I am currently having issues with this boiler. The boiler works
perfectly with the central heating however when using the hot taps in
the house the water that comes out is not fully heated and the
temperature varies from hot to luke warm.
The boiler has been reset and the taps turned at different rates to see
if the flow has any affect - it doesn't.
I am assuming that a thermostat or temperature gauge has gone. Is this
a common problem? Is the solution simple to implement.
Depends, I was surprised to find my Vaillant didn't heat the water to the
demanded temperature, it simply heated it up by X Celsius, depending on how
far you turned the dial. There are several temperature sense thermistors,
but the one in the DHW loop only senses an overheat condition ( >65C ) to
If your boiler works the same way (?) the thermistor would have to fail in
such a way as to make the boiler think that luke warm water was an
Thermistors are easy to test, there are standard versions, often 2000 Ohms
at 25C is a typical value. I think you ought to consider other possibilities
for your symptoms though. The diverter valve may not work properly on the
DHW side so allowing only a little heat through to the DHW heat exchanger
Have you a boiler manual? Have you a voltmeter? Do your CH pipes get hot
when you turn the HW on? Try that test when the CH is off so you can easily
feel if hot water is entering the CH system when it shouldn't be.
Now you mention it - I have noticed that the radiators have been warm
in the house when the heating has been turned off. At the time I had
thought it was some kind of maintainance function but you may have hit
the nail on the head.
I have downloaded boiler manual, I have volt meter. I will do the test
and check this weekend.
The standard thermistor used in most boilers is 10kohms, some use
100k,some use 1meg. Is yours a Sine 18 ? I can't remember what that
uses offhand, but I can't think of any other boiler which might possibly
use a 2k NTC.
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