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vented mains pressure hot water flow problem

View the thread, titled "vented mains pressure hot water flow problem" which is posted in Bathroom Advice on UK Plumbers Forums.

A

action jackson

Evening all.

I'm a regular user of the tiling forum but have popped over here to see if someone can help me work out what the problem is with my hot water flow issue? I have already had a plumber in but he is struggling to work out the problem.

I live in a first floor flat and believe i have a mains pressure, vented hot water system with 2 x electric immersion heaters. I'm by no means an expert but i believe the system consists of a water cylinder with internal coil, a separate header tank, an expansion vessel and a thermostatic mixing valve.

The issue i have is that the hot water flow has dropped off dramatically in the last couple of weeks, making it un-useable for running a bath or having a shower. Another issue that may be related is the header tank in the cylinder is continually overflowing and running through the overflow pipe. I haven't experienced this in the last 7 yrs.

About 2 months ago i had a whirlpool bath fitted with a concealed dornbrachet mixer unit and thermostatic control for bath and shower. No problems were experienced when initially installed.

The pressure in the expansion vessel was checked on Friday and was zero psi. I have pumped up to 1 bar and so far the pressure has remained stable, although it hasn't fixed the problem.

I was advised that maybe the thermostatic mixer valve for the shower/bath could be letting mains pressure pass into the hot water circuit due to the in balance with hot and cold? Is this possible and would this cause the water level in the cylinder to rise?

I should also add that the tank was removed 3 weeks ago because i though we had a crack in the cylinder, but we couldn't find any problems with the cylinder (blanked off and filled outside with cold water for 24 hours). We now think that the leak was actually coming from the overflow pipe in the header section of the cylinder which has now been fixed.

Lastly i recall checking the header part of the cylinder a couple of years ago and noticed it was bone dry! I wasn't sure why but as everything was working i didn't worry about it. (a guy at work is ex plumber and couldn't understand why i had a header tank in the cylinder for this type of system) Now the water is brimming out the top and continually overflowing!

Any sugestions or checks to carry out would be much welcomed! :icon4:

:icon4: IMG_20120226_214449.jpgIMG_20120226_214454.jpgIMG_20120125_233022.jpg
 
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Just to add the valve for the water pipe feeding the header part of the cylinder is closed (always has been), so the raising water level in the header must be caused by the water level in the main part of the tank. There is a small internal pipe linking the two parts of the the cylinder.
 
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You've got a thermal store hw cylinder, if your overflow is running then you either have a split coil, ball valve isn't shutting off or the thermostat on the immersion has failed causing the cylinder to overheat, I would suspect the blending valve or split coil for the pressure issue
 
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action jackson said:
Just to add the valve for the water pipe feeding the header part of the cylinder is closed (always has been), so the raising water level in the header must be caused by the water level in the main part of the tank. There is a small internal pipe linking the two parts of the the cylinder.
Click to expand...

Valve feeding the top part of the cylinder should be open
 
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action jackson said:
will it make any difference if i open the valve?

What is the purpose of the top part of the cylinder?
Click to expand...

Top part is a expansion & feed tank for the cylinder part. The overflow is almost certain to be coming from a burst coil as others have said. That probably also explains why you have reduced hot flow.
On some thermal stores, the mains pressure was too high & split the coil.
Hopefully you will get it under warranty. IMO a stainless steel unvented cylinder is better where suitable to fit.
 
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Since i had 10 minutes to kill while drinking my tea before i hit the sack, i did a wee drawing of basically how it works
untitled2.JPG

It works like a back to front cylinder. The elements heat the cylinder (store) and when you draw water the cold runs up through the coil picking up heat and comes out and runs through the blending valve (to cool it down a bit). The top part of the cylinder is only to top up the store for evaporation. If the top part empties it means the water level in the store will drop and your hot water will not be as hot.
 
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Agree, Tamz, but top part is better described as an expansion & feed tank, as it both tops up any water loss in store via a feed pipe from base & has a vent pipe to top of it.
IMO, the overflow should never be made of plastic on these units, as when the coil fails the water is hot running from o/flow, which could mean the fittings coming apart.
 
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Best said:
Top part is a expansion & feed tank for the cylinder part. The overflow is almost certain to be coming from a burst coil as others have said. That probably also explains why you have reduced hot flow.
On some thermal stores, the mains pressure was too high & split the coil.
Hopefully you will get it under warranty. IMO a stainless steel unvented cylinder is better where suitable to fit.
Click to expand...

Thanks for all the replies. The manufacturer has stated the cylinder only has a 2 year warranty. A new identical cylinder is £1,000!

Can the coil be repaired?
 
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Agree with what others have said, get a s.steel unvented unit, as it is cheaper & better in every way. Great flow of hot water if mains up to it. 25yr warranty usually also. The thermal store in your case, only advantages was that it didnt require unvented regs or a copper discharge pipe to a safe point.
Don't try to save on install, - have it done properly by qualified person for safety & warranty.
 
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Best said:
Agree with what others have said, get a s.steel unvented unit, as it is cheaper & better in every way. Great flow of hot water if mains up to it. 25yr warranty usually also. The thermal store in your case, only advantages was that it didnt require unvented regs or a copper discharge pipe to a safe point.
Don't try to save on install, - have it done properly by qualified person for safety & warranty.
Click to expand...

What is the minimum mains pressure for uvented systems? Mains pressure is not fantastic.

Ref copper discharge pipe to a safe point, what is required for an unvented install?

I live in a first floor flat and have no access to the outside etc.
 
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