Sizing a hot water cylinder

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Domestic hot water cylinders start at around 80L, and many makers do a range of sizes up to 300L or more. The smaller sizes being adequate for properties that have a single shower/bathroom. Larger sizes become more appropriate for larger properties with more baths and showers to cater for. Specifying the "right" size is not an exact science, and this article helps lead you through the decision process.

Factors to consider

The size required will be influenced by a number of factors:

  • Water storage temperature
  • Recovery time
  • Maximum likely "single use" demand.

Water temperature

The higher the water storage temperature, the more energy contained in the cylinder. A smaller cylinder full of water at 70°C will produce the same amount of water at final mix temperature as a larger cylinder using a lower storage temperature. However choice of storage temperature is not a case of higher always being better...

  • Temperatures over 60°C will tend to accelerate deposition of limescale inside the cylinder in hard water areas.
  • Higher temperatures represent a scalding risk (although modern installations should usually use thermostatic blending valves in accordance with the building regulations to protect users)
  • Lower temperatures will result in higher boiler efficiency when reheating with modern condensing boilers.
  • Higher temperatures will more effectively kill bacteria such as legionella

With modern boiler controls one may also opt to run a cylinder at around 60°C normally, with a once a week heating to a higher temperature to ensure sanitisation.

Recovery time

Recovery time is dictated by how quickly heat can be transferred into the water in the cylinder, and how soon after the temperature of the water in the cylinder starts to fall that recovery is started.

With a very fast recovery, cylinder size can be reduced and demand still met. (there is also a positive feed back effect here - smaller cylinders will also reheat faster than larger ones)

Electrically heated cylinder recovery time is limited by the power of the immersion heater(s) fitted. Larger cylinders may allow for three of them, with a combined output of 9kW however this is still short of the output available from even a modest boiler.

Indirect cylinders are fitted with a heating coil. This allows water from the boilers primary circuit to circulate through the water in the cylinder without mixing with it, but allowing heat to transfer. Older cylinders (usually the vented type) typically have a coil that can transfer heat at a maximum rate of around 5kW with a primary flow temperature of 85°C (the lower the flow temperature, the lower the transfer rate). Many modern cylinders, and in particular most unvented cylinders will have a "fast recovery" coil. This will often allow flow rates of over 20kW (boiler power permitting) for a significant proportion of the reheat time.

Fast recovery cylinders favour heating control systems that can divert the full output of the boiler to the cylinder (W Plan, and S Plan for example), rather than the Y Plan that suited slower recovery cylinders by allowing boiler output to be shared with the central heating at the same time as the cylinder reheating.

If the boiler is triggered to reheat the cylinder soon enough, then its also possible that even an almost depleted cylinder will be able to supply water for a lower delivery rate use such as a shower on an indefinite basis - the water being heated "on the fly".

A system where the controls allow the cylinder to recover at any time of day, can also work well with a smaller cylinder than if reheating times are restricted to certain times of day (i.e. via a CH timer/programmer or immersion timeswitch)

Meeting Maximum Demand

You can estimate the amount of hot water likely to be required in any given single use (i.e. without much time for recovery) and make sure the cylinder is large enough to hold this much.

A typically bath will require 120L or more. A shower may well be less (or indeed more, depending on how long the shower takes, and the water use rate of the shower in question). However that is "mixed" water - i.e. hot and cold water added together to get the desired temperature.

You can work out the amount of hot water required if you know the total volume of mixed water needed. The calculation is based on the ratio of temperature differences between the final mixed temperature, the hot and cold water temperatures:


  • is the Total Volume of water required
  • is Volume of cold water
  • is Volume of hot water

The actual volume ratios are calculated using:




  • is final mixed water Temperature
  • is the Temperature of the incoming cold water
  • is the Temperature of your hot water from the cylinder

So say you need 150L for a large bath, and you want it at 45 degrees (), while the cold water is at 10 degrees (), and the hot water is at 60 (). That gives you a ratio of:


So you need hot and cold water in the ratio of 35 parts hot to 15 parts cold. 

So if you split 150L total into that ratio:

 of cold water

 of hot water

So that shows a cylinder of capacity of 105L is about adequate for a single bath fill without reheating.

Estimating Yield

Alternatively you might want want to run the calculation in the other direction. Say you have a cylinder with 120L of water stored at 60°C, and want to know how much water you will get when mixed down to say 50°C

Work out your ratios again:

Now work out how much cold you can add if you are adding 10L of cold water to every 40L of hot:

Add that to the original 120, and you know you will get 150L of water at 50°C

Work out water storage temperature

The last estimation you might need, is to work out if its possible to meet a demand by adjusting the storage temperature. Let's repeat the above example, with the 120L cylinder, but this time see if we can meet a demand for 160L of water at 50°C rather than 150L without needing a larger cylinder.

So we know out total volume is 160L, and we also know our cylinder volume is 120L. So that means we must add 40L of 
cold water to get the right volume. So that means we know the mix ratio as well 120:40

So if we put what we know (the amounts, and cold and mix water temperatures) into the Cold Water Ratio sum we get:


So if we add 50 to both sides, we get:

So yes, it is possible to get another 10L of 50°C water using that cylinder by raising the storage temperature to 70°C.

See Also

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