Fitting Hot Tank Thermostat

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As the hot water on an S-plan system was far too hot, I worked through the following logic.

The immersion heater was definitely off.

There had been no recent work on the system.

The cylinder thermostat was set correctly and the Myson 2 port 28mm zone valve (Neon and motor) responded to manually turning the setting dial.

The actuator head was removed and was seen responding, with the motor turning the slot.

The now exposed valve stem on the body was free to move.
(At this point I suspected the brass valve internals to be worn & letting by; hence my earlier thread about obtaining replacements)

The shoes and arm in the internals of the brass valve were examined and OK, not stuck or broken.
Replaced everything but water continued to become too hot.

Turned hot water off at programmer and left system to run on heating only. Used up hot tank water and the contents became & remained cool.

Was that a sensible investigation pathway so far?

I reasoned that the only culprit must be the Potterton PTT100 thermostat. A replacement was not available but I got a Drayton HT53which is similar, although the Potterton had its wire connections soldered directly into the micro-switch and unmarked.(One live feed-redwire- from timer, one to zone valve motor-black sleeved red, - & one –grey-made safe unused. All via the wiring centre.)

The new one has 3 terminals. marked as C common ( which I have connected to live feed,) 1 Call for Heat & 2 Satisfied .

There is a sticker saying “On temperature rise C1 break, C2 make.”

Using a multimeter, with the thermostat setting at minimum (positive off according to instructions) the No1 terminal is live.

I am scratching my head to understand from those instructions which terminal to connect with the black wire that goes to the zone valve. Could somebody interpret it, please?

Thanks for reading
 
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1. Have you checked the microswitch in the motorised valve. If its faulted in the "On" position, the boiler will continue to run. Opening the valve manually doesn't necessarily actuate the microswitch.
2. I would interpret the instructions as:
2a. C1 makes contact when the temperature rises to the required level
2b. C2 breaks contact when the temperature rises to the required level
2c. On that basis, as you want the thermostat to stop the heating once temperature is reached I'd use C2 as the other side of the switch.
 
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Thanks Terry. I agree and that was what I tried this morning. However the water is still scalding hot even with thermostat set to 60c.
Oldbuffer---the boiler control is irrelevant as it is running from the radiator heating requirements in any case, so the only control relevant to the tank is whether the zone valve is on/off & this is OK.

Could it just be that the boiler circulated water is very hot and when it passes through the cylinder coil it heats the tank contents around the coil causing these to rise to the top, so that any water drawn is initially always at the tank top temp, rather than the thermostat temp 1/3 up the tank?
 
Could it just be that the boiler circulated water is very hot and when it passes through the cylinder coil it heats the tank contents around the coil causing these to rise to the top, so that any water drawn is initially always at the tank top temp, rather than the thermostat temp 1/3 up the tank?

Any hot liquid or gas will rise, so it will always be slightly hotter at the top, than where the stat is. With tank hot - if you turn the stat up and down, does it click at an appropriate temperature?

With HW enabled on the clock, when you click the stat, does the valve and actuator move position appropriately?
 
Thermal Stratification.
With The stat a third from the bottom there will be a big difference at the draw off , Depending on coil fitted .
 
Thanks Harry. Yes and Yes.
Thermal stratification looks the only remaining answer--but never noticed this situation in the past winters.

Incidentally, the thermostat bi-metal strip is kept about 1mm from the tank surface by the casing. Should this space be filled with any thermal paste?
 
Thanks Harry. Yes and Yes.
Thermal stratification looks the only remaining answer--but never noticed this situation in the past winters.

Incidentally, the thermostat bi-metal strip is kept about 1mm from the tank surface by the casing. Should this space be filled with any thermal paste?

It might be making a difference - Thermal paste only helps as a thin coating. If you mean the insulation is stopping the proper contact, could you not trim it? Failing that, use a piece of metal to fill the 1mm gap.

Have you tried actually checking the temperature at the outlet from the tank? It needs to be 60C to kill the legionella, at least once per week, but could you nudge it down the rest of the week?

I have my HW set to 60C BTW - the cylinder is only just big enough for a bath at that setting in winter. Its a bit too hot for putting your hands in, but great for the bath and for washing up. At the downstairs toilet w/basin, I use a thermostatic mixer and a single output tap - so the water comes out at the perfect temperature for hand washing.
 
I never look at the temp setting on tank. I just tweak it to my needs and season.
I live alone most of time and use shower never bath.
Cyl is 42 x 18 .
There was never a hot shortage when there was + 3 women living here

I have just checked and my stat is set at 50°
Temp at stat now is 26 and 34 at draw off, It has been off since last night 2100.
I will check again when heated as it has awoken MY curiosity.
 
I am still struggling with this one and have no remaining ideas to resolve the situation.
The DHW will only set to produce a tap exit temp that is far too hot or far too cool.
I have moved the tank thermostat so that it sits at 1/4 down from the tank top,( to see if stratification and low use was the problem.) and I have replaced the thermostat twice (in case I had a faulty batch of stats.)
The zone valve (swapped to check) and its microswitch are fully functioning and pump and boiler reacting normally, and "twiddling" the thermostat shows tham reacting.
None the less, I cannot find a thermostat setting that produces a reasonable temperature of water. (Currently its showing about a 55c setting) With only a tiny change of dial setting (literally fraction of a degree) the results vary from cool to scalding.
This does "scream" thermostat fault, but how many replacements are reasonable?

Any further suggestions would be more than welcome.
 
The stat needs to be moved much further down, to sense the temperature just above the lower of the two heating coil pipes - about 1/4 up from the bottom.
 
Thanks for the interest.
That's true and that was the original position of the stat. However the problem as described existed at that point and moving it higher was an attempt to check for and discount stratification, but neither changed the symptons. Therefore the problem is not arising from the position of the stat
 
Thanks for the interest.
That's true and that was the original position of the stat. However the problem as described existed at that point and moving it higher was an attempt to check for and discount stratification, but neither changed the symptons. Therefore the problem is not arising from the position of the stat
I suppose if your cylinder stat has a large differential it could cause those symptoms, though as you've tried two with no improvement it's unlikely. And I doubt you can adjust the differential.
 
Thanks. I can see that, but I would not expect a stat sold for this purpose would have such a massive differntial nor for so many to display a fault.
 

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