DIY Doctor UK DIY Forums

Could someone please help
I have a baxi solo boiler controlled by a Honeywell ST9400C. Hot water tank and a midway diverted valve. The problem I am having is, If I run the hot water only it runs fine regulated correctly by tank thermostat. If I run the heating only it runs fine on a room thermostat, all is good until I try to run water and heating together, the water heats up to temperature and cuts out with the tank thermostat but the heating will not work. I have to turn of the water and heating and run heating on its own. Any ideas would be appreciated

Are there three wires connected to the tank thermostat? (ignore any wire connected to an earth terminal.)

Hi Atticus thanks for replying
I was wondering what your thoughts were, I am not at home at the moment so can't check the wiring.

Hi Atticus
Have checked tank thermostat, it is 3 wire. I have live on com' also live on terminal 1 and water comes on, live on 2 when thermostat reaches temp. Also the room thermostat appears to be working correctly. Do you think it could be a faulty diverted valve.

I thought the wire from terminal 2 of the cylinder stat might be missing. When the cylinder reaches temperature the stat switches over so there is voltage on terminal 2. The wire goes to the grey wire on the mid-position valve. When there is voltage on the grey wire the valve should switch over to CH only.

You say that you can get CH only when HW is turned off, but not when HW is on and satisfied. This means that the grey wire is getting voltage from the programmer HW OFF terminal, but not from cylinder stat terminal 2.

There should be a large junction box where all the cables meet. Take the lid off - CAREFULLY 240v PRESENT - and locate the grey wire from the valve. There should be two wires connected to it, one from the programmer, the other from the cylinder stat. If you have a mulltimeter, check that there is 240V on it (a) when HW is OFF at the programmer, (b) when HW is ON at programmer and cylinder stat is turned right down.