Will it Fit? Cylinder Sizing and You

In our first post, we compared two different heat-sources, a heat-pump, and a condensing boiler. We took a look at the different performance characteristics, capabilities and expected running costs of each

In our second, we saw how the choice of heat-source could affect the temperatures we store water at, and what affect changing the storage temperature had on the amount of water we needed to store to get the same performance from the system.

Here in our final post, we will have a look at one of the most important components in a Greentherm central heating system; the buffer tank, Thermal Store or Combi-Cylinder.

We’ve already seen that to efficiently meet the hot water demands of a  modern home, the cylinder has to be far larger than the traditional copper tank tucked away in the hot-press.

The actual amount of storage required is calculated by us here at Greentherm as part of our design process. Based on the plans supplied to us by your architect, we calculate the heat load and hot water demands for your home and match them with a heat source capable of delivering the required performance. We then size the buffer tank based on the most efficient operating point of this source and the demands we have calculated.

This is a fairly involved process the details of which go far beyond this post, especially when modern regulations such as Energy Performance and Carbon Performance have to be accounted for.

For argument’s sake let’s assume that, based upon the plans and specifications we’ve received from your architect, we’ve calculated you need a 1000 Litre Thermal Storage tank to meet your new home’s hot-water and heating requirements.  How much space would that take up?

It certainly won’t fit in with the towels, so somewhere else must be found.

We recommend that a dedicated plant-room be set aside for your hot water storage and heating systems, both for ease of maintenance, ease of access and for your convenience.

This dedicated plant room will have to be large enough to accommodate the Thermal Store and its plumbing. So, how large will it have to be to fit our specified Store inside?

A simple way to estimate your cylinder dimensions

The Volume we require to store 1000 Litres of water is, conveniently, 1m³. A cube-shaped tank, 1 Metre by 1 Metre by 1 Metre will be large enough to hold 1000 Litres of water. Unfortunately for us it’s not that simple; the majority of Thermal Stores are cylindrical.

The basic geometrical formula for the volume of a cylinder is πR²H.

π is a constant that relates the circumference of a circle to its radius length. It can be taken for our purposes as having a value of 3.14

H, or the Height of the cylinder will generally be limited to the ceiling height of your home, less approximately 200mm for headspace to simplify installation.

R, is the radius of the cylinder – or the distance from the centre of the cylinder to its outer wall. It is important that the depth of insulation not be included in this.

To know the space required for the cylinder in the room, we re-arrange the above formula to give:

R = √(V/πH)

Assuming we have enough room to fit a 1.8 metre tall cylinder in the room

For a 1000L tank, R = √(1/3.14×1.8),

This calculates out to a cylinder radius of approximately 43cm.

This gives a total vessel diameter of nearly 90cm. On top of this, most cylinders will have an insulation blanket that also needs to be accounted for. An extra 5cm on each side of the cylinder for insulation means a 1000 Litre cylinder will be at least a metre wide when finished.

This is too wide to fit through most doors unless you opt for a tank such as the Jaspi Oval tank which has been specifically designed to fit through a standard doorway.

It will be necessary to allow for sufficient clearance around the sides of the cylinder for proper plumbing access. Depending on the works required, at minimum this could be another 30-50cm, meaning a 1000 Litre cylinder could require up to 1.3M of space in a room to comfortably install and plumb.

This will, of course, require some special arrangements to install, to ensure the cylinder will fit.

These special arrangements will need to be discussed with your architect in the design and specification phase.

The normal capacity we would usually install would be normally be in the range of 500 to 800 for a buffer tank, and typically up to 900 L for a combi-cylinder – so 1000 Litres is a little on the large side, but still within the realms of possibility.

That brings us to the end of this short series of postings, we hope you’ve found them informative. Contact us if you have any queries or questions on what we’ve gone through in this series, we’ll be more than happy to answer them.