How long do you heat your hot water tank?

Harnser

Harnser

Harnser

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For the first time I'm living in a house with a vented hot water tank (before I've always had combi boilers). The tank was unlagged when we moved in so I've added a fibreglass jacket to it, and the thermostat was programmed to heat the water for 2 hours morning/evening. I turned off the electric immersion heater as heating it via gas was cheaper.

With fuel prices going up and only two of us in the house, is heating the water for 4 hours a day normal? The shower uses hot water from the tank and that and washing up are the only uses most days. Baths occasionally.
 
It's almost irrelevant* how long it's programmed for as the thermostat on the tank (on the surface near the bottom) should indicate to the boiler that the water is up to temperature and stop the boiler.

*I say irrelevant because if you draw hot water from the tank then cold obviously replaces it and the temperature drops; if this is during the programmed on period then the boiler will heat it up again.

What you need to is determine yourself how much hot water the household uses and when. For example my other half could empty a 110L hot water tank when having a hot shower in boost mode if the boiler wasn't on, ridiculous but true! Programme it an hour or so before you need the hot water and reduce the duration until it's not hot when you need it, then change it until it is. It'll take a different amount of time to heat a full tank depending on the temperature of the water already in it and the temperature of the cold water feed.

You should never use an electric immersion heater unless in a dire need as even before the energy price increases it cost a fortune to run.
 
The amount of energy you will use will be made up of:
The amount of hot water you use
The amount of heat the tank looses naturally over time.

The amount of time you have the tank ‘heating’ is sort of irrelevant as the heater will turn off once the tank is up to temperature. Of course if you use any water in that 2 hour period it’s ‘on’ the boiler will automatically top it up.

A well lagged tank shouldn’t lose much heat over time but if your airing cupboard is a lot warmer when you open the door, you need more lagging.

If your heating it with a boiler, it should be able to recover the tank temperature within about half an hour unless you have completely emptied it.

I’d set it to come on before you would normally have any showers in the morning and then again in the early evening before you would normally use any hot water for baths and showers and call it done. If you find you run out of water then adjust accordingly.
 
I used to play the game of picking times for the hot water cylinder to be up to temperature, but got fed up of the occasions we ran out due to extra showers during the day and then having to wait.

Now I have the controller set to heat it if required from 05:30 to 23:30. Even though we have our HW set to 60 degrees the extra energy usage (both gas for boiler and electricality for pump/boiler) is far less than I expected based on some measurements I did, although we do have a largish cylinder that is well insulated and our boiler pipes are also well lagged so lose very little heat on their way to the cylinder.

You should try reducing your HW hours until it becomes annoying, but for overall costs I found reducing heating temperature a fraction and times far out weighed longer hours on the hot water. I just need to stop the other half leaving doors wide open in cold weather when unloading the car, or opening doors/windows when its a bit cooler than our normal heating temperature and letting the heating try to heat the garden!
 
I'm pretty worried as I'm also moving somewhere with an immersion heater for the first time ever after always having a combi. Ive got no idea how different it's going to be!
 
XeNoN89 said:
I'm pretty worried as I'm also moving somewhere with an immersion heater for the first time ever after always having a combi. Ive got no idea how different it's going to be!
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If that's purely electric HW heating, not a cylinder heated by a gas boiler with a backup electric immersion then yes that will be costly if you use much hot water.

However for combi vs cylinder heated by a boiler then its not that much more costly as manufacturers say. A combi has to keep firing up and isn't as efficient when running for shorter times and I would never one as I like the convenience of having a lot of mains pressure hot water ready to go, multiple showers at the same time, plus an airing cupboard. Combi boiler are more about simpler fitting and space saving.
 
Simon42 said:
If that's purely electric HW heating, not a cylinder heated by a gas boiler with a backup electric immersion then yes that will be costly if you use much hot water.

However for combi vs cylinder heated by a boiler then its not that much more costly as manufacturers say. A combi has to keep firing up and isn't as efficient when running for shorter times and I would never one as I like the convenience of having a lot of mains pressure hot water ready to go, multiple showers at the same time, plus an airing cupboard. Combi boiler are more about simpler fitting and space saving.
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The new place has no gas at all. Immersion heater for hot water and electric underfloor heating throughout.
 
b0rn2sk8 said:
A well lagged tank shouldn’t lose much heat over time but if your airing cupboard is a lot warmer when you open the door, you need more lagging.
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I find if I heat the tank using gas the airing cupboard is warm but using electric it’s not. Seems like usung gas it also warms the surrounding pipes with hot water which in turn heats the room and walls.

