Hot water problem

[redsturgeon]

We have an issue with the hot water from our system.

The hot water seems to only heat up when the central heating is on. When just the HW system is on the hot water doesn't heat up.

Any ideas?

We have a Vaillant boiler with megaflow HW tank if that is any help.

TIA

John

[BullDog]

If everything else is working normally but just a lack of water heating, then I suspect you have a faulty diverter valve. If it's the common Y Plan system there's an actuated three way valve there which in my experience is the most likely suspect. My own experience suggests also that the Honeywell valves are far more reliable than any of the other brands out there. I have previously swapped Drayton valves out for Honeywell due to much better reliability, for example.

[Maroochydore]

I suspect you have a faulty diverter valve. If it's the common Y Plan system there's an actuated three way valve there which in my experience is the most likely suspect.

Agree it is a problem with the diverter valve but not sure on your system. Some valves are located withing the boiler whereas some are located near the tank.
If it is a motorised valve - and I've had this problem before - then it is easier and cheaper to just replace the motor, about £30ish. On our system the motor is housed in a small rectangular metal box which sits on top of the valve.
A picture of motor and valve is here but choice of supplier up to you, DYOR. https://www.bes.co.uk/motorised-mid-pos ... 2mm-17391/
I hope this helps.

[jfgw]

I reckon it's more likely S-plan if the cylinder is unvented as there will already be a 2-port valve supplied with the cylinder (It is one of the safety features — if the water gets too hot, the valve is disconnected so that it closes). There are likely to be two or more two-port motorised valves in the vicinity of the cylinder. Follow the pipework — it should be pretty clear which one is for the hot water. (While it may be Y-plan (with a three-port valve), this 2-port valve must always be fitted nonetheless).

Assuming S-plan:

I think that the most likely fault is the microswitch in the motorised valve head. When the system is calling for hot water, power is applied to this valve so that it opens. Once it is fully open, the switch is actuated and this switches on the boiler. A faulty switch will mean that the valve opens but it will not tell the boiler to fire up. If the heating is also on, the boiler will be on for the radiators and, since the valve for the hot water is open, will also heat the water.

It is possibly a stiff valve. If the valve is seized, it may not open enough to operate the microswitch. If you take the head off, you should be able to turn the spindle on the valve body easily with pliers, if not with fingers.

You can replace just the valve head without draining down or getting wet.

Motors do fail, and they are easily replaced. Every brand that I have seen uses the same type of motor (get a genuine synchron one), https://www.screwfix.com/p/drayton-synchronous-motor/28670. However, if the motor was faulty, the water would not get hot at all.

I tried to replace a microswitch once but found that there are minor (but significant) dimensional differences between different manufacturers.


Julian F. G. W.

[quelquod]

I’ve had a similar problem a couple of times (in 30-40 years mind you!) and it turned out to be a failing, but not completely failed, motor in the valve to the HW. So the valve partly opened but not enough to close the micro switch. A new motor at a £5er and 15 minutes faffing was all that was needed in the more recent valves at any rate as long as you don’t drop a screw! The most recent time was on the CH circuit where although the radiators warmed up there was too little flow to heat them properly as the valve was only half open.