Hot water from tundish - unvented system

[Matt21]

I have an OSO unvented HW tank heated by an immersion heater and/or a gas-fired Keston 80 boiler. This morning I found hot water gushing from the PRV on the tank after the HW had been heated by the immersion heater (not from the boiler, though I need to check to see whether the zone valve for CH/HW is operating correctly - I suspect that I may have been inadvertently heating HW as well CH despite only setting for CH).

The radiators were very hot (unusually so), as was the hot water. The gushing stopped when I switched off the mains water, and restarted as soon as I turned the mains on again, eventually slowing down and stopping altogether as I ran hot water out of the taps and turned off the immersion and boiler.

So - seems like a thermostat/temperature problem somewhere that led to the PRV doing its job. Any ideas gratefully received! Thanks

 

[gremlin16]

Faulty pressure reducing valve. ?

 

[holty]

usually if its pressure thers a small drip,if its temp it will gush out till the water cools then stop then start again. if so its either the boiler stats gone but would usually shut the boiler off and a safety zone valve(if it works or has one) to the cylinder, or the cylinder stat has gone.just a few probs it could be. if i were you i would get someone compotent to have a look at it.

 

[muggles]

I would say most likely cause is a stat failure, quite possibly on the immersion heater. Assuming all the safety controls have been wired correctly then the high limit stat should have kicked in and shut down your zone valve and immersion, but as I say that does rely on it being wired correctly. If the water in the cylinder is hotter than the water in the central heating coil, you'll get reverse heat transfer which will cause the rads to heat up more (sounds to me like you've got a 3-port zone valve).

I would get it checked over ASAP, apart from wasting water, overheating isn't a particularly safe situation

 

[ajstoneservices]

Could be the bubble needs recharging and nothing to do with a faulty stat/immersion stuck on. But you need to check the controls to be certain.

 

[holty]

if it was a bubble or ev gone it would be a pressure issue. and you would only see a a small drip instead of a gush of water.its more a temp problem and so would seek to get this resolved asap.

 

[muggles]

A lost air bubble would cause a discharge of cold water, and only a small dribble, so no, I don't think it's that.

 

[ajstoneservices]

If the cylinder content is hot the t&p will discharge hot water, they do dribble if the bubble is lost or pressure too high, however they can also run at the t&p when the bubble is lost, pressure too high. You need to check the controls too.

http://www.osohotwater.com/uk/pdf/20_series_installation_manual.pdf

 

[muggles]

If the cylinder content is hot the t&p will discharge hot water, they do dribble if the bubble is lost or pressure too high, however they can also run at the t&p when the bubble is lost, pressure too high. You need to check the controls too.

http://www.osohotwater.com/uk/pdf/20_series_installation_manual.pdf[/QUOTE]

I disagree (having fitted several and gone to a hot OSO discharging cold water due to a lost bubble last wednesday). A lost bubble would cause a discharge of cold water, and in any case it would be the PRV and not the T&PRV that would be running with a lost bubble, unless the PRV was jammed shut. I still say overheating and not over-pressure

 

[ajstoneservices]

I disagree (having fitted several and gone to a hot OSO discharging cold water due to a lost bubble last wednesday). A lost bubble would cause a discharge of cold water, and in any case it would be the PRV and not the T&PRV that would be running with a lost bubble, unless the PRV was jammed shut. I still say overheating and not over-pressure

.


The T&P 7bar 90c, PRV 6bar so as you say the PRV should open before the T&P however that will depend on the system plumbing location of the combi block/CV.
We look after a number of OSO's on an estate in Norfolk where we have to recharge them some more than others, we don't have the same problems with Tribunes with an external EV on the same estate.

 

[Matt21]

Thanks gents. I have recharged the bubble just in case, and am now trying to heat the HW via the boiler. If it doesn't overheat, then I guess my problem is likely to be the immersion stat. If it does, then i don't know. Either way, sounds like I need to get someone in! Thanks for the help - I will post a solution if/when I find one!

 

[muggles]

I disagree (having fitted several and gone to a hot OSO discharging cold water due to a lost bubble last wednesday). A lost bubble would cause a discharge of cold water, and in any case it would be the PRV and not the T&PRV that would be running with a lost bubble, unless the PRV was jammed shut. I still say overheating and not over-pressure

.


The T&P 7bar 90c, PRV 6bar so as you say the PRV should open before the T&P however that will depend on the system plumbing location of the combi block/CV.
We look after a number of OSO's on an estate in Norfolk where we have to recharge them some more than others, we don't have the same problems with Tribunes with an external EV on the same estate.

Yeah I know there's a general issue with cylinders with internal air bubbles, to the point where the few manufacturers still using them are looking at moving to the more conventional external EV. I'm surprised at the ratings on your (T)PRV's though, pretty sure the ones we did were 8 bar PRV and 10 bar/90º TPRV...maybe they've changed them at some point. Anyhoo, we're getting very slightly off-topic here