Grundfos Pump

13 Jan 2005
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United Kingdom
I have recently replaced my grundfos circulating pump because the upstairs rads did not stay hot even after bleeding the system fully. The rads only stayed warm at the bottom when you switch on the pump again. The domestic HW also did not get hot from the boiler, only the immersion. One of the flow/return pipes to the tank is hot (bottom pipe), the upper pipe is cold. When you bleed the system, the top of the rad gets hot and the upper pipe to the HW tank. Switch on the pump again and after a while the top of the rad etc, cool down.
I have changed the pump, but it has no effect.

I think I may be at the max height for a 6.0m head circ pump.????
Any ideas.???
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sounds like you are bleeding the collected air out then switching the pump on and freeing some more air through the system, is it noisy in the pipes when the pump fires up?. Try stopping and starting the pump until you stop hearing air rushing through the pipes then bleeding it out the pipework and rads.
There is no noise at all, the pump sounds fine, and it does not matter how many times I stop/start pump and bleed system, it reverts back to the upstairs rads beeing cold at the top and no domestic hot water.

The height from the pump to the upstairs rads is just on the 6.0m limit, so I am wondering if a higher lift pump would be the answer.??

Should I have an automatic vent in my piping up at the expansion tank??
Regarding your radiator. I would think you have a restriction. This is preventing the water circulating through the radiator so it stays cold. As soon as you open the bleed valve and run a little water off it is replaced with hot water through one of the valves so the rad gets hot but then soon cools down again. The restriction could be sludge in the flow or return pipe, if water can't circulate through this pipe circuit then it can't enter or leave the radiator.
:rolleyes: :rolleyes: :rolleyes:
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You need to find out why you have no hot water.
You say pipe into cylinder is hot but the other exiting is cold. This suggests a blockage or pump failure.
Turn off your CH so that the boiler is only heating the DHW. Your 3 port valve should be in DHW mode, and check the pipes going through the cylinder to see that they are hot.

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