Track 5: Heat and cooling

7 diagrams.

DegreeDays

Heating degree days add up how far each day's average falls below 65 °F. A whole winter can total under 2,000 or well over 12,000.

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Left: a bar chart of one sample week. The 65 °F base temperature is a dashed line near the top. Each day's average temperature is a dot, and a bar hangs from the base line down to it. Monday averages 45 °F, giving 20 heating degree days; Tuesday 40 °F, 25; Wednesday 35 °F, 30; Thursday 50 °F, 15; Friday 55 °F, 10; Saturday 30 °F, 35; Sunday 38 °F, 27. The week adds up to 162 heating degree days. Right: whole-winter totals compared on bars. A mild southern or coastal site has under 2,000. The lesson's cabin example has 6,000. The coldest zones in the lower 48 states pass 9,000, and parts of Alaska go well past 12,000. A note gives the rule: winter heat is about hourly heat loss per °F times HDD times 24, so 300 BTU per hour per °F times 6,000 HDD times 24 is about 43 million BTU.

DiurnalSwing

Night flushing needs a big gap between day and night. In a dry climate the night air drops well below room temperature; in a humid one it stays warm.

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Two simplified charts of outdoor air temperature over 24 hours, from noon to noon. A dashed line in each marks a comfortable indoor temperature. Left, a dry climate with cool nights: the air is hottest in mid-afternoon and drops far below the comfort line from late evening to early morning. That shaded stretch is when open windows can cool the house and its thermal mass. The day-to-night swing is about 20 °F (11 °C) or more. Right, a humid climate: the air swings much less, and even at night it stays above the comfort line, near 78 °F (26 °C) in the lesson's example. Night air cannot cool the house, so shade, sealing, fans and a heat pump do the work.

HeatPerKwh

For each kWh of electricity, resistance heat gives 1 kWh of heat. A heat pump gives more, because it moves extra heat in from outdoors, but less in deep cold.

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A bar chart of the heat delivered for each 1 kWh of electricity. Electric resistance heat, with a COP of 1, delivers 1 kWh of heat, all of it from the electricity. A heat pump on a cold day, with a COP of 2.5, delivers 2.5 kWh: 1 kWh from the electricity and 1.5 kWh moved in from outdoor air. A heat pump on a milder day, with a COP of 3.5, delivers 3.5 kWh: 1 kWh from electricity and 2.5 kWh from outdoor air. A note says COP falls as outdoor air gets colder, and backup resistance heat runs at a COP of 1.

HeatPumpCycle

A heat pump moves heat from cold outdoor air into the house. Electricity runs the compressor; it does not make the heat directly.

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A simplified concept diagram of a heat pump in heating mode. A wall divides outside, with cold air, from the warm inside. Refrigerant flows in a closed loop. In the outdoor coil it boils and soaks up heat taken from the cold outdoor air. The compressor, run by electricity, squeezes the refrigerant so it gets hotter. In the indoor coil it gives off heat into the house. An expansion valve then lets the pressure drop, and the cold refrigerant returns to the outdoor coil. In summer the flow reverses to cool the house. Refrigerant and wiring work is for a licensed installer.

HeatPumpWaterHeater

A heat-pump water heater pulls heat out of the room air and moves it into the tank, so it uses about one-half to one-third the electricity of a resistance tank.

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Left: a section through a utility room with a heat-pump water heater. The heat pump sits on top of the storage tank. Warm room air is drawn in on one side, and cooler, drier air comes out the other. Electricity runs the fan and compressor. Heat taken from the air is moved down into the water in the tank. Cold water comes in and hot water goes out to the house. Right: a bar comparison of the electricity used to heat the same water. A standard electric resistance tank is the full bar. A heat-pump water heater uses about one-half to one-third as much, because it is two to three times as efficient. Notes list its needs: about 1,000 cubic feet (28 cubic meters) of air space, a room that stays between about 40 and 90 °F (4 and 32 °C) all year, it cools and dries its room, and it is slower to recover after heavy use.

NightFlushSection

Night flushing: open up at night so cool air chills the thermal mass, then close up and shade by day so the cool mass soaks up heat.

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Two cross-sections of the same small house with a concrete floor that acts as thermal mass. Left, at night: windows are open low on one side and high on the other. Cool night air flows in through the low window, across the room and out through the high window as warm air. The concrete floor gives up the heat it stored, so it cools down overnight. Right, by day: the windows are closed and an exterior shade over the window blocks the sun before it reaches the glass. Heat in the room flows down into the cool floor, so the indoor air stays cooler than outside. A key shows heat, air and sunlight arrows.

SolarThermalLoop

An active solar water heater: a pumped antifreeze loop carries heat from the roof collector to a coil in the tank, and a backup heater covers cloudy days.

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A simplified section of a house with a solar thermal collector on the south roof. Sunlight heats an antifreeze fluid in the collector. The hot fluid flows down to a coil near the bottom of a storage tank, gives its heat to the water, and the cooler fluid is pumped back up to the collector. A pump and controls run the loop. The antifreeze loop never touches the drinking water. Cold water enters the bottom of the tank, and hot water leaves the top for the taps. A backup heater in the top of the tank covers cloudy weeks and winter. Notes say the system needs a way to dump heat in summer if nobody uses the water, and that freeze, overheating and pressure protection are design work for a professional.