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Tim JA
Tim JA, Customer Service
Category: Appliance
Satisfied Customers: 2238
Experience:  Owner at Self-employed
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Hot water has got hotter. Does the thermostat need replacing

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Hot water has got hotter. Does the thermostat need replacing and if yes how much £.
JA: Have you replaced your thermostat or parts of it before? If so, how long ago?
Customer: no
JA: How long has this been going on with your thermostat? What have you tried so far?
Customer: tried to adjust temperature but display won’t permit
JA: Anything else we should know to help you best?
Customer: only that the boiler te

Good afternon, and welcome to the Technical section in JustAnswer, an independent paid question and answer platform that matches customers' questions to experts in various skills. My name is ***** ***** I'll be happy to help you today.

So that I can start to give you accurate guidance, please let me know the make and exact model number of the boiler, and whether or not it's a combi.
_____

There's no time pressure on this topic - it will stay open to you for many days, and you can reply at any time. I will be away from my desk sometimes, but will respond as soon as I can.

Customer: replied 13 days ago.
Vaillant ecoTEC plus. Not a combi

OK. My hunch is that you have a sticking motorised zone valve. Do you know what I mean by that?

Customer: replied 13 days ago.
Sorry Tim, no

I can guide you through some simple tests. Can you attach a photo showing the components near your hot water cylinder, so that I can see what make and model of MZVs you have?

Customer: replied 13 days ago.
File attached (QP3Z33T)

OK You have a Honeywell 3-port MZV - the silver box next to the cylinder, with the black flex connected, is the actuator.

Please do the following:

1. Turn off both hot water and central heating sessions at the programmer.

2. Touch the thin metal lever at the upper end of the MZV actuator, and attempt to push the lever along its slot. Tell me if you feel resistance, or if it's floppy along its slot.

3. Turn on (at the programmer) the central heating, turn up the room thermostat to maximum, wait 10 seconds, and feel the thin lever again. Tell me if you feel resistance, or if it's floppy along its slot. It should feel floppy all the way to the end of the slot.

4. Turn on (at the programmer) the hot water. and feel the thin lever again. It should feel floppy to about halfway along the slot.
_____

Please let me know the three results.

Customer: replied 13 days ago.
It resisted at first and now floppy. Have returned lever with mx temp and seems same as when started ie a little resistance then floppy
Customer: replied 13 days ago.
Display still shows cylinder charging off

I don't know which display you're looking at, or what charging means.

Could you tell me the three results of the three tests?

2. Is there resistance, or is the lever floppy along its slot?
3. Is the lever floppy all along its slot?
4. Is the lever floppy until half way along its slot?

Customer: replied 13 days ago.
Floppy until half way

OK. Which test result was that?

Customer: replied 13 days ago.
4

OK. What was the lever doing in tests 2 and 3?

Customer: replied 13 days ago.
Sorry am confused now. My last test was floppy till halfway so the other 2 tests are irrelevant aren't they

All of the steps are essential and relevant, and have to be done in the correct sequence. Please do the following, and note the result of steps 2, 3, and 4, and let me know those three results.

1. Turn off both hot water and central heating sessions at the programmer.

2. Touch the thin metal lever at the upper end of the MZV actuator, and attempt to push the lever along its slot. Tell me if you feel resistance, or if it's floppy along its slot.

3. Turn on (at the programmer) the central heating, turn up the room thermostat to maximum, wait 10 seconds, and feel the thin lever again. Tell me if you feel resistance, or if it's floppy along its slot. It should feel floppy all the way to the end of the slot.

4. Turn on (at the programmer) the hot water. and feel the thin lever again. It should feel floppy to about halfway along the slot.

Customer: replied 13 days ago.
2 no resistance 3 floppy all way 4 floppy all way now
Customer: replied 13 days ago.
When I turn heating off leaving hot water on it resists half way. Hope this helps

OK bear with me...

Please do the next steps:

5. Turn the room thermostat to minimum.

6. Note the position of the setting on the cylinder thermostat.
This is the white box strapped to the hot water cylinder, lower and to the right of the MZV.

7. Turn the cylinder thermostat to maximum, then wait a few seconds and feel the MZV lever again. Does it feel floppy, or do you feel resistance?

Customer: replied 13 days ago.
Resistance half way

OK. The MZV isn't behaving as I would expect. It might be sticking partly open - not seized, but not able to move to the hot water only position. There are two microswitches in the actuator, so if the position is wrong then the boiler won't necessarily fire when it should. If the valve is sticking then I would expect intermittent behaviour in the summer. in the winter if the valve has been sticking then you would have hot water by virtue of the central heating being on.

The next step involves unmounting the actuator and checking the valve spindle. I can give you detailed guidance. Are you up for that?

Customer: replied 13 days ago.
Tim I think this could be a step too far for me as I'm getting tired now. You have been very helpful but I think having tried the steps you requested I am minded to get someone in to fix. What do you reckon?

I understand, but why not sleep on it and you can always ask me in the morning when you're fresh :-)
All you would need are:
1. A screwdriver - medium sized, flat-bladed.
2. Knowledge of where to turn off the power for your heating system.
I think the switch is in the cupboard there.

I give this guidance a lot, by phone and text, and most people manage.  It's not difficult.

Customer: replied 13 days ago.
Ok. Let's try but do bear in mind I am pen pusher by trade!

Your success or failure is determined by the quality of my guidance ;-)

Customer: replied 13 days ago.
Ready
Customer: replied 13 days ago.
Are you there?
Customer: replied 13 days ago.
This is taking longer than I thought and it's late so going to close call. Thanks and goodnight

Close the call?

I'll send the guidance in a few minutes - I'm just in a Zoom meeting at the moment.

Tim JA and other Appliance Specialists are ready to help you

Here are the steps.

1. Power off the heating system, for safety.

2. With a suitable flat-bladed screwdriver (or a Torx bit if you have one), loosen by a couple of turns the screw that holds the actuator cover in place, and lift off the cover. On later models the cover is tethered by an earth cable inside, which can be a nuisance as it hangs in the way.

3. With the same screwdriver, loosen the two screws in the 1 o'clock and 7 o'clock positions on the baseplate of the actuator (when viewed with the short side of the casing at either 12 o'clock or 6 o'clock).

4. At this point, the actuator will lift away from the brass valve, but you might need to do some wiggling to persuade it. I advise against prising it off with any tools, as you might bend the baseplate.

5. With the actuator removed, try to wiggle the spindle of the valve. It has only about 35 degrees of travel. It should be possible to move it just with your fingers.

If it moves freely, then it tends to indicate that the intermittent behaviour of the valve is caused by a failing actuator. If you can't easily turn the spindle with fingers, then it's sticking - you can try to wiggle it free with a tool, but don't use anything with serrated jaws that might rough up the brass surface. A small adjustable spanner is good, but don't force the valve spindle - just gentle wiggling.

If the valve was sticking, the actuator motor might have become damaged, but you can turn the power back on with the actuator left unmounted, and check that it operates. The best test in your case is as follows:

a) With both central heating and hot water on, turn the room thermostat to minimum and the cylinder thermostat to maximum. The actuator should move fully to the position
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When the time comes to re-mount the actuator, turn the power off again. Then move the thin lever fully across, gently, and latch it in the hook at the end of the slot. When it has latched, put the actuator back onto the brass valve body, wiggling it as you go, until it's fully seated and doesn't rock at all. Then you can tighten up the two screws.

Good afternoon, how did you get on?