Boiler tripping on overheat

[Paulrsm]

Hi, hopefully I can get some advice.

My central heating boiler, Potterton Prima Profile 60E, keeps tripping on overheat. This is very regular, a lot of mornings the heating isn’t on because it tripped the night before.

The system is s(?) plan, two motorised valved.

I have checked the bypass valve (15mm gate) and it is well open and I have proved a flow.

This evening I did a few tests.

I wound the thermostat up on both the hot water tank and the boiler and started the hot water system up.

With the boiler very hot and still firing I shut off the program and the following happened.

1, Boiler shutdown (HW valve shuts), pump stops, boiler trips, pump starts on overrun.

2, Boiler shutdown (HW valve shuts), pump stops, boiler trips, pump does not start.

3, Boiler shutdown (HW valve shuts), pump stops, pump starts on overrun, boiler trips.

4, Boiler shutdown (HW valve shuts), pump continues on overrun, boiler trips.

The instructions say the pump overrun will initiate if the water is >80C. They do not state what temp the boiler will trip at although they state max temp is 82C.

My thoughts:

The trip temp and overrun temp are too close together meaning that if the HW valve closes and the boiler shuts down while it is near it’s upper temp range the boiler temp increases to trip temp before the overrun can cool it.
Can the overrun thermostat temp ‘drift’ if the thermostat is old, is this how they fail or would it just not work at all.
Either it’s this thermostat or a fault in the design with the overrun not allowing enough flow, being only 15mm.

Or am I way off the mark?

 

[HERTS P&D]

The first thing to do is fit a new pump and chemical flush.

(X800 for 6hrs then drain down also fit a magnetic filter.)

Andy

 

[Paulrsm]

I ran cleaning solution through it a couple of months back.

Why the new pump?

 

[HERTS P&D]

What cleaner did you use?

Your pump may be working but not a full strength and allowing the the boiler to over heat as it can not move the water around fast enough.

Andy

 

[Paulrsm]

It was either x600 or x800 can't remember. Two tubs poured into a radiator. System run for a week then ripped through each radiator one at a time before draining. Then inhibitor, x200 i think added.

how does poor performing pump cause results 1 and 2 above?

 

[TCCHeating]

Deleted: needd to learn to read properly.

 

[Paulrsm]

Oh, why do you think it isn't the pump thermostat?

 

[TCCHeating]

Oh, why do you think it isn't the pump thermostat?

Because it seams the boiler is tripping even though the pump is overrunning.

Are you sure the bypass valve is ok??

 

[TCCHeating]

Actually just re read your post. Sorry thought you were listing different scenarios at first but i realise now you are listing what happens after continually resetting.

Pump overrun stat as i said in deleted post

 

[Paulrsm]

Yes. It's a bit awkward because all the valves and pumps are under the floor. Squeezed in this evening before my tests and tried to take the bypass valve out to check it (had plugged the vent and feed from tank).
Couldn't get the valve out because the pipes were too rigid so instead closed it and started HW system. when pipes were getting warm (still cold at bypass) I opened it and it was obvious that there was flow, could hear it and bypass pipework quickly got hot.

I know the various results seem to contradict each other.
No 1 & 2 implies to me a thermostat problem either not starting the overrun or tripping too early.
No 3 implies to me overrun wasn't needed when boiler shutdown, water heats up, starts overrun but too late and boiler trips.
No4 implies that stats may be ok but overrun is not doing the required cooling.

 

[wyseass]

Hi, hopefully I can get some advice.

My central heating boiler, Potterton Prima Profile 60E, keeps tripping on overheat. This is very regular, a lot of mornings the heating isn’t on because it tripped the night before.

The system is s(?) plan, two motorised valved.

I have checked the bypass valve (15mm gate) and it is well open and I have proved a flow.

This evening I did a few tests.

I wound the thermostat up on both the hot water tank and the boiler and started the hot water system up.

With the boiler very hot and still firing I shut off the program and the following happened.

1, Boiler shutdown (HW valve shuts), pump stops, boiler trips, pump starts on overrun.

2, Boiler shutdown (HW valve shuts), pump stops, boiler trips, pump does not start.

3, Boiler shutdown (HW valve shuts), pump stops, pump starts on overrun, boiler trips.

4, Boiler shutdown (HW valve shuts), pump continues on overrun, boiler trips.

The instructions say the pump overrun will initiate if the water is >80C. They do not state what temp the boiler will trip at although they state max temp is 82C.

My thoughts:

The trip temp and overrun temp are too close together meaning that if the HW valve closes and the boiler shuts down while it is near it’s upper temp range the boiler temp increases to trip temp before the overrun can cool it.
Can the overrun thermostat temp ‘drift’ if the thermostat is old, is this how they fail or would it just not work at all.
Either it’s this thermostat or a fault in the design with the overrun not allowing enough flow, being only 15mm.

Or am I way off the mark?

Thy setting the boiler temp stat low as possible. should give you max 72degrees, carry out your tests again.
Also only have the bypass valve 1/2 open, else to much hot water may be getting back to the boiler.

 

[picasso]

@ wyseass if you want to reply to posts dont use the quote button, at the bottom of all the posts is a "post reply" button, this lets you reply without quoting.

 

[Tipper]

My old Profile 60 did this. At first I assumed it was a motorised valve fault but it was an intermittently stalling pump. A new pump fixed it.

 

[scoobydo123]

A timed relay connected to the pump will perhaps solve your issue. From reading your tests it sounds like the pump overrun does not cut in quick enough. Are you saying once the demand is switched off the pump is stopping, boiler trips but then the pump overrun starts but this is too late as the over heat has tripped.

With a timed relay connected to the pump you can be assured that the pump 100% will continue to run regardless of what the overrun stat says.

 

[AngelesPlumbing]

Fit a 22mm automatic bypass simple