Megaflow wired backwards

[flowjoh]

Hi, I recently bought a flat and have discovered that the Megaflow (Unvented Megaflow DD201HE) is wired backwards: the economy switch is hooked up to the top and the regular electricity to the bottom.

So I'm not going to be able to use the immersion during daytime which is a little bit of a pain but not the end of the world.

I'd like to understand more about the two power inputs. I believe they both heat elements in the tank and that the bottom element is a bit better to use as then it heats all the water from the bottom up. But is that it in terms of differences? If the top element also has a thermostat and turns off when the water reaches the specified temperature then I can live with the backways wiring for a while.

Does anyone know?

have a look at your meter and your consumer unit(s). Post photos of them and ALL the cables around and between them.

There is a good chance that, during the off-peak period, all the electricity in the house is charged at the lower price. In which case you can turn on your lower immersion element with a timer, or at bedtime.

The lower element heats the entire cylinder, which might be 200 litres or so, and is often enough for an entire day's usage.

For economy, the upper element can be turned off unless you run out of hot water during the day.

To re-connect the cables to the immersion heaters is a trfling job.

I am assuming that you have no gas boiler and must heat the cylinder with electricity.

Energy from electricity currently costs about six times as much as energy from gas, so a gas boiler is preferable.

Thanks for your reply!

My apartment is supplied by electricity only (no option to get gas). I've attached a photo of the consumer unit and the meter.

I think you are correct that all power during the night is at the lower rate (even if not from a dedicated 'night' circuit). The dedicated night circuits are the three on the right hand side of the consumer unit.

So I should get an electrician to swap out the two cables where they enter the tank (pictured)? Do they need to be Megaflow certified to do this work or can a regular electrician do this work?

it's only changing the electrical connection, so an electrician can do it.

plumbing work on an unvented cylinder has to be done by a plumber with a special qualification (G3 ticket). Many boiler menders have it now.

I can't see how the timing is done on yours. If you don't already have a timer for the immersion, consider getting one. The Horstmann is very good, and brings togethe the off-peak and the on-peak in one control. The Digital version costs the same as the older design with the timeclock, I prefer the older design, it is more intuitive to set, but have what you want.

That's really helpful, thanks for the info! Good to know that just a regular electrician can switch the connections for me.

I don't have timers currently so the tank is being heated up from 12:30am til 7:30am. Its costing roughly 4 units a night (or 60p) so not too crazy. I presume it'd be cheaper only doing it for the required 2 hours but maybe not massively so. Going to wake up at 5:30 some morning and switch it on to measure the difference on the meter, see if its worth getting that timer soon or if I can wait for a bit. My neighbour has the Horstmann and it does look like a good setup, will get one in at some point.

I also called a shop that sells Megaflow tanks to ask them if there's any real difference between the bottom and the top element on the tank, besides their position. They told me there's not: both heat up the water to about 60° and both turn off due to a thermostat when they hit that temperature. So seems like I don't have to rush with the rewiring either.

4 units a night

4kWh is the amount of electricity needed to heat about 75 litres of water, and will take one hour and 15 minutes. So that's probably all you are using (unless you have a different heater turned on during the day). If it is already partially hot, it only needs to warm the cold amount; once it is hot, the thermostat will turn it off, even if the electricity is still on. It will use a bit more electricity in winter when the incoming water from the main is colder.

Megalflo's are usually quite a lot bigger that that. Because the warm water rises and stratifies, they only heat water at or above their own level, so the lower heater will warm the whole tank, and the upper one maybe a quarter or a third, depending on its position.

(for the benefit of other modern cylinder owners who have a gas boiler, it will take around half an hour to heat a whole tank as they are much more powerful)

Right that all makes sense. Mine is a 210L, so heating with the hot element only it's only heating about 1/3 of the tank.

In my case using the top element only is working out ok as I'm not using that much water - one 3-4 minute shower and washing my hands mostly (and a tiny bit of washing up but most of that is done by the dishwasher). I have had hot water anytime I've needed it so far so I must be staying under that ~75L level.