Potterton Suprima 70 & Myson Power Extra Valves

11 Jan 2006
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United Kingdom
Hi guys

Anybody shed any light on this please...?

I've just had a new pcb fitted to my Potterton Suprima 70 (as seems to be the norm after a period of time with these things...!)

Boiler now fires up when required and heats hot water and heating well...

However, when I have either hot water only OR hot water and heating selected to run, the pump seems to run for a period of about 5 minutes, then stops for about a second then starts again.
The pump never seems to stop running during the whole 3 hours or so I have the heating or hot water timned to come on....just a continual cycling of 5mins (ish) on...2 secs off.

I have 2 Myson power extra valves. The central heating valve seems to be operating normally, cycling on/off for the heating demand.
The other valve (return valve back to the boiler (I presume) is constantly energised (LED ON always).

Should this valve be cycling ON/OFF as well, thus sending a signal to the pump to stop - or is it normal for the valve to be permanently ON and the pump cycling ON (5mins) Off (2 secs)...?!!

It sounds like a faulty myson to me...but would be grateful if anyone could give me a heads up on how the system should run before I get BG out.

Many thanks guys!
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It all depends on the system controls that you have!

With the correct full controls on heating the boiler would cycle on/off at about 4/6 minutes and the pump run until the room stat is satisfied then everything would turn off ( after a short pump over run. )

On hot water, the same thing, the boiler cycling on a shorter time constant `until the cylinder stat is satisfied. Check that the cylinder stat ( if fitted ) is set to about 55°C.

Thanks for the reply Agile

I have the tank thermostat set to 60 degrees.
The room stat is set to 20. Not entirely sure if it works though...!

If we took heating out of the equation, so the boiler was only producing hot water for the tank for taps etc, the 5mins on/5 secs off cycle of the pump continues for the total duration of the time hot water is timed on (ie:3 hours).

This suprises me, as I would have thought that the boiler should heat the tank up, then if not demand is made for hot water, turn everything off until either a) a demand is made , ie:eek:peneing a hot water tap, or b) the temp of the tank drops below the stat level and it needs topping up with hot water again.

...Confusing this...!
In that case it seems as if your cylinder stat is not switching off!

Or possibly its not correctly located


The boiler flow temperature is too low to let the cylinder reach the set temperature.

As I suggested turn the cylinder stat down to 55° and perhaps increase the boiler temperature a little.

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Suprima's always cylcle the pump when starting the ignition sequence so dont worry that the pump, starts stops.

As Tony suggests, tank stat down to 55, boiler stat up to the 2 or 3 o'clock position
Tried that....

no difference though.... :confused:

Don't get me wrong...the only reason why I think this cycling is odd is that it happens continuously for 3 hours.

The pump runs for approx. 5mins then stops for about 2 secs...then runs for another 5 mins...off for 2 sec....all the time that the timed cycle is active on the wall programmer.

...it just doesn't seem like it should be doing it...!!

How hot is you HW out the taps??

Could be a number of things, Tank stat, boiler thermistor, restriction in HW circuit.

Get out your friendly BG engineer, they will be happy to help solve your quandry!

Thanks mate...will do!

The hot water is plenty hot enough.

To be honest....I'm not as interested in fixing it myself, but more interested to know if "if it needs fixing" as it doesn't seem normal to me!

Anyway...if anyone else has any ideas, I'd be most grateful!

so far, I've had British Gas around to the house...FOUR times!!

Yup...going for the world record I think.

Problem seems to point to the PCB (for those who may have a similar problem)

The pump is being signalled to stop running momentarily for a second or two every time the boiler is signalled to fire up on a heating cycle.

New board (last one replaced 3 weeks ago) is arriving on Monday apparently.
We'll see.
1.Check the pump feed and switched live are correctly connected.

2.The Myson 2 port valve 'body' may be stiff and keeping the actuator held open.
Don't the mpe valves have an indicator light on to signify a demand at the valve?
Still think that its a system control issue and not the PCB.

I would be thinking Tank stat or as baxipot says stiff body of valve.

However I have been wrong in the past...not very often tho mind :LOL:
This is really frustrating...!

New replacement PCB fitted.

Fault still occurs.

British Gas called Potterton and eventually spoke to a "senior field engineer" who said that the system is designed to operate that way...(?!!)

I said to the BG guy: "so a system is designed to shut off a pump every time a call is made to the boiler to fire on a heat cycle...every 5 mins or so...?! Isn't that going to wear out the pump quicker than usual...?" ; He said not.

To be fair to the Brit Gas guy, he seemed foxed as to what to do next. His opinion was that the only reason why the pump is turning off is because power is being lost to the pump (which he confirmed) everytime the boiler is commanded to fire up. The only thing that provides power to the pump...is the PCB...!!

I've just had a chat with next door, who have exactly the same system with the new/improved PCB that these Suprima 70's need. They are not aware of their pump turning off momentarily every 5 mins for 2 seconds - so it can't be "normal operation" as Potterton suggest.

baxpoti / superdupergasman

I've taken the Myson power valve top off and turned the valve myself. It seems free & easy to move. Is that what you were referring to that may be stiff? This valve only cycles off when the stat on the tank has reached set temp (60 degrees).

Any more ideas would be really welcome as British Gas have effectively washed their hands of it...but I'm certain it ain't right...!

Thanks guys
If the cylinder stat is turned down there is no demand to the valve, the indicator light goes out and the pump over-runs for about 5 mins and then stops....how hard can it be to stick a multimeter on it and trace where the power is coming from when the demand is satisfied? (if it is)

Sorry - I'm not with you? What are you saying then?!
When the tank fills with water hot enough to turn the stat off, the power valve closes and the pump overrun runs for a period of time as you describe...

This cycling problem I'm referring to happens prior to the tank reaching the stat temperature.

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