I've recently replaced my room thermostat. This works fine, until the hot water comes on as programmed, all the radiators also come on !
The light on the thermostat comes on only when the heating is programmed as on, but does not come on when the hot water is on, but all the radiators are boiling !!
The old thermo had neutral, live etc but also had a wiring point for heating.
But the new thermo, as well as live, neutral etc has a wiring point for 1,2,3, but not one for heating?
I've tried all the possible combinations, obviously leaving the live and neutral as they are, but the radiators still come on when the hot water comes on.!
I've checked the programmer to make sure the rads shouldn't be on.
The boiler is a gas Homeglow BBU, and got the thermo as a direct replacement for the old one !!
There are many types of central heating from the combi boiler which has all the bits built into one box to the typical school room system but most follow one of the Honeywell plans or a combination of the plans. We have C, S, W and Y the latter being about the most popular. All of the plans would not allow the radiators to heat up if the programmer had turned them off. Since you have only changed the central heating thermostat I would suspect an un-related fault. Except for the C plan the central heating is controlled by motorised valves. The fault could be very likely one of these valves stuck and in most cases there are tell tail levers to show what position they and in and allow manual operation for bleeding the system. With the C plan the pump only runs when central heating is on. Sometimes there is also a motorised valve to turn hot water off to stop the hot water getting too hot but not always. The C plan does sometimes allow radiators to heat up with thermo siphon especially a problem if the radiators are not cold when boiler starts. All the plans have a cistern somewhere and most are vented although they can have expansion vessels. Most send hot water through a pipe in the cistern called a hot coil some work other way around and have then mains pressure water going through the coil to give instant hot water that is now becoming more common as it allows solar panels to be added to the system. On a Y plan if the valve fails it is common to latch the manual leaver to keep central heating running and this will cause it to continue to get radiators hot in the summer. Often you can work out what Plan the system is using by the type of motorised valve fitted. Two port or three port the latter has two types Mid way and change over which can often be found by part numbers. Most non combination boilers can be configured in many ways so if you tell us a little more about your system maybe we can help more.
Hi, If I had to guess you have replaced one of the old Swichmaster combined Room Stat / Hot Water controller, is your 3 port or 2 port valves in the airing cupboard blue with Switchmaster on them?
If this is what you have replaced the only way I have come accross to remedy this would be to rewire the airing cupboard wiring center to accomodate the different type of room stat, but I am only guesing with the info I have so far.