Megaflow Cold tap running hot

[DIY_Query]

Hello all

I wonder if anyone maybe able to shed some light for me.

Just moved and we have a mega flow cylinder

When ever we go away I always isolate the water in the road, I’ve seen a number of houses take some serious damage while occupants away

So this time, ive Isolated in the street, gone back in to do a sanity check in the kitchen and the tap is running. Went back out incase I’m loosing it and confirmed off

Back inside and the cold tap is now running boiling.

Slightly intrigued, go upstairs. Bathroom taps are also all running hot

Start to look around and jump in the lift. The cold feed in on the cylinder is still cold. The hot out on the cylinder is hot to touch

Thinking I’ve got a crossover I’ve isolated every tap and shower and go round. No fault found and the cold taps continue to produce hot water

The kitchen where the cold is mains fed is also producing hot water.

Water will run for 3-5 minutes

Everyone is a little baffled by this. I’ve got one bar tap to rule out but the body of this tap doesn’t feel like it’s causing the mix.

All I can think of is the water was turned off to remove a sink from an old room. At this point the hot took ages to drain down. 5-10 mins.

Air gap has been redone following guide and no change

Hopefully I have not missed anything but guaranteed too

Any ideas folks?

 

[terryplumb]

Do taps eventually cease to give water flow once street cock is turned off ? If they don't the stopcock isn't fully closing.
Hot water Coming out cold water taps is usually down to a fault in a mixer tap or shower valve.

 

[Madrab]

There should be an isolator at or prior to the combination set for the unvented? If I remember correctly, all models of the Megaflo's control set came with an ISO valve as part of it. Shut that down, that should stop the HW flow to the outlets within a min or 2 once the pressure is released.

 

[Lower]

If you've isolated the any mixer taps so that there can be no cross feeding, i wonder if the residual pressure in the megaflow is feeding back through the megaflow cold water inlet back into the cold water system in the house.

 

[DIY_Query]

Thanks all and sorry for the delay. Many projects on the go…

so if I shut the isolator down you mention madrab then hot water runs till tank is empty. And will run out of hot and cold taps

They do all stop eventually when street is off. And the dial stops spinning

Lower - any way to stop that back feed?

 

[Madrab]

No, it won't run until the cylinder is empty, just until the pressure is released from the cylinder, can be as quick as 10 secs or up to a min or 2, pipework dependent.

back through the megaflow cold water inlet

If the check valve has stuck open, then could be but they rarely do IME. If it was that, the cold inlet would go hot too as it heads down the balanced feed/cold mains if it's hot at the outlets.

Sounds like a crossover/backfeed somewhere.

 

[Johntheo5]

Dip tube in mega flow? probably extends downwards ~ 50/60 litres so dependent on air bubble partial depletion may flow 20 or 30 litres of hot water at least before pressure at zero?.

 

[Madrab]

There is a dip tube yup, to extend down past the internal baffle but that doesn't really factor into it.

When an unvented is heated the pressure increases inside the cylinder, that's then absorbed by the internal bubble/EV being compressed, it's that pressure that needs to be released. Once that's happened then the water will stop flowing, how much is really determined by the size of cylinder and pressure retained within the expansion space.

E.G. I have a 210L unvented, once the cold inlet is closed, the cylinder will release the pressure in ~10secs, losing about 10L of water.

 

[DIY_Query]

Thanks both, I can run the taps for a good 5 minutes.

I guess my bigger concern is that the hot outlet is the only one getting hot, but the cold taps are throwing out hot water…

Yet I can’t find a crossover anywhere. I guess I need to now consider whether someone has done something silly plumbing wise

 

[Johntheo5]

There is a dip tube yup, to extend down past the internal baffle but that doesn't really factor into it.

When an unvented is heated the pressure increases inside the cylinder, that's then absorbed by the internal bubble/EV being compressed, it's that pressure that needs to be released. Once that's happened then the water will stop flowing, how much is really determined by the size of cylinder and pressure retained within the expansion space.

E.G. I have a 210L unvented, once the cold inlet is closed, the cylinder will release the pressure in ~10secs, losing about 10L of water.

Maybe not quite as simple as that, if one assumes a 60L bubble with dip tube extending just to below this then my calcs show that when cold water is admitted to the cylinder at 3 bar that the air bubble will now will be compressed to 15L with a 45L water "reserve" if the cylinder is now heated to 60C then an additional 3.4L will further reduce the air bubble and give a cyl pressure when hot of ~ 4.2bar pressure, if you shut off the cold feed then over 48L will flow from the cylinder until the pressure falls to zero or 25L will flow in falling to 0.5bar?. If one measured the distance from the cyl top to the TPRV which is the bubble starting point then it is easy to calculate the air bubble volume.

 

[Madrab]

OK ... but maybe it doesn't need to be as complicated as that? It takes as long as it takes.

Yet I can’t find a crossover anywhere. I guess I need to now consider whether someone has done something silly plumbing wise

The only way the cold outlets could be delivering hot water is there is an internal crossover somewhere. If it was external then the combi-valve check valve has failed and there would be hot water flowing down through the cold feed into the cold water system. The fact that there isn't suggests there is a crossover at one of the outlets. Does the hot water from the cold outlet run when all the hot water taps are off?

Again do you have monobloc single valve type taps? That would be the first place to look IMO.

 

[Johntheo5]

No, it won't run until the cylinder is empty, just until the pressure is released from the cylinder, can be as quick as 10 secs or up to a min or 2, pipework dependent.

If the check valve has stuck open, then could be but they rarely do IME. If it was that, the cold inlet would go hot too as it heads down the balanced feed/cold mains if it's hot at the outlets.

Sounds like a crossover/backfeed somewhere.

That seems the most likely IMO, there may have been no nr valve installed originally, the 3 or 4 litres of expanded hot water probably wouldn't reach the cold taps
norramally, easy to check by feeling the balanced cold a few feet from its take off. the PRV would prevent it backing up any further.