I Need Help.
Whole system seems to behave in an odd manner. most the time it works fine but every few days one of the following happens.
1 whilst hot water is heating radiators dont heat.
2 radiators get VERY hot and dont stop even when heating is turned off. But if the hot tap is run for 5 mins everything settles down.

System is an old gas boiler (gravity fed) an electric immersion tank (electric element turned off) the two are connected together with a 3 way valve. The 3 way valve seems to work (when problem 1 occurs turn hot water off and can see valve switch position with no problems).

Please can you offer any suggestions.
regards
Neil

I think the problem is with the radiators since you had mentioned that they are quite old. Base on what you said you might be needing new ones or let them checked just to be sure whether they are still ideal to use or not for overheating them may cause some other problems.

I would hope after this long it has been cured? However there are three thermostats in the basic heating system.

Sometimes only 2 as with thermosyphon sometimes there is nothing on the tank.

So there is a thermostat in the boiler which should stop it boiling.

There are two types of three port valves some have centre position some don't. With the type with centre position the room thermostat does not go directly to the boiler but it goes to the valve. Once the valve moves then the valve sends a signal to the boiler to run.

The micro switches inside the valve can get stuck and I would guess that is what is going on here. So I will guess a new electric head on the valve is required.

I think it unlikely that the radiators are causing this problem although of course muck could have travelled from radiators to the valve and jammed it.

I do not see working on the old system is really a DIY job but this is one of the diagrams it shows just one switch inside the valve but really there are two or three in them.

Kindly check your radiator because base from your thread you said that it is quite old. Perhaps, you should replace it with a new one. Also, check your system closely their might some thing that you omit in checking.

thanks for the diagram.