boiler overheating????

[Slugbabydotcom]

Pump over-run stat is next on the list.

i,ve just replaced my pump

And there was me feeling guilty cos I thought it was the high stat Never mind, many engineers habitually change all the stats as a matter of course as they are relatively cheap and once one has gone the rest may soon follow. <<< Saves callbacks. Likewise eliminating the pump may not have been a bad idea as they don't last forever either.

in the summer i had 2 radiators off for decorating.....after they were put back on.....my other rads seemed to be out of balance, so i balanced them up so they were working ok...could this have anything to do with the boiler overheating problem??.maybe its started cutting out since they were off??

Unlikely .... You need to eliminate the POR stat 1st

why doesnt my boiler cut out on LOW....is it because its not as hot???

Yes
The limit stat kills the pilot when the water it senses is too hot. The solution lies in finding out why the water is too hot.
My strongest theory is that the pump is turning off too early as the POR stat wrongly cuts off its power supply before it should.

Is there any chance that when you changed the liimit stat that you accidentally put the same one back in?

 

[ChrisR]

You could eliminate the POR stat by hot -wiring it so it's always on. Not efficient to leave it like that, but free and would prove the point. Just move the wire in the boiler...

 

[barny3565]

having pump overun stat fitted at weekend......my old pump was getting very hot and now the new 1 is on its just as hot, making me think old pump was actually ok....never mind
i,m hoping pump is hot because of POR......iff not could system be blocked???
wiill let u know how i go on with POR....cheers

 

[barny3565]

well i,ve changed the pump overun stat......and i still have the same problem
i,m reluctant to give up yet as this as been a good boiler and still is i hope.
any more ideas plz

 

[Slugbabydotcom]

Ok you have eliminated the favourites now its time to look further down the field.
You seem to relate the problem to removing and refitting those rads. Balancing them down could reduce the flow to a nearby radiator which was acting as a by-pass. Work through the following so that I and others who read this can be more familiar with your system or hopefully the answer will jump out and slap you in the face along the way.

How far away from the boiler is your by-pass valve?
Has anyone been playing about with it?
Is it adjusted correctly to give you 11ºC temperature difference between flow and return pipe at the boiler? You may have to invest in one of those laser temp sensors that they sell in maplins for £30-£40 or a pair of pipe stats
Is the manual vent [if fitted]near the cylinder cold when the system is running? It could need bleeding!
Is the pump wired to the boiler properly so that it runs from the POR?
A thermocouple that is on its way out; can work, or not when it feels like it. These are not as cheap as other thermocouples because they have the interrupter in them
Has the correct value limit stat been fitted? It is rare but not impossible to get new parts that are faulty/ wrongly supplied.
When you took those rads off was there a lot of sludge in the system?
Was the system water rusty red, black or clear?
Is there a lot of kettling when the boiler is in operation?

The question you need to keep in your mind is. Why is the water near the limit stat too hot 2 minutes after the boiler has shut down

All good things come to an end so it's now time to remind you that your dreaded worst case scenario is that your heat exchanger is blocked with crud, lime or whatever.
A system design fault such as one I saw the other day where a CH and hot water system shared the same header tank would be spotted by an engineer.

 

[barny3565]

hi slug
i will run through all the things u have gone through, t o be honest i dont haver the expertise, to check through all the list, but i have a friend to help me, but it will be after xmas now.
i will let u know how i get on as soon as i can.
once again thankyou so much for your help.....i will be in touch soon.
merry xmas

 

[barny3565]

just a cpl of questions i,m hoping u can help me with while i wait to get this thing sorted 1 way or another.
1. is there anyway my room thermostat could be faulty?, i,m asking this because when i put my heating on HiGH my rads seem to get very hot before boileer cuts out even on a setting below 20c on thermostat.
2. i opened all my rads on both valves to see wat happened, my heating stayed on for about 5 hours, without cutting out. it eventually did cut out, but maybe im on to something? the next day i tried again bu2 rads were flat cold, so had to balance them out again.

 

[Slugbabydotcom]

1. The boiler should run in a stable fashion even if there was no room stat.
It's a boiler stat that controls the temp of the water in the radiators. In this case the Hi stat when Hi is selected.
2. You could be on to something but not anything that I havent already covered

my heating stayed on for about 5 hours, without cutting out.

Points towards the by-pass issue also with the weather we have just had it would take longer to reach a lock out temp situation

2 rads were flat cold, so had to balance them out again.

Points towards sludge in the system however by opening the valves at both ends you did unbalance the system so this could be a red herring.