Pump Pulling Air


Postby Whitcop » Thu Dec 09, 2010 3:31 pm

My heating system was installed in 1981 when the bungalow was built. It consists of a wall mounted Ideal Concord WCF 255A boiler with the flow and return (28mm) going up into the loft where the pump and 3 motorised valves are fitted (I think it was called an S plan plus). This allows separate temp and timer control of the living rooms, bedrooms and HW. In the attic a feed / expansion tank is fitted approx 1.2m above the pump and valves. There is a 15mm cold feed from the tank that tees into the 28mm return just above the boiler and a 22mm open vent (over the tank) that tee’s off the 28mm flow just before the pump.

The original pump was changed for a Wilo 15/50 pump about 7 years ago. Over the last few weeks I’ve noticed that the pump seemed to be getting very noisy so last week I changed it for a Wilo Smart pump and I’m running it on it’s slowest setting. The hole system is very noisy and I’ve noticed that the pump is pulling air in through the open vent pipe (I held a container of water over the end) and air is collecting in the rads which need bleeding each day. Any ideas why this should start happening? Can I blank off the open vent and allow the boiler to vent back through the cold water feed pipe?

Thanks

Whitcop
:(
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Postby plumbbob » Fri Dec 10, 2010 1:02 pm

Not sure why this should suddenly start happening unless the plumbing has been changed when the new pump was fitted. Maybe the new pump is more efficient or the system has been balanced properly.

Anyway, the fault is caused by a poorly designed system. It is a constant surprise everytime I see this because it is such a common problem caused by a fundamental basic error in pipe installation.

The point at which the expansion and feed pipes from the header tank connect to the system should be close together. No more than 150mm apart in fact. If they link further apart the pressure differential will cause "overpumping" or suction in the expansion pipe.

Move the point at where the feed from the bottom of the header tank connects to the main circuit to a point just below the expansion connection.

Your system will go quiet!
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Postby Whitcop » Fri Dec 10, 2010 7:26 pm

Thanks plumbbob. So is this the correct order? Boiler flow (28mm), Open expansion vent tee (22mm), within 150mm cold water feed tee (15mm) then pump inlet (sucktion)?

Whitcop
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Postby plumbbob » Sun Dec 12, 2010 12:36 pm

Whitcop wrote:So is this the correct order? Boiler flow (28mm), Open expansion vent tee (22mm), within 150mm cold water feed tee (15mm) then pump inlet (sucktion)?


Yes can be.

Points to take on board.

1. Boiling water/steam caused by a boiler fault must have clear unrestricted access up the expansion pipe to the header tank.

2. Adequate consideration should be given to avoid air becoming trapped when filling the system resulting in airlocks.

3. Position the feed and expansion in such a way as to ensure both are subject to an equal pressure (eg, not one in the flow and the other in the return or one on either side of the pump etc). It does not matter what that pressure is so long as it is the same.
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