Air locks in hot side gravity system.

[Radioman]

I have a repeat call back to a propert where after 4 to 5 weeks the hot stops flowing. All I do is back flow mains cold through a kitchen mixer and bingo it works. But what's causing it? We have 50 gal coffin tank in loft new 48 inch hot cylinder , triton power shower in en suite leccy shower in master fed from Teed off cold supply to tank, bath bog and basin in master. Only 2 occupants. Vent is clear all gates replaced with full flow ball lever, supply tank ball valve replaced. Tenant assures me only 2 residents living in.

My theories are. Supply of mains cold in the house is 15mm so when leccy shower running tank main is starved causing drop in level. Power showered is then run at same timecwhich drops level further causing low flow in hot due to no water in loft tank.
Or tenant is turning boiler and tank stat up causing a problem there. Any ideas

 

[SimonG]

Second tank. Had to do the same in a nursing home. About once every 3 months, turned out it was when the two baths were filled at the same time. Extra tank fitted about 18 months ago and never had to go back for that problem.

 

[jase158]

Try it!! Run shower and run bath and hey presto you will have answer!!
If not this then check vent is high enough above cwsc and hot feed is raising up until it goes to hot water supply pipe.

 

[kay-jay]

where does the hot come from for the power shower. is there a pump? connection via flange or if teed of hot distribution in which position?

 

[stevetheplumber]

i would check the ball valve orifice for crud a small bit will be the diffence between the tank keeping up and notive fitted two ballvalves or a three quarter one before now

 

[lame plumber]

plastic pipe with loads of high spots a common cause nowadays

 

[Radioman]

where does the hot come from for the power shower. is there a pump? connection via flange or if teed of hot distribution in which position?

Hot comes from T on 1st draw off hot supply 1/4 below top of the cylinder. Shower is Triton with built in pump so gravity to shower which then pumps. Second shower is leccy with 15mm teed off 3m from tank fill ball valve. (Previous install by non qualified bathroom installer now retired)

 

[Radioman]

If not this then check vent is high enough above cwsc and hot feed is raising up until it goes to hot water supply pipe.

Vent runs in 22mm up into roof pitch 1m above cold tank then down to tank which is by law 30. Hot feed comes up from top of cylinder 40mm then horizontal to hot supply.

 

[Radioman]

Worth noting that this problem went away for 7 weeks after we fitted new 48 in cylinder in lieu of old 36. At the same time the power shower was re plumbed as it had really low feeds. Also we reset the boiler and tank stats to give correct water temps for ch and HW with thermometer. Tenant now admits that directly before this event he turned boiler up from 3 to 6 and tank stat up 10 deg as the power shower was not getting hot enough for the missus. Next thing that happened was air cavitation in shower followed by no hot out of taps. I reset temps on Saturday.