Siemens Cylinder Thermostat Faulty? Testing Advice?

[Yip Yo]

Hello Peeps,

I have the following Cylinder Stat on my HW Tank:
https://www.amazon.co.uk/Siemens-RAM1-Cylinder-Pipe-Thermostat/dp/B00BJ721PA

But even though I have it set for 80 degrees, it cuts out at around 60-65.

Is there a maximum set on these for safety, or is it faulty - is there a way to test it?

Cheers,
YYo

 

[MeldrewsMate]

What cuts out at 60 to 65, and how have you measured 'it'?

Could it be you're reading a display on the boiler, and 'it' is the boiler cutting out?

In any case domestic hot water should not be as hot as 80C, in fact in hard water areas it is recommended NOT to exceed much over 60C else rapid scale formation will result.

 

[Yip Yo]

Sorry, I wasn't clear. This is about the Cylinder Stat (boiler is working fine)

In normal operation, when the Cylinder Stat reaches temperature there is an audible click from it (the Cylinder Stat) and the boiler stops heating the water (or the Y Plan switches to CH Only)

I have set the Cylinder Stat to 80 (Soft water area), but the Cylinder Stat always clicks at 60-65 Degrees and the boiler stops heating the water.

If I turn the Cylinder Stat down from 80 to 60, it clicks and the boiler fires. If I turn the stat back up to 80, the boiler heats for 5-10 mins and shuts off again (at about 65 degrees). Hot water at the bath tap is about 60-65 degrees, so it looks like a problem with the Cylinder Stat.

So I wanted to find out how to test the Cylinder Stat or find out if there was a factory preset which prevented it from going above 65 degrees, this is the Cylinder Stat:
https://www.amazon.co.uk/Siemens-RAM1-Cylinder-Pipe-Thermostat/dp/B00BJ721PA

I've got it located about a third of the way up from the bottom of HW Cylinder (as per installation instructions) < not sure if this would make a difference.

 

[oilhead]

If the cylinder stat is switching at 65o a third the way up the tank, then I would have expected the temperature at the top of the tank to be nearer 80o. They are cheap, replace it.

 

[stem]

Still one odd point about your enquiry.

If I turn the Cylinder Stat down from 80 to 60, it clicks and the boiler fires.

If that is correct the thermostat has its NO and NC wiring transposed. But from your other comments, I will assume that it's your typing that is transposed instead

So I think your question is why your cylinder thermostats isn't accurate. First of all most use 40 year old mechanical technology a 5 to 10 degree difference between switching on / off when the dial is moved manually is not unusual. They get away with it to some extent because a few degrees variation on a hot water temperature of 65 degrees is not all that noticeable. (unlike a few degrees variation in a room temperature for example) There's no real point improving this accuracy, which is why it still exists, because there is a bigger influence out there.

The temperature marked on the thermostat dial is there to provide an indication of the usable hot water temperature, but the problem is that the temperature right at the bottom of the cylinder where cold water comes in could be 10 degrees, and at the top it could be 65 degrees, so in reality, the temperature of the water ~one third of the way up where the thermostat measures it, will be cooler than at the outlet to the taps at the top. This will vary from property to property, installation to installation and fluctuate depending upon your water usage pattern, the temperature of the incoming cold water and even a slight variation in mounting height.

So, the manufacturers have to take an average 'punt' and at the same time for safety, ensure that the cylinder doesn't overheat and scald anyone. So, I find the vast majority of cylinder thermostats function exactly like yours does. Having been asked this question a few times now, I usually advise users to turn their thermostat up if their water is too cold or down if it's too hot, regardless of the temperature indicated on the dial.

One third of the way up is the correct location for the thermostat for it to provide a reasonably full cylinder of hot water before it switches off, and for it not unduly effected by the cold inlet at the bottom.

 

[Yip Yo]

If the cylinder stat is switching at 65o a third the way up the tank, then I would have expected the temperature at the top of the tank to be nearer 80o. They are cheap, replace it.

I agree, but it should still be sending a 'call for heat' as it has not reached 80 degrees yet and it stops at 65 degrees.

If that is correct the thermostat has its NO and NC wiring transposed. But from your other comments, I will assume that it's your typing that is transposed instead

Not really, I turn the stat down from 80 to 60 to see what the stat thinks the temperature is - it should be 80 but the stat thinks its 60-65 and clicks to call for heat, firing the boiler.

