Steel radiators show their mettle for low-temperature heating
Can steel-panel radiators meet the needs of low-temperature heating systems? MARC, the Manufacturers Association of Radiators & Convectors, is more than confident of their ability.
Radiators and boilers have been stalwarts of the space-market for decades — especially in homes. The market is dominated by steel panels, but decorative radiators can add a definite touch of style where required. However, as energy-efficient heat sources such as condensing boilers and heat pumps, which are characterised by lower flow temperatures than traditional boilers, have come onto the market, the use of steel-panel radiators has been questioned.
Traditionally, radiators have been associated with high water temperatures such as a flow of 82°C and return of 71°C. In more recent times explains Martin Wright, vice-chairman of the Manufacturers Association of Radiators & Convectors (MARC) and sales and marketing director with Myson, a temperature difference of 50 K between mean radiator temperature and room air has become the norm.
Data from manufacturers quote outputs based on that 50 K temperature difference. The output of a radiator is significantly affected by water temperature, with the required size increasing for lower water temperatures. Correction factors for lower temperatures is included in manufacturers’ technical data, but its quality varies significantly.
The fact that the heat output of radiators falls off with lower water temperatures has led to concerns that they are not well suited to the lower flow temperatures associated with condensing boilers and heat pumps.
A return of 71°C is definitely not suited to condensing boilers, which require a return temperature below about 55°C to achieve the benefit of condensation in the flue gases. Nor is a ∆t of 50 K, which suggests a return temperature about 65°C. The efficiency of heat pumps is even more dependent on low water temperatures, with a flow temperature of 50°C being a reasonable objective.
Those discrepancies in temperature between what heat sources operate at most efficiently and the temperatures at which radiator heat outputs are usually quoted has led to a growing perception, possibly encouraged by those with vested interests in other types of heat emitters such as underfloor heating, that radiators are not suitable for low flow/return temperatures.
Now MARC has hit back, with Martin Wright arguing, ‘Radiators and convectors are the best way of introducing energy-efficient generators into existing properties.’ On the side of MARC is what is described as a ‘myth-busting report’ initiated by Rettig ICC’s R&D team and supported by six academic and research institutions. MARC chairman John Colling highlights four main findings.
• Because of their flexibility, steel-panel radiators and convectors are the best way of introducing energy-efficient heat generators into existing properties, with renovation and retro-fit accounting for over 90% of the UK and other major western-European markets.
• Steel-panel radiators work well with low water temperatures.
• Steel-panel radiators have quick reactions to temperature control, with small heat losses, meaning efficiency is not lost with long heat-up times. This is a benefit for the lower thermal mass of steel-panel radiators.
• The total energy efficiency of buildings heated by steel-panel radiators is excellent.
What is lacking from MARC’s argument of the case for linking radiators with energy -efficient heat generators is how the output of radiators relates to their temperature. One rule-of-thumb suggestion is a 2% fall in output for every 1 K drop in ∆T.
There is a useful table in technical data from Stelrad, which is presented here as a graph. Note how a ∆T of 50K is the reference point with its relative heat output of 1.0.
While that table indicates a halving of output if the ∆T is reduced to 30 K (flow/return of 55/45°C perhaps), radiator designs can readily maintain the required heat output from a given wall area. Adding another panel to a single-panel radiator with one set of convector fins increases the output by about 50% for the same wall area. Adding another set of convector fins achieves an 80% greater heat output than a single-panel radiator with one set of fins. Steel-panel radiators are clearly quite versatile beasts.
But as Martin Wright correctly explains, the heating requirement of new-build is falling all the time. Likewise, adding insulation to existing buildings reduces their heat losses — so it highly likely that existing radiators matched with an energy-efficient heat source such as a condensing boiler or heat pump will emit enough heat.
As well as addressing technical issues such as adequate heat emissions from lower water temperatures, steel-panel radiators have numerous other economic and installation benefits.
One of the most important benefits is that steel-panel radiators are the most cost-efficient ways of delivering heat into a space.
Looking to the longer term, 70% of the housing stock of 2050 has already been built — and most of it has radiators. Given a radiator life of 20 to 25 years, there is a major refurbishment market for which underfloor heating is regarded as unrealistic because of its cost and the disruption it causes. Little wonder then that Martin Wright comments, ‘Our business depends upon the very real benefits of radiator-based heating continuing to be recognised by our customers as the first choice, irrespective of the building involved.’
And as a final point, radiators need not be regarded as a complete alternative to underfloor heating. In a low-temperature heating system it is possible to use the same heating water for radiators and floor heating.