Hot water cylinder too hot

[Djlm12]

Having a hot water issue.

Y plan boiler system. Hot water cylinder stat does not seem to work corrrectly.

It does click when adjusted with but seems to be extremely hot for 60 degrees.

The main issue im having though is it doesnt seem to turn the boiler off when temp is reached. I pressume it should turn it off regardless of timer setting?

Recently changed timer comtrols to a Nest however i do not think this is related as i dont think it ever worked as intended.

Any help appreciated.

 

[JohnD]

Might be the 3 port valve

Post photos please.

 

[Djlm12]

Might be the 3 port valve


Post photos please.

Okay heres some pictures. The three port seems to work okay

 

[stem]

Try operating the thermostat manually. With the cylinder hot, turn the thermostat down to it's lowest setting and leave it there. If the water eventually cools (this may take up to a day or more depending upon how much hot water you use) then it's wired in OK and the valve is working as it should. If it's still overheating something is wrong. If it doesn't overheat, the the cylinder thermostat may be inaccurate. In reality none are perfect, sensing the water temperature one third of the way up the cylinder is different to the temperature where it leaves for the taps from the top.

Cylinder thermostats can fail but they are usually quite robust and there are other faults to look for:

1. The 'hot water off' signal from the hot water cylinder thermostat may be missing. This is unique to a Y-Plan and is a wire that is made 'live' when the water is hot and it moves the motorised valve to shut the hot water port off. This can be tricky to detect, but a pointer is that the hot water works OK during the summer when the central heating isn't on, but overheats the hot water when the heating is on. This wire goes from the hot water satisfied contact of the cylinder thermostat and connects to the grey wire of the motorised valve.

I have shown the wire is question in purple below, it should also be connected to the Nest terminal 4 'Hot Water Satisfied'. [called HW OFF in the diagram below]

2. The cylinder thermostat is not making good contact with the metal of the hot water cylinder, either the tensioning device is slack, or on a couple of occasions I have found the cylinder thermostat laying on the floor and not in contact with the cylinder metal at all.

3. Faulty motorised valve that doesn't close off the hot water port when it should.

EDIT.
Your photos arrived whilst I was typing, so as far as I can see you need not worry about 2. above. Lets see what @JohnD makes of them.

The thermostat looks to be quite an old one so maybe is past its best. The manual test I mentioned will highlight if the rest of the system is working OK or not, and if it is then a new stat maybe the way to go.

 

[Djlm12]

Hi Stem, @stem

1)To be certain of what your saying for the test. Once the hot water cylinder is hot. Leave the hot water ON but turn the stat right down and see if it cools? Otherwise with the hot water setting off it would not eventually cool anyway no?

2)I have checked the position of the thermostat and it does seem correctly positioned against the tank. The only thing I could think of, like you said is it is an old one. I can change this with little hassle.

3)The motorised valve will happily move when setting either hot water, or heating on or both (middle position). When the hot water is satisfied should this move out of the hot water position (if heating is off). The only thing that changes this is if I turn the (red) pump up to max power it seems to bypass both sides but I think thats unrelated


Finally the diagram you supplied (thank you). I will check the wiring links correctly. Once hot water is satisfied should this send a signal to the boiler to turn off?



Edit: Turning the cylinder stat all the way down to the left (below 50) did make it click but it didnt seem to turn the boiler off. Perhaps I didnt allow for enough over run?


Also the tank has a SEPERATE digital temperature reader. That says the top of the tank reads at 70+ once heated but that roughly the same position as the thermostat (1/4 of way up) was only around 20-22. Are these temps correct? Maybe the lower half of the tank is not heating correctly

 

[stem]

1) To be certain of what your saying for the test. Once the hot water cylinder is hot. Leave the hot water ON but turn the stat right down and see if it cools? Otherwise with the hot water setting off it would not eventually cool anyway no?

Correct.

2) I have checked the position of the thermostat and it does seem correctly positioned against the tank. The only thing I could think of, like you said is it is an old one. I can change this with little hassle.

OK

3) The motorised valve will happily move when setting either hot water, or heating on or both (middle position). When the hot water is satisfied should this move out of the hot water position (if heating is off).

