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Ask Jerry Your Own Question
Jerry, HVAC Technician
Category: HVAC
Satisfied Customers: 422
Experience:  Epa universal certification, hearth products specialized.
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Everything functions when first fired up but hot water to heating

Customer Question

Everything functions when first fired up but hot water to heating system does not circulate. Only get one cycle before (overheat?) shutdown, boiler just sits with demand light running. Grundfos pump is working, have checked impeller, tight on shaft and not blocked up. All radiators have been bled. Is it possible/probable that there could be an airlock elsewhere in the system? Hot water is fine throughout the house. Any clues would be helpful thanks.
Submitted: 4 years ago.
Category: HVAC
Expert:  Andrew Smith replied 4 years ago.
Have you had the plug out of the pump when the system is one just to make sure the pump is spinning?
Customer: replied 4 years ago.
Yes, it runs at hi speed lo speed and switches off on its on board switch.
Expert:  Andrew Smith replied 4 years ago.
It could be, are you getting heat from the boiler (trace the pipework from the boiler and see how far it goes?)
Customer: replied 4 years ago.
Heat exchanger runs initially and heats water along approx fifteen inches of pipe towards a 90 degree downward bend. (This boiler is in the roofspace of the house to simplify pipe work and has been running ok for years). Presumably because something is preventing circulation the heat exchanger shuts down when water temp too high. All rads bled three times no air pockets, what else can prevent circulation??
Expert:  Andrew Smith replied 4 years ago.
If the pump was running correctly I would expect much more circulaion, is there a speed selector on the top?
Customer: replied 4 years ago.

I think i already answered that!!!

Expert:  Andrew Smith replied 4 years ago.
The pump should run at a constant speed, is it not the fan that runs at 2 different speeds?
Customer: replied 4 years ago.

Pump seems to run at constant speed . It has three switch settings, Off, low speed and high speed. it is set at high speed and always has been. I have no way of knowing if the speed currently is correct to the manufacturers spec.

Expert:  Andrew Smith replied 4 years ago.
With the cover off, can you tell if it is the fan that is changing speed?