Away for a week, hot water on or off?

lstar337

I'm going away for a week and can't decide if it will save me to turn the hot water off. It is heated via economy7 overnight.

It seems to use ~3.5kWh/night to keep it ticking over, which would be 24.5kWh or £1.67 on our tarrif.

What I don't know, is how much it will cost to heat from cold.

I know it's never going to be a huge saving, but would I be wasting my time with this?

retiredin2011

I always turn my gas off when I go on holiday in the summer.


I used 5 kWh of gas per day just to maintain my hot water tank at its 60 deg C temperature without using any hot water.


That includes 3 kWh for the pilot light.


Found that out when I forgot to turn it off a couple of months ago.


It takes 18 kWh approx. to heat my tank up again when it has been off for a week.


However the water was still warm after a week when the gas is usually off when on holiday.



How long does it normally take to heat your water back up to temperature if you use it all, say for a bath?

ic

Turn it off seeing as its the middle of summer. You could always ask someone to pop round the day before you're home to switch it back on.

lstar337

retiredin2011 wrote: »

How long does it normally take to heat your water back up to temperature if you use it all, say for a bath?

You couldn't bath in it, the tank is too small. It's 90L, which is why we don't have a bath in our flat. Its only used for washing up and washing hands/face etc.

I wish I knew how much it cost to heat it from cold. That would make it easy to work out if it was cost effective. I don't even know how much heat is lost over a day, maybe it won't go cold in one week?

lstar337

ic wrote: »

Turn it off seeing as its the middle of summer. You could always ask someone to pop round the day before you're home to switch it back on.

Not worried about it not being on when we get back, just wondering if the cost to heat it from cold, will be greater than the cost of keeping it ticking over for the week.

sheffield_lad

well if its 90l 3kw immersion your looking at max of 4hrs from cold so say eco 7 rate is 6p x 12 = 72p max you will pay.

So not worth the worry.

martinsurrey

turn it off.

to heat 90 litres of water by 40 degrees (18 ambiant house temp to 58 hot water supply) will take 15.1million joules of energy.

working - 90,000g of water X 40 (required temp change) x 4.2 (specific heat capacity of water, 4.2joules per gram per degree)

this is around 4kwh ( working - 15.2m / 1000*3600)

there are losses to consider, but still, turn it off, its the cheapest option by a factor of 6.

Cardew

lstar337 wrote: »

It seems to use ~3.5kWh/night to keep it ticking over, which would be 24.5kWh or £1.67 on our tarrif.

Is that with no hot water use?

A well insulated 90 litre tank should not 'lose' 3.5kWh overnight.

Modern tanks are tested to a BS(British Standard) and a typical loss for a 150 litre tank with water at 65C is approx 2kWh every 24 hours.

lstar337

martinsurrey wrote: »

turn it off.

to heat 90 litres of water by 40 degrees (18 ambiant house temp to 58 hot water supply) will take 15.1million joules of energy.

working - 90,000g of water X 40 (required temp change) x 4.2 (specific heat capacity of water, 4.2joules per gram per degree)

this is around 4kwh ( working - 15.2m / 1000*3600)

there are losses to consider, but still, turn it off, its the cheapest option by a factor of 6.

Only 4kWh to heat it from 18c to 58c?

That's scary, why is it taking almost that to keep it ticking over when we use hardly any hot water at all?

Night stores are off, so I may have to investigate where that power is going.

lstar337

Cardew wrote: »

Is that with no hot water use?

A well insulated 90 litre tank should not 'lose' 3.5kWh overnight.

Modern tanks are tested to a BS(British Standard) and a typical loss for a 150 litre tank with water at 65C is approx 2kWh every 24 hours.

Yeah, I'm starting to see a problem here.

The night stores are all off. There are the flat appliances, but it is only our fridge freezer (A+ and uses hardly anything), 2 PVR's and a TV (the rest are on RF sockets which are off overnight), so something is not quite right.

The tank seems pretty old. It is a combination tank with a cold section at the top. It had a very thin yellow jacket on it (no foam), and I added a homemade jacket from a duvet folded four layers thick. I assumed it was pretty well insulated now. It did used to lose quite a bit of heat before added my jacket.

I think I will turn it off tonight, and measure the consumption without it. That might give a clearer picture.

Granariesgirl

My policy is:
HW off - anything more than 2 days away but I have a "holiday mode" on thermostat so it switches back automatically.
CH to no frost - as above.
Fridge freezer off - anything over 5 days: that's a big power guzzler, especially in Summer! Need a kind friend/ neighbour to turn it back on though & get milk & bread.
:cool: