What’s the Difference Between Direct and Indirect Cylinders?

Direct, or Indirect… That is the question!

Here at Brimble, we often receive calls from confused home owners wanting to know the difference between Direct and Indirect cylinders. So to help you decipher what you already have installed, or what you’re going to need we’ve outlined the differences below with some handy cutaways from out main Cylinder Supplier, Joule UK.

As you probably know, a hot water cylinder is just a store of, you guessed it, hot water, that is then pumped around your house and comes out of your taps. The “Direct” or “Indirect” name is given to a cylinder to describe how that water is heated.


The clue with Direct Cylinders is in the name. The water inside that cylinder has a heating element or more usually two, called Immersion Heaters, think of these immersions as the metal element inside your kettle. Electricity is used to heat up the immersion element, which is submerged DIRECTLY into the water inside the tank, heating it to the required temperature. You can see on the cutaway the white heads of two immersion elements. The bottom is usually what people describe as using Economy 7, cheap off peak electricity to heat the entire cylinder from the bottom up. The second element located in the middle of the cylinder is the “Boost” option, allowing the user the ability to heat only a small part of the cylinder for a quicker boost of hot water when required.


In contrast to direct, Indirect cylinders use an external heat source, almost always a gas boiler, to heat water. This hot water is then piped into a metal coil sat submerged inside the cylinder. The heat from the boiler water transfers via the coil to heat the cylinders water without the two ever meeting. This allows water travelling through your boiler and radiators (usually a dirty brown colour if not treated properly) to remain separate from water in the cylinder which will come out of your taps.

The cut away on the left shows the metal coil sat inside the cylinder, it’s this coil that contains the hot water from your boiler, acting as a radiator to heat the separate cylinder water. You may have also noticed the white emersion heater showing on the bottom right of the cylinder in the picture. These are included in an Indirect Cylinder (even though they are a form of direct heating) in case of emergency boiler break downs or if you have solar PV on your property, allowing you to use free electricity to heat your cylinder instead of the gas boiler.


Hopefully this clears things up for anyone unclear on which cylinder you have / need, and why. If you need any further assistance please feel free to contact us via phone or email!

Team Brimble