Hot water too hot.

As Chris mentioned the cylinder stat is not in contact with the cylinder so either get some net curtain wire (and hooks) or get another stat...
Then go from there.
 
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Jumping in both feet resolves nothing. Get the meter out and go from there. There is an issue that has been overlooked that will cause malfunction and strange operation even when programmer is off.

Systematic approach will win the day
 
Two contacts the made when cold turns boiler on, the made when hot turns the water flow from valve off, so faulty made when hot can cause DHW not to turn off, but has no effect on causing boiler to turn on.
I don't understand that. "Two contacts" is that the DPST contact in the cylinder stat? What does "the made" mean?
As I said, I don't think it's likely the cylinder stat is faulty and always calling for HW, more likely the 3-port valve is sending water to the cylinder even when the cylinder stat is satisfied.
 
As Chris mentioned the cylinder stat is not in contact with the cylinder so either get some net curtain wire (and hooks) or get another stat...
Then go from there.
Agreed there's nothing obvious in the pics to keep the stat firm against the cylinder, but the OP said this problem has only started since the weather turned cold and the heating came on, so presumably the stat was working before.
 
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I don't understand that. "Two contacts" is that the DPST contact in the cylinder stat? What does "the made" mean?
As I said, I don't think it's likely the cylinder stat is faulty and always calling for HW, more likely the 3-port valve is sending water to the cylinder even when the cylinder stat is satisfied.
I suppose to prove this the op could use/drain all the hot water from he cylinder then with hot water set to off and central heating on, if he gets hot water in the tank then the 3 way valve is faulty
Op may find this useful Boiler & Central Heating FAQs | Page 2 | DIYnot Forums
 
I don't understand that. "Two contacts" is that the DPST contact in the cylinder stat? What does "the made" mean?
As I said, I don't think it's likely the cylinder stat is faulty and always calling for HW, more likely the 3-port valve is sending water to the cylinder even when the cylinder stat is satisfied.
I think understanding 3 port mid position valve operation is paramount to under how the system works.

Three port mid position valve in not powered, HW port is open, boiler runs when HW channel on timer is operated and thermostat on the cylinder is calling for heat
 
Agreed there's nothing obvious in the pics to keep the stat firm against the cylinder, but the OP said this problem has only started since the weather turned cold and the heating came on, so presumably the stat was working before.

i think you missed what op said about raising boiler thermostat setting to better heat the rads.
 
I think understanding 3 port mid position valve operation is paramount to under how the system works.

Three port mid position valve in not powered, HW port is open, boiler runs when HW channel on timer is operated and thermostat on the cylinder is calling for heat
In normal operation, yes, but isn't it possible for a faulty valve to leave the port to the cylinder open, although the cylinder stat is satisfied?
 
On the cylinder stat there are two sets of contacts, one set normally open, one set normally closed, don't like that terminology as you need to define normal.

With power to make some thing happen easy to understand, but with the Y Plan the grey wire is powered to stop hot water, so a fault with time controller (programmer), cylinder thermostat, three port valve, or wiring can all cause the DHW to get too hot when the central heating is running.

When central heating not running then must be the cylinder thermostat, but when running so many things can cause over heating.

With my C Plan I simply have not control anyway, so not such a big deal.

I have shown the Y Plan Y-Plan.jpg however not easy to test, as the valve is complex. mid-position-valve.jpg The diodes and resistors in the valve mean need a volt meter to fault find, first time I came across Y plan I also had problems, I felt likely problem was the V3 micro switch, but not sure, Sunday so could not buy a V3 micro switch, so only option was whole valve, and it fixed it.
 
Ok, thanks everyone. Yesterday we cleaned up the contact between the stat and the cylinder and it is now held in place very tightly with a bungee cord.

The problem initially became apparent when we turned up the temperature in the boiler which (no surprise) made the radiators hot enough, and the hot water too hot.

Turning the dial on the cylinder thermostat doesn't seem to do anything, no 'clicks' from the thermostat or the MPV. I can also turn the dial beyond 360 degrees.

As suggested, tomorrow we will turn heating and hot water off, use all the hot water, and then turn the CH on and see if we get hot water.

Thanks.
 
In normal operation, yes, but isn't it possible for a faulty valve to leave the port to the cylinder open, although the cylinder stat is satisfied?
That port is always open for sole hot water heating. It is the cylinder thermostat that switches the boiler off not the motorised valve
 
That port is always open for sole hot water heating. It is the cylinder thermostat that switches the boiler off not the motorised valve

Correct! The actuator is in the 'at rest' position, the position it takes when the power is turned completely off.
 
That port is always open for sole hot water heating. It is the cylinder thermostat that switches the boiler off not the motorised valve
That is, or could be, the problem. Without studying the mid-position valve operation in detail, I would think if something goes wrong it defaults to the HW position, like the either/or W-plan valve. So if CH is calling, the cylinder gets overheated.
 
On the cylinder stat there are two sets of contacts, one set normally open, one set normally closed, don't like that terminology as you need to define normal.
I meant SPDT, not DPST.
I have shown the Y Plan however not easy to test, as the valve is complex. The diodes and resistors in the valve mean need a volt meter to fault find, first time I came across Y plan I also had problems, I felt likely problem was the V3 micro switch, but not sure, Sunday so could not buy a V3 micro switch, so only option was whole valve, and it fixed it.
Agreed, I never saw significant advantage in Y-plan. With weather compensation becoming more popular, where mid-position is not an option, perhaps we'll see fewer of them in future. Discussed on here only a couple of weeks ago.
 

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