Central Heating tripped out

[sfj]

Boiler.. Glow worm 38 hxi
Domestic system using OSO unvented pressurised system
High and low pressure sensors set apart from main boiler.

I have dropped the pressure in the system whilst bleeding radiators. Despite increasing the pressure to 2 bars the boiler will not relight. I think the low pressure sensor has tripped but I do not know how to reset. The pressure valves are Johnson Controls P735 AAA.

Can anyone help or is it going to be a cold Easter?

 

[gas4you]

Try draining boiler and re-filling again. Could be air locked. Have you made sure auto air vent cap is loose?

 

[sfj]

Thanks. Not sure which is the auto vent cap? The whole system has been trouble free until I took off the rads. I thought the boilers had low pressure cut outs within.. why would I have separate low and high sensors in line? Any idea how I could reset them if they were the cause? Draining the boiler could test the plumbing skills! Many thanks

 

[gas4you]

Just re-read your post and note you have an HXi. I misread this somehow as an SXi

You will not have a low pressure sensor on this as it is an open vent heat only boiler.

 

[Livesgas]

Was the boiler running when the pressure dropped? If so it may have overheated. Remove the front panels and you will find a overheat stat next to the heat exchanger. If its overheated the pin will have poppped out and you will need to push the pin back in to reset it. The boiler should then run
Thanks

Livesgas

 

[gas4you]

Have you got an auto by-pass valve fitted to your system?

Put your pump on setting 3, these HE's need a fast flow through them.

 

[Livesgas]

These boilers being heat only boilers dont have any low pressure cut out. So if your pressure drops the water fails to flow through the boiler and it overheats. There only safety device is a high limit (overheat) stat that needs to be manually reset. Stupid i know seeing as its inside the boiler.

 

[sfj]

Thanks gents for your help.. but not solved yet!
Have found the stat and all looks well.
The boiler was not running when the pressure dropped but would have tried to come on on timed.
Am I right in decribing these Johnson controls as pressure switches?
From your comments should they be fitted to the system or just making things difficult?
Perhaps a plumbing course is called for!

 

[gas4you]

Sorry, I haven't a clue what you mean by 'Johnson controls'

There is always a chance that your HXi has been fitted with an external pressure vessel to make it a sealed system. Have you got a large round red 'football' sized metal vessel anywhere?

 

[sfj]

A chilly night tonight!

Yes the system is sealed and the red pressure tank is present. It is this that had the pressure dial that fell and has now been restored.

The Johnson Controls refer only to two guages (blue boxes about 3 X 3 inches) that each have 2 ranges on them ... 1 "pressure" and the other "diff". The installer wrote "high" and "low" above them about 5 years ago!. Are these causing a problem ...should they be in the system?
If its important the water runs a pressurised OSO unvented tank.

 

[gas4you]

Any chance of a pic of these boxes?

 

[sfj]

Hi again.. have uploaded a pic to my profile... I think you can access this?

 

[gas4you]

I have never used or even seen these before

They look like a type of pressure sensor or flow meter, obviously wired into the box above.

I think it could be safe to say it will be one of these causing the problem.

 

[sfj]

Ok Thanks very much for your help
SFJ