Cylinder stat issue and recommendation

[peredelkin]

Hello fellow DIYers.

I have a system boiler with UFH, the UFH manifold is set in an S-plan with a hot water cylinder. The cylinder has its own thermostat to switch hot water on/off as needed, which fits on a pocket. The stat looks like this:

As you can see, the dial is set to 55C. And yet, water comes out at 66C at the tap. I took the stat out, put a meat thermometer in the pocket, and the meat thermometer measured 55C (even though it didn't reach all the way to the end of the pocket). Put the stat back in, turned the dial down, and it would only switch off the system at 35-40C.

Questions for my fellow DIYers:
1. My stat's broken right? Is there any other reason why it would behave like I described? The cylinder is in a cold loft so could the low external temperature affect the reading of the stat?
2. If it is indeed broken, what do I replace it with? Is there any particular brand/model you would recommend?
3. One frustrating issue with rotating dial thermostats is that you never know what temperature you're actually setting them to and what temperature they're measuring. Is there any good digital cylinder thermostat that shows the current temperature and allows me to select the temperature on a display? (The ESI ESCTDEB seems to be sold on quite a few plumbers' websites, but I have seen lots of negative reviews)

Thanks!

 

[stem]

That sounds fairly normal to me. Cylinder thermostats are notoriously inaccurate. The problem is that cold water comes in at the bottom of the cylinder at say 5C and is heated by the boiler. Hot water rises to the top where it is drawn off to the taps at say 65C. So the temperature range of the water inside the cylinder is 60C. The thermostat is usually a third of the way up. What's the temperature there? who knows, somewhere between 5C and 65C. And it will change depending if the boiler is heating the cylinder or not, and how cold the incoming cold water is, both of which can change.

Users want to know the temperature of the water going to the taps, but because the cylinder thermostat is much lower down, they are derated to take into account the temperature that they are measuring much lower down will be less. But because it's a moving goal post and even hot water cylinders vary due to their size the thermostats are rarely right. In my experience, the 10C difference you refer to actually about par for the course.

My advice would be to ignore the number on the dial, and simply increase the setting if the water isn't hot enough and decrease it if it is too hot. Even then, there is still usually a swing of 10C on the outlet, because when hot water is used, the thermostat being lower down can be cooled by the cold water coming in at the bottom quickly and so start to heat the water, even when the water at the top may still be hot, and so will now get hotter.

 

[peredelkin]

That sounds fairly normal to me. Cylinder thermostats are notoriously inaccurate. The problem is that cold water comes in at the bottom of the cylinder at say 5C and is heated by the boiler. Hot water rises to the top where it is drawn off to the taps at say 65C. So the temperature range of the water inside the cylinder is 60C. The thermostat is usually a third of the way up. What's the temperature there? who knows, somewhere between 5C and 65C. And it will change depending if the boiler is heating the cylinder or not, and how cold the incoming cold water is, both of which can change.

What you say makes sense. However there is one important detail I omitted from my original description: I have a timer in series with the stat. The timer switches on 1 1/2 hour in the morning and 1 1/2 hour in the evening. So the cylinder stat is not constantly live, switching on/off, but only does so at specific times of the day.

What I had noticed was that when the timer was on, the call for DHW from the cylinder was constant. I thought okay, the flow temperature should be roughly 15C above the cylinder stat set point, and so I raised the flow temperature to 70C in the hope that at some point the cylinder stat would cut off. Yet what actually happened was that the cylinder was still constantly calling for heat for 1.5 hours continuously and my DHW was coming out of taps at 70C.

So yeah, my issue is that with this particular stat the limiting factor seems to be the boiler flow temperature, because if it was for the stat, it would be constantly on.

Are you suggesting that I should just make peace that the stat is sh*t, and use it as if the dial had no numbers on it?

And if that's really what you are suggesting, then would this calibration protocol be correct?
1. Stick the timer to constantly on.
2. Set the stat to, say, 30C.
3. Let it run until it cuts off the boiler.
4. See how hot the water that comes out of the tap is:
4a. If the water is too cold, repeat from step 2, increasing the stat by 5C.
4b. If it is at the right temperature, done! Put the timer back to its original automatic setting.

 

[Johntheo5]

I would first check that the stat is switching, ie clicking on/off at a difference of 6C to 10C max, (the hysteresis). turn the stat up or down until you hear a click, then turn in the opposite direction until you hear another click, note the two settings from the index.

Assuming the cylinder is heated with a HW coil then, yes, you can get a difference of up to 10C between the HW temp at the top and at the stat as the hottest (heating) water enters at the coil top and exits at the bottom, there may be 10 to 15C difference between the temperature at the coil entry (top) and the coil exit (bottom), depending on the flowrate through the coil, this is reflected in the cyliner temperature gradient. The reason for having the stat located low down is to start reheating the cylinder as soon as 15 or 20 litres of HW is drawn off. You will not see a cylinder temperature gradient if the heating is done by a electric heating element in the bottom of the cylinder or, as I have seen in some heating applications, where the heating hot water enters through the coil bottom and exits from the top.

 

[ianmcd]

If as i suspect that is an unvented cylinder the thermostat doubles as a safety device and can only be replaced with an identical one by a competent person

 

[Harry Bloomfield]

1. My stat's broken right?

No, wrong. If you turn the dial, and it clicks cleanly turning it up and down - then it is fine, doing what it should. Just tweak the setting, to get the water coming out of the hot taps, at the temperature you desire.

What I had noticed was that when the timer was on, the call for DHW from the cylinder was constant.

Your boiler will run constantly, if it's flow temperature is lower than the cylinder stat temperature. It can never satisfy the cylinder stat, so will keep cycling the boiler endlessly.

 

[peredelkin]

No, wrong. If you turn the dial, and it clicks cleanly turning it up and down - then it is fine, doing what it should. Just tweak the setting, to get the water coming out of the hot taps, at the temperature you desire.

Got it.

Your boiler will run constantly, if it's flow temperature is lower than the cylinder stat temperature. It can never satisfy the cylinder stat, so will keep cycling the boiler endlessly.

Yes, I figured out as much. My problem seems to be that the stat is set to 50 but actually switches at 80, then a boiler flow temperature of 70 will never satisfy the cylinder stat.