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Need to calibrate a cylinder thermostat?

Discussion in 'Plumbing and Central Heating' started by Hamslay, 26 Feb 2012.

  1. Hamslay

    Hamslay

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    My Mum's been in her current house for 3 years, and is now saying that since she moved in the hot water has been extremely hot - almost unsafe.

    I checked the immersion heater is not on, checked the motor and 3 port valve and all that side is good. There was no clicking from the old HTS2 cylinder thermostat though, so I replaced that with a new HTS3. However, the water still remained extremely hot, and the HTS3 would only click on/off at 45C. I queried this with Drayton and the supplier, and the supplier kindly sent another HTS3 to try. I tried that last night and same results.

    Now, what I have found is that the thermostat is actually working. I can put it on the tank, get the strip to bend, wind the temperature down to 45C so the boiler and pump switch off, get the thermostat off and blow on it and it cools, clicks and restarts the boiler and pump. So the thermostat and wiring are all fine, but the temperature range on the thermostat is nowhere near the temperature of the water coming out the taps.

    I have no idea if this due to limescale in the tank or something changing the temperature at the thermostat (one third up the tank), but the tank feels damn hot to me!!!! I don't think I can turn the thermostat low enough to achieve 60C, so maybe I will need to recalibrate the thermostat?

    I can see from the HTS2 how the screws can calibrate the metal strip deflection, so I'm thinking with a thermometer under the tap and some tweaking of the calibration, I can probably get the thermostat working roughly in the right range.

    So.... after all that rambling, I'm just wondering whether this is something a professional would normally need to do. It seems from talking to Drayton that the units are assembled based on a one-off calibration based on a perfect tank surface and no limescale etc. Surely after several years, the default calibration is that unlikely to be correct?

    Am I barking up the wrong tree? Has hours of sitting in my Mum's airing cupboard finally driven me mad??
     
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  3. DP

    DP

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    First thing to look at is the position of said thermostat on the cylinder.

    Next, you have to appreciate water at the base of the cylinder can be cold and 60 at the top and 45 where the stat is fitted.

    If present position of thermostat gives you 60 at the delivery point, then either fit the thermostat at a higher point on the cylinder, or instal blending valve to temper the water at the hot tap.
     
  4. Hamslay

    Hamslay

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    Thanks DP.

    The thermostat is currently one third of the way up. I always read it should be one quarter to one third up. Does one sometimes move up to halfway or even higher then to get an accurate reading compared to the tap temperature?

    If I have to reposition the thermostat on the tank higher up, what does one use to fill the old hole where the copper is exposed?

    It still seems easier to recalibrate the thermostat! :D
     
  5. 45yearsagasman

    45yearsagasman

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    Are you 100% sure that the 3 port valve is not passing to heat the cylinder when the valve is on CH only. Just because the motor works does not prove the valve is shutting the HW port off. Just a thought.
     
  6. Hamslay

    Hamslay

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    I did wonder that myself, so I took the motor off of the valve and was able to move it freely with my fingers. There wasn't much movement, like 7 o'clock, 6 o'clock and 5 o'clock positions, but it moved easily. To prove it though, I turned the DHW off completely for 48 hours and asked my Mum to keep the heating on so that if the hot water was being warmed at all by the central heating I would know. After the 48 hours, the hot water was stone cold.

    So, accepting I'm more of dabbler than a plumber, I am fairly confident the problem is solely with the thermostat "calibration". :D
     
  7. D_Hailsham

    D_Hailsham

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    Have you measured the temperature?
     
  8. Hamslay

    Hamslay

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    No. I have a thermometer coming tomorrow, but I know it's not 45C. You can't put your hands under the hot water once it's been running a few seconds. I'll update the thread with the temperature tomorrow.
     
  9. Agile

    Agile

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    The thermostat is fine! Dont dabble with it.

    Either its wrongly positioned or the other aspects of the system are correct.

    If you want to do it properly you need a contact thermometer to measure the HW and the CH temps.

    First check to see if the boiler flow temperature is not too high. Ideally 70 c or lower.

    Tony
     
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  11. Agile

    Agile

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    The thermostat is fine! Dont dabble with it.

    Either its wrongly positioned or the other aspects of the system are correct.

    If you want to do it properly you need a contact thermometer to measure the HW and the CH temps.

    First check to see if the boiler flow temperature is not too high. Ideally 70 c or lower.

    Tony
     
  12. Hamslay

    Hamslay

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    Thanks for the reply Tony.

    Can I just ask why the boiler temperature really matters? Assuming the valve is working and the feed to the hot water tank is closing off, isn't "the hotter the better" reasonable for the central heating in the winter? I thought that was the whole reason for having the cylinder thermostat - that the boiler water is hotter than you would want it stored in the tank.

    (Sorry if this demonstrates my stupidity) :LOL:
     
  13. Agile

    Agile

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    A standard size cylinder works OK by heating the contents to about 55 C using CH water at about 70 C and measuring the turn off temp 1/3 of the way up the cylinder.

    But that relies on the temp in the cylinder being fairly uniform. In your case there just MIGHT be something different from normal and the top may be getting too hot.

    A photo of the cylinder and the stat position might immediately tell us whats wrong.

    It could also just be your assessment of temperatures without proper measurements.

    Already you have said you think the hotter the CH the better! The correct answer is the cooler the better as that enables the boiler to be the most efficient.

    One of my customers heats her hot water to 45 C and uses it directly without adding any cold. VERY efficient that.

    Tony
     
  14. DP

    DP

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    OP, you would need to take temperature reading AT the thermostat contact point, which will be about 45 degrees. A tolerance figure will apply. You tweaking the stat to exact point will make no difference to water at the tap. If anything, you will probably introduce error, so operate/ release of bimetallic strip will then at other temperatures, react differently.

    The sensing component is not a precsion component. If this is what you want, you will need to go electronic but end result will still be hot to scalding water at the tap. Invest in circulation pump to mix the water in the cylinder. The cylinder thermostat will now at present fixing point, will switch the heat source at precise temperature give or take the tolerance figure.
     
  15. ALEC1

    ALEC1

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    the primaries are too hot, thats the problem....turn that down a fraction, but be aware that it must be at least 10c higher than the cylinder temp...
     
  16. DP

    DP

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    Alec, why should that make a difference?

    If what you suggest be the problem, what is the purpose of the motorised valve + cylinder thermostat?

    Combi boilers (now that is a separate issue) are fitted with temperature control that seldom kicks in to control the system unless the flow rate at the tap is less or cold water in is plus 20 degree C (assuming HW stat is at max which it will be in this weaher)
     
  17. Hamslay

    Hamslay

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    Well, having left the thermostat set to 45C thinking it might bring the tap temperature down nearer 60C, I measured the water today at 47C. It's possible to wash your hands under the running hot water alone now, so Mum is happy! :D

    I don't know what temperature the water is being stored at, as there's obviously a heat loss in the pipework, but I'm guessing it can't be far below the recommended 60C if it comes out of a kitchen tap at 47C?

    I think I'll tweak the cylinder thermostat up to 50C and work on the basis that the water is probably stored at a safe temperature, but cool enough that Mum's guest will stop scalding themselves when they wash their hands! :D

    Thanks everyone for your help!
     
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