Central heating terminology

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?
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Hugo Nebula
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wrote:

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.
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.andy

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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
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Yes and no.
It would make a difference on condensing boilers because a continuous flow would raise the return temperature and reduce efficiency.
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.andy

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On Wed, 24 Nov 2004 22:30:02 +0000, a particular chimpanzee named Andy

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

If there wasn't a room thermostat, what would happen then?
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Hugo Nebula
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wrote:

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.

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.
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.andy

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On Wed, 24 Nov 2004 23:40:01 +0000, a particular chimpanzee named Andy

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.
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Hugo Nebula
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