Can you get an Airlock in a hot water pipe ?

[jtaunton]

My power shower is fed by the cold water tank and a tee-off from the hot water pipes. Some mornings, a few seconds after switching on, the pressure drops, the water runs cold and you can hear the pump labouring, as it doesn't seem to be getting any water from the hot feed.

The best way to solve this is to turn off the shower, run the hot bath tap for a few seconds, and then turn back on, as if there is some funny airlock but I can't understand it.

For different reasons, I have had to replace the pump and the thermostatic mixer recently, so they are both out of the frame as the problems happened with old and new parts

Any ideas how to solve this problem ?

 

[Bahco]

Check your hot water pipe run and put a bleed valve at its highest point.

 

[felix]

It's a pretty good bet that the hot water pipe run already has a permanently open bleed valve at the highest point, namely the vent. Is your shower's hot supply teed off the vent pipe (a common arrangement with non-powered mixer showers). If it is then I suggest that the pump is sucking water out of the vent pipe faster than it can be replaced by the cold feed into the cylinder. This will be aggravated by any other demand on the hot water system. If the water level in the vent pipe drops below the tee off point the pump will suck in air.

Check your cold feed for any signs of partial blockage (eg sludge in tank) but the only cure might be to raise the cold tank or take the shower's hot supply from a point lower down in the system. The second solution will likely leave you with an unvented inverted loop of pipe and then you WILL need a bleed valve!

 

[ChrisR]

Don't use an Automatic air vent - they'll allow air to be sucked in!
Best way around your air sucking problem is to take a pipe back to the HW cylinder. An Essex flange is easy to fit, and 15mm plastic pipe seems to be OK with pumps, even where you might think it would be a bit small.