A bit of advice if any plumbers out there.
I have a solid fuel stove with a wrap around boiler. The boiler uses 1" copper and has 2 in's and two out's. The first circuit goes to the hot water tank and the second to the rads. There is a pump on the in from the rads and a stat on the out to the hot water tank.
Basically this means that the hot water tank is heated by convection and the rads are run by pump.
Would the stat be better on the out to the rads, as this cools down much more quickly than the out to the tank.
I hope Jim or "Pipers" comes in here but I presume your hot water cylinder is upstairs ?
If it I can't figure out why there is a stat on the flow (top pipe) to your cylinder as the water will only rise when hot, do you have another boiler in the house like an oil fired boiler as your main heating ? that may explain it.
Usually the stat (if any) is on the flow to the rads to switch on the pump, I can't see it serving any useful purpose on the cylinder side unless it is on the bottom pipe (return) to allow the cylinder to heat first then it switches on the heating.
Tank is upstairs and no oil fired boiler involved.
The only reason I can see for the stat being on the out to the boiler is lazyness/convenience. To put the stat on the out to the rads would have needed a longer cable(which he didn't have) and would have been more awkward to fit. I just wanted to check that there wasn't something blindingly obvious that I was missing.