In the field... Robert O'Brien Technical Heating Co. Mt. Sinai , New York What size boiler do I need if I’m using an indirect water heater? Quite a common question and for the vast majority of residential applications, the answer is: the same size as without an indirect. Whenever boiler sizing is in question, over-sizing is never the right answer. The short cycling and reduced efficiency with over-sizing are common knowledge. “Yes, but I’m using a XX gallon indirect and the I&O manual says you need 180K BTU/Hr to get the rated output.” True enough, but how do you know that’s the right size indirect and that you need the full rated output? Not many do it, but the correct procedure is to do the math rather than just guessing the size indirect tank that is needed. You need to know the Gallons Per Minute (GPM) and temp rise that is needed, the wild card is the duration; how long do you want larger than the heat loss of the structure? Quite a few. Oversize the indirect, for starters; most indirect manufacturers rate their tanks at varying BTU/hr inputs. For example, a 30 gallon tank paired with a 160K boiler may yield the same first hour rating as a 60 gallon tank and a 90K input boiler. Manufacturers’ specs are the place to start. Every water heater should have a mixing valve, correct? Absolutely! And what does maintaining the tank temp at 150°F vs. 120°F do for DHW production? Going from 120°F to 150°F will increase the effective size of the tank by 25% and just like that a 40 gallon tank can give the initial output of a 50 gallon tank and help prevent Legionnaires’ Disease as a bonus If needed, multiple indirects can be used or a storage tank on a single indirect. If none of these will give the desired quantity of DHW needed, a tankless water heater or multiples are the way to go; these will provide their rated GPM output forever and not require a boiler three or four times the heat load of the home. There is no good excuse to oversize a boiler and I know many will think this is heresy, but you don’t have to do a full blown heat loss calculation on every job. After you do quite a few heat loss calculations, you start to realize that similar sized and designed homes have nearly identical heat loads and that this number is usually less than the smallest boilers available. Of course, homes that don’t fit the standard ranch, cape or colonials will require a calculation. Don’t add a “fudge factor” to the inputs or the final number, the software has plenty of fudge already baked in. Over-sizing is a disservice to the consumer and the hydronic industry as a whole. Homeowners depend on us to provide efficient solutions to their problems and it is incumbent on us to do so. ICM Boiler Sizing and Indirect Water Heaters to maintain that GPM flow rate? At this point, a thorough site inspection and customer interview are vital. Two identical 1500 sq/ft Cape Cod-type homes with identical heat losses can have wildly varying Domestic Hot Water (DHW) loads. For instance, there will be a few teenagers in one home and a retired couple in the other; this means a huge difference in requirement. In one case, a 30 gallon load is more than enough and in the other an 80 gallon load may be insufficient. Without doing the math, you just don’t know. Ask the customer what they want and need and then size to that, but keep in mind there are many ways to get to the same end result, and all of them will be more efficient than over-sizing the boiler. What ways can you deliver the required DHW without a boiler 14 ICM/May/June 2016
ICM May-June 2016
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