In the field... Robert O'Brien Technical Heating Co. Mt. Sinai , New York Heat-loss calculation on every boiler replacement Do you perform a heat loss calculation on every replacement job? Should you? Everyone will say you absolutely, positively must! At the risk of being excommunicated from the Hydronic Brethren, I’ll say not always and, depending on the type of work you do, maybe never. How can that be possible? I live and work on that lengthy, narrow atoll that industry expert Dan Holohan refers to as the “Isle of Long,” aka Long Island, NY. It’s full of subdivisions built mostly between 1950 and 1980. These developments are invariably comprised of a very few models of homes: ranches, capes and colonials primarily with a smattering of variations on that theme, splanches, high ranches and split levels. Approximately 90% of the homes will fit one of these categories. Back in the early 80s, I was told a Manual J was the proper way to size boilers and I decided that I would use that method. The Manual J is very aptly named, because it entails a lot of manual labor to gather the data and then calculate the old school way, on worksheets. After slightly more than four hours into it, I arrived at a number that was approximately one third that of the existing boiler. Impossible! I did the math again with the same result. I thought my scientific approach would impress the prospective client and win me the job, but not quite. I presented my proposal to the homeowner along with the heat loss worksheet showing a boiler size of 59,000 British Thermal Units per hour (BTU/hr). It wasn’t received very well. After he stopped laughing and regained his composure, he showed me three other quotes he had received. The existing boiler was 160k and the quotes were from 160k-225k. All from reputable local companies who, at that point in time, had been in business a lot longer than me. Long story short, he went with the company proposing the 225k boiler. “That way it doesn’t have to work as hard.” That turned me off heat loss calculations for quite a while— until I got my first computer, an IBM XT. Some very expensive heat loss software and I was back in the game, only now with very official looking reports printed with my daisy wheel printer. I had a little better luck this time around and, while not everyone was buying this method of boiler sizing, enough were. I’d be lying if I wasn’t nervous when the first stretch of design temp weather set in but, amazingly enough, no complaints. I started doing quite a few with the new computer but after a while I noticed something: the numbers were almost all the same. No sense spending time measuring another ranch when you’ve already done ten of them and they are all within 5% of each other. There was, and still is, a very substantial proportion of oil heated homes on Long Island. Unfortunately, there aren’t a lot of choices in size for oil boilers. Even today, 65k is about the smallest and many manufacturers’ smallest are in the 85k range. It doesn’t make a lot of sense measuring a house that you know, from doing many heat loss calculations on similar homes, is less than 50k when the smallest boiler available is 85k. Yes, but what about the oddball home? The mid–80s contemporaries or the circa homes? I have an answer to those as well, with no measuring required. You need some information, such as the heating degree days over a known period of time; I’d recommend a year and the fuel consumption over that same time period. Degree days can be found at weatherdepot.com. Step 1. Add up all the BTUs of the fuel you used and multiply by the AFUE rating of your heating appliance. A gallon of oil has 139,000 BTUs/gallon, a therm of natural gas has 100,000 BTUs/ therm, and a gallon of propane contains 91,000 BTUs/gallon. Some gas utilities measure their consumption in CCF, which is 100 cubic feet, which is 102,000 BTUs. Step 2. Divide the total number of BTUs by the number of heating degree days. The result is a measure of how many BTUs your home lost per Heating-Degree-Day. Now divide that result by 24 to obtain the number of BTUs your home loses per heating-degreehour. Lastly, multiply the result by the difference between 70°F and the design temp. For example, if the design temp is 0°F, then the difference you multiply your heating-degree-hours by is 70°F. An example: Home using 1,000 therms/year in an 80% boiler in a 6,000 Degree Day climate per year 1000 x 100,000 ( BTU per therm) = 100,000,000 BTUs 100,000,000 x .80 (AFUE) = 80,000,000 80,000,000 ÷ 6,000 (Degree Days) = 13,333.33 13,333.33 ÷ 24 (hours per day) = 55.555 555.555 x 70 (desired indoor delta T) = 8,888 BTU/hr 38,888 = heat loss of home A couple of big advantages to this system is that it gives you an actual heat loss on a real home, not a calculation based on factors that may or may not be correct. How do you know the infiltration or do anything other than guess at an R value? No tape measure required. However, there are a few caveats: if they use supplemental heating, mini split heat pumps, pellet stoves, etc. the number will not be correct and, depending on how much they supplemented, may not even be close. Caveat number two: they kept the house colder than they would have liked due to fuel cost—this one is pretty common in oil and propane heated homes and will skew the number lower because of the abnormally low fuel consumption. Ask these questions upfront and when you get the final number, ask yourself if it makes sense. The average number around here is 20 BTU/sq ft. If I am far off that in either direction, I look for a reason why. As a last resort you can always break out the tape measure! ICM 14 ICM/March/April 2016
ICM March-April 2016
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