Puma 100e DHW fault

[accyis]

Hi, i've got a problem with the domestic hot water on a potterton puma 100e and wondered if anyone could point me in the right direction?
The problem being that the hot water intermittently reduces to a low flow rate and losses temperature for about 45 seconds before returning upto a constant 58 degrees.
I've studied the flow rates and temperature rates on all taps in the house without finding a regular pattern. However, it does occur more regular when a thermostatic mixer shower is being operated.
I've just replaced the DHW thermostatic/ flow limiter thinking that may cure the problem.
Any ideas please?

 

[Agile]

Its not possible to usefully consider the operation of a combi boiler when feeding a thermostatic shower.

If it works OK at the taps then the shower/boiler combination is to blame.

Only test the boiler at a tap then tell us what it does quoting flow rates involved please.

Tony

 

[accyis]

Hi Tony.
This problem has only just started after possibly seven years since installation of the boiler. It's at my parents house and until now it's been compatible with the existing taps and thermostatic shower.
I've conducted flow and temp tests at all domestic hot taps as well as the shower, each running for a ten min period and recording my findings throughout. I did this because i suspected that the showers thermostatic mixer valve was to blame.
However, after several tests the fault finally occurred on the dhw tap in the bathroom.
The flow rate suddenly drops from a constant 8 l/m @ 60` temp to 4.5 l/m with the temp dropping to 25` temp. The burner will extinguish and then within a 30 second period will be lit again and back to normal.
The frustrating thing is that there is no regular pattern to when this happens.
I hope you can help. Thanks!

 

[Agile]

I am wondering if you are being confused by the normal operation of the boiler.

As you already know, when prompted, that the flow rate is reduced when the water temperature is low.

You have associated a reduction in flow with a reduction in temperature!

However, I am going to suggest that a reduction in temperature occurs and as a normal consequence the flow rate reduces.

To confirm that you will need to monitor the flame versus the temperature/flow.

If I am right then the only real fault is a reduction in temperature. That could be a fault on the PCB or a fault on the DHW sensor. Since the sensors are cheap then if you have inspected it and its not wet and the resistance is normal then either replace it or swap the two sensors over and see if that fixes the problem.

Tony

 

[Baxmax]

Check the condition of the tubes to and from the air pressure switch for cracking or splits.

 

[kevplumb]

I am wondering if you are being confused by the normal operation of the boiler.

As you already know, when prompted, that the flow rate is reduced when the water temperature is low.

You have associated a reduction in flow with a reduction in temperature!

However, I am going to suggest that a reduction in temperature occurs and as a normal consequence the flow rate reduces.

To confirm that you will need to monitor the flame versus the temperature/flow.

If I am right then the only real fault is a reduction in temperature. That could be a fault on the PCB or a fault on the DHW sensor. Since the sensors are cheap then if you have inspected it and its not wet and the resistance is normal then either replace it or swap the two sensors over and see if that fixes the problem.

Tony

CHRIST is this still a DIY forum

 

[accyis]

Thanks so far for your help. I've just checked the resistance on the dhw temp sensor at room temp and it's 7.5k ohms. Should this be ok? . Also changed the pcb for a spare and the fault is still happening.
The cold water flow rate at the tap is approx 15 l/m yet i've noticed that the flow rate at the hot tap is only 8 l/min. Has anyone ever had problems with the inline cold water isolation valve filter being blocked or possibly the heat exchanger? We live in a soft water?
It's very frustrating, i'll try everything. I look forward to a winning reply.

 

[ChrisR]

It would have to be warmish to be as low as that. If you check the fault-finding, iirc you'll find it says 7.5 - 15k, in small letters.
Not expensive so worth changing. (or swap with the other one)