Sometimes the valve can be seen to move, but may not fully close off the HW port. The pipe from the valve to the hot water cylinder should cool when the hot water is off, but this can sometimes be difficult to confirm unless it has a long run of pipe between the two. When the valve and cylinder connection are close, the pipe can still get warm from conduction via the valve which will be hot if the heating is still on.

The valve will sit at the hot water only position when unpowered and it is held there by a spring. It will only move to the middle or heating only position when instructed to do so by the central heating controls. Whilst it looks quite simple, it is quite complicated in how it works. Under certain circumstances it can remain in a powered (heating) position when both the heating and hot water are actually off. It will only move from that when a new demand requires it.

Finally the diagram you supplied (thank you). I will check the wiring links correctly. Once hot water is satisfied should this send a signal to the boiler to turn off?

There are three wires at the cylinder thermostat. A 'HW ON' wire. (in your case from the Nest 6 'Hot water call for heat') One wire that controls the boiler, and the one I highlighted in purple that tells the valve to close the hot water outlet.

This wire is sometimes missing when a system has been upgraded to a Y-Plan. (other systems don't have this connection) However if it is missing, it would never have worked properly in the past.

Turning the cylinder stat all the way down to the left (below 50) did make it click but it didn't seem to turn the boiler off. Perhaps I didn't allow for enough over run?

Maybe, or maybe the central heating was still running.

Also, the tank has a SEPERATE digital temperature reader. That says the top of the tank reads at 70+ once heated but that roughly the same position as the thermostat (1/4 of way up) was only around 20-22. Are these temps correct? Maybe the lower half of the tank is not heating correctly

Heating hot water in a cylinder is not an exact science. There will be a significant difference between the temperature at the top of the cylinder (+60 degrees) and the bottom where the cold water enters. (at approx. 5 to 20 degrees) Situating the hot water cylinder thermostat about one 1/4 to 1/3 of the way up from the bottom is usually the recommended place to get an average reading and the thermostat should have been calibrated to take this into account to some extent, but as cylinders vary in size and shape, the way a cylinder cools will be different if it cools naturally through heat loss, or through use when cold water is fed into the bottom. Also the cold water inlet temperature will vary. All of this will have an effect.

 

[JohnD]

@Djlm12

In your point (3) I understand you to say the the 3-port valve moves correctly.

To check, do you mean that by operating the cylinder stat manually, and making it click, it makes the 3-port valve move?

Does it also move when you operate the room stat or its substitute?

You also say "it bypasses both sides"

Do you mean that when it should have closed off one of the pipes, hot water still flows through it?

 

[Djlm12]

Lets give this multi quote thing a go

3) The motorised valve will happily move when setting either hot water, or heating on or both (middle position). When the hot water is satisfied should this move out of the hot water position (if heating is off).
Sometimes the valve can be seen to move, but may not fully close off the HW port. The pipe from the valve to the hot water cylinder should cool when the hot water is off, but this can sometimes be difficult to confirm unless it has a long run of pipe between the two. When the valve and cylinder connection are close, the pipe can still get warm from conduction via the valve which will be hot if the heating is still on.

The valve will sit at the hot water only position when unpowered and it is held there by a spring. It will only move to the middle or heating only position when instructed to do so by the central heating controls. Whilst it looks quite simple, it is quite complicated in how it works. Under certain circumstances it can remain in a powered (heating) position when both the heating and hot water are actually off. It will only move from that when a new demand requires it.

This is what confuses me. The three port has three modes. W - Water / WH - Water & Heating / Heating. Obviously depending on the demand you give it goes to one of these three thats the easy bit. For arguments sake if im JUST heating water via boiler to tank. If the boiler is on and heating; the port is open on Water. If I turn this off, that water port should shut? Or just remain as is with the boiler off?

If I currently tell the hot water to stop manually via the nest controls the boiler shuts off like it should but im not aware that the three port valve changes in anyway.

Finally the diagram you supplied (thank you). I will check the wiring links correctly. Once hot water is satisfied should this send a signal to the boiler to turn off?
There are three wires at the cylinder thermostat. A 'HW ON' wire. (in your case from the Nest 6 'Hot water call for heat') One wire that controls the boiler, and the one I highlighted in purple that tells the valve to close the hot water outlet.

This wire is sometimes missing when a system has been upgraded to a Y-Plan. (other systems don't have this connection) However if it is missing, it would never have worked properly in the past.

