baxi combi instant 105e luke warm DHW prob please help

4 Feb 2011
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United Kingdom
Orig problem was having to reduce DHW flow to retain hot water temp, gradually this became worse until no DHW at all. CH is all working fine.

So to be honest left it and used electric shower. However wifey not happy so need to fix it.

I am only a DIYer but noticed burner not firing up when hot tap opened. The differential valve was not operating the microswitch as the rods inside the valve were scaled up so I sorted this.

Now I have luke warm DHW although all the temp neons are lit, also the preheat function goes on forever and cannot light all the temp neons.

So as a test I swapped the leads over on the CH and DHW temp sensors with the following result.

CH rads cannot get fully hot, temp neons light up only to 50/60 degrees.

On the DHW side of things the tap water is still luke warm but the temp neons only light to 40/50 degrees. However the preheat function works normally lighting all the temp neons and provides a small amount of very hot DHW.

does this point to the DHW temp sensor being faulty.

thanks for any help / ideas
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You check sensors by measuring the resistance with the boiler cold!

If all heat lights are lit on dhw and you still get little hot water then your plate h/x is scaled/blocked or your diverter valve has gone. I bet the later as it doesn't kettle when all lights are lit
if its not firing on hw its either the hw diaphragm or microswitch faulty,if it is firing up but quickly reaching temp and shutting off (before suspecting plate h-exchanger) it is more likely the pressure differential valve spring seized not letting the water out of the expansion vessel,common fault on the pre heat baxi 105's :)
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(orig post 4th feb 2011)

Thanks for the replies chaps.

Before I start working through the possible remedys I have some more info to add.

After sorting the DHW diff valve as mentioned before, if I leave the preheat function on it warms the rads when there is no demand for heating.

Also, The divertor valve has a CH differential valve on the front of it. I have found that when the CH thermostat is operated the pushrod operates and the heating comes on all OK. But the microswitch seems knackered, no click when manually operated, took the little black box apart containing the microswitch and the travel of the microswitch plunger is short and spongy compared to the DHW microswitch.

Does the CH differential valve microswitch have any involement in the DHW ?

I will get a new switch anyway and fit

thanks again to everyone who replied

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