Unvented hot water issue

[hand91]

Hi all!. First post here so bear with me. I have searched for my issue, but i have not found an answer as of yet. I also know that i can't do any work on an unvented system. i just want some ammunition for when the plumber comes round.


We have an unvented system, megaflo cylinder, indirect with boiler, but have a problem with the hot water. So the problem is that hot water comes from the taps only when we open the hot side all the way i.e., all the way to the left and nearly all the way up. Once the hot water reaches the mixer tap, and we reduce the flow slightly (tap down) or reduce the heat (turning tap right slightly) or both even slightly, stone cold water starts to flow when we do this. If we don't open fully, we don't get hot water flow! This is for all taps in the house.We got told that the problem was the pressure reducing valve which is before the cylinder (we have not had it serviced a good 10 years), so we had that changed (old PRV was faulty) with all original parts (3 bar PRV).
This has not solved the issue. Its only the hot water side that has the pressure reduced, cold is still mains pressure to all the taps (could this be the issue? unbalanced hot and cold pressure? should i check my mains pressure and possibly fit a PRV near the mains entrance?)


Maybe there is a blockage somewhere? Cold water inlet? Hot outlet on top of cylinder? There is good pressure from all hot taps though.
I hope this makes sense. Any advice will be amazing. Have you come across something like this before.



Any help will be appreciated!


Hubert.

 

[hand91]

or maybe cold water flowing from cold side pipe into hot side pipe? fitting a non return valve on the hot could help possibly?

 

[AWheating]

Prob a pressure balance issue. Non returns won't help. Get the g3 plumber who did the service and changed your pressure reducing valve to take another look.

 

[kay-jay]

unbalanced cold supply and a knackered mixer tap is probable

 

[DrainMedic]

Has this problem just started suddenly? Or has it always been this way?

it sounds like the strainer to the inlet of the system is blocked/partially blocked?

Has your plumber re-commissioned the system after the prv replacement? Ie: has he/she pressure tested the outlets/taps etc to ensure the prv is actually working? It's also possible that it's a flow issue rather than pressure (inlet strainer etc).

Its also worth noting that the prv, although brand new, might be faulty, or more likely mis-diagnosed as the problem.

 

[DrainMedic]

Ps:

mixer tap tap fault is ruled out as its happening throughout the house.

 

[Reg Man]

Should be equal pressure to the taps on both hot and cold. Cold pressure is probably to high.

 

[DrainMedic]

The cold supply to the taps should come from after the p.red.v.

If if its just started happening, then it's either p.red.v or strainer.

If if it's been happening all the time, it's possibly a design issue, but unlikely as surely there would've been a complaint in ten years?

after the strainer and p.red.v and check valve, there are no other valves before the taps, so it's really all it can be? It's possible the check valve is stuck shut partially

 

[chris watkins]

Hi Hubert & welcome along.
I think you have already come to the correct conclusion through your own research.
It is most likely the unequal pressure between the hot & cold water sides, you could check the pressures on both but an easer way might be to have a look at the hot & cold supply to the mixer fittings hopefully at least one will be fitted with a Ball-o-fix type valve/s, the ones you operate with a screw driver.
If you can turn the cold off & open up the hot tap if it the hot flow is now correct turn it right over too the cold only & open it. Then crack open the cold ball-o-fix so only a small amount of cold water comes out, try the tap to see if it works.
If the above does indeed work & your cold water main pressure is very much higher than the 3 Bar of the hot you could install an adjustable pressure reducing valve on the cold water into your property & set it for around 3 bar.
I would recommend you use a plumber who understands this type of work because there could be some setting up but its not a problem re the un-vented.

Although most house systems are installed as balanced (both cold & hot taken off after the Megaflo PRV) it is not a requirement.

 

[Best]

I am not sure about Megaflow, but some unvented recommend you have a check valve fitted to hot outlet pipe from unit to prevent back pressure if you have any taps with unbalanced supply.
You probably have too much pressure to cold taps & a PRV on the mains into house will be needed if you can't do it at unvented combination valve.

 

[hand91]

Thank you all for your responses. Really appreciated.

Drain Medic-We used to use the Megaflo as part of an open vented system so (until about 2 years ago), cold water supply to the taps is directly from the mains and not after the 3bar PRV. This was never changed.Im not sure if the plumber took readings after the PRV to the Megaflo was changed... :S
So i went to Screwfix today and bought a mains pressure tester. Went to my outside cold tap and took a reading of 4 bar precisely. Assuming that the PRV to the MEgaflo works and reduces to 3 bar, would the difference of one bar between hot and cold make mixer taps in this way?

As Chris Watkins suggested,I tried turning the ball valve off under the sink to the cold water, and the hot was as it is suppose to be...hot. Reducing the flow did not have an impact on the temp, obvs. Then i unscrewed the cold slowly and all was good until it was fully open, then the same symptoms occurred. The flow rate on both hot and cold is very strong. When fully open, they splash everywhere. Should it be like that? Can the flow rates be an issue?

Best- that makes sense. Could i not just put check valves under the sinks on the hot pipes? Has anybody else come across check valves on the hot outlet on the cylinder to prevent back pressure??

Many thanks,

Hubert

 

[Best]

Firstly, - the cold needs to be same pressure as hot really. Only place that there is no risk of back pressure is to a single cold tap, like an outside tap. Modern kitchen sink mixers now have a double chamber spout (pipe in pipe type of spout) to prevent hot & cold supplies mixing. But this will NOT guarantee the highest pressure water mixing as there can be a fault inside a mixer tap - like a tiny hole or crack between the hot & cold inlet points or worn O ring seals in spout base.
Check valves on sink taps will help but then you have the problem of shower mixers etc.
Best way is cold supplies all coming from combination valve cold draw off and next best is a second prv fitted on incoming mains & set to unvented unit max required pressure - usually 3bar

 

[hand91]

Old PRV:
tinypic.com/r/14n1oz/8
tinypic.com/r/10r4vn5/8
tinypic.com/r/2h4jpms/8

New PRV:
tinypic.com/r/6p4tvo/8

 

[hand91]

Thanks Best. I think a 3 bar PRV on the mains inlet is something we have to consider. Is it possible that the taps installed in the property are not meant for high pressure, but for gravity fed pressure as it has been until 2 years ago?

Looks like 1 bar difference between hot and cold does make a difference?

 

[hand91]

I have just bought a Reliance 15mm adjustable PRV from Unvented Components Europe eBay store for £20 I will to fit it this week or the week after, where the mains enters the house. I will report back on progress.

Thanks for all your advice so far!