Masonry Heaters by Ken Matesz

Author:Ken Matesz [Matesz, Ken]
Language: eng
Format: epub
Publisher: Chelsea Green Publishing
Published: 0101-01-01T00:00:00+00:00

Heating Requirements

The first step in designing a masonry heater is to know how much heat is required for the given space over a particular time period. In terms of a home in a cold climate, the heating requirements of that space are equal to the amount of heat that the building loses over a period of time at a specific target indoor temperature. This heating requirement calculation is often referred to as the heat-loss calculation. Obviously, if a structure never lost any heat that was produced or collected within its confines, it would forever stay at or near the temperatures produced by that heat.

Practically speaking, zero heat loss is impossible. Every building material, including insulation, has some combination of porosity, conductivity, and a degree of leak-proneness. In addition, thick insulation requires thick walls that have typically high labor and material costs. Window and door openings are prone to tiny leaks, while glass itself is a poor insulator. And of course, people go in and out of houses, exposing the indoors to the cold outdoors. Thus, there are two primary ways in which heat is lost from a building. Either it is lost because heat conducts through materials to the outside, or it is lost because of some kind of leak, often called infiltration. There really is no way to completely prevent heat loss from a dwelling from time to time. This continuous winter heat loss is the whole reason for heating.

WHAT ARE BTUS AND KILOWATTS?

BTU stands for “British Thermal Unit,” which is simply the amount of energy required to raise the temperature of 1 pound of water by 1°F. (This definition assumes liquid water and standard atmospheric pressure.) When speaking about masonry heaters, BTUs are used to refer to the power produced by the heater. Power is energy (or work) delivered over a period of time. You might say, for example, that a particular masonry heater outputs 10,000 BTUs per hour. In this context, you’ll often simply see the measure in “BTUs”; the hour units are presumed.

A kilowatt (kW) is 1,000 watts. Again, a watt is a unit of power. Most people know that a 100-watt lightbulb gives off more light than a 40-watt bulb—the one is more powerful than the other. Similarly, a masonry heater that throws more kWs of heat is a more powerful heater.

Since both units are often used in North America as measures of power, they also relate directly to each other. One kilowatt is equal to 3,412 BTU/hr. One BTU/hr equals about 0.293 watts (.000293 kilowatts). A fairly large masonry heater would output 6 kW, which is then 20,472 BTUs/hr (6 × 3,412 = 20,472).

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