Heat Loss



As of 2012, the European Commission have imposed the Energy Efficiency Directive which are a set of binding measures to help the EU reach it's 20% energy efficiency target by 2020. This is done through the efficient use of energy and in the case of our hot water cylinders, the efficiency relates to the amount of heat the unit loses over time.


Units can be heated through a variety of means be it using an immersion heater, boiler, wood burner or solar, the idea is to keep the heat inside the unit for as long as possible or else energy will have to be used to re-heat the unit up to its storage temperature. Keeping heat inside the unit is done through the use of insulation; there are a few ways to ensure this such as applying foam insulation as we do or wrapping the unit in a jacket however as cylinder manufacturers we need to ensure our products abide by these regulations as they go out the door. That's why we use a high-density polyurethane foam to insulate our units; its high density ensures that heat remains in the unit for as long as possible and of course the more layers of foam that is applied the longer the heat is retained.


As part of the directive, all manufacturers must test their range of products and assign an efficiency label to each of them. As we are a manufacturer of bespoke products with a virtually limitless range, we calculated how long it would take for us to do this.


Our standard range of direct units from 300-600mm diameter and 900-2000mm height gives us 161 separate units, 322 with indirect units, 644 including combination types, and with our standard insulation sizes being 35mm, 50mm, 75mm and 100mm, this gives us a total of 2,576 copper units that need testing, and if we were to include our stainless range that would leave us with 3,864 total units that need testing. Since each unit takes 3 days to test, that would take around 31 YEARS to have heat loss just for our standard range, that isn't taking into account the special units we produce.


The Process

We log the heat loss of the unit over three days with an addition day for the cylinder to reach full temperature. The probes measure the temperature inside and outside the unit as well as the ambient temperature.

The data can be mapped on a graph to show the changes in temperature as the cylinder is heated by the immersion heater. The dips and peaks in the graph help display the times of the heat up cycles.

The meter readings are logged at the start and end of each test, these display the amount of energy used by the immersion heaters. This value is compared with the average temperatures of the unit and room to give a heat loss figure per 24 hours.

The Lab

Due to the massive number of units we require testing, we determined it to be more economically viable to construct our own testing lab on-site. This lab is a controlled room heated to a constant 20°C to ensure minimal discrepancies in the ambient room temperature of the tests. Units as tall as 2.3m and as wide as 1050mm can be tested in the room through a removable wall section.


The tank is filled though the cold feed until water is discharged from the top pipework into a bucket (or floor) to indicate the unit is full. The immersion heater and logging software is then activated to log the initial heat up time of 24 hours. The log is kept running for an additional 72 hours to ensure an accurate result can be gathered.





After gathering heat loss data on a wide range of units, we can start to develop an accurate estimate on the heat loss reduction of our units depending on the thickness of insulation.


We have calculated that on average, each additional 25mm of insulation there is a 25-35% reduction in heat loss.


Capacity Insulation Thickness Heat Loss Watts ERP Banding
120 50mm 1.38kw 58 C
144 50mm 1.51kw 63 C
166 50mm 1.71kw 71 C
210 50mm 1.97kw 82 C
250 50mm 2.23kw 93 D
120 75mm
1.02kw 43 B
144 75mm 1.10kw 46 B
166 75mm 1.22kw 51 B
210 75mm 1.38kw 58 B
250 75mm 1.54kw 64 B
120 100mm 0.66kw 28 A
144 100mm 0.72kw 28 A
166 100mm 0.81kw 34 A
210 100mm 0.93kw 39 A
250 100mm 1.05kw 44 A

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