Potterton Puma 80e Overheat on DHW

[GeeTee]

Hi

Into my 4th year keeping this lump running thanks to you guys' help

Latest problem is a very rapid overheating (kettles, steam blows out of AAV) followed by lockout when running DHW. The CH is working absolutely perfectly.

Similar happened a couple of years back and turned out to be badly scaled DHW side to the heat exchanger - all solved with a pain in the a$$ job to get the HX out and some descaler.

Before I go about taking the HX out again (and I'm not asking about that as I know the forum rules have tightened up recently) a question....

Is it possible for the DV to stick in such a position to cause this? I can kind of picture it in my head - the way the wax head expands to close off the DHW loop to force the system water around the CH circuit. But in DHW mode could it maybe not retract enough and block the system water entry into the DV from the HX?

I know we all love diagrams so below is one I prepared earlier I can't remember where the DV plunger seals against - is it against the narrow section of the DV body, on the wax cap side of where the system water enters from the HX? If so then there would be no way it could block the output to the CH at all, so no way it could cause an overheat.

Only reason I ask is that the CH pressure switch (which attaches to the small lug on the DV) failed not so long ago because it was blocked with black crud - and just thinking the DV way be sticking in the body for the same reason.

Just wondering if it's worth the few minutes pulling the DV apart before getting stuck into the major dissasembly required to get the HX out really

As always - apologies for the stupidly long post for a simple question - I have typorrhea!!

Thanks
Graham[/img]

 

[baxpoti]

Is the DHW switch 'sticking' on after the hot tap is shut off?

Check the red led on the mod board, should go off straight after the tap is turned off.

 

[GeeTee]

The overheat occurs while the DHW is still running (tap open).

The switch does move properly when the tap is turned off and the red light on the board goes out as expected.

 

[baxpoti]

Does the boiler modulate?

 

[GeeTee]

While running DHW, in the short time before it overheats, if I turn the HW temp control down the flames reduce in size and vice versa. Beyond that there doesn't seem to be any kind of modulation i.e. it fires up with a certain flame size which remains constant until it overheats.

But I seem to remember that's how it has always been. But then my memory may not be that great Should it be modulating down as the outgoing DHW temp increases towards the set point? If so it's not.

Ta
GT

 

[GeeTee]

OK embarassment embarassment

The DHW switch was sticking on intermittently. It seems the diaphragm / rod assembly is getting lazy - i.e. it's not moving very much at all. I've adjusted the switch out a bit and it seems to be making and breaking reliably now. But I'm sure not for too long.

Turns out it was only overheating after the DHW had been turned off.

Also noticed the DV is letting by while on DHW (scalding hot CH flow 3 feet away after about 30 seconds). I replaced the wax cap on this a couple of years back as it was doing the usual no / cool DHW thing. Fixed that problem, but I'm suspecting general filthiness of the system as a whole is underlying a lot of these problems.

I did run X400 through it for 4 weeks when I changed the DV wax cap and got loads of black crud out and it has had X100 in since - however it clearly is still far from clean

Time for a proper clean me thinks!

Thanks for the help - much appreciated.