Potterton Puma 80- pressure problems

[kat23]

Hello,

I have a Potterton Pum 80. Its a good age and until now, has been quite trustworthy. However, even though the central heating and the water are hot, the pressure goes off the dial. Its meant to be around 2-3 and as soon as the central heating goes on, it sky rockets to over 4 in about 20mins and then drops back to the top end of 3 when off.
Can anyone help or suggest what it might be so we have an idea what we're dealing with. I just don't want to be ripped off by anyone who comes to have a look at it.
Cheers Kat

 

[Gasguru]

Presure should normally be between 1 and 1.5 Bar with the radiators COLD. When rads are hot pressure should ideally not be higher than 2 bar.

Read FAQ's on expansion vessels, safety valve will also need replacing/cleaning and filling loop disconnected.

 

[bengasman]

If it goes anywhere over 3, you have more than one problem. automatic safety should prevent going over 3. Apart from that your expansion vessel is not working. Cold pressure should be around 1, certainly not 2 or higher