So, Gas is less efficient but helpful in winter to heat surrounding area and dry clothes / towels.
 
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SDK^ said:
I find if I heat the tank using gas the airing cupboard is warm but using electric it’s not. Seems like usung gas it also warms the surrounding pipes with hot water which in turn heats the room and walls.

So, Gas is less efficient but helpful in winter to heat surrounding area and dry clothes / towels.
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The heat will travel to the cylinder via the pipes so you also need to lag the pipes to stop the heat loss into the rooms.
 
Personally I have it coming on for 2x 30min periods each day. In summer it's kept hot via solar divert to immersion.

Worth checking the boiler thermostat on the tank is actually working and in the right place on the tank to work - the last 2 houses I've moved to they have been broken.
 
Left on permanently and stat set to 65c, last service the engineer recommended this setup. Not sure if its relevant but both me and the misses are shift workers so hot water usage varies every week.
 
About 40mins in the morning in summer for our pressurised unvented cylinder (think it's quite smaller, either 120l or 180l)
Tend to up to an hour in winter.

Never had an issue with just 2 of us, showering up to twice a day
 
We have a oil combi - I really do miss the hot water tanks in old houses we had - I kept it on 24-7 and I don't think there was much difference in costs trying to fiddle with settings. Once up to heat it didn't take long to reheat - I also had a Tee off radiator from hot water pipes to tank and that bathroom was never cold.
My oil usage per year was 1200 ltrs the same as it is now with a combi which I hate.
 
I only heat ours once a week.

The day of cleaning the house!

Don't use it for anything else at the moment.

(shower is electric)
 
30mins in the morning and evening. Family of 4.

Kids often have a bath (the youngest hates showers for some reason), and useful for washing up pans and bits that don't go in the dishwasher.
 
I recently changed a faulty Grundfos pump to a cheap BritTherm pump which is noticeably louder, whiney even (it's going to be replaced soon). Since then I've discovered that the system is wired in such a way that when the Hive signals for hot water it closes the zone valve for the hot water circuit and the zone valve's n/o contact closes and signals to the boiler to fire and the pump to run. Now, the oil boiler cuts off when the hot water tank has reached it's temperature but the pump continues to run. It runs for as long as the hot water zone valve is actuated which depends on how long it's programmed for and it's noisy and quite inefficient. I really need to check the wiring to see if it can be improved in any way.

Anyway, the longer you have hot water set for the more electricity you are using as well as oil or gas.
 
The hot water boiler is always on 24/7. It's a new cylinder with minimal heat loss. Probably costs 20p extra a day to never run out of hot water or worry about setting timers.
 
24/7 hot water. Most modern tanks (mine included, from 2000) will have negligible heat loss at 60c compared to the energy that you take out via using that hot water.
Lets assume your tank loses 2kWh per day (pretty average, leaning on conservative figure) at 60c in ambient temp. At todays price caps of around 7.3p per Kwh and also assuming a 80% efficiency for boiler/pipe loss it would cost you a grand total of £5.66 a month in gas to simply heat the tank for a month and keep it heated but never use the hot water.

SDK^ said:
I find if I heat the tank using gas the airing cupboard is warm but using electric it’s not. Seems like usung gas it also warms the surrounding pipes with hot water which in turn heats the room and walls.

So, Gas is less efficient but helpful in winter to heat surrounding area and dry clothes / towels.
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Its more efficient and cheaper to use gas to heat your tank rather than resistive electric unless you are producing the electric yourself for "free". Even then its debatable as you could be using the electric for more useful work than heat. Most the of the electric from the grid is created with gas and there are larger losses in doing so than the bit you lose heating the cupboard.
 
Gammawolf said:
Its more efficient and cheaper to use gas to heat your tank rather than resistive electric unless you are producing the electric yourself for "free". Even then its debatable as you could be using the electric for more useful work than heat. Most the of the electric from the grid is created with gas and there are larger losses in doing so than the bit you lose heating the cupboard.
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Cost : using gas isn’t cheaper for me
- The electric rate I use for heating water is less than my gas rate. Elec : 7.5p kWh and Gas : 8p kWh now but soon will be ~17p (Oct price cap increase)
- Also, my excess solar heats water, if I don’t use it for this it gets sent to the grid at half (or 4x less) what I buy it at.

Efficiency : gas is less efficient at heating water than electric
- Electric converts nearly all the energy it intakes into heat for the water in the tank. Gas has loses through heat in other parts of the system

Grid side
- Demand also has an element in pricing energy, not just the cost of what is used to create it.
 
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