Changing Cylinder Thermostat

[comsen]

I have a very old central heating system that seems to work very well but the tap water has been getting very hot of late and it appeared to be the old Honeywell L641A1005 strap on cylinder thermostat that was not switching off once the set temp was reached.

I have just purchased a replacement thermostat, which is a Sunvic SA2452. There were two wires going to the old thermostat, one red and one black. The red went to terminal marked "C" and the black to terminal marked "1". There is a terminal "2" that was not used. The replacement thermostat has three positions "1" for call for heat, "2" for satisfied and "3" common. I have wired the red wire to "3" as I assume "C" stands for common and the black wire to "1" as it was wired to "1" on the old switch. Can anyone tell me if this is correct? When I turn the thermostat down, the CH pump and motorised vale switch off but they come on again at 30c, which seems low. Any advice would be gratefully received.

 

[DetlefSchmitz]

Your wiring sounds fine. The setting at which the heating comes on while you turn the knob obviously depends what temperature it's seeing. Set it to 60 and see how hot your water gets. Then fine-tune it as they're not that accurate in my experience.

If I set mine to 60 it heats the water up to 51.

 

[seco services]

If I set mine to 60 it heats the water up to 51.

51 where at the taps ?

 

[DetlefSchmitz]

51 where at the taps ?

No, I have temperature probes inserted into the cylinder at top and bottom as part of the solar heating control, so I can see what the boiler achieves when it is used.

 

[comsen]

The boiler has not fired up in hours and the pump has stopped running with the stting on the stat at 30c. The water at the taps is still very hot but assume that is just what is in system before turning down. Will monitor for 24 hours and see what happens as hot water becomes exhausted. Thanks for your advice.

 

[EFLImpudence]

If the water is very hot and the stat is set to only 30 then boiler/pump will not be on.

See if boiler/pump come on by turning up the stat and that they stop when you turn it back down. They will 'run on' for a minute or so

The temperature of the water can be roughly judged by where the stat is when it clicks.

 

[comsen]

If you turn the new stat to past 30c, it clicks and the pump, motorised valve and boiler start up and as you turn it down past 30c they stop, however, the water is still pretty hot, despite it being turned down overnight and the boiler and pump still kick in as it needs to draw more hot water it seems, it's just that it is running a little hot. Maybe the setting on the front of the boiler needs to be turned down a little, although it's set mid to low at the moment.
Would it be right for the pump to continually run even when the central heating room thermostat is switched off, as it tends to once you place the cylinder stat at past 30c. I assumed that there is still a need for central heating hot water to circulate in the cylinder, to provide heat to the tap water?

 

[holmslaw]

..

 

[bernardgreen]

The contact area between the tank and the base plate of the thermostat needs to be clean and with some pressure to press the plate against the tank to ensure good thermal transfer. Sometimes the hole cut in the insulation is not large enough or the wrong shape for the new thermostat and this prevents the thermostat making adequate thermal contact so it cannot read the temperature of the tank.

Also the water at the top of the tank might be a lot hotter than water lower down in the tank where the thermostat is located. This depends a lot on the type of internal coil and the direction the boiler heated water is pumped through it.

 

[comsen]

There is an immersion heater but I checked and it is switched off, so that is not heating the water. The contact area may be worth looking into, as although the area is clean for good contact, the position is quite low on the cylinder and I may try moving it up to say almost half way, to see if this the problem. Unfortunately it means making a further cutout in the insulated covering but may try to re-fill the old hole, just to get max insulation.
Many thanks to one and all.

 

[Stoday]

The correct position of the thermostat is just above the internal coils. Hot water rises and the top of the cylinder will heat up first. As the heat from the boiler continues, the interface between the hot and cold layers moves down and eventually reaches the thermostat.

If the boiler heats water to (say) 80° then the water at the top of the cylinder will be heated to somewhat less than 80°, not to the setting on the thermostat. In fact, a strap-on cylinder thermostat is pretty useless at controlling the cylinder temperature unless the water's pumped around the cylinder to mix the hot and cold layers.

Reducing the boiler temperature would be a cure for the OP's problem, but might cause another problem in the winter when he wants space heating.

 

[comsen]

Still have to move stat further up the cylinder to see if that helps. At the moment the water is still pretty hot with the stat set at 30c lower down the cylinder but hoping moving the stat higher may fix the problem. I read somewhere that the boiler has a stat, which after a period of times becomes in-operative. Problem is getting spares for a 30+ year old glow worm boiler. Thanks for your post.

 

[DetlefSchmitz]

The correct position of the thermostat is just above the internal coils. Hot water rises and the top of the cylinder will heat up first. As the heat from the boiler continues, the interface between the hot and cold layers moves down and eventually reaches the thermostat.

What actually happens is that the body of water between the coil and the top of the cylinder heats up at a fairly homogeneous temperature, (below the coil stays pretty much unaffected.)

This is because the water heated by the coil rises, and colder water falls to replace it, and this circulation mixes the hot and cold so the entire body of water's temperature rises as one.

In my opinion the best place for the thermostat in halfway up the coil.