We had been using a 170lt Megaflo for about 23 years and it has been fantastic. Now that the house is larger I have been told I must use a 300lt. But it seems extremely wasteful to heat up 300lt for just taking a shower. Is there a solution? Maybe have two cylinders? One say 100lt and a larger at say 200lt with separate switches on/off?
Choosing H/W cylinder size
- Thread starter akist
- Start date
If you've added a bathroom or ensuite shower or similar then it would be wise to increase the volume of stored hot water.
If you've got the space then 2 separate cylinders can work (but bear in mind you are increasing your annual maintenance costs).
You say switch on and off- is the cylinder not heated by the boiler? If it is and you do go the 2nd cylinder option you'll need another motorised valve and associated control gear.
If you've got the space then 2 separate cylinders can work (but bear in mind you are increasing your annual maintenance costs).
You say switch on and off- is the cylinder not heated by the boiler? If it is and you do go the 2nd cylinder option you'll need another motorised valve and associated control gear.
How many bathrooms? Baths or showers (or both) in each?
How many regular occupants / users of those facilities?
What is their use? (Short or long slow showers? Deep hot baths, topped up as they cool?
Now, how is it heated?
Direct electric Immersion element(s)?
Indirect Heat Pump, Natural or LPG Gas or Kerosene?
https://www.hotwater.org.uk/sizing-a-hot-water-cylinder/ seems to cover it and for 4 average water use occupants suggests 160 litres indirect... 4 bedrooms it'd be 200 litres.
Are you having issues with the current cylinder capacity? Do you have HW reheat timed to coincide with the draw off(s) of water?
Sounds to me like you may be being up-sold by someone with a lack of easy work at the moment.
Unless they have grilled you about your household's hot water use and determined that 300 litres they recommend with very good reason?
How many regular occupants / users of those facilities?
What is their use? (Short or long slow showers? Deep hot baths, topped up as they cool?
Now, how is it heated?
Direct electric Immersion element(s)?
Indirect Heat Pump, Natural or LPG Gas or Kerosene?
https://www.hotwater.org.uk/sizing-a-hot-water-cylinder/ seems to cover it and for 4 average water use occupants suggests 160 litres indirect... 4 bedrooms it'd be 200 litres.
Are you having issues with the current cylinder capacity? Do you have HW reheat timed to coincide with the draw off(s) of water?
Sounds to me like you may be being up-sold by someone with a lack of easy work at the moment.
Unless they have grilled you about your household's hot water use and determined that 300 litres they recommend with very good reason?
Simply comes down to usage and if anything else what the potential usage could be given the make up of the property.
On the days when you don't use 300 litres, you don't have to heat 300 litres from cold. It's like having a big petrol tank in your car.
The amount you use, you have to replenish and heat.
The cylinders (and, I hope, your pipes) are so well insulated that you will find your airing cupboard doesn't get very warm from escaping heat, becausd heat loss is quite low.
170 litres is enough for two reasonable baths. Unless you have another immediately, your boiler, if turned on, will heat another bathful in around half an hour.
The amount you use, you have to replenish and heat.
The cylinders (and, I hope, your pipes) are so well insulated that you will find your airing cupboard doesn't get very warm from escaping heat, becausd heat loss is quite low.
170 litres is enough for two reasonable baths. Unless you have another immediately, your boiler, if turned on, will heat another bathful in around half an hour.
I was thinking I could have two cylinders, a small one and a larger one, when we are alone in the house (2 people), operate only the small cylinder while the other is bypassed/off. If we have lots of guests in the house, then we can switch on the larger cylinder. By "switch on" I mean have some valve that will direct the HW pump to it and some other valve to allow the cylinder's output into the circuit.
Now I am thinking more about it though, and the larger cylinder may spend months in the off setting, which means old/stale water in it?
Now I am thinking more about it though, and the larger cylinder may spend months in the off setting, which means old/stale water in it?
If you want a hot shower, it will take much longer to achieve on a 300lt than on a 100lt. I know hot water rises but even so you'd have to heat up a larger quantity of water with the larger cylinder and that means more energy spent. The hot water not used would slowly dissipate
its heat overnight, and the 300lt would lose more heat than the 100lt one. Unless I am missing something?
No it wouldn't.
Pointless. There will be no savings and depending on use patterns 2 separate cylinders could actually cost you more in losses than a single cylinder.
The difference in heat loss between a smaller and larger cylinder is so small as to be insignificant.
However when both cylinders are in use, losses are 2x.
Example losses: https://osohotwater.co.uk/product/delta-coil/
Smallest cylinder loss 0.94kWh per day
Largest cylinder 1.18kWh per day.
A tiny 0.24 kWh per day difference, or less than 3p per day if using gas as the fuel.
I would only use one or the other, depending on how many guests would be in the house.
I was considering inefficiency while heating up a larger quantity of water than needed, like boiling a full kettle only to use a cup after.
A cylinder is highly insulated, a kettle is not.
There really isn't any significant loss from heating up a larger cylinder - it's the 3p/day as above assuming a cylinder of 145l compared to a 285l
About £10 per year, and as that waste heat will end up heating the house for about half that, you would only save around £5 per year, compared to paying £100s for the second cylinder in the first place.
The myth about cylinders being inefficient is just that - lies spread around by those who want to sell and install combi boilers.
It may have been valid decades ago when uninsulated bare copper hot water cylinders were a thing.
Most hot water losses occur in the pipework, not where the water is heated/stored.
If you want to spend extra to reduce losses, make the pipework between the cylinder and outlets as short as possible and ensure that all of it is insulated.
There really isn't any significant loss from heating up a larger cylinder - it's the 3p/day as above assuming a cylinder of 145l compared to a 285l
About £10 per year, and as that waste heat will end up heating the house for about half that, you would only save around £5 per year, compared to paying £100s for the second cylinder in the first place.
The myth about cylinders being inefficient is just that - lies spread around by those who want to sell and install combi boilers.
It may have been valid decades ago when uninsulated bare copper hot water cylinders were a thing.
Most hot water losses occur in the pipework, not where the water is heated/stored.
If you want to spend extra to reduce losses, make the pipework between the cylinder and outlets as short as possible and ensure that all of it is insulated.
Is it better to have a LONG run to the outlets but short run to the boiler? Because this is my current setup.
No, that's the worst option.
Long pipes to the outlets means they are filled with hot water every time a tap is turned on, that water then cools and the heat is wasted.
It also wastes water as people flush out all of the cold water in those pipes every time they want hot water.
Losses also apply to the pipes from the boiler to the cylinder, but taps are used far more often than the boiler will be heating the cylinder.
The ideal would be the boiler, cylinder and all outlets close together, but that is rarely practical.
Long pipes to the outlets means they are filled with hot water every time a tap is turned on, that water then cools and the heat is wasted.
It also wastes water as people flush out all of the cold water in those pipes every time they want hot water.
Losses also apply to the pipes from the boiler to the cylinder, but taps are used far more often than the boiler will be heating the cylinder.
The ideal would be the boiler, cylinder and all outlets close together, but that is rarely practical.
Yes, but it's not likely to make any significant difference to the cost of operation.
Once heated, a modern cylinder will stay hot for days. In normal use, that water will be used in less than one day.
With a gas or oil boiler, a more likely use of a timer or other controls is to have the cylinder heating while hot water is in use (such as someone using a shower) and also have it with priority over the central heating, so the whole output of the boiler can be used to heat the cylinder. That way, the cylinder is never cold, hot water is always available, and the cylinder can be smaller than without those features.
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