Find the energy-saving “sweet spot” with condensing boilers BY ROY COLLVER
The spring 2009 issue of Ops Talk had a very good article by Art McDonald on boiler system design to save energy. I urge you to dig out your old copy and give it a good read. Recent research and innovation in system and boiler design fully support this article, and in addition, have further identified some especially fertile ground for using condensing boilers in both retrofit and new installations.
Put simply, we need 100 per cent of the heating system’s capacity for only 10 per cent of the heating season. Sixty per cent of the season, we only need 40 per cent capacity; and for 30 per cent of the season, we run at 15 per cent. These results confirm that we have all been looking at the wrong side of the sizing spectrum in designing systems. We get the most from the LEAST. One result of the increased use of
Condensing boilers used in warmer weather with high temperature boiler.
The real low hanging fruit, as Art identified, is in using condensing boilers to look after the base load in an existing “high temperature” system. One or more condensing boilers can be configured to heat the building in the spring and fall, with high temperature boilers reserved for the extreme weather. But it turns out, the numbers are even better than we thought. Recent research commissioned by Natural Resources Canada during development of the CSA P10.07 performance standard resulted in a simplified, threepiece approximation of the heat load versus system capacity for much of Canada (the purpose of this approximation is to evaluate efficiency of heating equipment).
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Ops Talk Fall 2009
condensing boilers is that baseboard manufacturers are now rating their products at ever lower temperatures – one manufacturer is now publishing down to 110°F average water temperature – a little over 25 per cent of the rated 180°F output – and the ratings are encouraging us to re-think our previous minimum temperature requirements. We are adding more insulation to these older buildings, tightening them up and installing controlled mechanical ventilation. Better windows, better controls – all this stuff adds up to big reductions in required water temperatures to existing radiation, good news for condensing boilers. In many cases, the high temperature “cold weather warrior” boiler is no
longer needed, and even if the water temperature occasionally has to creep above the condensing point (approximately 125°F return water for a natural gas boiler), 87 or 88 per cent combustion efficiency is still much better than what we have expected from boilers in the past. Most high quality condensing boilers now use pre-mix combustion systems that provide the precision needed for higher turn down ratios to meet minimal seasonal loads at maximum efficiencies. Larger systems take advantage of the deep modulation resulting from staging multiple boilers. It all adds up to outstanding savings. Some school boards in the Lower Mainland are getting paybacks in less than two years for boiler room retrofits using this technology. New buildings, of course, can and should be designed from the outset for low enough temperatures so that boilers condense all year - which is all good – but if we really want to see some serious reduction in the carbon footprint, we have to dig deeper and squeeze a little harder. New buildings, new ideas – how low can you go? Some designers are having great success putting condensing boilers at the other end of heating season – the high temperature cold weather side. How does this make sense? Well, if we add heat pumps to the mix and we look at the colder weather to bring the condensing boilers into play, we have some really exciting numbers to play with. With current insulation standards, and properly designed terminal units (radiant floor or panel radiators), our building’s base load can be handled by geothermal or air to water heat pumps at water temperatures well below 110°F, and condensing boilers can be used to top things up (110 to 140°F supply water) by burning fossil fuels on those difficult winter days – oh yes, the back-up is nice too. Insufficient real estate for the geothermal field is no
