air in domestic hot water

[alex3rd]

I have a problem with air in the hot water system, when I open the tap in the bathroom upstairs the pressure drops off, if I leave the tap open then open the downstairs hot tap, it will eventually start to splutter and cough air, then the water will run OK. This cures the problem for a few days then it starts over. Can you help me find a permanent cure. Many Thanks.

 

[Softus]

This sounds like an air lock.

Do you have an open vented system?

For how long has this symptom been evident?

What is the moste recent plumbing or heating or boiler work you've had done?

 

[oilman]

Sounds like the vent pipe does not rise all the way from the hot water tank.

 

[alex3rd]

Thank you for your replies, the system is an open vented system, and this has been going on for some time, occasionally it will run for a couple of months without problem. as to boiler work there has been none for a few years.
Thank you
Alex.

 

[Softus]

OK. What's the path of the pipework from the hot cylinder to the hot taps, and the pipework from the cold storage cistern to the DHW cylinder?

And what valve is on the DHW outlet from the cold storage cistern?

 

[alex3rd]

Hi Softus, The cold water Header / storage tank is in the loft, there is an inlet valve & ball cock, but no outlet valves either for the Hot or Cold just a freeflow from the bottom of the tank, one either end. the flow from the Hot water tank to the taps, leaves the tank and goes back up into the loft, there is a branch on this pipe where the hot water leaves to flow back down into the house, the rest of the pipe continues up through the roof to vent. I have previously connected a hose from the mains cold water to the hot tap downstairs to blow the air from the system, and the water flows backwards into the Loft tank. I have wondered if I should block the flow back into the tank while doing this, which I think should then blow the water out of the vent pipe onto the roof, and clear this pipe incase it is blocked, thus not allowing air to clear through the vent.
Sorry to go on a bit.
Regards
Alex

 

[Agile]

Its the bit going through the loft which is going to cause the problem.

Perhaps the cistern is slow to fill and sometimes drains allowing air to be sucked into the outlet when you have a higher demand for water use.

Your float valve should fill at at least 12 li/min if you have a reasonably sized cistern, say 200 litres.

A fill rate of 20 li/min would probably be needed to reduce the chance of this happening often.

Tony

 

[Softus]

You can certainly clear the air in the way that you've tried, although you will need to block the vent first, but your pipework is incorrect, which is why I asked about it.

Your air problem is likely to be the height at which your vent departs from the DHW supply. This needs to be as far below the base of the cistern as is reasonably possible, and it's traditional for this to be in the airing cupboard just above the take-off from the cylinder.

If you're not confident with modifying the pipework, just get a heating engineer to come look see, show him the vent, and see how quickly he gets his tools out!

 

[alex3rd]

Hi Softus, I may have misled you with the pipework. The Hot water tank is as you say in the airing cupboard in the bedroom and the feed to the taps leaves from the tank top. after about 10" the branch to the taps leaves this pipe, and both then go up into the loft, the vent keeping going to the roof while the pipe to the taps crosses over the loft and down a boxed in section to the taps.
Thanks again
Alex