Softus said:
Then can you work out what will happen when the cylinder thermostat detects a rise in temperature and opens its contacts?
If the answer to the second question is "nothing", then why would you want to fit a cylinder thermostat?
Correct, with the CH on the cylinder thermostat would have no effect, I have no problem with the tank being heated when the CH is on, it does that currently so theres no change there.
The problem is that this time of the year the CH isn't needed very often, but with the current setup either I leave the system on for a number of hours in case the house temperature falls, causing the boiler to run continuously just through the tank, cutting out on its own thermostat, and as its in the garage that cycle happens often. Or I cut it down to say 1H per day just to heat water, and have to keep manually flipping the override onto the system to get some CH when needed. In summer its fine, in winter its fine, its just the bits inbetween that causes the problem. What I want is to leave the system on a sensible timing, just relying on both the thermostats to determine when to start.
I can see the reason for having a zone valve in standard plans, but the setup here would require a complete end to end rebuild to support it. The system has a froststat and independent pump fitted, so relies on a loop through the tank as it stands.