Repeated air locks in hot water system?

[griffic]

I've come across a problem that's got me stumped and wondered if anyone can help. I was called to a customer who had no hot water. I suspected an air lock and back-flushed the hot supply with mains cold and got the hot flow restored. The gravity system in the flat had a cold water tank in the airing cupboard that was slow filling and I suspected that this had caused the tank to run dry and air to get in the system. I changed the float valve and thought I'd sorted the problem.
Approx 2-3 weeks later the customer called me to tell me the hot water had gone again. The cold tank was full and the float valve filling fast when required. I gave the hot water system a longer backflush and this seemed to sort the matter again.
After another 2-3 weeks the symptoms returned - the customer reporting that the hot water flow started to get slower for a few days before stopping almost completely. This time I suspected debris in the hot pipe-work. I cut into the hot water outlet on the top of the hot cylinder. This produced a very healthy flow of water when I turned on the supply from the cw tank and no debris was flushed out. I also back-flushed the hot from the kitchen sink whilst the pipe was cut and this produced a very healthy flow with no debris displaced into my large bucket. I re-assembled the pipes and the hot flow had been restored.
This has lasted for another 2-3 weeks and the problem has now re-occurred.
The hot water outlet pipe on the top of the cylinder is sloping upwards slightly to allow air to vent to vent pipe. The flat is about 20 years old and the problem has only occurred recently. There are only two hot taps, a basin and kitchen sink. Both hot taps are affected. The shower is electric.
I'm stumped ! Has anyone any ideas to help please.

 

[Neil K]

How big is the cold water tank feeding the cylinder?

 

[gassafe]

Is this a fortic tank or a regular system... Those gravity systems sometimes have loads of scale in the tanks, maybe it's all built up on the base & restricting the cold feed into the tank...

 

[griffic]

Its approx. 60 x 60 x 60 cm. Its in the airing cupboard above the hw tank. Although small, it only supplies h & c basin taps and kitchen hot. The system has apparently worked fine for several years until recently.

 

[tolly]

Possibly scale in cylinder blocking inlet causing air to be drawn down open vent when tap opened?

 

[griffic]

Its a regular system with separate h & c tanks. It could be a build up of debris in base of the hot tank, I hadn't thought of that. I checked the cold tank and its very clean.