Even though the Cylinder Stat is set to 80 degrees, it stops calling for heat at 65 degrees, very odd. I think its faulty and replacement may be the best option.

 

[D_Hailsham]

Not really, I turn the stat down from 80 to 60 to see what the stat thinks the temperature is - it should be 80 but the stat thinks its 60-65 and clicks to call for heat, firing the boiler.

What happens if you turn the stat below 60, say 40?

The stat should not be at 80C, even if the water is soft, it's DANGEROUS; and you have to use even more cold water to bring the water down to a safe temperature (40C) for showering etc.

 

[Yip Yo]

What happens if you turn the stat below 60, say 40?

The stat should not be at 80C, even if the water is soft, it's DANGEROUS; and you have to use even more cold water to bring the water down to a safe temperature (40C) for showering etc.

If turned down to 40, the Stat will heat the water to 40 degrees, ending the call for heat.

RE Showering: It is a gravity fed shower with a Showerforce Pump < hotter water meaning more showering time (but this is a whole different topic).

 

[D_Hailsham]

What is the flow temperature of your boiler?

 

[Yip Yo]

Not sure, its a Gloworm Fuelsaver Mk2 - the manual says 15.2 L/min > but thats the flow rate through the boiler.

 

[oilhead]

I agree, but it should still be sending a 'call for heat' as it has not reached 80 degrees yet and it stops at 65 degrees.


Not really, I turn the stat down from 80 to 60 to see what the stat thinks the temperature is - it should be 80 but the stat thinks its 60-65 and clicks to call for heat, firing the boiler.

Even though the Cylinder Stat is set to 80 degrees, it stops calling for heat at 65 degrees, very odd. I think its faulty and replacement may be the best option.

This is not a valid test for the crude technology employed on these units. Bi metallic switches will always show a differential of 10-20 degrees when operated thus. Attach a thermometer adjacent to the thermostat and monitor the actual temperature adjusting the thermostat marginally. You will need patience.

 

[D_Hailsham]

Is the boiler thermostat set to Max?

 

[Yip Yo]

Is the boiler thermostat set to Max?

Its set at about 80% (approx 8KW), but that should be enough to heat 100L of water to 80 deg in 1 hour, HW is set for 2 hours in the morning and 2 hours in the evening.

My thinking is this:
If I turn the dial up to maximum (90degs - all dangers aside), the cylinder stat should continuously call for heat until it reaches that 90 degs on the dial - or until the set time expires, but it stops calling for heat at 60-65 degs on the dial - during the 2 hour time setting for HW. At this point the actual temperature is irrelevant, if I set the Cylinder Stat dial to a high value, it should continue calling for heat until it reaches that value - but it stops calling for heat a number of steps before that set value.
So I think I have a faulty Cylinder Stat (or some factory safety limiter set at 65 degs).

I'm going to have to buy another one and test < my only worry is if it is set to 65 degs by the factory somehow.

 

[oilhead]

There is no factory limit. You're only problem will be that if your boiler heats the cylinder to 90, you will trip the immersion heater limit, and oo when you come to use it, it will not work. Is your cylinder too cool? Are you sure your cylinder stat is cutting out and it is not the boiler cutting out on its own thermostat?

 

[D_Hailsham]

Its set at about 80% (approx 8KW), but that should be enough to heat 100L of water to 80 deg in 1 hour, HW is set for 2 hours in the morning and 2 hours in the evening.

1. An 8kW electric immersion heater would be able to heat 100 litres of water from 15C to 80C in about an hour; but we are talking about using hot water travelling through a coil to heat the 100 litres.
2. Heat travels from the hotter water to the cooler water.
3. The water in the cylinder can never be heated above the temperature of the water passing through the coil.
4. The boiler thermostat sets a temperature, not the boiler output which is fixed.
5. The maximum temperature available is 82C, not 90C.
6. If the thermostat is set to 80% the boiler water temperature will be about 75C (0% is approx 50C).
7. A 75C flow will give a cylinder temperature between 60C and 65C, which is what you have.
8. If you really want the cylinder water at 80C, you will need the boiler set well above that, say 90C, which is impossible.

Try turning the boiler stat to max and see what effect it has.