Kingswell: 'It's about long-term savings gains'

When it comes to the claimed sustainability of conventional panel radiators working in conjunction with heat pumps at low water flow temperatures, there are several drawbacks which are often overlooked or understated, but must be addressed in order to make an informed decision. Fortunately, there is a more energy efficient, not to mention practical, alternative - namely, the fan-assisted low water content heat emitter.

First, let us look at the question of physical size. It is a well-established law of physics that in order to maintain a specific heat output when lowering the water flow temperature, the radiator size must be increased. In a modern, well-insulated house, the heat loads may be relatively small and, hence, 'oversized' radiators may still physically fit. However, in older, less well-insulated houses with higher heat losses, the oversized radiators may not fit and, even if they do, a smaller fan assisted radiator will almost certainly be more aesthetically pleasing.

The recently published Domestic Heat Emitter Design Guide which forms part of the MCS Installation Standards for heat pumps is conclusive on the matter. Developed by the Heat Pump Association, this guide is endorsed by the Department of Energy and Climate Change, the Energy Saving Trust and others, and clearly illustrates that fan assisted radiators are suitable for a much wider range of applications than either conventional radiators or, indeed, underfloor heating.

Next, let's turn to claims regarding 'rapid response times' and 'reduced water content' that many manufacturers of conventional radiators are keen to stress.

Rapid response times relate directly to the mass of water contained within the radiator; the lower the mass of water, the quicker the response time. As fast response times are directly associated with overall efficiency, this is clearly a critical issue.

Modern radiators that claim to have 'lower water content' still typically contain up to eight times the water content of the equivalent fan-assisted heat emitter. As an example, to give 1kW heat output with water at 45 deg C flow temperature, a fan-assisted low water content emitter would be 1,400mm long by 500mm high. It would contain less than two litres of water and have a total mass of approximately 16kg.

The equivalent twin-panel radiator would need to be 2,000mm long by 600mm high, would contain around 13 litres of water and have a total mass in excess of 80kg!

Such a high thermal mass cannot be energy efficient. Independent tests carried out by BRE confirm that low water content heat emitters can be up to 10 per cent more energy efficient than conventional radiators. When it comes to conventional radiators, once the room temperature is achieved the energy stored within the radiator continues to heat the room unnecessarily. Low water content heat emitters maintain room temperatures much more accurately by avoiding over-heating. Therefore, claims that conventional radiators 'deliver high efficiency' can be misleading.

An additional benefit of fan assisted radiators that often goes unacknowledged is the added 'comfort factor'. Users tend to report even temperature distribution throughout the room, achieved by the better circulation of air, meaning no more hot and cold spots.

However, there are some misconceptions and concerns about fan-assisted heat emitters. For example, while you will need a power supply, it is important to remember that this only needs to be 3 Amp. Most of the fans used are low voltage and consume minimal power, substantially less than a low energy light bulb, for example. Another concern is likely to be the 'noise' generated by the fans. Some fan-assisted radiators are simply modified fan coil units, which can be unacceptably noisy.

So, it is vital to look specifically for fan-assisted radiators, which are virtually silent in operation, as most fans modulate and thus simply do not run unless they are needed.

Once room temperature is achieved, generally the static heat output from the emitter is enough to maintain it without the use of fans to 'boost' the output.

Conventional radiators do have advantages - low prices and simple installation. But remember, always look carefully at the overall heat pump system design and select the heat emitter that will give the highest system efficiency as well as being practical to install. Good heat pump installations provide lower long-term running costs and attractive payback times. It is not simply about low initial capital cost, but the long-term savings.

// The author is commercial manager of Jaga Heating Products (UK) //