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Even though in the last year they have doubled their share of commercial sales, less than one in four boilers sold today is a condensing boiler. In Germany, last year the comparable figure was double that. Evidence is that the caution in the UK is based on a technically shaky myth. Retrofitting condensing boilers onto systems that were designed for conventional boilers is not worthwhile, says the perceived wisdom. Theoretically the system is only condensing friendly if it has a low temperature difference (typically flows of under 70°C and returns under 55°C). Yet savings between 20% to 25% are common from a wide variety of retrofitted systems where condensing boilers have been installed as straight replacements. What is happening?

Many existing systems are over designed. Radiator sizes may well have been selected on the basis of the next highest up for safety, adding percentage points of extra surface in each room. Additionally, many buildings over the last twenty years, have had their heat losses cut by double-glazing or other energy saving measures. On top of that, heat gains from electronic equipment have risen. Just think of all those pcs humming away at 40 W, eight hours a day. All this over-sizing means that flow temperatures can be reduced anyway, without reducing comfort levels. Dropping the flow temperatures can be positive if the system is oversized, as it will reduce cycling.

Higher efficiencies
Next is the operating efficiency of a condensing boiler – at any loading and temperature difference. Condensing boilers operate, even in non-condensing mode, at higher efficiencies than conventional boilers. Even under adverse operating conditions, short cycling in warmer ends of the heating season, they are outperforming a conventional boiler by several percentage points. Weather compensation of course improves the picture, bringing good condensing flow temperatures for most of the year, even on conventionally sized systems.

What if there are parts of the system that must have 82°C/71°C flow and return? Over the last 15 years many of the arguments we hear in the UK were raised in Germany and the Netherlands. Many systems there, as in the UK, had circuits operating on different water temperatures or operating on floating water temperatures. There might be, for example, a standard ahu, a perimeter heating circuit and even an underfloor circuit, all operating via mixing valves at different water temperatures.

This challenge led to condensing boiler designs that allow return water to enter the boiler via two tappings, one low one high. High temperature returns come into the top. Weather compensated and low temperature returns come to the bottom. This allows the boiler to maximise condensing efficiencies.

Control is another key factor. The Vitotronic control system, when employed on these circuits, is constantly trying to push the boiler into condensing mode by reducing the boiler flow temperature.

When the ahu is off for example, but the underfloor is on, the control will drop the flow temperature from 82°C to 55°C, maximising fuel savings. This is achieved by fitting controllers to the mixing circuits that report required temperatures constantly back to the boiler via a two-wire Lonworks bus.

Results
Typical of the customers where we have seen improved results with retrofitting is the city of Salford. They have a policy of fitting condensing boilers on every boiler change possible and have fitted them in over 40 major building and hundreds of homes.

Has the city of Salford's policy delivered the savings expected? "Since gas deregulation our energy audit staff have been able to monitor each individual building's energy use. This shows that condensing boilers are giving savings in a range of 20% to 25%," says Graham Piper the city's mechanical engineer.

"This compares with an estimate at the start of the programme of 15%. In some cases, where we have replaced really old and inefficient boiler we have recorded savings of up to 30%," he concludes.