How does a hot water tank work?

[WhatsitWiggle]

I've lived in my house for 15 years, always using gas boiler to heat hot water for 2 hours in the morning and 2 in the evening.

But with fuel prices rising and me now being a single parent, it's made me question if this is necessary? I have a daily morning shower (4 minutes most days, 10 minutes twice a week to wash hair), washing up done every evening. Washing machine is a cold water fill. Every other week, DD stays with me - she takes longer showers or baths in the evening.

If I put the hot water on for 1 hour in the morning, is that enough to heat water for my shower? And will the hot water stay hot until the evening or do I still need to put it on again? If the hot water is set to come on again in the evening, but there's still hot water in the tank, does anything happen? Ie am I using gas or not?

With hindsight, I wish I'd changed to a combi boiler when the old one went kapput two years ago, but at the time a straight replacement was the quickest and cheapest option.

Ours is on for just an hour in the morning - four people showering daily, and we never run out. We have a Megoflo cylinder thing, so we can run multiple showers at the same time with no loss of pressure which makes it better than a combi boiler for us, and the water stays hot all day for hand washing etc.

It's more cost effective to keep my hot water tank constantly hot. It takes a long time to heat up from cold, and is rather large. Too big for the house really.

It will depend how big your tank is and how fast it heats up. Also consider what temperature it is heating to, do you need it at 70 or could it be at 55 etc.

So to be clear my hot water is on constantly. Sadly

Also worth considering that your radiators will be taking hot water from the water tank too. So if you WfH or use the heating a lot in the day, you'll need water hot in the tank for that. My heating won't come on without the hot water being on, and the tank being hot and ready means my house does warm up quickly.

Ours is on for an hour a day and that does a couple of showers, DD's bath and the odd but of washing up. You def don't need four hours a day!

Also radiators don't take from the water tank in a normal system boiler setup. The boiler heats the water that goes directly to rads. The tank is just for hot water from taps/shower.

And yes water will stay hot. The tanks are very well insulated. We heat once per day and the water is still hot 24 hours later.

Our showers are electric so don't use up the water in the hot water cylinder. Our hot water is on 1 hour per day and that seems to be sufficient for our needs.

P.s. combi boilers don't suit every household so when we replaced our boiler last year we kept our tanks in the loft and the hot water cylinder as it suits us.

Mine's on for 30 mins a day - that's plenty for us as a Family of 3. We sometimes run out and will put on again for 30 mins but that's about once to twice a week.

Ah sadly this is not the case for mine. It is a complicated system (and was set up by the previous owner who was a plumber!) 🤦🏽‍♀️

Just reduce by 30 minutes every week and see how you do. We've got ours down to 1 hour in the morning and 1h30 in the evenings for 4 of us.

It depends on the size of your tank I think so just push it down until you can't anymore.

Glad I asked! Right, I'll try changing it to 1 hour in the morning and see how I get on. No idea on size of tank, I'm going to guess at 100 litres going by the dimensions and it's set to 60 degrees being the minimum recommended to avoid legionnaires.

Are the extra (unnecessary) hours using gas though? I'm trying to understand how it works. The boiler heats hot water, that's carried inside the tank in pipes to heat the cold water in the tank, then cold water goes back to the boiler. But if the tank water is still warm, no heat will leave the pipe water so hot water goes back to the boiler. Google is not helping but I'm probably not asking the right question!

And thank you 😊

Let's say your hot water is timed to be on between 6 and 8am. At 6am the boiler will fire up and start heating the water in the cylinder. Once the water reaches the temperature set on the thermostat (say, at 6.40am) the boiler will stop, even though the hot water is still on, so you won't be using any gas.

If you then get a shower at 7am, this will obviously use up some of the hot water in the cylinder and the temp will drop. When it drops below the thermostat setting, the boiler will fire up again and heat the water until the set temp is reached.

In this example, at 8am when your hot water turns off, you've got a tank full of hot water for the rest of the day. It should stay hot into the evening. If you're not showering/bathing at night, I very much doubt you need to be heating the water again in the evening. All you are doing is using up gas to heat water which is going to sit there unused overnight.

It really depends on timings. Eg, if instead of showering at 7am, you took a long shower at 8am, the hot water has just turned off so the boiler won't be working to replace what you use and you'll therefore have less hot water available for the rest of the day, and may need an evening top up.

