vented direct hot water cylinder expansion problems

[alpina1970]

hi this question will baffle most experienced plumbers,
i have a normal direct vented d/h/w system in a small hotel that im working on, vent on cylinder via roof and not back into storage tank, it is continuing water escape , ive put in two new thermostats at immersion heater and tested hot water temp ...sitting at 50c....water escape via expansion pipe only stops when cold feed to cylinder is shut down, ive even taken off any shower mixing valves to eliminate any faulty pass of water to hot water side. hoping ther might be one of yous guys that can help as im tearing my hair out .

 

[gigz]

Have you tried tying the ball valve to the cold feed tank up ? Just a thought as if you are turning stop tap off it might tee off to a washing machine/mixer tap which may be passing.

 

[alpina1970]

thanks mate , but only taps supplied are single and ive blanked off all mixers, i did forget to mention that o fitted a swing non return valve to cold feed from storage to cylinder. it does only start to escape via roof when cold feed is on to cylinder.

 

[Richardthe3rd]

Skoolboy physics mate, your expansion pipe is not high enough. When the water expands it creeps up the expansion pipe higher than the header tank.
Or you have a crossed pipe that's feeding into the header tank, to check; tie up the ballvalve, shut off the cold feed to the cylinder & see if the header tank overflows.
Next!!!.......

 

[stardanny]

Where exactly do the vent goes to? Is it over the tank? It is possible the vent is lower than the water level in cold water tank.

Daniel

 

[ALCPlumbing]

Most likely as someone else said is that the water level of the cws tank is slightly higher than the vent or maybe the mains is teed into the hot supply somewhere. It doesn't have to be on the cold feed to the cylinder it could be on any hot water pipe and there may be a valve linking the two that is shut off but letting by. Unlikely but it would have the same effect.

 

[alpina1970]

Thanks guys but everything seems normal ie height of cws to vent . Dhw is being vented through roof and is approx 7 ft above head of hot water level ..I have fitted a swing non return valve to cold feed outlet to hot water cylinder this has only moved the problem from back filling cws tank to pushing over the expansion. ..

 

[bellairaphon]

How do you shut off the CW? If stop tap you checked any mixer taps, showers? Passing back into the cylinder?

Nath

 

[alpina1970]

Thanks mate there are only two other mixing valves we have taken them off and blanked tails so only the cylinder is being fed via cws aftee a ling investigation we have found that when the hot water is being used it does temporally stop coming out expansion pipe..absolutely baffling as ive bn in the game now for 28 years and only came across similar circumstances once when it was a mixing valve failure but fiest we did check heiggt of expansion compared to height of cws and expansion pipe is def roughly 5/6 ft hugher than cws

 

[alpina1970]

Please excuse spelling as im still racking my brains at 3.15am thinking about this lol

 

[stardanny]

Morning.

You say it only stopped when the cold feed to hot water cylinder from cws is turned off, yes?

You put in a non return valve on cold feed to cylinder to stop backfilling into cws, but it end up backfilling through the vent?

So how is that you put in non return valve in to stop water from somewhere backfilling to the cws yet before, the overflowing stopped when you turned off cold feed to cylinder which is other way flowing?

Are you sure it is the only one cws? What about cold feed to bathroom?

Is the ballvalve in cws for hot water cylinder running when the vent pipe is overflowing?

If the ballvalve is shut and isn't running, vent pipe is overflowing, yet cold feed to cylinder is turned off, vent pipe not overflowing, ballvalve is still shut off ( still on but not running ) then there is something connected to pipe between the cold feed valve and the non return valve?

Daniel.

 

[alpina1970]

Hi Daniel thanks...all cold feeds to taps and showers are mains fed ..I thought that the cwsfeed to ccylinder is 28mm might have a impact on outlet pressure at draw off on cylinder do you think it is possible to raise expansion pipe higher , even although ive approx 5/6 ft from top of cws to top of expansion pipe

 

[alpina1970]

Might add that we've also replaced ball cock on cws,

 

[mogget]

If the expansion pipe is 7 feet higher than the top of the CWSC then clearly the pressure in the expansion pipe is a fair bit higher than the pressure in the cold feed. Clearly this pressure is not coming from the CWSC. Are you absolutely sure that there is no other supply connected to the cold feed / expansion pipe? And that no mains is passing?

 

[Gasguru]

Isn't this just a really basic plumbing problem...when the outlets are open there's a back flow of cold mains pressure water back to the cylinder.

The cold water takes the easiest route (lowest head loss) and rises up through the vent pipe. Single check valves on the hot outlets (or hot outlet of the cylinder) as a minimum for a start but would be better to feed the cold outlets from the CWSC.

Take the non return off the cylinder feed for a start...where's the water going to expand....