I hope picture works, other than TRV the control was the pump for house was plugged into a socket, and the pump for flat was connected to a thermostat, and there was a timer to switch whole system off/on.
This resulted in the house pump needing one to go outside and go down to flat under main house to turn it off/on, and heating domestic hot water also heated house just a little slower than if pump was running, and if a pump was started in either flat or house the water in the other would flow in reverse direction.
I am told the domestic hot water must be allowed to thermo syphon with no valves to stop it as the boiler needs to cool down and also pipes both work DHW and fill the system and vent the system, so all I can do is add motorised valves and add a better control, the cables are near impossible to run so trying to reuse the existing.
It depends on the system pipework setup rather than the wiring, if a gravity system you cannot have any valves on the primary circuit.
Most sealed systems will have built in bypass or bypass valves, so pipework and valve configuration can be altered to suit what you're trying to achieve, using 2 port or 3 ports, older gravity systems can be altered to pump assisted, to give better primary circulation and efficiency for heating the water.
[img]https://www.diynot.com/diy/attachments/pipework-jpg.166913/[/img]This is to date what I have worked out with plumbing, I have installed Nest but waiting for plumber to fit motorised valves, he is due to arrive Monday, while waiting bottom wiring centre has a link to work central heating pump, but not needed central heating so not tested.
DHW works although it does some times cause the bedroom radiators to get warm if cool enough for the TRV to be open, it is set for 1/2 hour every other day heating DHW and when it comes on boiler runs for around 19 minutes, then off for 4 minutes, back on for 5 minutes then off and before it fires again the 1/2 hour has lapsed so stays off.
My worry is if all the TRV's are closed will the pump cavitate and be damaged? Where the pump is on supply one would fit a by-pass valve, however on the return not sure if fitted it would actually work.
The guy who is coming is a plumber, not a heating engineer, on his first visit I asked about motorised valves, and was told they would not help, I suggested a test, turn off the maintenance valve on the pump, if the radiators still got hot then clearly a motorised valve would not help, however if they stayed cold then it would help, and they stayed cold.
I know in theory the TRV should do the same thing, however only 3 out of 10 are paired with Nest so they could be open when we don't want heating, setting a valve *, 1, 2, 3, 4, 5, 6 does not really tell you what temperature they will work at, and even set to 18°C the motors in the TRV heads take quite a time to open and close, plus they have an exercise time, so could be open when DHW is called for, so felt a motorised valve was more positive.
I do have a TRV in same room as wall thermostat, but it's linked to the thermostat so they should work together.
The towel rail does not have a TRV but it's on the thermal syphon circuit so would not protect the pump, and in summer closed anyway.
I am an electrical engineer, but not a plumber or heating engineer, and although I am sure my circuit will work, not sure if it will damage pumps, or not should there be a demand for heat and the TRV's are closed? Unless the micro switch on motorised valve sticks the pump should not run with valve closed, but that does no hold true for TRV's.
[attachment=0]Pipework-question2.jpg[/attachment]Sorry my photo did not work on last post, I have tried to work out what exists and what will happen, as it stands the motorised valves are wired up, but not fitted, neither is the by-pass valve, not sure if by-pass valve required, and note plumber has only left one valve.
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