CH Design question - and potterton tip

[David P]

I'm trying to understand my very unpredictable (old) potterton Puma combi and could do with having a very basic question answered.

3 upstairs rads 3 downstairs. the one in the bathroom is small and has a short pipe underneath that bypasses it, with an inline valve that was set only partially open. this pipe runs from rad feed to rad out directly beneath the rad.
question is why, exactly is it there? I guess it is so there is return flow even if all rads are closed but if so why the valve?

And the reason im asking is that this combi seems to mistakenly believe that the whole system is hot sometimes, when only the bathroom rad is luke-warm.
I have discovered an obscure prob with these combis that goes something like this: the HW out passes through the CH distrubution thermostat, so that whenever HW is drawn it cools the CH thermostat and cuts the CH. until the HW stops then it fires up again.

the only thing is that the thermostat is far too sensitive, so if the HW tap is dripping (like 1 drip a minute) the CH thermostat believes that HW is being drawn and wont kick in.

It took a lot of detective work to figure this out - but I have since seen it posted on other boards - so if your potterton puma is refusing to deliver CH - check your hot taps for drips.

my current prob though is far weirder - sometimes all rads get hot - then they wont for hours - just as i make up my mind to rip it out and get a new one it behaves for a day or so. recently it seems it wont supply CH until the timer has turned on CH for 3 hours or so - its a real pain when you get back to a cold house in the evening.

any ideas on either the design/pipe thing or what may be wrong with the system would be gratefully received.

thanks in advance
David

 

[kevplumb]

the pipe is a bypass to give a minimum flow through the boiler hence the valve to stop all the heat going in a circle

dripping tap bit is common

if the bathroom rad wont heat properly open the locksheild valve a 1/4 turn and make sure its vented(bled)

timer problem pass

 

[David P]

thanks kevplumb
suspected as much with the valve - perhaps its too open, and thats why the other rads arent getting hot sometimes.
whats the usual amount of opening for them?

 

[kevplumb]

minimum 9 litres per min i think

rule of thumb close it and open 1/2 a turn and see how it goes if the boiler gets noisy open it a bit more

do you have the book