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Change always comes bearing gifts. ~Price Pritchett

12 Commandments of Efficient System Design

An Article written by Richard Trethewey,
mechanical systems expert of "This Old House"

From fireplaces to thermostats in less than seventy years!

M ethods for warming people have come a long way in a relatively short time. The historical progress from open fires in caves, through fireplaces and wood stoves to gravity warm air systems was dramatic. It was the advent of steam and gravity hot water heating boilers in the 1890’s that made central heating systems possible. Then came the modern hot water boiler with copper fin baseboard (circa 1947) which made for convenience beyond people’s wildest dreams. Just think about it – from fireplace to thermostats in less than seventy years! It was a fantastic period in our industry’s history. There is more innovation to be had beyond the technology of 1947. We can learn about what lies ahead by exploring some of the basic principles of comfort and heating. Every major industry continues to innovate: the computer industry, the automobile, the kitchen appliance. (Can you imagine Ben Franklin working with a Cuisinart? He’d wish he had never discovered electricity!) To help you keep your share of the market that is looking for comfort and innovation, here are some basic and very general rules for your review.
Principle One:
Heat always goes to cold.

U nder normal conditions, the human body maintains an internal temperature around 98 degrees with 70 degree skin temperature. If we lose heat too quickly to a colder space, the blood vessels constrict, thus reducing blood flow to our hands, feet and heads, and we shiver. If it’s too warm and our bodies can’t shed heat fast enough, we start to sweat. Thus, we should not be trying to lightly “toast” the human body. What we should be doing is controlling how much of our body heat goes to cold. In other words, we are not trying to heat or cool the human body; it can do that it’s self. We are controlling the external factors that would pull the heat from us or put too much in us. There are six factors that determine human comfort: air temperature, air velocity, relative humidity, radiant environment activity level and insulating value of clothing. Most people tend to think only about air temperature. In fact, it seems as though some people don’t know if they are comfortable unless they look at the thermostat. That doesn’t tell the whole story. If air were blowing across our body in winter, we might want our environment to be 74 degrees to be comfortable. In a radiant environment, we might feel extremely comfortable at 68 degrees. In the low relative humidity of winter, an even higher air temperature might be required for comfort in both of these cases. There certainly is more to this than setting the thermostat at 70 degrees!
Principle Two:
Heat Loss changes all winter.

W hen the outside air temperature drops below 68 degrees, Principle One kicks in with heat trying to get from hot (inside) to cold (outside). This amount of heat loss, changes from day to day and even hour to hour, all season long according to the difference between inside and outside temperature. Engineers call this difference Delta T or (Temperature Difference). The traditional approach for sizing heating equipment has been to determine the coldest weather you would ever expect (design conditions), add a safety factor, some pixie dust and one more boiler section. The end result is a boiler that is probably oversized by a lot the rest of the season. Ideally, we should have a boiler that changes size with the weather. Think about this: if you design for optimum system efficiency, the boiler would never shut off on the coldest day of the year.