Boiler cuts out before radiators get hot

[dominoman]

I have recently had the fan and PCB replaced on my boiler. Ever since then (and I don't know if it is related or just coincidence) the radiators do not get hot.

The boiler is a Suprima Potterton 100L, and it is on an open-vented Y plan system with a Honeywell divertor valve and a new Grundfoss pump, which seems to work well.

The boiler fires up normally and I can feel the the flow pipe getting nice and hot. Within a few minutes the return pipe starts to get warm too, but then 5 or 6 minutes later the boiler cuts out. It fires up 5 minutes later and then cuts out again within a few minutes, so the radiators are never more than lukewarm.

I've watched it when it closes off. I get a red flashing LED and green flashing LED (flash around 1 times per second) together for around 6 flashes, then the red turns off and the green stays flashing until the boiler fires again 5 mins later.

Any ideas what this could be? I already replaced the PCB and that didn't fix it.

 

[JohnD]

All of them?

 

[dominoman]

Yes. All radiators. The ones nearest the flow get hot while it runs, but then the boiler cuts off before the furthest radiators are hot.

Placing my hand on the return pipe to the boiler it never gets properly hot before the boiler stops

 

[stardanny]

What's the setting of boiler thermostat?

What setting is the pump on?

Sound like it is not pumping away from boiler quick enough.

Daniel.

 

[JohnD]

Turn off the radiator that gets hot.

You need to balance your radiators.

 

[dominoman]

What's the setting of boiler thermostat?

What setting is the pump on?

Sound like it is not pumping away from boiler quick enough.

Daniel.

Boiler thermostat is just a touch off maximum. The pump is on setting three, which is the highest setting.

 

[dominoman]

Turn off the radiator that gets hot.

You need to balance your radiators.

I don't think it's that because I had them balanced fairly recently.

If the water was all going via one shortcut route I'd expect the return pipe to the boiler to get hot, but it never gets more than warm.

It does sound like a circulation problem. But the pump is new.

Could it be a problem with the thermostat that measures the return temperature? Is there a way of testing if its that?

It's frustrating, as the system seems to be working well now apart from the boiler switching off every five minutes before radiators have a chance to get hot.

 

[Scarum]

Does the boiler heat hot water? If so what happens when it does?

 

[dominoman]

Does the boiler heat hot water? If so what happens when it does?

Yes it does. Exactly the same thing. It cuts out after running for 5 minutes or so, with the boiler return pipe still not very hot.

Eventually the hot water does get hot.

I usually run my system with hot water and radiators on together. I've tried running it on heating only. Made no difference. I haven't tried running it on hot water only.

 

[JohnD]

open-vented

I wonder how much sediment is in it.

Are any of the radiators cold in the middle?

Has it ever been cleaned?

 

[Hugh Jaleak]

Did this problem start when the new pump was fitted, or did it work ok initially when you had the new pump?

 

[dominoman]

Yes - it worked after the new pump was fitted, for around 6 weeks.

The whole system was powerflushed when the new pump was fitted, and the F&E tank was thoroughly cleaned out too. The radiators get hot evenly all over. I bled them all last week and some had air in, but now they are all full.

 

[Hugh Jaleak]

Have you tried bleeding the pump? Still points to a circulation issue, i.e. the heat is not getting away from the boiler.

 

[dominoman]

The pump is a new Grundfos Alph 2 15-60 which apparently doesn't need bleeding.

Just in case there was air in it or a blockage I disconnected it tonight and took it apart. It was clean and seemed to work perfectly.

When I started the system from cold it stayed on a lot longer, but as soon as the water returning to the boiler is at 40 degrees or so (I am measuring by touch so not very exact) the boiler switches off.

I'm thinking now that this could be a faulty temperature thermistor?

 

[StudWall]

Turn your boiler stat to minimum then turn the heating or water on until it cuts off, increase boiler stat until it cuts in, wait till it cuts off etc until you reach maximum ... Tell us what you find