Hot water timed "On" period not always activated.

22 Sep 2015
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United Kingdom
Boiler: Worcester GS 30cdi installed August 2013. Programmer: Potterton EP2002. Hot tank thermostat: Honeywell L641A1039 set to 55 deg.
Programmer settings: Boiler ON, Hot Water, TIMED (Mon-Fri 06.15 to 09.00, and 16.30 to 17.15, Sat/Sun 06.30 to 10.30, and 16.00 to 1900.)
PROBLEM: Hot water TIMED on period sometimes fails to activate boiler. Usual action: C/H room thermostat turned up to 18, C/H D/Valve opens and boiler lights.
With boiler on, H/W D/Valve still closed, even though programmer light still on for H/W and water cold, so I open valve manually to let hot water through from C/H to hot tank.
Pipes above and below both D/Valves now hot, and H/W now heats up to normal temperature. H/W D/V then closed manually afterwards.
This problem nearly always occurs when the hot water in the tank has been used up (e.g. after a bath), and the water fails to heat up again, even though programmer is still in one of the timed on periods.
If we need the central heating on, we just turn up the room thermostat in the hall, without altering the programmer. This brings the boiler on. No problem with this at all, as the boiler always lights if we do this.
I think that the H/W D/Valve may be faulty. I have a newer Potterton EP2 Programmer I could fit. Any suggestions please?
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Your on the right track, you need a multimeter to confirm

Chase the live from clock, to cylinder stat, to valve
If the programmer is giving an output then the problem is either going to be the thermostat or the motor valve.

You seem sure the valve is not opening. That means either the stat faulty or the motor is failing to open the valve.

A common fault is the valve opens but does not bring on the boiler. I had an intermittent one like that last week.

When they are intermittent, the contact resistance of the switch is often high.

Could be the case on the stat switch in your case.

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Basically exactly as I've just said without the nonsense in between

Yes 100% very good advice.

But BigAl2 you need to understand that your system is tailor made for your property,it could have even been installed/wired incorrectly who the feck knows ?.

Diy advice is offered here,


Get an expert in,your property insurance will not cover for many diy repairs going wrong.
Last edited by a moderator:
Many thanks for your replies.
When I said the H/W D/Valve sometimes does not open, I meant that on those occasions the lever on the top of the motor valve remains sprung after the "timed on" period has started. I then open the valve manually by moving the lever against the spring action, and hook it in place until the water has heated up.
I cannot recall the problem occurring after the H/W D/Valve has opened, i.e. the lever is unsprung.
the clock will send power to the cylinder stat, if the cylinder stat isn't satisfied the cylinder stat will send power to the motorised valve (brown wire). Then from valve to boiler switch live (orange)

you need to be competent with a multimeter to test and prove the fault.

If you don't feel confident, get someone in.
As said: check with mm that you are getting a feed from the cylinder stat into the dhwmv. From your description it sounds like the cylinder stat but needs confirming with testing. If you're up to it, you could loop a live over to the brown wire of the dhwmv, in the wcc, to see if it travels and boiler fires but this is given with usual warnings about diy and electricity!!
Thanks guys.
It appears that the programmer clock was failing to send a signal to the tank stat, and so the H/W motorised valve was not getting a feed from the stat, and it was staying closed.
I replaced the EP2002 with the new, more flexible, EP2 yesterday, and the hot water problem seems to have been solved. Using the factory-set timed periods for H/W (and On for the C/H, but with the room thermostat turned right down to 13 deg), the water in the tank heated up normally last night.
This morning the H/W valve opened at 6.30am when the programmer H/W light came on, and it closed correctly at 9.30am when that light went off. On the evidence so far, the EP2002 was the cause of the problem.

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