A Buyers' Guide to Slimline Hot Water Cylinders

Slimline Hot Water Cylinders UKSlimline Hot Water Cylinders have seen a dramatic growth in popularity as architects, specifiers and end-clients try to reduce the floor space required to accomodate the hot water storage solution.

However, before you assume that a slimline cylinder is just a thinner version of a standard cylinder, think again.  Our buyers' guide to slimline hot water cylinders provides the essential information you require to ensure you get the most efficient hot water storage solution for your project.


The obvious consideration for a slimline cylinder is the diameter of the cylinder itself, however you need to be clear about the shell size and the insulation.

Typically, most companies offer standard slimline cylinders measuring 430-491mm including insulation, however not all specify the level of insulation supplied.

At McDonald Engineers, we offer complete flexibility when it comes to your slimline unvented cylinder, starting from a minimum 300mm diameter shell size.  We then recommend 50mm insulation to all sides of the cylinder for maximum heat retention, if the space is very limited then we can reduce this to 35mm.

Connection Layout

With space being the main driving factor in the purchase of a slimline hot water cylinder, it’s not simply a case of the diameter of the cylinder you need to consider but the total floor space required.

If connections are located to the side or the rear of the cylinder, additional space will be required to accommodate the pipework as well space to work in the area.

We therefore tend to recommend that all connections are located to one face of a slimline cylinder, ideally in a line, ensuring all pipework is located in one area.


With slimline cylinders tending to have a marginally greater surface area than standard cylinders, insulation is even more critical to minimise heat losses and therefore the efficiency of the cylinder. It is therefore very important to know the amount of insulation applied to the cylinder. 

As a guide, McDonald Engineers UK will apply 50mm of high density polyurethane foam to all of our slimline hot water cylinders unless specified otherwise by the client.

Position of Coil

The optimum placement of the coil in any hot water cylinder is as low to the bottom as possible. With copper it allows us to configure the coil so that it heats right to the bottom of the tank meaning a full store of hot water.


Wherever the immersion is positioned in the tank will dictate the volume of hot water it can provide.  Ideally you want it as low as possible so if the boiler ever goes down, then you have full electric capacity. If the immersion is located half way up the cylinder, you will effectively only be heating half of the cylinder

However, due to the flexibility of copper, McDonald Engineers are able to locate the immersion at low level, or if the client only wants 50% hot water on electric backup, that is what we will provide.

Direct or Indirect

We can manufacture and supply both direct and indirect slimline hot water cylinders but not everyone does. For more information on direct and indirect, check out our cylinder tips article at  https://www.mcdonald-engineers.com/Direct-or-Indirect-Hot-Water-Cylinders

In fact, we can also offer slimline tanks suitable for renewable applications with heat pump and solar thermal. Our range of THERMflow can also come as a slimline unit allowing multiple heat sources to be used simultaneously. 

Material – Copper or Stainless Steel.

Copper offers significant benefits over stainless steel when it comes to hot water cylinders, including heat transfer properties to improve efficiency and heat up time, bacteria killing properties to reduce the threat of legionella and E coli, and recyclability to reduce the impact on the environment.

However, one of the main benefits of copper when it comes to slimline hot water cylinders is its flexibility which is critical when you are trying to achieve a lot in a relatively small area.  The flexibility of copper means that we are able to manufacture and supply the slimmest hot water cylinders on the market today.

Click here to find out more about our Slimline Cylinders.