Potterton Puma 80 keeps losing pressure

9 Oct 2004
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United Kingdom
I have recently fitted a new pressure relief valve and pressure gauge. Upon opening the fill tap, the needle moves much faster than the old one and when the CH fires up the gauge moves quickly into the red. I release sufficient to 1.5 but it quickly goes into the red again. I have to do this 4 or 5 times before it remains at 1.5 to 2. However, every time the CH is off, within a couple of hours the pressure is at zero. There are no obvious leaks anywhere, not even the pressure relief valve. What else could cause this??
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A non-obvious leak!

Check your pressure vessel precharge.
Drain enough water (not through prv) to release all pressure,, and a bit more. Pressur (aka expansion) vessel at the back should be at 10 - 15 psi (use a car foot pump by hand).
Refill system to 1 bar

Now the expanding water has somewhere to go the pressure shouldn't shoot up so much.

It has probably been leaking through the prv - it evaporates when hot.
The clue is in the answer:
10 - 15 psi (use a car foot pump by hand).

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what I meant was how do I change it if its not at 10-15psi??
after you check the pressure with the system press released it should be between 1/1.5 bar
if not pump it up there is a car type valve on the vessel
back to my boiler after 2 days break and seems i have another problem in that the ch fires up for about 30 secs then goes off. hot water ok, pilot ok. pump seems hot to touch, could it be its jammed and thats why the burner goes out. Sorted pressure thanks for that
SOunds likely. Undo the screw in the end, it dribbles. Usually withthem the bit you unscrew is the bit you can turn the shaft with - see if it turns. If so, switch on and you'll see shaft turniing. Turniong pumps don't always pump, but usually!
right, sorted. Forget all that about pump business, discovered someone left hot tap dripping in bath, just slightly but obviously enough for boiler to give demand to hw supply. problem solved there, BUT pressure still goes into red and has to be released a few times while ch on, and when off returns to zero when system cold. faulty valve somewhere in expansion vessel maybe???
Try isolating boiler from system and drain boiler.When boiler empty of water disconnect expansion flexible hose from boss at side of pressure release valve.Whilst holding flexible hose over bowl or bucket remove slug of sludge from elbow on flexible hose with small scew driver.Also clean out boss where hose connects onto. re assemble,repressurizeand check.
one thing after another. heating ok apart from pressure problem but hot water demand causes shut down. got gradually worse and even with ch off it doesnt work. what it does do is heat the flow pipes for ch so guess my diverter valve has had it. could that cause shut down after 30 secs of hot water demand. can it be repaired or better to fit new one? is it easy job for a better than average diy plumber?
Now is the time to call in the heavy team. Pick up the BIG yellow book and look under Central Heating. Man comes out and LOOK at his CORGI card. Turn it over and look for tick against central heating. Ask him if you can watch and set him loose.

If he knows your boiler, repairs will be painless.

Majority of the times boilers stop functioning because they do not get serviced. Problems referred to in your post would suggest this. EV is one of the first things that gets checked during service.

Now that you are thinking diverter is faulty, it is going to start getting expensive with no light at the end of DIY tunnel
Not much help DP. BIG yellow book full of rip off merchants and cowboys, 2 weeks ago man from respectable looking ad in BIG book came and fiddled with my thermocoupler, took it off and said 'theres yer problem mate'. said he would go and get new one from nearest dealer, over half hour each way. total cost for £5 thermocoupler wouldve been over £200 if I hadnt told him where to go. 1 week ago man from same book, nice ad comes and says, with hands not left his pockets, ' give it up mate, 9 years old? you need a new boiler, I can supply and fit for £1400.' goodbye I said. All I want is for some decent honest plumber to look listen fiddle and genuinely say what problem is and how much, like you do when your car plays up... (garage, not a plumber though)
Anyway, I let CH get cold and with it switched off I opened up hot tap, burner fired up, 30 secs hot water flowing down correct pipe but flame out 30 secs later. this time diverter valve seems ok

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