Hot Water troubleshooting help please

[axis]

Hi - I'd really appreciate some help on an issue I'm having with my hot water.
It's a conventional boiler system (pretty old). Symptom is less hot water than before and I only really get good hot water in the cylinder when the HW and CW programmes have been running for a fair time. CH is OK. This is what I've observed:

• Programmer on for HW - boiler fires OK, pump runs, 2way motorised valve appears to open OK (lever is loose/floppy - ie moves freely left and right)
• But after a minute or 2 the boiler stops
• Pump is still running, motorised valve lever is still loose
• After a while (several minutes) the boiler will fire again for a min or 2 and go off again
• Adjusting the cylinder thermostat higher or lower does not change the behaviour
• Therefore getting adequate hot water in the cylinder takes a long time

I was thinking maybe it's the cylinder thermostat that's faulty but then why does the 2way valve not shut when the boiler stops firing (or have I mis-understood). A few points of note:
1. 2way valve was replaced earlier in the year as HW stopped working but was OK after initial install
2. The top of the 2way valve does get quite warm/hot to touch - is this normal?
3. CH is OK - ie when on the boiler is firing continuously until room thermostat reaches setting. I have however noticed that if I just have the CH programme selected it does not fire the boiler and boiler only fires when HW is also turned on - so poss valve fault on the CH ? Not sure of the two issues coincide or a related as I generally have HW and CH on at the same time anyway.
4. I don't have a multi-meter at the mo but could poss get hold of one.

Thanks very much - your input would be much appreciated.
Regards

 

[flameport]

Partial blockage in the cylinder coil, or air in there, or the valve has failed internally and is only partly open.
When the valve was replaced, was it just the actuator head (a 5 minute job) or was the whole thing replaced (which would have required draining down first and taken far longer?)

Valves do get hot when in use, the motor is on continuously while it's open, plus there is hot water flowing through the valve body.
CH not working on it's own could be by design, or someone has wired the system incorrectly, or if there is a separate valve for the CH it could be defective.

 

[axis]

Thanks for the response. When the valve was replaced it was the whole thing, drain down etc.
Any advise on how to check/deal with air in the cylinder ?