Ch works but no hot water. Valve and motor work potterton heat max 28he

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Dan182

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Hi all,

I am working on a potterton heat max 28HE that has no hot water (hot tap runs cold but boiler fires up when the hot tap is on and hot water is sent to the valve) this boiler seems to be identical to the baxi deo tech?

I have looked over many threads and had a convocation with a guy on YouTube (HeatingGeek) about it but I want to tap into other people' knowledge before I replace the pcb.
Am I right in thinking if the DHW sensor was at fault the boiler would not fire up?( I saw in another thread on this forum that had a similar problem it was the DHC sensor but there boiler did not fire up when demanding hot water)

The fault with the boiler is that the CH works fine. However when the hot tap is on the water flows but there is no hot water (but the boiler fires up and the light on the display indicates a demand for HW) . With the CH off and the hot tap on still no hot water. The boiler fires up when the hot tap is on and CH off, however the water does not get hot. If i remove the Motor that operates the 3 way Valve the pin in the valve rises and the water becomes hot to the hot tap. If I connect voltage from a plug socket (not from the boiler) the motor operates and the pin goes in and out (with the Live and neutral on the corresponding pins, I have to change the wires around to make it go in and then to make it go out, but this test shows me that the motor works). If i leave the motor in the hot water mode (after testing it) and re fit it to the boiler, the hot water works when the tap is on. When i put the CH on the motor switches the Valve to CH mode and the CH works. When I then put the hot tap on again the boiler fires up and the light on the boiler indicates a demand for hot water however the water remains cold at the hot tap as the valve is still in the CH position as it does not change back. I checked the hall effect sensor by doing a voltage test on the neutral from the sensor and the power to the sensor, that was 15v I then checked the voltage on the white signal wire from the sensor, I exoected this to be about half volts but it was 15v still. With the hall effect sensor unplugged the boiler does not fire up when the Hot tap is on. Any help would be appreciated massively. Thanx for reading this Dan
 
The valve changes between centertal heating and hot water when voltage is provided from an external sauce to the motor. At the moment I have removed the motor so that the hot water can be used. If the motor is installed in the hot water position the hot water works, when the Ch is turned on the motor puts the valve on to ch mode but when the hot tap is put back on it will not change back to dhw. Then I used an external power source to operate the motor and put the hot water on, the hot water worked again but once the boiler wiering was re connected and heating put in the boiler would not put the hot water back on.
When I first looked at the boiler a fuse had gone, the valve had a leek and the leek had got into the motor causing it to fail I replaced the valve and the motor and the boiler worked fine, but a week later the air realise valve (bleed valve) on the top of the boiler leeked. (Totally unrelated I presume?) But then this new fault happened. The fuse had blown due to the leek but all electrics appeared dry from what I could see. This is why I tested the motor with a external power sauce. I also had a spare motor I tried fitting that but the problem remained
 
Just to try and clarify if there's any confusion with my explanation with the hall sensor test I carried out this is the test I did
. The video shows baxi duotech Im working in potterton heat max. My findings were different to the ones in the vision oncc his problem was fixed. The white wire had no volts with the tap off and 15 volts with the tap on (I kept the mester in the same position and had someone else operate the tap) 15 volts was also on the wires for power to the sensor so I was expecting 7.5v ISH on the white wire but I found 15
 
Just reread your post (not easy):D if you have 240vac between 5 and 7 on x3 on the pcb during hot water demand and the diverter head isint moving check the lead, if no 240 change pcb.
 
Picasso, Sorry I understand that my posts can be difficult to read, I have dislexia and sometimes spell check does not fully help :-) thank you for spening the time to work out what I am trying to say. what did you bean by on times three? I checked the cable from the PCB to the motor for resistance and they show no resistance. There is voltage to the CH wire that sends volts to the motor for the CH but none at the PCB or the other end of the cable for the Hot water demand. Many thanx
 
Hi dan I meant x3 on the pcb, you need to check if you have 240vac between 5 and 7 when you have a demand for hot water, if the boiler is showing a call for hot water and you don't have 240 then the board is stuffed.

low resistance (0 ohms) or no resistance (OL ) on the cable?
 
0 mhms (the same as having both Leeds of the multimeter touching) thanx
 
So between five and seven on the PCB? Is there anything else that could cause these to not get 240v other than the PCB? Many thanx again
 

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Dan have you checked mains voltage coming in at the pcb? I once had a vaillant that was a similar issue and was about to change the pcb. When I carried out my dead test, I found it was only a small voltage to pcb. I then found the wiring in the fused spur connection was incorrectly wired to the from stat and pcb would only get 240vac when heating was on switched live. :confused:
 
That is a really good idea I will check the mains voltage coming in at the PCB. out of interest is there any way i can get hold of a Component Layout for the PCB? many thanx
 
So check between these two to see if mains voltage? Should there be mains voltage at the fuses also thank you
 

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Dan182 said:
That is a really good idea I will check the mains voltage coming in at the PCB. out of interest is there any way i can get hold of a Component Layout for the PCB? many thanx
Click to expand...

The component layout should be in the manufacturers installation instructions, and I would be testing the mains voltage coming into the boiler also. Out of interest has the fault always existed? Have they had any work carried out recently? Etc.
 
I replaced the motor and valve as the valve had leeked into the motor and set the wires on fire (the wtong fuse had been put in previous my guess is the leek had been there someone put s bigger fuse in so it didn't blow the fuse with water in the moror. I meant the componant layout of the actual PCB as I want to check which resistors and diodes eccetra are on that circuit if it is the PCB they are usually simple to re solder componants. I checked the capacitor's none are bent up at the top (often when they blow the tops bend up) I'll check the incoming voltage tomorrow and everything else mentioned hear as I'm re visiting tomorrow many thanx
 
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