I've got a combi boiler but also a hot water tank. I've never really understood why I've got a tank because I thought a combi boiler just heated the water when you need it but have set the hot water to come on for an hour every morning 'just in case'. Does anyone know if or why I need to do that?

1. It's a cylinder not a tank

2. it has nothing to do with heating the radiators

3. for the last 50 years or so, it should have a thermostat, meaning the boiler turns off and stops trying to heat it once it is hot, so not wasting gas

4. properly timed and insulated, it is more economical on gas than a combi.

What colour is yours (this is important) and what are its dimensions?

Some photos, including the cables, pipes and devices around it, would be helpful. Also of your timer/programmer and thermostat.

I am so glad you posted this! I having been trying to figure this out too and nearly posted one myself but felt silly for not knowing! We have a Worcester cylinder. It’s on constant at the moment and always has been. There is two of us in a four bed house (why cylinder is so big) shower in the morning around 7/8 and again 7/8! What should I do @PigletJohn

In your case I can see from the pic you have a large, unvented cylinder. The efficiency and insulation are very good. If you set the timer to come on 20 minutes before usual shower start time, and to go off 20 mins after usual shower end time, it will be fully hot and unlikely ever to run out. It will turn off the boiler demand when hot so it will not be using gas all that time. Assuming you have a modern boiler, it could probably heat the entire cylinder from cold in about half an hour.

Turning the timer to off outside shower times will save a bit of gas, because it will not turn on the boiler for short periods to top up when you use smaller amounts of hot water. Multiple short tops are less efficient than one long full heat. I think I can see the hot pipes are insulated, which also reduces wastage.

Other people will have different sorts of cylinder and the quantities and times will be different.

Gosh that’s so helpful, thank you so much for your help, it’s very much appreciated 😊 I’m off to do that now.

@PigletJohn

Sorry, have always had family refer to cold and hot water tanks!

The cylinder is about 90cm high and 45cm wide. Was installed just over 14 years ago (original one had a red 'duvet' round it and it kicked out so much heat it was untrue). The pipes in the cupboard aren't insulated though, would it help to do that?

The timer was new about 3 years ago with the boiler and I'll need to dig out instructions. I thought I could do water and heat separately but it's not obvious.

Thank you for any guidance.

I thought I could do water and heat separately but it's not obvious.

Mine's a different make to yours but the layout is similar so I would guess you can put heating and hot water on separately.

I have an electric shower so I only put the hot water on when I need it for washing up (just by pushing the boost button generally). Conversely in winter I use the timer to set my heating on/off.

Your cylinder holds about 100 litres, which is enough for a bath, more than that when you add cold, especially if your water displacement is substantial. It looks like you have a shower pump. A blue cylinder is the current standard, with good insulation and quite fast reheat. You will have a cold tank somewhere above, probably in the lift. It ought to be well bigger than the cylinder so it will not run dry.

Your programmer is capable of heating the cylinder and the radiators separately. Older systems with different plumbing put the HW on even when you only want radiators, but if you have a modern boiler yours is unlikely to be like that.

I think that in summer you probably use between half and one of cubic metre of gas per day, for hot water. Check your gas meter. The cost of hot water is small. One cubic metre of gas is about 11 kWh. I think you ought to set the timer to start half an hour before shower or bath time, and off half an hour afterwards. It will probably hold enough HW for your other daily needs, though it is much smaller than Wiggles' white one.

@PigletJohn thank you so much. Yes pump for the shower, cold water tank in the loft.

I have a smart meter and yesterday it used 11kwh for gas which would have all been hot water. That would have been heating the full cylinder as I switched everything off for the week we were away. I'm 7p per kwh currently for gas and not on fixed, hence wanting to understand if I could reduce usage - it all helps.

I'll play with the programmer - it seems the timings can't be different for heating and hot water (I'd need a different timer at approx £70 plus fitting) but what I can do is turn the hw off and use the boost function when i get in from work - that heats for one hour. I'll try that this week and compare the usage and if it's a big difference then it'll be worth changing the timer.

As I recall the Drayton Lifestyle you can.

You can set either or both to on/off twice a day, or on all day, or on 24 hours, or off 24 hours. And depending on model you can set times for all seven days, or each day individually, or weekdays one pattern and weekends a different pattern.

Have you got the instructions?

You can also get a programmable room thermostat which has its own timings and temperature settings.

Sorry, I just looked up the LP112 which you have, and you are right, it says the and ch times are linked.

How tiresome.