Air in hot water system

[clan184]

We have a standard cold tank in the loft feeding a hot cylinder on the first floor.
Our oil boiler heats up the water via a coil in the cylinder or we use an immersion.
Over the last 6 months or so, after leaving the system for a while (hours) when turning on any hot tap slightly we are getting some air coming out with the water and if we turn on the tap more than slightly we get water hammer in the pipes until the air has been expelled. Water flows out OK so there's not a complete air lock. After a short while you can turn the tap on full without more noise. After a while the air must build up again and the process is repeated when you next run hot water.

The hot water takeoff just above the cylinder continues up the expansion pipe to overflow into the cold tank above. So any air would vent you'd think not come down the pipes. With my limited knowledge of water systems with the standard cold tank presurrising the cylinder I can't understand where the air is coming from.
We have no obvious leaks or dripping taps etc.
I did wonder if the heating coil may be leaking air into the cylinder but that should vent thorugh the expansion pipe???
Any ideas?, Thanks

 

[ecowarm]

Welcome to the forum Clan, it sounds like air is being sucked in through the vent pipe for some reason. Possibly the outlet from the cylinder is getting blocked with scale

 

[diamondgas]

Does this happen at all the hot taps or one in particular clan? :welcome: by-the-way

Does the 'air' give off any odour? If it's not drawing air in down the vent then you may have something bacterial going on in the pipework?

 

[clan184]

Thanks for the welcome.It's nice to have somewhere to get advice.
It's happens with all the hot taps and there's no smell.

 

[clan184]

We don't get much scale in our kettle so I assume the water is not very hard.
About 5 years ago we had a period of getting brown water and wood bits from the hot tap. I found someone (not guilty) had used a piece of chipboard for the cold tank lid. It had got damp and collapsed into the tank. I replaced the lid with a plastic one and the problem disappeared. It may be what ended up staying in the cylinder is now causing a partial vent pipe blockage.
Wierd that the water flow seems good all the time.
Thanks

 

[diamondgas]

We don't get much scale in our kettle so I assume the water is not very hard.
About 5 years ago we had a period of getting brown water and wood bits from the hot tap. I found someone (not guilty) had used a piece of chipboard for the cold tank lid. It had got damp and collapsed into the tank. I replaced the lid with a plastic one and the problem disappeared. It may be what ended up staying in the cylinder is now causing a partial vent pipe blockage.
Wierd that the water flow seems good all the time.
Thanks

It's possible that there is a restriction in the feed or draw-off at the cylinder. If you want to find out whether it's drawing air in or there's a flow issue there's a pretty simple way to test. It'll take two people however, one at the tank and another at the bath tap .... If you can access the tank and the openvent pipe take a clear glass fill it with water and immerse the open vent into it. open the bath tap and see what happens to the water in the glass. If the water at the tap is being drawn out faster than it can be replaced you'll see the water level in the glass drop. If the vent drinks the lot then you've got quite a big restriction!

 

[clan184]

I'll have a go doing that this weekend, Thanks