Specifying the correct size of any hot water cylinder is key to ensuring you receive the most efficient cylinder possible. However when it comes to thermal store sizing, it is even more important.
The size calculation of a thermal store can be made more complicated when the system is responsible for more than simply the hot water supply and when it incorporates multiple fuel sources.
Hot Water Demand
As with most hot water cylinders, the starting point in determining the correct size is to understand the likely demand on the system.
As a general rule of thumb, we tend to allow 40-50 litres per person per day, however as the number of people in a home will vary over the lifetime of the home, it is generally easier to calculate the likely demand from the number of bedrooms and bathrooms.
Where the thermal store is responsible for supplying the central heating in the home, this needs to be taken into consideration.
This will include the number and size of radiators and the heat loss factor of the home.
The more radiators and the higher the heat loss factor, the more demand that is placed on the thermal store and therefore the more capacity required.
Output of Heat Sources
One of the key benefits of the thermal store is its ability to incorporate multiple energy sources, however the kW output of all the heat sources need to be taken into account to maximise the efficiency of the system.
Where renewable energy sources such as solar thermal or solid fuel stoves are incorporated, it is vital that an expansion tank is added to allow the stored water to expand during periods of high temperature.
However, if the storage tank isn’t sized correctly, the excess energy will cause the tank to overheat and wasted via the overflow pipe.
The optimum size of a thermal store is therefore dependent on achieving a balance between energy input and maximum potential demand.
Cylinder Space Available
The final consideration to bear in mind when sizing a thermal store is the space available for the location of the cylinder.
Tight spaces in cupboards may limit the capacity or require a slimline cylinder to achieve the required capacity.
Where height is an issue, for example, in loft spaces, it may be more beneficial to use a rectangular thermal store which can deliver the same capacity in up to 27% less space.
But the best way to ensure to get the optimum sizing of your thermal store is to contact our technical sales team on 01592 611123 who can discuss your requirements and provide you with advice on all the options available.
Specifying a Thermal Store
When specifying your thermal store firstly choose the type of UNIT then the appropriate MODEL.
Combination Unit - Use in new-build situations or where no remote primary feed & expansion tank exists. Primary Feed and Expansion Tank must be 500mm above top of radiators.
Cylinder Unit: Use when there is an existing remote primary feed & expansion tank. Working head of system must be given when specifying/pricing.
THEN SELECT THE APPROPRIATE MODEL
Boiler Model: Use when space heating and hot water are required. Suitable for oil, gas and certain solid fuel boilers. Specify if boiler is Vented or Sealed System.
Electric Model: Use when no space heating is required. Supplied with twin immersions to take advantage of low cost tariffs, e.g., a location with no loft space and a gas boiler would need a Combination-type, boiler model.
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Boiler Model - Bedrooms
Boiler Model - Baths & Showers
Heating Load Max (kW)
Hot Water Allowance (kW)
Electric Model - Bedrooms
Electric Model - Baths & Showers
Other Thermal Store Sizes and Larger Units available on request.
Contact our technical sales team on 01592 611123 to discuss your thermal store sizing.
THERMflow: Thermal Store sizing made easy by McDonald Engineers.