Boiler only responding to room stats, no hot water - valve failure, or something else?

[AVR2]

We have a Worcester 14/19 CBi downstairs with a cylinder upstairs. We've suddenly lost our hot water, but the CH still works fine.

Googling around the subject, this immediately sounds like a motorised valve failure. When I adjust one of the room stats upwards, the "demand" light on the boiler comes on and it fires up, and then shuts off when the temp is reached. However, there's never a response from the boiler when I run a hot tap. Should the "demand" light come on then as well?

If it's a valve failure, is there any way to identify which one, and then manually force it to provide hot water rather than CH?

There are three motorised valves in the upstairs cupboard along with the cylinder - a Honeywell 272848 on the horizontal input pipe from the boiler to the cylinder, and two Drayton ZA5's on vertical pipes running up the wall to the side of the cylinder. They all seem to be two-zone valves with just A and B, no midpoint positions, and they're all on A.

 

[D_Hailsham]

Any chance of pic of the cupboard showing the general layout of pipes, valves and cylinder?

 

[MeldrewsMate]

Not familiar with that model of Honeywell valve, but most will have a manual lever that you can gently move to open the valve, and a detent for the lever to hold it in the open position. In that way a demand for heating will help generate hot water, but beware that the hot water may be much hotter than normal using this stopgap.
There are four possibilities, cylinder stat failure, valve failure (more than one mode), timer failure, and wiring failure.
Turn on the HW at the timer, and pull the tank thermostat away from the cylinder (if you can, or simply confirm that the tank is cold). Does the HW valve drive open, so that the manual lever becomes slack? Is a click heard at the end of its travel, after about 10 seconds? Does it audibly spring return when the HW timer is turned OFF? If not I suspect the HW valve may have failed. A competant electrician or heating fitter should be able to diagnose, or confirm your initial diagnosis.

Hope this helps, MM

 

[Agile]

Whichever motor valve supplies the cylinder seems to have failed.

The boiler will only fire up to heat the water whenever the cylinder has cooled or water has been drawn off.

The Honeywell and copies have a manual latching lever. I think the ZA5s might do as well.

But the correct repair is to identify whatever has failed and replace it. Probably enough to replace the valve head.

Many would probably suggest considering replacing the ZA5, if failed, with a Honeywell type as these are easier to repair/replace.

 

[AVR2]

Thanks for the replies so far - here's a pic I happen to have on my phone. I can take more detailed ones when I get home if necessary.

You can see the Honeywell valve on top of the horizontal pipe that connects to the front centre of the cylinder. According to the diagram in the front of the cylinder, this is the input from the boiler.

The two ZA5's are attached at floor level to the two pipes you can just see branching vertically downwards off the short horizontal pipe on the the wall on the centre right of the picture.

So theoretically, I can move and latch the lever on the Honeywell, and that should cause the boiler to fire up (assuming the time switch is on)?

 

[Steelmasons]

So theoretically, I can move and latch the lever on the Honeywell, and that should cause the boiler to fire up (assuming the time switch is on)?

Not unless the valve end switch makes which it won't when manually latching valve over.
This is an unvented cylinder which requires a G3 ticket in order to carry out any work on it.

 

[D_Hailsham]

Thanks for the pic; without it I wouldn't have known you have an unvented cylinder.

However, there's never a response from the boiler when I run a hot tap. Should the "demand" light come on then as well?

No! That only happens on combi (instant hot water) boilers; you have an unvented indirect cylinder. The water in the cylinder will only be reheated when it reaches the temperature set on the cylinder's thermostat. There is normally a 10C difference between on and off, so the water will heat up to 60C, but not reheat until the temperature drops to 50C. You set the times when you want the water to be reheated on the programmer.

The valve on the horizontal pipe may be faulty, or it could be the safety cut out in the box attached to the cylinder. You really need an engineer who is qualified to work on unvented cylinders (G3 qualification) to diagnose and rectify the problem.

 

[AVR2]

Thanks for your help, everyone!

 

[mcmoby69]

You have an immersion heater element installed, is it working?

 

[AVR2]

It is, but it's not heating the water as well as it used to. It used to be able to get it steaming hot in a big enough quantity to fill the bath, but simultaneous with the loss of non-immersion hot water, it now only gets it warm, and the water starts running cold before the bath is half full, even if we give it an hour to heat before we run the bath.

 

[D_Hailsham]

Then you definitely need someone with a G3 certificate.

 

[stem]

So theoretically, I can move and latch the lever on the Honeywell, and that should cause the boiler to fire up (assuming the time switch is on)?

As Steelmasons says that will open the valve, but not trigger the boiler. However, if you leave the valve latched open, then when the central heating switches the boiler and pump on, it should also flow through the manually open valve and heat the hot water too. You won't have any thermostatic control and the water may get hotter than normal depending on how long the radiators are running for.

If it is the motorised valve it's a simple job to replace the powerhead without touching the plumbing.

The immersion however is another matter.

 

[Steelmasons]

If it is the motorised valve it's a simple job to replace the powerhead without touching the plumbing.

The immersion however is another matter.

Naughty naughty stem...
The MV will be interlocked via the cylinder thermal cut-out (OH stat)..
G3 being a requirement in order to carry out any work on these inclusive of immersion heater.