Very thick question re: heating water.

[AdamAnt]

We have a system boiler with tank (i.e. not a combi-boiler).

We were told by a plumber that it would be better to leave the boiler on constantly, rather than setting it for, say, an hour in the morning and/or evening, as it takes a lot of energy to heat the tank up from scratch. He said if you leave it on constantly it just gives the tank a wee boost of heat whenever the thermostat prompts it.

Our gas bill seems big this month (first month in this house) and I'm wondering if it's because we're doing as he suggested. Previous house had a combi-boiler, so can't make direct comparison.

what heating controls do you have?

We have a thermostat and a timer. (Is that what you mean? )

do you have a thermostat on the hot water cylinder aprox 1/3 way up from bottom? trv's on the radiators?

Ah, the hot water tank is virtually inaccessible (short of dismantling the (rented) kitchen). I think there's a thermostat on the boiler though. I think we have trvs on the radiators (the things you twiddle to make the rads hotter or colder, right?)

You could test it. Take a reading from your meter for one week with it on all the time and one week as you are using it now and see if there's a difference.

I only have the hot water on for 1 hour a day, I normally get enough hot water to last all day, sometimes more.

I think OP is only talking about hot water, not water for heating rads.

I reckon it might partly depend how you use the hot water.

For instance, we have dishwasher, washing machine that both take only cold water input. The few dishes that need washing by hand we do only ever other day -- as often as DH showers. And rest of us bathe less often.

So we heat up the tank only for a few hours every other day or so. That's enough for our needs -- and bound to be less than keeping it to temp all day every day.

But we only started this regime recently, so we don't have enough historical info to go on. However, it only takes half an hour of the immersion heater to get enough water for shower/dishes.

The advice your plumber gave you is right, given that you have the correct heating controls, it's v important that you have a stat on the hot water tank. Why not try putting it on a timer this month and compare the two, if your not convinced.

Thank you! I'd much rather keep it on all the time, so that's good to hear scanner. However, I will do a couple of week-long trials just to check.

Also if you only need a tiny amount of hot water (to wash a few dishes/wipe worktops) then I think it is cheaper to boil a kettle or even a pan if you have gas.

It must depend on your routine to some extent. If everyone showers in the morning for example, then no need to have hot water available at night. In our case we only use hot water from the boiler at night (dd's baths).

Whether it's cheaper to keep it on all the time or just have it on when you need it also depends on how well insulated it is. The best advice is to try different things out. This is probably the best time of year to try as your heating won't be on, confusing things. Read your meter, try one way for a couple of days, read your meter again, then try a different way for the same period, read your meter again.
Check that the thermostat is set to 60 degC. I take it it's a gas boiler? If it's electric, then it might be worth just heating it overnight on the economy 7 cheaper tariff.

Ages ago there was a thread a little like this (but about heating rather than water.)

Similar theory: leaving on low all day v several bursts of it working hard morning and evening. I think the consensus was it is cheaper to have it on twice rather than all day. (but much cosier to have on all day!)

You will ALWAYS use more energy keeping the water hot all the time than heating when you need it, since the heat loss (wasted energy) from the tank is higher the hotter it is (however good the lagging - but much more marked if it's not so good). If it's hot all the time you waste that much energy per hour 24/7 and that WILL be more than heating the whole tank up from whatever temp it's cooled to over the same time.

You will ALWAYS use more energy keeping the water hot all the time than heating when you need it, since the heat loss (wasted energy) from the tank is higher the hotter it is (however good the lagging - but much more marked if it's not so good). If it's hot all the time you waste that much energy per hour 24/7 and that WILL be more than heating the whole tank up from whatever temp it's cooled to over the same time.

oops

This is really helpful info - thank you! I'm guessing that the tank isn't well insulated (tis old).

I give energy advice and support to people in the area and the myth still exists, perpetrated by plumbers electricians and the like, who should know better, that its more efficient to leave immersion heaters on all the time.
It makes no difference who you are and what your lifestyle. If you just heat water for when you need it you will save energy and money. Who leaves a kettle on all the time in case they need to make tea? An immersion heater + tank is just a big kettle!

www.moneysavingexpert.com/utilities/energy-saving-myths - it will be cheaper to have it on a timer rather than all the time.

Is way cheaper to heat when you need it. Way way way cheaper.