sized on/off boiler will short cycle for over 90 per cent of the heating season. If we are over sizing above the maximum input needed, we are making the problem even worse. Number two: match the heat output to the actual load by staging or modulating the input to the boiler plant. A 5:1 turn-down ratio will reduce short cycling to only 14 per cent of the heating season. There are more and more boilers on the market today that can reach down to a 20 per cent (5:1) firing rate, and even lower, and in larger schools, the staging of multiple boilers has the same effect as modulating a single boiler. See how powerful the P.10 chart is in illustrating some of these principles. The variation illustrated below is a perfect example of how we can utilize this tool. Number three – Use outdoor reset to lower water temperatures as low as possible, whenever possible. This is a “nobrainer� and should be required of any boiler. Some movement in this country is afoot to legislate mandatory outdoor reset control for all boilers; however, there is significant push-back from some stakeholders who have yet to embrace the concept of low temperature hydronic boilers, and therefore who do not have products that can easily comply. Reducing water temperature based on warmer outdoor temperatures works for just about any heating application you can imagine. Many doubters deny this strategy can work with baseboard radiators or fan coils, but they are just plain wrong. The chart below is from one of
34 Ops Talk • Fall 2010
the biggest baseboard manufacturers in North America, and every manufacturer has similar specifications. Based on these types of charts, a general rule for estimating is that a 10 degree Fahrenheit reduction in water temperature reduces heat output by 11 per cent, which just about perfectly tracks the reduction in heat loss from a space as the outdoor temperature warms up from design conditions. So you doubters out there have a hard look at the information presented here – this is not blue sky and wishful thinking, it is based on science and sound engineering principles, as well as many years of experience. Outdoor reset is a proven strategy that saves energy big time for a very minor outlay – many boilers today, especially low temperature and condensing boilers, simply throw it in to their onboard control for free.
And lastly, number four: use condensing boilers to maximize the benefit from outdoor reset and to squeeze the last bit of energy from the flue gasses. We used to throw hot flue gasses up the chimney (500°F at the flue collar of an atmospheric boiler is not uncommon), along with a whole bunch of dilution air and water vapour. Now we can cool flue gasses down to as low as the boiler’s return water temperature and condense the water vapour right in the boiler, recovering the latent heat from the condensate – there’s nothing left to recover – we can get it all.
For further information, please visit the IBC Technologies website at http://www.ibcboiler.com/ or call (604) 877-0277. íľż
