We recently needed to replace head valve due to pump over run problem.(Boiler is Potterton Profile) On putting Potterton EP6000 programmer back onto backplate, screen was blank,so replaced this with new EP6002 onto existing backplate.
All worked fine for an hour after engineer left with CH & HW set to once. Then when I put both back to timed, found we could only get heating with hot water on at same time. Engineer thought it must be faulty head, so he came and replaced head,but no change. He then got his electrician pal to come and check all wiring to head just in case and they found all was OK. They checked and rechecked everything, moving room stat up and down and sliding cotrols on programmer seperately etc and all then worked fine for 8 days. Came home one day and pressed advance on CH, but nothing happened, tried turning up room stat but again,no joy.advanced HW, boiler fired up. So replaced the new programmer with another, but again no difference. Then replaced old Tower room stat with new Honeywell T6360 in case it was this. This controls temperature fine, but still only when CH & HW on together. As soon as you take off the HW and leave CH only, boiler shuts down immediately, put HW back on, boiler fires up again. Prior to original Head replacement, system had worked fine for 19 years ? Is there something we're missing. Any suggestions very much appreciated

JT, February 2010

Thanks Peccavi I'll mention this to my man. In the meantime I rang Potterton technical and they suggest that we need to make sure that programmer is set up correctly. To do this they say to get engineer/electrician to come back, put multimeter on back of programmer whilst Potterton are on the phone and run through the readings that he's getting and what is happening and this should give them an idea as to what is causing our problem. Many thanks to all for your advice and will keep you posted.

JT, February 2010

You might still consider setting the system up to make Central Heating only and check the Orange Wire from the Head of the MPV - It needs to be live for the boiler to turn on - it comes from a microswitch in the valve head. And work backwards from there.... the White wire supplies the signal via the switch.

The valve should have been moved over to full travel for CH only making the switch and sending out the fire-up signal to the boiler on Orange.

Good luck...

Peccavi, February 2010

Hi Ian, many thanks your suggestion,have looked on back of programmer but there is nothing like you advise. Also why would it have worked OK for that 8 days and then reverted to needing both CH & HW again when I tried to advance CH ?
In respone to P, problem showed up hour after motorised valve was replaced and new programmer fitted. Have now had two valves and two programmers in case faulty but doesn't look like they were.

JT, February 2010

check that the programmer is set for fully pumped, there is usually a tab or jumper on the rear of the programmer. otherwise heating will only operate with hot water as the programmer may be set as if it's a gravity system

Ian, February 2010

Q: When did the problem first show up?
A: When the Mid Position Valve was changed

P, February 2010

http://www.howtomendit.com/answers.php?id=319917

Similar problem to the one you describe...

Peccavi, February 2010