Potterton Performa 24 not modulating

[JonBoy001]

I have my combi covered by British Gas. There have been a series of issues recently around intermittent hot water. The PCB has been changed and this seemed to resolve matters but it now seems that the wiring looms are to blame and these are being replaced tomorrow.
However, the hot water now cycles on and off with no apparent flame modulation. This started after the PCB replacement and has continued after a further PCB replacement. The plate heat exchanger has also been replaced. So has the pump.
The hot water gets very hot and then the flame cuts out. On the maximum heat setting, the flame remains on with lots of very hot water delivered. Turning down the temperature setting results in the on / off behaviour on the burner. (The lower the set temperature the sooner the burner cuts out having got the HW less hot.)
Similar behaviour with CH i.e. water gets very hot and then cuts out with no apparent modulation.
I know the engineer tomorrow will say that this is OK because if the HW setting is at max, there is continuous HW but I disagree that this is OK. I do not want to have to have only high flow rate scalding water.
I understand that the requested (by the PCB) modulation can be measured by checking the voltage. So my questions are:

1. Where exactly should the modulating voltage by read? On the PCB? Which pins? Or elsewhere? I want to be able to clearly tell the engineer what he needs to measure where.

2. If the PCB is correctly modulating, then this presumably can be compared with the measured gas pressure? (Which I know is not modulating down because the engineer was measuring it on last visit.)

3. Is the issue likely to be incorrect setup of the gas valve or does it need to be replaced? (I think the values he measured were 12mb when running and possibly 20mb on the input.)

4. Am I barking up the wrong tree entirely?

I would very much tomorrow's visit to be the last for some time so any help in making me an informed customer would be much appreciated.

 

[gas112]

tell him to check the high and lows they will know what it means bet the low is far too high

 

[AGAS]

why are you worried about measuring boiler parts when you are on a care plan , they will keep changing parts until its correct , highs and lows sound incorrect or sensors ??

 

[JonBoy001]

I'm worried because I have to be at home for each visit. And there have been 5 visits to date. This is not helpful when I should be working elsewhere.

I think understand what high and low are with respect to gas pressure. I mentioned high being 12mb. Should he just know how to check the low?

I should have mentioned that the last engineer did also call Potterton / Baxi for advice but they had no ideas either.

It would also be helpful to have confirmation that I am not being unreasonable in expecting to be able to set the HW temperature to whatever I want and having a constant(ish) HW temperature produced? Should there be visible change in flame height when modulating? I will be standing over his shoulder tomorrow as I really want this resolved once and for all.

 

[Dan Robinson]

You're not being unreasonable in expecting it to work properly.

Chances are they haven't set the board up properly (if its the one I'm thinking of).

Standing over his shoulder will achieve nothing. Let him do his job but make sure it is tested thoroughly until it is working. @petit_pablo might have some advice too.

 

[AGAS]

I will be standing over his shoulder tomorrow as I really want this resolved once and for all.

ye he'll love that " not "

you should see flame height difference when modulating .let him sort it out you'll get there , check its operation before he leaves

edit , some boilers can be a pain in the ass and need return visits TBH

 

[ianmcd]

I agree with Agas dont stand over him that will just **** him off , wait until he is finished then test that it is sorted before you sign anything, and dont let him leave till you are convinced that it is sorted, dont be confrontational when the engineer arrives say something positive like I really hope you manage to get to the bottom of this as it has been ongoing for a while

 

[petit_pablo]

Basically the guy hasnt got a scooby if he's replacing a loom. Pretty sure that's where the laptop fires you when it's running out of possibilities
If the jobs on 5 visits you should be expecting at least the local technical support engineer to be assisting. The local service manager should also have made a curtesy call to you too.

I wouldn't stand over the guy tomorrow, he will be stressed enough anyway at what's going on behind the scene.

Gas valve does sound like it needs setting up correctly. Hopefully he will do this tomorrow.
Is this the only problem with it?

 

[AGAS]

was thinking that petit , honestly can't see the only problem was not setting the valve . more going on before hand i expect but maybe the op don't know about .

 

[ianmcd]

I have been a gas engineer for over 30 years and have never replaced a wiring harness yet, have come across a few situations where a part of a wiring harness needed replacing but always because of trapped wires or melted wires , sometimes plugs melted etc but I have never replaced a complete wiring harness and agree with Petit Pablo, it seems to be a get out when they cant really work out what is wrong on a fault finding flow chart, then say it is the wiring harness, cant ever see why you would have too unless the boiler had been on fire and then you would just fit a new boiler

 

[Agile]

On most boilers, both a replacement PCB and the gas valve have to be set up properly.

You might be surprised that so many engineers really don't know how to set these parameters up on boilers.

The boiler is not capable of total modulation without limits.

That model probably only carries out modulation of 3:1.

In simple terms that means it should be able to give hot water at a flow rate of 9 li/min and reduce the power input when the flow rate is reduced to 3 li/min. At a lower flow rate the boiler's minimum power input is too much and the boiler turns off the burner.

Tony

 

[bernardgreen]

Basically the guy hasnt got a scooby if he's replacing a loom.

it does sound like a desperate "last thing to left to try" action.

That said on some older boilers the loom could cause faults on the PCB. Solder joints where the loom connected to the PCB were failing due to vibrations in the loom which were being absorbed by the soldered joint where the connector was fitted to the PCB. One manufacturer designed out the connectors, ( the revised PCB came complete with loom ) and more importanly added strain relief fixings to prevent vibration reaching to where the wires in the loom were soldered directly to the PCB

 

[Agile]

Well Jonboy, what was done to fix your boiler?

We do like to know the outcomes of these situstions!

Tony

 

[AGAS]

Well Jonboy, what was done to fix your boiler?

We do like to know the outcomes of these situstions!

Tony

don't think he cares as he probably don't have a problem now , jonboy thinks as a one boy . would certainly like to know

 

[Phill Peck]

why are you worried about measuring boiler parts when you are on a care plan , they will keep changing parts until its correct , highs and lows sound incorrect or sensors ??

His premium will be higher next year