Puma 80 DHW problem

Well, that's where this page of answers came in.

I understand P saying check the burner pressure - if the boiler is at half power for some reason you wouldn't get full HW performance.
You can check it yourself though - put the HW on full and watch the gas meter. Time how long it takes for 1 cu ft of gas, in seconds. Call that T

then (3600/T) x 1035 gives Btu/hr. So if T was 36 you'd have 103,500 Btu/hr.
If it's a Puma 80, expect something like 90,000 cos you're measuringthe input not the output. 1kW = 3412 Btu/hr.

You've fixed the leak on the wax cap then?
Is that chrome pipe going up into the H/E hotter now?

Is it a hard water area?

and how can you tell from

http://www.partsarena.com/

that there's stuff "underneath"? Still haven't found where it gives the sensor resistances!

Hi Chris

Thanks for that - you were right about the 'O' rings - it turned out that one of them was completely missing, and also its brass backing washer!

Sadly, although I believe the old wax capsule was faulty, the fault with no DHW still persists. So, we investigated further, and found that the supply to the gas valve remains at zero volts when the DHW demand LED is lit. According to Potterton this is a sure sign that the modulation control board is faulty.

Since I have some electrical skills I might just get Mr. CORGI to isolate the services so that I can remove the board and do some fault-finding on it.

Thanks again for all your help so far; we're not quite there yet but I think there was more than one underlying fault, so working systematically is the only way to reach the goal.

Cheers
W.

Sorry Chris - I neglected to answer some of the points you raised.

ChrisR said:

Well, that's where this page of answers came in.

I understand P saying check the burner pressure - if the boiler is at half power for some reason you wouldn't get full HW performance.
You can check it yourself though - put the HW on full and watch the gas meter. Time how long it takes for 1 cu ft of gas, in seconds. Call that T

then (3600/T) x 1035 gives Btu/hr. So if T was 36 you'd have 103,500 Btu/hr.
If it's a Puma 80, expect something like 90,000 cos you're measuringthe input not the output. 1kW = 3412 Btu/hr.

Click to expand...

This was very helpful - thank you. As in my previous reply the latest news is that Mr. CORGI found the pressure stayed at the minimum when we demanded DHW. This of course led to the GV, which appeared not to be operating. The modulation control board appears to be the culprit. However, I believe that the DV was also faulty, and I KNOW that the system had no inhibitor in it so was just waiting to go wrong somewhere.

ChrisR said:

You've fixed the leak on the wax cap then?

Click to expand...

Yup - thanks.

ChrisR said:

Is that chrome pipe going up into the H/E hotter now?

Click to expand...

Nope.

ChrisR said:

Is it a hard water area?

Click to expand...

Yup, but until the GV is made to work we won't know whether or not it's scaled.

ChrisR said:

and how can you tell from

http://www.partsarena.com/

that there's stuff "underneath"? Still haven't found where it gives the sensor resistances!

Click to expand...

I now can't recollect how I discovered the nether regions of Parts Arena, but the route to finding the resistance values is as follows:

1. Stab your browser window firmly towards http://www.partsarena.com/baxi/System/index.htm

2. Navigate down through these links:
Potterton -> Boiler -> Puma -> 80 -> Installation Instructions ->
-> Puma 80 & 80e

3. Scroll down to page 38, on which you should see the fault-finding chart.

4. Print the chart, and use Supervision to read the tiny writing.

Or you can call P. of course, and they'll give you the specified resistance at room temperature.

Regards
W.

I hadn't realised that the PartsArena thing is pointed at by both Potterton and Baxi websites! It seems bizarre that you can't get to the data from the top site.

Sounds like I'll have to get a new PCB to you, though yours might only need something resoldering. Relay contacts go bad too though.

Ah yes - now I remember!

For the benefit of anyone else reading this, the link sequence is:

go to http://www.potterton.co.uk/

then click on "After Sales",
then on "Spares - interpart",
then on "What Part Do I Need?",
then on "Search the Technical Documentation Library".

The site works better with Internet Explorer than with Mozilla / Firefox

Here's the next chapter in this story....

I took out the modulation control board, after which it was VERY obvious that there were two dry joints and a third suspect one.

I resoldered all three, and gleefully reinstalled the board, but my work had cured absolutely nothing! I then noticed that a couple of the relays are partially blackened inside, so something has been arcing or overheated.

Conclusion - new PCB need!

Thanks are due once more to ChrisR for being very supportive and helpful with this one.

Final phase of work: replace the PCB with a brand spanking new one.

When I took the old one out I could see that my repair had already been "damaged" - resistor R1 had disconnected itself again.

Before fitting the new PCB I took the precaution of testing both gas valve solenoids in case one of them had gone short circuit. GV3 was the suspect but I tested both; the Potterton specification says:

GV2 : 4K ohms
GV3 : 120 ohms

Fortunately these were both spot on, so in went the PCB and on went the hot water. And one very grateful customer danced around.

Once again, thanks are due to ChrisR for his help with this.