Heating water efficiently with a Megaflo tank

[33goingon64]

Now we have a smart meter I am shocked at how much we're paying for gas. Even when we were on holiday last week with thermostat set to 16 we used £5 per day. The water heating is set to be on for an hour in the morning, an hour in the middle of the day and an hour in the evening. We have a Megaflo tank and combi boiler. The house is quite big compared to average which may be relevant.

My question is: is that a really inefficient method of water heating? If the heating was hardly on last week we must be spending most of that fiver a day on heating water. Is it unnecessary to have it on at lunchtime? Is it better to have it on all the time but lower (though would that make water colder?).

Anyone in a similar situation who has worked out a cheaper solution? Thanks.

You don't have a tank/cylinder with a combi boiler.

You have a system boiler with a hot water cylinder, and the hot water cylinder is megaflow - known as an unvented cylinder because it is pressurised.

The most efficient way to use hotwater in this setup is to create a cylinder full of hot water, use it all, then do not refill it with hotwater again until you need it.

What you don't want to do is create the hot water, use it, but while you are using it continue to heat the water. What this will do is make sure that when you have finished all your showers/washing, there is a full tank of hot water left, which you don't need.

My megaflow cylinder is 125 litres, and holds enough hot water for 2x showers and 2x washing up (although now I have the bosch dishwasher I do not use it for washing up). I turn it on for 30mins every 2 days.

TBH you probably won't save that much money by optimising your hot water. The reason is that megaflow tanks are normally really well insulated, my one only loses 1kWh of energy every 24 hours (about 10p in gas terms). Where you will save money is by using less hot water.

I have a megaflo. I heat it for 20 minutes in the morning and that is enough for 3 showers and the washing up. If someone wants a bath in the evening I top up another 15 mins of heat.

When the heating is off we only use about 5kw gas a day.

@Ridingthegravytrain

I am about 2-3 kWh, same ballpark but less showers per day of course.

I guess looking at our numbers the OPs do look excessive.

@ 10p per kWh £5 of gas should get you 50 kWh per day, so 30 showers per day by your usage.

Something not quite right there, unless the OP is hosting the local football team.

We have a megaflo cylinder. Recently went away for 2-3 days and completely switched off the hot water. The water in the cylinder was still hot when we returned. So as long as you're not using all the hot water in the cylinder the actual heating time for the hot water each day can be very short.

How old is the Megaflow and do you live in a hardwater area? Chalk build up is an insulator and it will take a longer burn time to heat the water.

You need to heat the cylinder above 60C.

Ours is on for an hour in an evening. That's it. It's a massive tank, but we have yet to run out of hot water.

I'd suggest scrapping the lunchtime heating period. And maybe one if the other two if you don't have large numbers of baths.

Wow, I feel very ignorant. Thank you for the advice. I understand the idea of hot water usage, I was surprised that even when we were away it was costing £5 per day. I'll definitely kill the lunchtime hour and consider the evening one too.

@33goingon64 thank you for starting this thread. We inherited settings for our megaflo and kept them, but it looks like we can reduce by st least an hour a day.

Well, I killed off the midday heating and have saved several pounds today! Hooray!

To illustrate, the daily rate was about £11 and yesterday it was £6.40. But that was without heating on at all and water heated for only 2 hours. Wow, what a lot of money I've been wasting!

Doesn't sound right at all.

Let's say you have a 30kW boiler (large) and it heats for one hour. That will use 30 kWh gas absolute max (and most probably much less). At 10p per kWh that would be about £3 (30x0.1) max - you are apparently "saving" £4.60 by turning off for an hour, almost half again as much.

It feels like here is something wrong somewhere with either your metering or the way your system is working.

How many litres of water is your cylinder (it probably says on the front) ?

How many kWh per day do you use at the meter (not the smart meter) ? If you post your readings and units (m3 or ft3) we can work out whether you are using a correct amount of gas.

I have a 250L megaflow and heat it for one hour in the early morning every day before we wake up and it gives us enough hot water all day, 4 powers showers etc. I would only flick it back on for a bit if I wanted a bath in the evening (its a big bath).

So the plumber came today for another reason and I asked him about it. He says it's cheapest to have the hot water set to constant on the timer because it turns itself off when at max temperature and heats itself again when needed rather than the timer deciding to fire up when it may not be needed. So I'm going to try this now! Gas usage so far today £1.35 with no heating on and hot water now set to constant.