After having googled for the answer, I'm a bit more confused than I should be ...
My boiler (Potterton Puma 80) has two dial controls, meant to control the temperature of (a) the hot water, and (b) the central heating water. The hot water dial is meant to go from 35 degrees C to 65 C. However, changing the dial seems to make no difference to the temperature at the taps. It's always too hot, but not hot enough to cut out the boiler (the central heating is probably the same, but I'm not as bothered aboput that).
So, I thought, faulty thermostat? But looking at various part websites, I don't think it's called a thermostat. The thermostats I see mentioned are only for the boiler overheat. Looking at the
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website (from potterton.co.uk) in the maintenance documentation, it is called a 'temperature sensor' but I can't see that mentioned in the parts list for the boiler.
Does anyone have a part number (and perhaps a source) for what I need? Also, it looks easy to change (no gas to fiddle with, but you have to drain the boiler), but is it?
This boiler has electronic temperature sensing. The sensors are usually thermistors and can cost up to £30 each. If "raden" is around he'll probably be able to tell you what the resistance of each one should be when it's hot and cold. Thus you can work out whether they are the fault.
Just a note of caution here. I presumed my Vaillant controlled the hot water temperature in a closed-loop fashion, since one of the three temperature sensors it has is in is to do with the DHW temperature.
However, I once set the DHW temperature to be non-scalding ( to be precise, you could just hold your hand in it without being scalded ) one day early on in the year, so I was surprised to find that by summer the DHW temperature was too hot to use. My digital display even allows me to apparently set a precise DHW temperature, but the measured DHW temperature as measured is only loosely correlated with this.
I decided in the end the effect was caused by varying groundwater temperature, and that the DHW thermistor only enabled the boiler to detect an overheat ( i.e.
65C or more ), and that the way it worked was that the boiler simply looked at DHW flowrate, and modulated the gas flow to heat that flowrate of water up by a fixed amount. Varying the DHW temperature dial simply varied the amount of gas the burner consumed for a given flowrate of water, so if the groundwater was very cold, or the groundwater was warm, the boiler simply dumped enough energy into it to heat it up by an approximate amount ( say, +30 C, depending on how you'd set the dial ).
All of which is a longwinded way of saying that your temperature sensor may be there simply to look out for an overheat, not to automatically control DHW temperature.
If that is the case ( and I do't know your boiler is designed in the same way as mine ) then your overly-hot DHW may result from some other problem, for instance a defective gas burner modulator, or a buggered DHW flowrate sensor.
My Vaillant can be checked to see if it is modulating the flame, either by measuring the gas pressure at a test point ( need a manometer, but they're easy to make ), or by reading off the modulator valve current on the front display. Dunno how the flow rate sensor can be tested.
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