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What are the kW ratings of coils for indirect cylinders?

As covered in a previous article, the difference between direct and indirect boilers is the heat source used. Indirect water heating cylinders use an internal copper coil through which hot water is pumped to heat the water surrounding it. This can be a very efficient way of heating and maintaining the temperature of water, and all these copper coils have a kilowatt rating depending upon their size (the surface area of the copper). The higher the kW rating, the faster the recovery time.

Having said that, this technical detail isn’t always obvious or easy to find from the specification of many indirect cylinders. However, having looked at several ranges, and without touching on twin coil or solar variants, here are some typical examples for popular sizes of indirect cylinders.

One thing that stands out from these figures is that the Megaflo Eco Plus range has the greatest (highest) kW ratings amongst these examples. Double the typical rating in some cases, so the recovery time could well be halved. Of course these ‘ratings’ should perhaps be looked at as capabilities, as the performance of the coil will depend entirely upon the power and limits of the heat source being used. (eg: gas boiler) So one may ask how a kW rating can be stated with such heat source variables, and the answer is that there is a standardised form to calculate the rating of any coil of this type: The kW rating is calculated based on the surface area of the coil and the assumption that water is flowing at a rate of 15L per minute at 80 degrees Celsius.

To summarise, indirect cylinders will almost certainly have a better recovery rate than most electric only direct cylinders that would typically have 3, 6, 9, or 12kw elements (the latter of which are only ever seen in very large capacity boilers).

For further information or help selecting the right product for you, feel free to give one of our water heating advisors a call on 01934 253600, drop us an email at or you could use the chat facility on our site for a virtual chin wag.