Hot Water Supply Issue

[RacingTortoise]

I’m a user, not a plumber! Apologies for the long post.

I have a traditional boiler, gravity fed c.h. system and unvented h. w. using a Megaflo. The timer for h.w. is set to 3 periods, moning, mid-day top up and evening. The h.w. tank used to retain sufficient heat so that after 10 mins of heating in the morning the hot water was too hot for the hand.

The main symptom is the hot water is suddenly no longer too hot for the hand, takes a long time to reach the washbasin and can go cold after a short while. A shower isn’t feasible as it goes cold after about a minute. The tank outlet remains hot.

An engineer concentrated on making sure the tank had its proper air reservoir, all diverter valves operated properly and the balance was correct between the flow to the c.h. and the indirect h.w. coil. That made the h.w in the tank hotter but failed to resolve the issue.

Subsequenty I carried out further tests. I have four mixers upstairs and one downstairs and two sets of taps downstairs where the hot and cold are controlled separately. Turning off the hot water outflow from the tank, cold water flows from the hot taps, even though any mixers are set to “hot”. Cold water also flows from a downstairs hot tap which is entirely separate from its adjacent cold tap.

I have no facilities to control the pressures of hot and cold separately, nor anything to measure the true temperature of the hot water at the tank.

Suggestions please for further diagnosis, anything I should have done, or even possible solutions. In lockdown I’m reluctant to have an engineer in the house again (vulnerable occupant) until I’ve done as much diagnosis myself as is reasonably possible.

 

[sxturbo]

wht type of cylinder do you have?

is the hot water cylinder to hot to touch behind the insulation?

 

[oldbuffer]

Possibly one or more of your mixers are letting cold into the hot supply.

 

[RacingTortoise]

It's a Megaflo so cannot tell, but the pipe from the top is deffo too hot (or gets that way when the tank is being heated).....

Its all a bit confusing (to me). I need to carry out the tap test again, to be certain. All basin taps (2 Vado up, 1 Grohe downsairs) are single pillar. Shower (Vado) is thermostatic with independent temp and flow controls. Bath single mixer embedded in the wall (Vado). Utiliity is separate taps and kitchen is an old type mixer with separate controls to a spout in the middle.

 

[RacingTortoise]

Megaflo tank, indirect, 5 yrs old.

Yes, I'm now suspecting that somehow the cold is getting into the hot flow, but I'm undertain what the normal operation shoud be. For example, the shower has independent temp and flow controls so presumably when set to a mid position, cold can flow into hot unless the pressures of each are equal? In my test I set the temp control in the shower to fully hot but not sure is that fully switches off the cold - that's why I mntioned the manufacturer in the reply above.......

 

[RacingTortoise]

Basic question to someone who knows.....should the mixer taps and bath / shower controls I've mentioned above, allow a significant flow from cold to hot when the levers are set to "maximum hot"?

 

[sxturbo]

my kitchen tap if i run the cold at the same time it goes into the hot side, i put this down to the cold water pressure (cold tap connected to mains) being higher than the hot.

 

[oldbuffer]

1. In any thermostatically controlled mixer, cold is always present to reduce the temperature of the hot water. If they allowed hot only, there would be no cold to reduce the temperature, and either no flow at all or scalding water.
2. Your unvented cylinder will have what is known as a combination valve. This is where the cold supply from the mains enters, and its pressure reduced to a pressure suitable for the cylinder, normally around 3 bar. These valve normally have provision for the cold for the cold taps to be taken at the same pressure, thus avoiding unequal pressure problems. However, this facility is often not used on installation, as it can complicate pipe runs.
3. Many mixer appliances, particularly shower valves, have non-return valves in both hot and cold feeds to obviate the problem. However, these tend to be on the cheap and cheerful side, and can jam in the open or closed position.
4. For each mixer:
4a. Turn on around the mid-point for temperature.
4b. Feel the feed pipes.
4c. If both are cold, then the cold is feeding back into the hot.

 

[RacingTortoise]

Oldbuffer - thanks!

I can (will) do that, but the shower control and bath mixer are embedded in the wall so unable t feel. However for the three other mixers I will feel the supply.

Update:
Before explaining the result, I should emphasise the flow temp at a basin mixer varies between warm and cold. One oddity is that if I turn off the mixer after about a minute, the flow can be completely cold before getting warm again (note: warm, not hot).

Now the result: All mixers set to mid-point. I used an upstairs mixer for the test (one in the ensuite). The pipes can be touched with a couple of fingers, but not fully grasped (accessibiity). The temp of the hot inlet varies and is sometimes momentarily cold, so my conclusion is cold getting into the hot.

Do I need to test more extensively or is that enough confirmation?
Obvious questions:
- what are the next steps to identify where the rogue transfer is happening?
- Is this a common issue with mixers (which gut feel makes me think is the most likely source)?

Now see my later response...!

apols for the typos - keyboard is the next failure item!

 

[RacingTortoise]

oldbuffer,

Many thanks - you set me on the right track. Circumstantially I can now recreate the problem at will (well, I've done it six times).
I'll describe the situation for the benefit of all.

First an admission: I forgot we had two showers (the second is just for visitors). Both have identical thermstatic controls - the first control sets temp and the second is flow.
After my initial test I noticed the temp control on the second shower had not been set to its mid position (which yor test requires). With the basin mixer tap in our ensuite set to "hot" and the temp control on the second (forgotten) shower then moved to its mid position as you suggested, the water in the ensuite basin became scorching hot. Turning the shower's temp control back to "cold" (as it had been left), the flow in the basin reduced to luke warm. The variation is repeatable - set the shower temp control to mid position and all is ok. Set to cold and the ensuite basin flow s luke warm.

In short, I've discovered how to recreate the problem and can repeat it. I need now to just understand it fully and get the faulty bit confirmed and replaced.

My thanks agian.

 

[oldbuffer]

1. If you live in a hard water area, the rarely used shower is the more likely to have lime scale affecting its non-return valve if fitted.
2. If you know the make and model of the showers, look up the installation instructions or spare parts diagrams on the internet.
3. This may show you where any non-return valves are fitted, and you may be able to access them and free them up.

 

[RacingTortoise]

Thanks again. I don't know the model and the invoice shows only a retailer reference, so have asked Vado. On their site, there seem to be two different types - one with brass cartridge and one with a polymer insert. I fear the valves might be hidden in the inlets that may be less accessible....


Update:
Instructions found (I keep most things). Non-return valves integral with isolation valves and should be accessible if the fitter did his job well, but isolation therefore needed elsewhere. Sounds straightforward but......

 

[oldbuffer]

1. The Vado "Elements" concealed shower has non-return valves which should be uncovered by removal of the circular chrome cover plate. They are screwdriver slot valves either side of the main body, level with where the inlet pipes are.
2. As you say, you will have to isolate the supplies first before removing and cleaning.
3. When replacing, make sure the head of the screwdriver slotted shank is level with the body of the non-return valve.

 

[RacingTortoise]

oldbuffer,

Yes, clear what needs doing after finding the documentation. Tried briefly today but the valves are just a little too recessed to grip safely using an adjustable spanner, so better to use use a socket as Vado recommend. I don't have a 27mm socket .....so capital investment needed to complete the job (said it wouldn't be straightforward). Meanwhile, suitable positioning of the temp valve means everything else works ok, so hot showers already.