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Replaced a radiator in bathroom with towel rail which is 12 inches taller. Now get air in the hw/heating system. Raised the header tank for the heating by 14 inches, turned the Grundfos Selectric pump to lowest setting and managed to bleed air from the system and it runs silently. When turned up pump to medium setting it immediately draws air into the system again and on high sucks it in faster. Pipes gurgle and upstairs rads draw in air. There are no leaks only a vent pipe above the header tank and one above the static tank. There is no apparent other way in for air. Turn the pump to lowest setting and the air can be bled out and system is quiet again. Any suggestions for cause/cure would be most welcome.

The problem is where the cold water feed and the expansion pipe connect in to the heating circuit. If the pipes are too far apart, then either air is sucked down the expansion into the circuit or the reverse, water is pumped out of the expansion. The distance between the two must be no grater than 150mm.

To prove the point, stick your thumb over the expansion pipe then get someone to turn the pump on. No doubt you will feel a vacuum suck your thumb!

Why do you think air is being drawn in? you have added a new towel rail, so there will be air in the system, it can take a while to get rid of the air. Altering the pump settings will cause different noises until the air is cleared.

Where is the header tank? in the loft? why have you altered the height of the tank?

For htg engineer: When the pump is on low setting air is clear and system runs so silent you don't know its on. Turn the pump up one notch and air is drawn in. I know this because one of the rads upstairs partially empties and when bled a lot of air comes out and also air the gurgles loudly through the pipes. Put the pump back one notch and then system can then bled clear of any air noise and the rad stays full.The header tank is in the loft and when the pump is on second setting air bubbles back into the header tank. I raised the header tank because when a new boiler was put in some years ago I experienced the same problem. The engineer came back and raised the header tank which cured the problem whatever setting the pump was on. When I first changed the rad for a towel rail which is about 12 inches higher I was getting the air problem on the lowest pump setting. I assumed the problem was similar so raised the header tank. This improved the situation so the system became noiseless on lowest pump setting. I know I can continue to run on the lowest pump setting without problems but there must be an answer I would like to find. I have yet to get in the loft and measure the distances described by 'plum bob', but I am very grateful for your interest and would like to hear your views. Thankyou.

Is this a gravity HW system? which way is the pump fitted (from/to boiler)?

htg