Heating and Cooling

This is where you need to “put your thinking hat on!” Here’s a little help from the experts:

The thermal energy in a volume of air has “sensible” heat and “latent” heat. Latent heat is the thermal content due to the moisture content. Sensible heat is due to the temperature alone. When an AC system cools down air, moisture is first extracted – before the temperature drops.

Locate window fans for house ventilation on the downwind side of the house. Open a window in every room and keep interior doors open. Don’t ventilate if it’s humid outside.

A two-stage furnace operates at high speed only during peak load conditions (which happen only a few weeks each year), while the low speed runs the rest of the time. This scheme matches the heating load to the furnace output more closely, using less energy. (The same idea holds for two-stage air conditioning and heat pump motors.)

Multiple return air grills located strategically around a large house need less fan horsepower to operate without adding substantially to the up-front installation cost.

There must be a return air path from every room back to the air handler, either as an open door or a sizeable crack under a closed door.

Run the fireplace insert blower to get extra heat from the fire.

Most return air ducts in residential systems are too small. The return-air opening (where the filter is located) should be larger than the supply air main duct.

Put a box fan in a window on the opposite side of the house from your bedroom. Draw air through the house and into the bedroom for some refreshing nighttime cooling while you rest with (no fan noise). It feels just like a natural breeze.

Hot summertime air rises in a two-story house (by convection) and draws cool outside air into the building at ground level. Make the downstairs inlet area smaller than the sum of the outlet areas upstairs. This forces the cool air at ground level through an orifice, to increase air velocity.

Radiant heating systems achieve similar comfort to forced-air heating systems with a 5° to 7° F lower setpoint, and lower energy usage.

Hybrid systems are sized to provide both space heating and hot water from solar water panels on the roof.

Roughly 70 percent of all U.S. homes have HVAC systems that operate inefficiently or are close to a breakdown and significant repairs.

Close the blinds when air conditioning is on to eliminate some of the cooling load in each room. At a minimum, close the blinds when you’re at work all day, even if the AC is setback during that time. (Less heat will accumulate in every room with an exterior wall.)

Solar screens keep your house cooler in the summer by blocking or reflecting up to 90 percent of the sunlight. Their dark, heavy fabric adds a little insulation to the window assembly. (At R-2, even a little insulation has a big impact on windows.)

Electrostatic filters for the HVAC return can filter out smoke and other allergens that might influence asthma. Of course, the unit has to be running for the filters to do their thing.

Purchase a sturdy plastic comb to straighten out smashed fins on the outside AC condensing unit. Those are heat-transfer surfaces, and the greater the exposed surface area, the more efficient the process.

Adjust air-supply registers in the winter so that more air goes downstairs, which will then rise to the upper floor. Conversely, more cool air to the upper floor in the summer ends up downstairs as the day goes by.

A humidifier in the winter lets you decrease the load on the heating system because you don’t need the temperature set as high for comfort.

Airflow across the cooling coils decreases over the years because of poor maintenance.

Increasing that airflow to the rated value can improve the AC efficiency by 5 to 10 percent.

Both air conditioners and heat pumps use the refrigeration cycle to move heat in and out of the house. The only difference is that heat pumps have a reversing valve that; well, operates the system in reverse.

If your HVAC contractor wants to size your new heating or cooling system based only on the size of the old one, take your business elsewhere. A detailed heat-loss analysis (“Manual J Residential Load Calculation”) is the only way to size a new system. The contractor only needs to measure the outside wall, roof, and window area and plug the values into a computer program. Half a day’s work.

What is a BTU?

• 1 BTU = a kitchen match
• 1 kWh = 3413 BTU

A well-sealed vapor barrier on the outside of the insulation on cooling-air ducts avoids condensation.

Insulate HVAC ducts that pass through unconditioned spaces (attic, crawl space, garage) with R-8 foil-faced fiberglass batt insulation (foil facing outward and visible) for round ducts and rigid R-8 duct board for rectangular ducts. Seal joints with UL-181b duct tape. (Not regular “duck tape”; it will dry up and peel off.)

Be sure air can’t bypass the HVAC return air filter via gaps around the filterframe fitting.

