Baxi Combi Instant 105e - intermittent DHW

24 Nov 2010
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United Kingdom
Intermittent DHW. No real obvious symptoms to suggest what it could be. Tried Preheat on and off.. Tried just DHW with CH off... Changed both microswitches for secondhand spares...

One thing i have noticed is that the RESET function doesn't seem to work anymore... I'm sure it used to make the boiler shut down and then restart.. Could this point to it being the PCB??

Any help greatly appreciated...
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Remove the DHW microswitch and note the position of the pin that activates it. Turn on a tap and see how much it moves. You may have to do this a few times to get a feel for it, or mark it with a pen. If it doesn't move much you most likely have a perforated diaphragm - it could be that it's only got a small hole and so it works sometimes and not others.
This model does not have the most robust diverter valve in the world. Membranes are know to split more than others, and also very sensitive to crap in the system.
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thanks for the feedback.. i have ordered a replacement diaphragm, which will be here soon. however today i was playing with the boiler and tested both pressure switches (DHW demand and 3 way valve) and they seemed to function perfectly well, as did the microswitches.

What i have noticed is that when the CH is on (thermostat is requesting heat) the DHW just does not seem to get up to temperature. Is this because the 3 way valve is not diverting the heated water from the CH in to the plate heat exchanger? Would this also explain why sometimes when the pre heat comes on, instead of heating the retained DHW in the PRE HEAT STORE it is pumping the water around the CH system? I presume that this function would be controlled by the PCB..?

If the PCB is faulty, would that also explain the fact that the RESET switch function is not working...
hi baxpoti..

thank you for your suggestion of replacing the DHW diverter valve.. I am a little hesitant at doing this as I have tested the function of the valve and it seems to be working fine ie. when the hot water tap is switched on the pushrod clearly moves out and in turn activates the micro switch, which i have also tested with a multimeter and is definately working.. please could you look at my previous post regarding the DHW not working properly when the CH is on and also the fact that when the pre heat is on it often sets the CH rather than heating the stored water.

I look forward to hearing back from you..

Many thanks again..
sounds, to me, as though your diverter valve has some dirt in it so cannot close the CH off when there is a HW demand.

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