Turn off Hot Water Tank to save money?

sparky0138

FreeBear said:

Legionella is a very small risk in this country. Some 250 cases reported annually. Having an uncovered cold water storage tank with birds flying around will increase the risk, but most people will have a covered tank in the loft (so no birds). If you do have any concerns about disease, heat your tank to ~55°C for a couple of hours once a week. At 60°C, bacteria will be killed within minutes. At 55°C, legionella bacteria will be killed in an hour or so.

If you have a combi boiler, none of the above should matter - The incoming water will have been treated, and it doesn't sit around in the pipes long enough for bacteria to breed.

Thanks. It's not a combi boiler. 2 hours a week sound like a good compromise and should still save us £70 a year if my sums are right.

[Deleted User]

save us £70 a year if my sums are right.

And your sums are……..? We are a 2-person household and we heat our 216L unvented hot water cylinder using the immersion heater. Average daily energy usage is about 2.5kWh/day. 

Alnat1

Are you using all the 216L daily or is the 2.5kWh just topping up the heat in what is already fairly hot water?

sparky0138

Dolor said:

And your sums are……..? We are a 2-person household and we heat our 216L unvented hot water cylinder using the immersion heater. Average daily energy usage is about 2.5kWh/day. 

Boiler (40 minutes per day):
18kWh x 3.999p per kWh = 71.98p per hour
71.98p ÷ 60 minutes x 40 minutes = 47.99p per day
47.99p x 365 days = £175.16 per year

Kettle (2 boils per day):
0.225kWh (average 4-minute boil) x 2 = 0.45kWh per day
0.45kWh x 18.028p per kWh = 8.11p per day
8.11p x 365 days = £29.60 per year

£175.16 - £29.60 = £145.86 saving

Boiler (2 hours per week):
18kWh x 3.999p per kWh = 71.98p per hour
71.98p x 2 hours = £1.44 per week
£1.44 x 52 weeks = £74.88 per year

£175.16 - (£74.88 + £29.60) = £70.68 saving

You said you only use 2.5kWh/day so I've probably gone wrong somewhere.

Apodemus

[Deleted User] said:

Legionella is a very small risk in this country

Arguably, so is Scarlet Fever which is in the news at the moment. I would say that it is a given that if people stop heating their hot water cylinders there will be an increase in Legionella infections. The risk is magnified if there is very little water flow.

Not even close!  Very roughly, the number of cases of Legionella in a year in the UK equates to the number of cases of Scarlet fever in a week. 

k_man

sparky0138 said:

Dolor said:

And your sums are……..? We are a 2-person household and we heat our 216L unvented hot water cylinder using the immersion heater. Average daily energy usage is about 2.5kWh/day. 

Boiler (40 minutes per day):
18kWh x 3.999p per kWh = 71.98p per hour
71.98p ÷ 60 minutes x 40 minutes = 47.99p per day
47.99p x 365 days = £175.16 per year

Kettle (2 boils per day):
0.225kWh (average 4-minute boil) x 2 = 0.45kWh per day
0.45kWh x 18.028p per kWh = 8.11p per day
8.11p x 365 days = £29.60 per year

£175.16 - £29.60 = £145.86 saving

Boiler (2 hours per week):
18kWh x 3.999p per kWh = 71.98p per hour
71.98p x 2 hours = £1.44 per week
£1.44 x 52 weeks = £74.88 per year

£175.16 - (£74.88 + £29.60) = £70.68 saving

You said you only use 2.5kWh/day so I've probably gone wrong somewhere.

How big is your tank, and how well insulated is it?
And do you know what the tank temperature is set to?
How much hot water do you use to wash up?

The boiler won't heat the water constantly, only until it has reached the set temperature.

Our water heating is timed on for 90mins per day, but is only heating for about 45. That is for showers, washing up, and hand washing. And our boiler is lower output than yours.

18kW for 40 minutes would heat about 150 -200 litres, which is a lot of washing up 😁

The losses from the tank (if full) are 1 - 2kWh per day if well insulated, and the losses from the heating system getting up to temp to heat the water, could be another 1 - 2kWh per instance of heating.

So you may find you can heat the water once every 3 or 4 days to 60°C for 40 mins (for the legionella cycle), and still have hot (enough) water all week for washing up.

FreeBear

Apodemus said:

[Deleted User] said:

Legionella is a very small risk in this country

Arguably, so is Scarlet Fever which is in the news at the moment. I would say that it is a given that if people stop heating their hot water cylinders there will be an increase in Legionella infections. The risk is magnified if there is very little water flow.

Not even close!  Very roughly, the number of cases of Legionella in a year in the UK equates to the number of cases of Scarlet fever in a week. 

Scarlet fever - Spread via close contact with an infected person. Highly contagious.

Legionella - Contracted via breathing in infected water droplets. Can not be spread by person to person contact or drinking contaminated water.

Mpox (another one that has been in the news lately) - Spread by close contact with an infected person or animal.

[Deleted User]

Alnat1 said:

Are you using all the 216L daily or is the 2.5kWh just topping up the heat in what is already fairly hot water?

My immersion heater is on constantly with a 2 hour boost window in the evening. Using gas, it averages at 8 kWh a day. No solar surplus yesterday, all the reheating came from the Grid. I had a shower and my wife had a bath. The immersion heater thermostat is set to 65C.



OfWAT uses 45L as the average consumption per person per day.

Magnitio

An insulated hot water cylinder will lose heat at a rate of 50 to 100 Watts. Even more if old and poorly insulated. So that's 1.2 to 2.4kWh per day. Indirect heating of the cylinder from a gas boiler has inefficiencies, so it could take 1.5 to 3kWh of gas just to maintain the same temperature without any hot water being used. So if you are going away for a while, it is worth turning off.

EssexHebridean

During the BST months we heat the tank fully on a Sunday night, then top up with a 20 minute or so burst on weekday mornings, it then goes off altogether on a Friday. That gives us enough hot water to deal with washing up, face washing and MrEH shaving. At this time of year that doesn’t work because the timing on the E7 rate makes that first thing in the morning burst too difficult to manage so it just runs overnight. For us, it’s a balance between quality of life - having hot water coming out of the tap when we want it - and energy saving/cost effectiveness. If we were in a position where we couldn’t afford the bills, the overnight heating of the tank would stop in favour of using the kettle.