Hot water - on Constant or Twice?

The_Governor

I've always subscribed to the view that keeping the water on constant is cheaper than on Twice, as it takes less heat to keep a tank warm than it does to heat it up from cold twice a day.

However, bill increases have made me question this and I'm wondering whether anyone has done a comparison between the two setting to see?

In my head my theory still holds true, but I wonder is the practical reality different?

penrhyn

You are only going to be heating the tank from cold if you run off all the water.
I suspect with a well lagged tank, with a tank thermostat, it does not make much difference either way.

Magentasue

I'm sure on the gas and elec board, it is recommended to heat the water you need when you need it and no more.

The_Governor

Well I've taken a reading this AM and have put it on Twice for water (heating isn't on yet anyway). I'll see how my usage changes and report back

Dave2112

I only heat water when I need it. I've taken the view that it'll take more energy to keep the water at a constant temperature than to let it cool down and then heat it back up when needed. Haven't got figures but with the following scenario ....if you heated water up on a Friday, turned off the boiler, used the water and then went away until Sunday evening when you turned on the boiler again to heat the water. This would be better than leaving the water on constant.

My boiler is 20+ years old but still heats up a full tank in well under 30 minutes, the tank is well lagged and if water is heated up one evening it's still warm the next. Though the more you use, the more cold water enters the system so cooling the remaining hot water.

bigpaws23

I used to think the same as you, but did meter readings both ways and saved a considerable amount by having it come on for an hour in the morning and an hour early evening.........

Let us know what you find out. Our tank is quite old, so probably not very well lagged, although I have packed some old duvets around it.

Cardew

This has been covered many many times on MSE.

Without question it is more expensive to leave hot water, CH etc on constantly.

2 extreme examples to illustrate the principle:

If you went away on holiday for a month, would you leave the water on constantly? How about a week? a day?

If it was cheaper, then why not leave your kettle simmering all day?

That said for a well lagged HW tank the losses are not great and leaving it on constantly will not make a huge difference. Tests have shown a full tank of hot water will lose approx 3 kWh in 24 hours.

Sirbendy

I agree with Dave...on when you need it, off otherwise. Ours cuts in at 7am for 30 minutes (shower) midday for 30 minutes (probably can remove this), 5pm for the same (washing up).

Works nicely.

The_Governor

You've obviusly got a fancy control box there SirBendy as it allows you to have three settings?

Andy_Hamilton

Mine was set for 30 mins in morning BEFORE I woke up, the morning shower uses some of the water so it's not sitting warming a cupboard all day. It also came on at 5pm for 30 mins.
Since my lodger left I've dropped this down to just 30 mins at the night time (any heat loss works as central heating). Even before I double lagged the cylinder and cupboard the water stayed HOT for 2-3 days.
If my system is loosing 3-4Kw per day that works out as 10-12p. My daily usage for 2 people was only 60-70p per day

tomstickland

The_Governor wrote: »

it takes less heat to keep a tank warm than it does to heat it up from cold twice a day.

That is plain wrong.

There are many ways to explain it.

The rate of heat loss from the tank is proportional to the temperature difference between it and the building. So if you keep the tank hot all day more heat will be lost compared with allowing it to cool when not needed.

Or, thinking of the kettle. This is incorrect: "it takes less energy to keep the water boiling all day than it takes to heat it up when I need it"