routes around the circuits are closed. This means that there is
no bypass flow except when needed.

THe normal arrangement is then for a signal wire from the motorised
valve to go to the boiler switched live. This wire is live if either
thermostat demands and the motorised valve is open accordingly, and
fires up the boiler. Once this live goes away because the
thermostats are satisfied, the boiler is shut off apart from being
able to run the pump to disperse heat.

You could incorporate a flow switch, but normally the other controls
do the job.





--

..andy

To email, substitute .nospam with .gl

"Andy Hall" wrote in message
...
On Wed, 24 Nov 2004 21:59:30 +0000, Hugo Nebula abuse@localhost
wrote:

Can someone please explain the meaning and function of different bits
of a central heating system:
-Is there a difference between a by-pass and an automatic by-pass
valve?
-Do all new boilers turn off if the return is the same as the water
going out (which I presume is what would happen with a by-pass)?
-If not, is there such a thing as a flow switch to turn the system off
in conjunction with the by-pass?

A bypass is used with some types of boiler to cover the case when both
(or the active)heating and HW thermostats are satisfied and the boiler
is in full output.

Typically there is an over-run arrangement on the boiler internal
thermostat so that it continues to power the pump when this happens.
The idea is that water continues to be circulated around the system
until the water cools enough to stop it actually boiling in the heat
exchanger because the heat was not dispersed.

If the motorised valve(s) are closed at this point, there isn't a path
for the water, so hence a bypass is used. This goes after the pump
but before the motorised valve and in the simple case is a lockshield
valve slightly open.

An automatic bypass is pressure sensitive and so doesn't open until
the routes around the circuits are closed. This means that there is
no bypass flow except when needed.

Also all bypasses on new or upgraded systems must ow be automatic. Not sure
why as there is no real loss of heat due to it but that's the rulezzzzz

On Wed, 24 Nov 2004 22:35:27 -0000, "Mike" wrote:




Also all bypasses on new or upgraded systems must ow be automatic. Not sure
why as there is no real loss of heat due to it but that's the rulezzzzz


Yes and no.

It would make a difference on condensing boilers because a continuous
flow would raise the return temperature and reduce efficiency.



--

..andy

To email, substitute .nospam with .gl

On Wed, 24 Nov 2004 22:30:02 +0000, a particular chimpanzee named Andy
Hall randomly hit the keyboard and produced:

THe normal arrangement is then for a signal wire from the motorised
valve to go to the boiler switched live. This wire is live if either
thermostat demands and the motorised valve is open accordingly, and
fires up the boiler. Once this live goes away because the
thermostats are satisfied, the boiler is shut off apart from being
able to run the pump to disperse heat.

I think from the behaviour of the boiler when the programmer goes off
there isn't a pump overrun.

You could incorporate a flow switch, but normally the other controls
do the job.

If there wasn't a room thermostat, what would happen then?
--
Hugo Nebula
'What you have to ask yourself is, "if no-one on the internet wants
a piece of this, just how far from the pack have you strayed?"'

On Wed, 24 Nov 2004 23:22:27 +0000, Hugo Nebula abuse@localhost
wrote:

On Wed, 24 Nov 2004 22:30:02 +0000, a particular chimpanzee named Andy
Hall randomly hit the keyboard and produced:

THe normal arrangement is then for a signal wire from the motorised
valve to go to the boiler switched live. This wire is live if either
thermostat demands and the motorised valve is open accordingly, and
fires up the boiler. Once this live goes away because the
thermostats are satisfied, the boiler is shut off apart from being
able to run the pump to disperse heat.

I think from the behaviour of the boiler when the programmer goes off
there isn't a pump overrun.

You mean it tends to sing and make bumping noises? If there is a
pump over-run it may not operate for three reasons

- Boiler thermostat is buggered but pump is still powered via
motorised valve or controller.

- Over-run not connected

- Over-run not needed because water cools quickly or is cool enough
when burner goes off.


You could incorporate a flow switch, but normally the other controls
do the job.

If there wasn't a room thermostat, what would happen then?

If you are just relying on the boiler thermostat to control the room
temperature then there is noting apart from the timer to turn off the
heating. This isn't much different from the over-run point of view to
a room thermostat turning off the boiler when the room is warm.
Either way the burner can be turned off when at full power and the
system is hot. Possibly though, if you are using the boiler
thermostat to control room temperature, the boiler water temperature
will be some way below the intended 80 degrees.



--

..andy

To email, substitute .nospam with .gl

On Wed, 24 Nov 2004 23:40:01 +0000, a particular chimpanzee named Andy
Hall randomly hit the keyboard and produced:

On Wed, 24 Nov 2004 23:22:27 +0000, Hugo Nebula abuse@localhost
wrote:

On Wed, 24 Nov 2004 22:30:02 +0000, a particular chimpanzee named Andy
Hall randomly hit the keyboard and produced:

You could incorporate a flow switch, but normally the other controls
do the job.

If there wasn't a room thermostat, what would happen then?

If you are just relying on the boiler thermostat to control the room
temperature then there is noting apart from the timer to turn off the
heating. This isn't much different from the over-run point of view to
a room thermostat turning off the boiler when the room is warm.
Either way the burner can be turned off when at full power and the
system is hot. Possibly though, if you are using the boiler
thermostat to control room temperature, the boiler water temperature
will be some way below the intended 80 degrees.

It does exactly what you describe. It's my mother's system. The
boiler was replaced recently, but the controls weren't upgraded, and a
bypass wasn't fitted. Now, clearly this doesn't comply with the
Building Regulations, but I've had a hell of a job to even get the
installer to fit a bypass (which he's supposed to be doing next
Tuesday).

I was wondering if some sort of flow switch could be incorporated in
the bypass to act to turn the boiler off when it's activated. Is this
the role of an 'automatic bypass valve'? From the photos on various
websites, they just look like mechanical affairs, and I don't know
where the 'automatic' part comes in.
--
Hugo Nebula
'What you have to ask yourself is, "if no-one on the internet wants
a piece of this, just how far from the pack have you strayed?"'