Central heating switches off after 30 minutes - why?

[stephc]

Hi, I have a problem with my central heating. Basically, even if I put the thermostat up to 30, the heating switches off after around 30 minutes or so. The house is only luke warm at this stage hence I need it on for longer. Any idea what is causing this? I did previously have an issue with air in the system but think this is fixed now - could there be any link to that? I have a regular boiler system with a tank by the way.

Initial thoughts are that the pump is on the way out and needs replacing as the living room radiator only gets warm if I turn all the other radiators off. Even after balancing the living room radiator is still cold and only gets warm when others are completely off. I'll try putting the pump on max tonight and see if that helps.
I read that the pump is supposed to get warm - mine however gets very hot and you can only touch it for half a second.

Many thanks.

 

[JohnD]

pump ... gets very hot and you can only touch it for half a second.

obstructed circulation; or pump jammed; or air in pump; or pump faulty.

turn it all off, take out the screw in the middle of the pump end, insert a long small screwdriver, see if you can rotate the pump spindle easily. A few drops of water should drip out - no air should come out.

How old is it? Post some photos of the pump. How deep is the mud in your feed and expansion tank? When you bleed the highest radiators in the house, does water squirt out forcefully?

What did you do that seemed to have cured your air in system problem?

 

[stephc]

obstructed circulation; or pump jammed; or air in pump; or pump faulty.

turn it all off, take out the screw in the middle of the pump end, insert a long small screwdriver, see if you can rotate the pump spindle easily.

How old is it?

Thanks I'll try that. No idea how old but it does look old. There is no screw to bleed it, it has a hexagon shaped cover. I just unscrew that?

 

[JohnD]

no.

post a photo.

 

[stephc]

no.

post a photo.

I will take a snap when I get home.

The air in system was fixed by bleeding a valve in the airing cupboard on the tank and by bleeding the rads.

 

[stephc]

Here is a photo

 

[JohnD]

OK, in your case, yes, the hex head does seem to be the central screw I meant.

Have you ever turned the pump valves above and below the pump?

 

[stephc]

OK, in your case, yes, the hex head does seem to be the central screw I meant.

Have you ever turned the pump valves above and below the pump?

No but I did wonder about these. What do they do?

 

[JohnD]

They allow you to isolate the pump so you can remove it for maintenance or replacement without needing to drain down.

Unfortunately when they have been untouched for many years at high temperatures they may be seized, jammed or scaled up and fail to work properly.

Again, how deep is the mud in your feed and expansion tank?

Have you given the system a chemical clean yet? If not, that is one of the easiest and cheapest DIY plumbing jobs and worth doing on an old, open-vented system that's performing badly.

 

[stephc]

They allow you to isolate the pump so you can remove it for maintenance or replacement without needing to drain down.

Unfortunately when they have been untouched for many years at high temperatures they may be seized, jammed or scaled up and fail to work properly.

Again, how deep is the mud in your feed and expansion tank?

Have you given the system a chemical clean yet? If not, that is one of the easiest and cheapest DIY plumbing jobs and worth doing on an old, open-vented system that's performing badly.

No I haven't tried a chemical clean - I'll google for a guide. I'll have a look in the f&e tank tonight.

 

[JohnD]

please also check your highest radiators to see if water squirts out forcefully when you bleed them.

 

[JohnD]

https://www.diynot.com/diy/threads/...snt-flow-when-heating-on.489033/#post-3989998

 

[DaveHerns]

Which room is your thermostat in ?

 

[stephc]

Which room is your thermostat in ?

living room

 

[stephc]

please also check your highest radiators to see if water squirts out forcefully when you bleed them.

It doesn't come out with great force. More than just a dribble though, it goes up in the air before down. Downstairs rads are just a dribble.