ON / OFF for water on the Nest is wired correctly as that works. When I get home tonight I think the first thing to do is check the other OFF connections you marked above are correctly wired as well, as im assuming if one is missing I would still be manually able to turn it OFF or ON but the rest would obviously not function. Can you confirm if this is a suitable replacement for my current thermostat as it would be easy enough to try changing it assuming the rest of the wiring is correct https://www.screwfix.com/p/drayton-...CrNv7IkfPEZpbEtMR0gaAswwEALw_wcB&gclsrc=aw.ds

Turning the cylinder stat all the way down to the left (below 50) did make it click but it didn't seem to turn the boiler off. Perhaps I didn't allow for enough over run?
Maybe, or maybe the central heating was still running.

Ive been testing with central heating off to stop this. Central heating works perfectly on the nest thermostat comes on and goes off as desired from the thermostat so thats no issue.


I appreciate all your help!

 

[Djlm12]

@Djlm12

In your point (3) I understand you to say the the 3-port valve moves correctly.

To check, do you mean that by operating the cylinder stat manually, and making it click, it makes the 3-port valve move?

Does it also move when you operate the room stat or its substitute?

You also say "it bypasses both sides"

Do you mean that when it should have closed off one of the pipes, hot water still flows through it?

Okay so. Ignoring the cylinder stat for a second. If you turn the heating on, it will go over to heating fine, just heating the heating pipe. Turn that off and turn water on it then moves to the water side then heats the water fine. Leave both on it goes to the middle position and heats both pipes.

The red grundfos pump was set up to full power which regardless of position of the three way port was forcing water through both positions when either one was turned on. I rectified that by turning the power down one mode. Thats now fine.


Now to the cylinder stat. If I have BOTH heating and hot water on (middle position of the three way port) changing the stat does not effect the position it stays central and continues heating both. If I have just the hot water on (in water position of three way port) changing the stat does not move the position or stop the flow or turn the boiler off. (So that leave the issue being with the stat or wiring?).

 

[JohnD]

"That's now fine"

No it's not.

 

[JohnD]

The cylinder stat operates the motor in the 3-port valve.

When the valve mechanism reaches the "hot water off" position, it operates an electric switch, within the valve head, that turns off the "hot water demand" to the pump and boiler.

I think you are saying that the pump and boiler continue to run.

This is additional to the fault in the valve (probably a worn-out rubber ball) that fails to shut off flow correctly. Though it might just be the valve is not moving right.

 

[Djlm12]

@JohnD Also yes the three port valve moves fine with the room stat. So if heating comes on via the wall stat the port moves and heating comes on, once the house has warmed up to the wall stat setting the boiler turns off as expected.

 

[Djlm12]

The cylinder stat operates the motor in the 3-port valve.

When the valve mechanism reaches the "hot water off" position, it operates an electric switch, within the valve head, that turns off the "hot water demand" to the pump and boiler.

I think you are saying that the pump and boiler continue to run.

This is additional to the fault in the valve (probably a worn-out rubber ball) that fails to shut off flow correctly. Though it might just be the valve is not moving right.

Ok to get this straight (if we work back a little so I can get my head around it)


If the pump is the only item in the loop that tells the Boiler to come on or off.

And the wall thermostat works correctly, and I can manually turn the water on and off from the programer fine. That leaves the cylinder thermostat at fault whether it be wiring or whatever

 

[JohnD]

I think it's more likely the 3-port valve.

You've already told us it is allowing water leakage.

There is not much to go wrong in a cylinder stat, if it is correctly wired.

If you want, you can try taking the motorised head off and turning the spindle by hand to see if it is jammed or stiff.


Once you understand that the thermostats operate the valve, and the valve controls the pump and boiler, I think things start to fall into place.

 

[Djlm12]

There is not much to go wrong in a cylinder stat, if it is correctly wired.

I will double check this to be 100% sure as my first port of call

If you want, you can try taking the motorised head off and turning the spindle by hand to see if it is jammed or stiff.

Does this just pop off then? Best to have the power off etc to do this? How stiff should it be or should it turn pretty freely.



Even though I can manually turn the water on and off fine using the programmer all things point to the valve?


Last thing I thought I would mention. Even though the tank was reading 70+ at top the thermostat when manually adjusted only clicked under 50. Does this just mean the tank could heat more? Or unrelated the thermostat could be not reading correctly.