Hot water not coming out Megaflow

17 Jul 2022
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United Kingdom

We have two linked megaflwo tanks as in picture. Recently we noticed we were running out of hot water very quickly. It turned out the left hand tank was not giving out any hot water, just the right. Before the plumber came to look at it the hot water outs were connected in series at the top and you could feel the left hand out be cold when a hot tap was run and then come back to feel hot when the tap was turned off.

The plumber said either the cold water ins or hot water outs should be crossed. So he's crossed the hot water outs and therefore they are no longer connected at top. This did not solve the issue. He also installed two leavers on the cold ins so as to seperate the tanks allowing them to be tested independently. It was then noted the pressure reducing valve, second picture was not allowing correct flow of water. He said 100% a new prv would sort the issue. So done that.

Now we are still no further forward. When running hot tap we get hot water for a while then the hot out on the left hand tanks turns cold and water at tap drops in temp. Once tap is stopped the pipework goes back to being hot, I assume this mean there is still hot water in the tank.

All elements on both tanks are fine, been tested by electrician and plumber, giving correct resistence and the two on left hand tank are new July 2020. We have no mixer showers, isolating the mixer tap has made no difference.

One thing I'm not clear about is I thought it was the cold water coming in bottom of tank that help push hot water out the top. If I close off both the cold water ins using the leavers installed hot water still flows quite normally out the taps.

Any thoughts much appreciated please and thanks.

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Not sure what you mean having the two HW outlets in series, the two cold water inlets are in parallel after the (one) PRV so the two HW outlets must be in series as well?.

You could (theoretically anyway) run the two cylinders in series whereby you feed the cold water to one cylinder, the HW outlet from this cylinder then is fed in to and replaces the cold water inlet to the other cylinder, with the (one) HW outlet from this cylinder. You then have two cylinders of hot water available, disadvantage is you don't have the two cylinders heating together if the water was being drawn off uniformly from both, what will happen is the first cylinder will start reheating as soon as a small vol of HW is drawn off and its cylinder stat closes, you will then continue to draw off hot water until the first cylinder HW is almost completely exhausted and cooler water from its top will enter the second cylinder and close that cylinde's stat (with still almost a full cylinder of HW remaining (second cylinder), I would be surprised if both cylinders connected in parallel (like now) will ever result in equal volumes of HW being drawn off from each.
Thanks for reply John.

This drawing shows how the hot outs were when problem first discovered. You can see both hot outs connected at top of tanks then pipe taking hot water to bathrooms etc. We were getting about 180 or so litres of hot water then it would run out - both tanks are 210 litre capacity. The area circled in red was cold, properly fresh water cold. The plumber said cold water was coming out top of right hand tank and basically stopping the hot water from left hand tank passing and travelling onto bathrooms. He told us that crossing the outs would solve this. The left hand tank now connects into hot supply pipe below the right hand tank. It didn't. As mentioned above he then thought the prv was 'choking' the water going into lefthand tank and thus preventing it from leaving. Both tanks have a prv on cold supply. The plumber said I could isolate a tank at a time by closing the installed levers (blue handles) - doing so is currently giving me about 35 litres of hot water out of left hand tank it then runs cold but within a couple of minutes the out pipe at top is hot again and I get a little more hot water. I've tried to get you a better picture, you can see the newer shiny pipe work at right hand side where hot outlets meet, the right hand pipe comes from left hand tank.

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I think you have, i would use one PRV with a T as close to and equivalent distant from both cold water inlets.
Okay, thanks for prv advice.

more info - I ran the water totally cold, switched off emements on both tanks, ran water totally cold whilst isolating each tank in turn and together. I then heated water in the right hand tank only. With the cold supply to left hand tank closed I ran the hot tap. The hot outlet pipework from left hand tank is now properly hot, all the hot out pipework is properly hot. This does seem to suggest hot water from right hand tank is travelling back towards the left hand tank?
It should/might put a few liters back into the left hand cylinder to re pressurize the air bubble but I can't see it doing anything else. Assuming the left cylinder is still cold, if you then re open the left hand cylinder CW isol valve does (very) hot water still flow to/through the HW taps, if cold water is also flowing through then the mixed temperature will be much lower than from the RH cylinder alone, then shut off the RH cylinder CW isolating valve and cold water only should flow from the HW tap?. If that happens during those tests then I think it probably proves that the RH cylinder PRV is set slightly higher than the LH one (assuming two PRVs), If so then I think you should just feed both cylinders from one PRV.
Okay, reopened left lhand cylinder cw isol valve, so both cw isol valves are open - water at tap is cold.

close re close left hand isol valve - water is properly hot coming from right hand tank

do the reverse and just take from left hand tank and water is properly cold.

open both again and water is properly cold.

would this indicate a) the water is not mixing and b) left hand tank is 'overpowering' the right hand tank? The prv on left hand tank is the new one, prv on right hand tank is 20 yrs old and makes a fair noise when water going into the tank. the left hand tank is much quiter when filling.

I am contemplating adjusting the configuration to have them in series, this seems the least rejigging of pipework and could use the new prv on left tank but I feel bothered by fact that both heating elements on left hand tank being on overnight only resulted in 6 buckets of hot water (normally 18) from left hand tank, before going cold.
Re Heating electric ??elements, assume you have one at the bottom of the tank and one at the "top", do both actually come on at night?, I thought that the bottom one only came on at night, night rate cheaper electricity? are you sure that this bottom element or its thermostat is OK which I feel has nothing to do with the flow rates.

When you say connecting in series, are you saying connecting them like I suggested originally or simply feeding both cold water inlets from the new PRV?, if the latter then I would think a pretty reasonable chance that both will work in parallel as there must be lots connected like this??. There is no way IMO that they can work as configured ie with separate PRVs where, as you say, the LH is overpowering the right, even if adjusted, I still don't think it would work properly but either of the two options above, should.
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Hello, re heating elements, when the storage heaters were removed years back everything was put onto normal meter so there is power to elements all day. We have them set on timers but can override and have them permanently on which is how things have been recently. Couple of weeks back I was getting expected capacity of hot water from left tank on its own. Last night I switched on bottom element only and set thermostat a little lower, later today I will see where the thermostat is clicking on/off and see what volume of water I get. Both element and thermostat are 2 years old max.

Connecting in series - think I prefer how you described in your first reply - the hot out of first tank going into bottom of second tank. For me this ( I think) eliminates balancing issues, pipe lengths etc also allowing first tank to be switched off and using second only at certain times of year and if one element goes the other 3 will support it.

I had initially thought I could heat the water in first tank a little lower say 45 celcius and then have second tank bring it up to just over 60 so as to not have first tank doing all the work but then I wondered about legionella being an issue in first tank.
If you close the cold inlet to both the cylinders.
You should not get any flow from a hot tap.
Is this the case?
thanks for help folks, I'll get a chance to have a look at things again this afternoon.

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