Megaflo dd145 Economy 7 water tank issue

17 Feb 2022
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United Kingdom
Hey all.
I've just moved into a new flat with economy 7 so we have night storage heaters and a night heating water cylinder. It's 145L, but we only get half, maybe not even that before the water is freezing. The whole tank seems to be heated in the morning as it feels slightly warm on the outside but it will run out very fast, and by the end of the day feels cold other than right at the top. When we have got water it's really hot, so it's definitely heating to hot enough, there just isn't enough of it. It ran out the other day after two extremely shallow baths, as in water just covering legs, and they weren't run really warm either. We hadn't used any other hot water all day purposely to test it. We leave the night heating switch on the whole time, so I know it's heating for the full amount of time every night when we get the off peak rate. We have an immersion boost but that's charged at day rate and we can't afford to run that every day. It's a very old cylinder, over 25 years, but has apparently had the elements changed before. Will Changing them again fix the issue? I can't see how it would because it is heating water, it just seems to heat a bit and not the whole lot for some reason. Does this sound like an element issue?
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Please post a photo if the cylinder, and the two cables coming out if the immersion heaters, and the switches or timers that control them. It may be that the lower one is not working.

Sometimes people wire the upper one to the night supply.

A bath holds around 100 litres.
Thanks for replying!
So I'm pretty sure they are wired the right way around. When it is heating overnight I can feel warmth from the cable for the night heating and at the bottom where this cable attaches to the bottom element. When I switch on the day rate immersion I can feel this cable become warm and the area around the top element becoming warm.
I ran the baths probably 1/4 full, so maybe using 20-30L of hot water. I could do this twice and that would be all of it gone. I've just done the same and can feel that about half the tank has gone cold now but there's no way I used half of a 145l tank.
Sorry meant to add - there's no timer. With the night heating switch on it automatically just heats during the economy 7 off peak hours, something like 1am-6am.
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Feel the pipes. When you are not running a hot tap they should be around room temperature. Maybe you have a dripping tap or a hot water leak.

Have you got a water meter?
Ah ok, so yeah the pipe that carries the hot water does feel much hotter than room temp, even if we aren't using any hot water.
We have a water meter but we can't get into it.
Checked for any leaks and definitely none, we only have two sinks and the bath and there's nothing that's dripping.
145L isn't very much in the grand scheme of things. That can be used up very quickly, especially if it isn't being replaced.

Economy 7 is just that, you get cheap electricity for 7 hours. Usually between 12am and 7am or thereabouts, it changes slightly depending on location.

I think you need to check what the HW temp is, 1st thing in the morning when it's been on all night and then check what the stats on the elements are set to, may need the settings increased.
Is this flat new to you or brand new? If the latter it is not unknown for toilets to be misplumbed onto the hot water...and you really need to be able to access that water meter if it is supplying your flat exclusively.
...the pipe that carries the hot water does feel much hotter than room temp, even if we aren't using any hot water.

We have a water meter but we can't get into it.

Checked for any leaks and definitely none....

1. The pipe immediately off the top of the cylinder is no gauge, you'll need to feel it at least 3 feet away for a reasonable gauge. You can save energy and money by lagging that very pipe.
2. No explanation as to why you can't get to read your water meter. It would be very useful to you, not least of which is that one day you may need to turnoff the water supply in a hurry.
3. Do you use 'definitely' [no leaks] in the same context as [the water was] 'freezing'?

If you say the first drawings of hot water are very hot then there is nothing to gain by turning up the immersion heater thermostat.
Your bathwater consumption can be better quantified by making a mark at the 'normal' water level of your bath (use masking tape). Once emptied you can refil to the same level with cold water from a measured source, such as a 15 litre bucket; then you'll have a reasonably credible figure to quote, helping all to come to the right solution faster.

Be aware that, as hot water is drawn off from the cylinder, cold water replaces it, and to some extent will mix with and cool the remaining contents. Manufacturers make efforts to reduce this mixing, but with a 25 year-old cylinder whatever baffles were in place to stop this may have rotted off and be ineffective.
Another consideration is scale. If you live in a hard water area scale will form, and over time will take up space in your cylinder, so your 145 litre may in reality be less than 100 litre. There is a test...but it is probably better to do this in summer, when the storage heaters are off.
1. Turn off all other uses of night-time electricity.
2. Have a night-time bath and run the hot water until it goes cold.
3. Read the economy 7 meter at 11pm.
3. Read it again at 7am, and before any other power is used.

Each litre of water takes about 4.2 kJ to heat up 1C, so for 100 litre of water to be raised from 10C (cold) to 60C (hot) you will need 4.2 x 100 x (60 -10) kiloJoules, which is 21,000 kJ. There are 3600 kJ in one kWh, so that will be 5.8 kWh. Allowing for tank losses you should expect the morning meter reading to be 6 units higher than the evening reading. If the volume of water is less then the electrical consumption will be proportionately lower....thus you can approximate the contents of the cylinder. To get more accurate results measure the water temperature before and after.
I know it's a faff, and it can't be done when heating is needed elsewhere, but it's something you can do for free.

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