gravity hw problems

[chrissyposs]

evening all, new to this site and having a right headache need to pick some of your brains,

baxi bbu, gravity primaries in inch working of the boiler stat, with no other temp control. 22mm open vent to cws (50 gallon) 22mm feed to hwc with full bore isolater.

problem: hw backing up the cold feed, raising the level of cws 5" then overflowing to outside. this generally happens around 30mins after timer comes on and then seems to stop after cylinder reaches temp via the boiler stat. (set on 2) cwst only luke warm not hot

tried: lowered the level in the cwst, gas rate, new boiler stat, non return valve the cold feed (now gravatates over the open vent) extending the open vent higher then down, made sure the open vent was not blocked or limescaled, checked the outlet at the top of the cylinder, removed the non return valve and fitted a new gravity coiled cylinder, back to original problem of backing up the cold feed ond overflowing.

HELP PLEASE my brain hurts!

 

[danielp]

what about where the vent comes into the system but really for pumping over would only occur when the heating is on

 

[chrissyposs]

hi, the header feed is on the primary return and the open vent is on the primery flow as should be, the problem is the hw from the cylinder backing up the cold feed to the cwst the heating is fine, no pumping over into the f & E


sorry if ive explained wrong

 

[bobpape]

Hi Chrissyposs
Is the surface of the water in the CWSC and F&E tank at the same level.

 

[Blackcatgas]

Coil in cylider pinholed? water from the heating tank filling up the cylinder via the hole in the coil?.

 

[chrissyposs]

hi all,

the cwst and the f & e are same level, new cylinder fitted and still same problem of hot water from the cylinder backing up the cold feed to the cwst and overflowing

 

[stevetheplumber]

hi all,

the cwst and the f & e are same level, new cylinder fitted and still same problem of hot water from the cylinder backing up the cold feed to the cwst and overflowing

only thing i can think would cause this is mains water getting in to the hot water cicuit i would be checking any mixers showers or even your washing machine
logic being to raise higher than existing water level you must have water supplied from a source with more head you say water isnt hot so that rules out exspantion
try this then come backon to tell us the result
what i cant understand is why only when boiler is switched on?maybe turn of the main to your house drain via the lowest tap and switch boiler on to heat only see if it happens then to eliminate the possibility

 

[Smcart5]

I'm not experienced but from college class this sounds like a thermostat failure on the cylinder?

 

[REDSAW]

i go with the coil pinholed, is this not why the temp is tepid cause its cooling it.
do an air test?.

 

[Blackcatgas]

only thing i can think would cause this is mains water getting in to the hot water cicuit i would be checking any mixers showers or even your washing machine
logic being to raise higher than existing water level you must have water supplied from a source with more head you say water isnt hot so that rules out exspantion
try this then come backon to tell us the result
what i cant understand is why only when boiler is switched on?maybe turn of the main to your house drain via the lowest tap and switch boiler on to heat only see if it happens then to eliminate the possibility

+1! If youve replaced the cylinder and the coil is fine, then this is the only explanation left!.

 

[chrissyposs]

thanks for your thoughts,

dont you just love being a plumber!!