New pump but boiler stops firing before room temperature is reached.

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Hi. After the combi boiler pump failed I was feeling pleased with myself for flushing the CH system and changing the pump head (Grundfos UPS2 15 60). I would have liked to have changed the whole pump because the pipe was a bit cruddy but access to the nuts was difficult. Overall though the drained water was very clear so I didn't bother with any cleaning fluid, just inhibitor.

It was a relief to have the heating on and get hot water again. However even with the room thermostat set to 30 the boiler switches itself off well before then (I can hear the thermostat dial 'clicking' at 20). It fires up again after a while but only for a few minutes before switching off. It's maintaining warm but not hot radiators. All have been bled and TRV valves are fully open. The radiator and hot water dials on the boiler are on max. I don't have a thermometer but the flow and return pipes are similarly warm to touch. The hot water is working okay. Does the pump drive HW and CH or just CH?

Before changing the pump I hadn't noticed a firing up problem but I knew the radiators weren't getting hot and assumed it was all pump related.

The boiler is a Halstead Finest Platinum (12 years old). I know it will need changing soon but I'm trying to eke it out through the summer. Any thoughts on the likely cause and what I can do to resolve it? Thanks.
 
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I would love to make a suggestion but that is difficult without knowing what the flow temperature is.

Ideally the temperature in degrees. But failing that a hand touch comment, like can only keep hand on top of radiator for two seconds.

The pump operates for both CH and DHW.

When was boiler last serviced?

Tony
 
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It was last serviced three years ago (I know!). I can keep my hand on the pipes indefinitely, they're not too hot to touch.
 
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How did you flush the system with a flushing machine or chemicals and a hot flush
 
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I just emptied and refilled twice using the filling loop. I couldn't believe the water was so clear.
 
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I'd like to buy a thermometer to see what temperature the pipes are. Is any digital thermometer (as found in Boots or kitchenware departments) okay to use? Thanks.
 

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If you're lucky enough to have a good-quality multimeter, it may have a socket for a thermocouple. They are simple and fairly cheap.
 
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I'd like to buy a thermometer to see what temperature the pipes are. Is any digital thermometer (as found in Boots or kitchenware departments) okay to use? Thanks.

I use a digital laser one. It was about £15 from Screwfix. I believe it is quite accurate. Works a treat on white painted radiators.
 
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I use a digital laser one. It was about £15 from Screwfix. I believe it is quite accurate. Works a treat on white painted radiators.
But will be waaaay out if pointed at bare copper pipe. The trick is to wind some insulating tape round the pipe (any colour) and hold the IR gun close to the tape.
 
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How did you establish that the pump needed replacement? A symptom of a faulty pump is the boiler running until the water in it gets to the preset boiler stat temp and then shutting down. Without the pump running the water in the boiler is more or less dead-headed. The water in the boiler will heat up rapidly but the heat will only spread slowly through the water because there is no flow, so the rads will not reach working temp before the boiler cries enough and shuts down.

It sounds like it may not have been the actual pump but maybe an electrical fault that caused the initial fault and is still there.
 
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Sorry, thought replies had run out on this one

therambler - a plumber told me I needed a new pump. I could hear him bashing it which got a couple more weeks out of it but after that it stopped working altogether. After I put the new one in everything fired up again. If the hot water heats okay but the central heating doesn't reach its temperature, does that suggest any particular fault? Thanks
 
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If the hot water is OK, and the boiler fires up in the normal way, the problem is likely be one of poor circulation. The boiler fires, the flow side heats up, but the water can't circulate properly, the difference between flow and return temperatures at the boiler is outside the normal range and the boiler shuts down. Flow side cools and off it all goes again. Try and see what the difference in temperature is between the flow and return pipes, at the boiler, when the boiler shuts off.
 
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To give you proper advice we need to know the flow and return temperatures!

Tony
 
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Thanks for the quick reply. Could you explain the theory of flow and return temperatures (it's a combi boiler)? What should I be looking for and is it anything to do with balancing the system? I've not been able to work out the order in radiators heating up.
 
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Ok, I haven't bought a thermometer yet. At the risk of ridicule the flow pipe is 'quite hot but not scalding' and the return pipe is 'warmish'!
 

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