CH Hot water not coming on or delayed response to do so

21 Apr 2013
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United Kingdom

Lately my CH how water has been failing to respond to the honeywell timer schedule. So I have made following tests:-

- if I turn the setting to continuous it may come on immediately or after 20mins or so (like today)

- Tested that Central heating warms up the rads OK

- Tested the drayton diverter valve motor works.

- Checked header tank has water in and ballcock replenishes it

- Bled the rads finding that last week the rad in the spare room had a lot of air to release before water appeared. Today I did the same thing in main bedroom and again it was a minute before water came out - lots of air! Downstairs rads are fine and don't need bleeding. Maybe a leak here but no obvious signs of water egress from rad circuit anywhere?

Both Heating and how water worked fine over the winter so wondering whats happened. Obvs. not using CH atm but I did turn the HW schedule off for few a days 2 weeks back which precipitated this issue. Details of my CH system below - comments welcome

System:- indirect with Worcester Greenstar RI Boiler.
CH controller:- Honeywell st6400c CH
Diverter:- Drayton MA1 Mid-PostionValve Actuator
Honeywell L641A1039 cylinder thermostat

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I’d still be looking at the 3 port. You say you’ve tested it, how exactly?
I powered off CH and removed Drayton Actuator from the 3way valve. I then saw the actual valve head arm move when I turn the power on / off (carefully). Admittedly when it was powered on and I moved the HW to "cont" on the honeywell I didn't see it move. Which to mind it should do. I also checked that I could turn the valve itself between the positions proving its not jammed. I fitted Drayton Actuator back and decided to leave it for the day the following morning I was able to get the HW to work be turning HW to "cont" on the honeywell. Subsequent day the HW came on according to the timer. Just tested now and nothing when I switch it to cont but in 30mins maybe...

Other candidate could be that the Honeywell L641A1039 cylinder thermostat is faulty?
You need to test voltages present at the wiring centre. If voltage from the cylinder stat is getting to the terminal containing the switched line to the boiler it should fire up, when your programmer is set for domestic hot water only.
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Tests I am making are suggesting its the cylinder stat. I have ordered a new one and will post back with my findings after installing observing safety by turning power off. Thanks for help will report back but I guess if its still broken I will get a CH engineer in
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Confirming that replacement of cylinder stat did the trick & fixed the fault. Thankful it didn't cost much!
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