Potterton Puma 80 no hot water DHW for 5min but rads CH fine

[arakrazy]

I have a Potterton Puma 80, quite old I know.
A few days ago we noticed that hot water became very rare.

If I open the hot water tap, it flows slowly (the boiler is meant to restrict flow until the right temperature is reached) and stays cold for about 5 minutes. I hear the pump going while waiting.

After about 5 minutes, the boiler fires up properly I hear the relays click, and hot water starts being produced. The tap's flow becomes strong, and after a few seconds the water runs hot.

I also noticed that at the same time the radiators will (slowly) start to warm (tap on for 10-15 min or so) Once I turn the tap off, the boiler will pump for a few more minutes.

Manually pushing the pin causes the light to go on, so the flow switch seems alright. However relay 1 on the PCB only switches (providing hot water) after 3-5 min of running.
Pressure is fine - just about 1.2

What strikes me as bizarre is that once up and running, the boiler is fine...until the hot taps are completely stopped again The central heating is great (at the moment), and heats up immediately.

Have to say that in the winter, there was no boiler action if it was really cold for some reason, so boiler stayed on/warm round the clock.
Occasionally hot water will be produced the first time the tap is opened.
I can't really afford a plumber (£80/hr here in SW11?!) but any ideas?

Thank you in advance!"

 

[arakrazy]

I posted the links so that others could perhaps save themselves some time.

I spent hours searching the forums for info that could help and it's all confusing and contradictory...

It also lets people know I have searched the forums. Any chance I could get the original post back? I don't have a backup of the post with the links, and I don't want to spend half a day on it again.

 

[seco services]

they can never leave anything alone, and wonder why people post twice.

 

[twgas]

could be the diaphragm in flow switch, which may be slow to react to water flow

 

[steve32]

turn heating off, open hot tap does red hot water demand light on p c b come on and combi fire or is there a delay if so approx how long
with tap on does heating flow pipe get hot left 22mm pipe

 

[arakrazy]

turn heating off, open hot tap does red hot water demand light on p c b come on and combi fire or is there a delay if so approx how long
with tap on does heating flow pipe get hot left 22mm pipe

Red demand light comes on...the only thing that doesn't "click" is one of the relays as mentioned...

The left 22mm pipe doesn't heat (DHW water outlet) until the relay turns over....3-5 min after the red demand light comes on.

Possibly could be the diaphragm in the flow switch but as I said the diaphragm acts on a pin on a microswitch - manually pushing the switch activates the red demand light but does not start hot water production until 3-5min afterwards...even though the boiler fires up

 

[steve32]

does burner fire as red hw demand light comes on or is there a delay, if so how long
if red hw demand light comes on as a hw tap is turned on then the hw flow switch and micro switch are working ok
is pump and fan running at full speed as hw tap is opened

 

[arakrazy]

Ok was watching and listening very carefully

The pump and fan seem fine (i.e. full speed at the first sign of demand)

The burner doesn't fire until after that 3-5min delay

Thanks for the help again guys