Thermo-syphon, how to control, so CH works well?

[ericmark]

I have an oil boiler 19 kW I think, and house is very slow reheating in the morning, the boiler is cycling on/off, so can't really blame the boiler, the radiators are not transferring the heat to rooms, what I am wondering is if the DHW is turning off the boiler premature, all but one radiator have TRV's, the bathroom is thermo-syphon with DHW, two pumps, and two motorised valves, each with their own thermostat. One pair works the flat under the house which is rarely used in winter, so in the main set to 10ºC so does not run, in the flat radiators have mechanical TRV's, and the flat when switched on heats well.

However the main house has 9 electronic TRV heads, and one mechanical (the toilet and shower room), and a Nest Gen 3 wall thermostat in the hall, 4 of the TRV's should have linked to Nest, but it did not work, so not now linked. The idea was to have the same schedule on the TRV to the wall thermostat, however since the system is not warming up to a point where anything turns off, or down, this idea has for time being been abandoned.

For the amount of time the thermostat switches off, could just as easy have an on/off switch or timer, but the return pipe feels hot, but no radiator seems to have a hot return, I had blamed some panelled in control like a by-pass valve, which I can't see, the flat use to be a garage, and has been plaster boarded coving fuse boxes etc.

However I am now wondering if it is the thermo-syphon and if some tank thermostat can be fitted to a valve to reduce flow in cistern hot coil when it is warm enough. Some thing like this but for a cistern not underfloor heating.

But open to ideas, there is no easy route cistern to boiler or would use a motorised valve, and turn into S plan, so looking for ideas please.

 

[oilhead]

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This will do the job. On gravity circulation you will need the big valve. 15mm equivalents will be too restrictive.

 

[oldbutnotdead]

I'm not sure that the thermo syphon is causing your problem (boiler cycling)- problem being that there isn't enough flow through the boiler to remove the energy being chucked at the heat exchanger. If there is a cylinder stat, has it been wired properly (there was a useful post on here the other day showing the best way to do it, I'll try and find it.
Are the feed pipes to the rads hot or lukewarm?