Maximising Efficiency for Radiators

From a single radiator change to a completely new heating system, taking a holistic approach presents opportunities for greater energy efficiency.

Plan

First and foremost, the needs of the properry must be defined, and a full analysis should take place. If this is a new build property requiring a new heating system, all the required design data can be obtained from the architect. This data will cover architectural plans, sections and elevations of the property along with thermal insulation characteristics (U values). With an existing property, a site survey is needed, measuring each room to be heated, and the insulation levels ascertained from the householder.

Assess

Consideration must be given to the amount of insulation in the property. Obviously, a well-insulated house is easier to heat. If it's not well insulated then this installation may provide the incentive the homeowner needs, after all no-one wants to install an efficient heating system and then lose heat through poor insulation.

Calculate

Heat loss calculations can then be carried out. Many people think that heat losses are the result of complex calculations, however, they are actually quite simple.

There are some publications that give superb advice on the preparation of heat loss calculations. For example, the Domestic Heating Design Guide by the Chartered Institute of Building Services Engineers (CIBSE) was produced to assist professional heating engineers to understand, specifir and design wet central heating systems.

This booklet also suggests internal design temperatures and air changes for rooms with various uses. Heat losses are calculated on a room by room basis. The amount of heat lost through each individual room surface is calculated, and then added to the ventilation heat loss to give a room total.

A factor for intermittenry may then be added to ensure the desired room temperature can be attained in a sensible time from switching the system on. This factor can be around 15%. Once the building's heat losses have established, the boiler and radiators can be sized.

Size

We can now consider sizing the radiators using the radiator's Delta T. This is simply the difference between the average radiator temperature and the room's air temperature. It is calculated by taking the average of the design flow and return temperature, and deducting the room's design air temperature.

Delta T

All radiators in the UK have their temperatures published at a Delta T of 50. This equates to a 75°C flow temperature, a 65°C return temperature and a 20°C air temperature. So the average radiator temperature is 70°C which, when we deduct 20°C, gives us the published Delta T output. This enables the output of all radiators available in the UK to be accurately compared.

However, heating systems do not generally operate at a Delta T of 50. If the Delta T is higher than 50, the radiator output will increase.  Conversely if the Delta T is lower than 50, the radiator output will decrease. All MARC radiator manufacturers produce a table of correction factors so the published Delta T outputs can be corrected to show the actual radiator output at a different Delta T.

Modern condensing boilers are more efficient when their return temperature is less than 50°C, so to maximise system efficiency, consideration should be made to designing a system to operate with flow/return/air temperatures of 70°C/50°C/20°C, which results in a Delta T of 40, and a radiator correction factor of 0.75.

In an existing building, the increased insulation levels and improved energy efficiency of buildings over the last 30 years may allow an old heating system's Delta T of 56 to be dropped to the modern Delta T of 40. New high efficiency radiators may then be around the same physical size, if not actually smaller, than those originally installed, so upgrading old radiators can be a very easy job.

Location

The most sensible position to locate a radiator is near the coldest surface in the room, which is why it is common to see a radiator either under or next to a window. Depending on the amount of insulation and the layout of the room, a single radiator may heat a floor area of up to around 20m2. Above this area, two or more radiators should be considered to ensure even heat distribution.

The required boiler output is based on the total heat losses, with additions for domestic heated water and allowances for distribution pipe loss (normally ten per cent). If working with a combi boiler, then no additional domestic hot water load needs to be added. If the proposed system requires a separate domestic hot water storage cylinder then the aforementioned CIBSE guide recommends an additional load of between 2.0kW and 3.OkW, depending on the amount of bedrooms and bathrooms in the properry. This guide also recommends a minimum hot water capacity depending on the size of the dwelling.

Controls

Heating controls are extremely important, and the Building Regulations have specific requirements that must be complied with in the new-build environment. Specifically the government's Domestic Building Services Compliance Guide requires thermostatic valves, time and temperature zone control, and a boiler interlock. In existing dwellings, this document encourages controls to be upgraded wherever possible, from a simple repair, for example, if a radiator is replaced, then a thermostatic valve should be added, to fully upgrading the control system if a major planned renovation takes place.

Demonstrate

Once the system is fully installed, it is important to explain to the end user not only how the system works, but how it is best operated to ensure maximum efficiency. This should include showing the end user where shut off valves and fuses are, how to vent the system, and how to rebalance the system if radiators are removed for decoration purposes. A written user guide should also be given to the end user, along with operating instructions for the boiler and controls. The importance of regular servicing and system checks should also be highlighted to ensure the installed heating system continues to operate in a safe and economical manner.

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