Megaflo - temperature at tap is only hot when other cold taps on

[chrisd1967]

I have a peculiar problem and I wonder if anybody has any ideas.

I've experimented thus:

The megaflo tank is full of hot water.

Put the kitchen tap on hot, with no other taps on. Temperature at tap is 38ºC (i.e. poor).

Now, with the kitchen tap still running, turn another random *cold* tap on full elsewhere in the house.

Kitchen tap instantly rises to 53ºC (i.e. good - it's quite a way from the cylinder).

Turn the cold tap off elsewhere. Kitchen tap drops like a stone to 38ºC.

I have isolated the hot supply and have confirmed that you don't get a single drip out of the kitchen tap on hot when the hot is isolated - confirming that cold isn't mixing with the hot.

It isn't specific to the kitchen tap - it happens with any hot tap.

Any ideas what may be causing it or how I can rectify the issue?! My cold taps are all fed from the mains cold and the hot is fed from the megaflo, with the standard PRV on the input. Many thanks for any insight.

 

[picasso]

Your supplies are unbalanced, all the hot and cold outlets should be taken from the pressure reducing valve.

 

[chrisd1967]

Thanks for your reply.

The bit I don't understand is how this causes the water temperature to be low when only the hot is on. Surely the pressure of the cold at that point should be irrelevant?

Would you say that fitting a 3 bar (i.e. the same as on the megaflo) limiter in the main cold incomer would be a good idea?

 

[jeff the gasman]

You are getting mixed water probably through a shower mixer or blending valve.

 

[ianmcd]

As above a thermostatic mixing valve is blending somewhere

 

[dilalio]

+1 for crossover.

 

[chrisd1967]

Thanks all.

Annoyingly there are no signs of any isolators for my thermostatic showers, almost certainly buried somewhere, so I can't isolate the showers to try and work out where the culprit is.

My mains water incomer immediately splits into two 15mm arms which service various parts of the house. I can fairly easily put a PRV in each of these arms, limited to 3 bar, thereby limiting the pressure going to the megaflo (although that already has its own PRV anyway) and the taps in house to no more than 3 bar.

Would this in all likelyhood fix the issue?

 

[jeff the gasman]

Run all the showers until hot.
See which one goes cold quickly when you run your sink.

 

[fezster]

Are you able to isolate your HW supply? Try a hot tap after doing so and you'll probably find cold water coming through it. As already said above, this means you have a thermostatic mixer somewhere where the NRV is stuck open and allowing cold water to pass into the HW pipe.

The only way to identify which one would be to isolate them. Otherwise, you'd have to strip each one to try and clear the blockage.

When I had a similar problem, the culprit shower was not getting hot at all. In your case, it looks like you are having this problem on other outlets, so may not help you.

 

[ianmcd]

turn the inlet valve to the megaflow off, then turn the showers to max hot and see which one gives cold water out

 

[chrisd1967]

Are you able to isolate your HW supply? Try a hot tap after doing so and you'll probably find cold water coming through it.

I did isolate the hot and ran my mixer taps and didn't get a drop out of any mixer taps at all on the 'hot' setting (I didn't try showers though, I'll try them too). I took this to mean that there isn't any backfeeding going on, as surely I'd have seen cold water come out of the tap when I did that test? Unless the backfeeding only happens when there is also a flow of hot water? Puzzling.

 

[fezster]

I did isolate the hot and ran my mixer taps and didn't get a drop out of any mixer taps at all on the 'hot' setting (I didn't try showers though, I'll try them too). I took this to mean that there isn't any backfeeding going on, as surely I'd have seen cold water come out of the tap when I did that test? Unless the backfeeding only happens when there is also a flow of hot water? Puzzling.

In that case, it could be unbalanced supplies, as said by a previous poster. Do you know what your incoming cold pressure is? Putting a PRedV on the incoming cold is one solution.

 

[chrisd1967]

I don't know for sure, but I've ordered a pressure tester so I'll have the answer soon hopefully.

There is a t piece butted right up to my mains incomer stopcock that would require a lot of fiddling to move, so if it transpires that my mains water is a mega high pressure, then is my theory of putting two PRedVs in, one on each leg of the t piece from my mains incomer, OK? It sounds OK in my head, but you never know!

 

[ianmcd]

If this is something that has just recently happened you are on the wrong track, you have crossover

 

[chrisd1967]

There has always been a bit of an issue with tepid water in the hot taps, which we have always just put down to water taking ages to arrive at the tap. Recently it appears to be worse, so perhaps the mains pressure has increased or something like that.

My only hesitation with the crossover theory is that when the hot is isolated, we don't get any water at all out of the hot tap. If it were crossover then you'd expect some cold to come out?