Change the AC filter more frequently if you have the windows open a lot (pollen season!), burn fires in the hearth, or don’t dust and vacuum regularly.

Set the AC speed to “low” (or “on”) rather than “auto” to control humidity. Lower air volume may not cool quite as well, but it dehumidifies much better. (Note: dryer air feels cooler in the summer.)

Locate the AC condensing unit on the north side of the house (more shade from the sun), or in an area shaded by trees and shrubs (but nothing closer than 6 feet away to block the airflow).

Check the accuracy of the HVAC thermostat with a small digital thermometer (get one that measures humidity, too).

 “Passive cooling” methods to cool without refrigeration:

• “Cool” (reflective) roofs
• Natural ventilation
• Closing curtains and blinds
• “Cool” exterior paint colors

A solar chimney uses the principle that “hot air rises” to divert the breeze into a house without using a fan (or fan-assisted). “Architectural towers” do the same thing for commercial buildings.

Circulating air from a ceiling fan feels 5° to 8° F cooler in the summer because air moved away from the body carries heat away, too.

Run the ceiling fan and turn the AC up 5° to 10° F.

Energy Star fans with efficient motors and blade designs use 20 percent less energy.

For best cooling, a ceiling fan should be eight to nine feet above the floor, so they’re ideally suited for rooms with a tall ceiling.

High ceilings trap heat at the top of the room, and a ceiling fan (going clockwise) can push that air down to where it’s needed in the winter.

There are two loads on the HVAC system: “internal loads” (lights, people, appliances) and “external loads” (envelope, exhaust, ventilation).

If you tilt Venetian blinds at 45°, you increase the insulating value of double pane windows to having a storm window (triple panes). What’s not to like?

Close blinds and curtains in unused bedrooms to lower the heating and cooling load.

Do not close off air registers. Only close air vents halfway. Every room needs regular air circulation to dissipate humidity that infiltrates through the outside walls.

Trim back shrubs around the HVAC condensing unit. The system works by transferring heat from the coils to the air when the fan’s on. Shrubs planted around the unit can block that airflow.

Hose off the heat-exchange surfaces for the HVAC condensing unit outside. Dirt, pollen, grass blown from the lawnmower—anything on those metal heat-exchange surfaces retards heat transfer and lowers the efficiency of the unit.

Close the fireplace damper. The chimney, by design, draws smoky air up and out of the hearth. Even a light breeze pulls house air up the chimney. Talk about wasted energy!

Bedroom doors have a big crack at the bottom for a reason. That allows air (entering via supply grills in the floor or ceiling) to return to the AC unit. If you block that return air path, the space won’t get enough supply air.

Ductless mini-splits are suitable for:

1. Houses with no existing ductwork
2. Houses with bad ductwork, ducts outside the building envelope, or located in an inaccessible area
3. To heat and cool isolated rooms, e.g., a guest room or addition
4. If the existing central system is to be replaced

Change the furnace filter regularly just like you do with the central air filter.

Remove obstructions from floor registers that can block airflow: rugs, carpet, furniture, curtains.

Clean the air supply and return grills periodically.

Close interior doors to limit the area cooled by window or wall AC units. (All rooms served by the central HVAC should be open and their supply registers open.)

Operating the AC blower fan continuously increases leakage from the ductwork and also air leakage from the building envelope.

Unbalanced airflow (because of duct leakage) between the supply and return air ductwork pressurizes and depressurizes zones inside the house.

One way to be sure the supply air registers remain open is to install non-operable grills.

Use the official “Manual J” method to determine if the AC and furnace are the correct size.

Put a fire-rated floor protector under space heaters that sit on a rug or carpet. Situate space heaters away from furniture, drapes, anything flammable—and traffic.

Do not use a space heater to dry clothes or for any purpose other than heating the house.

Don’t put anything near a space heater that could limit the airflow near it.

All heat exchange surfaces inside a wood-burning stove should be cleaned at the end of each winter season, free of soot build-up.

Remove dirt stuck to the fins on the outside condensing unit with water coil cleaner and rinse with the garden hose until the water is clear.

Inspect the AC blower fan and blades. If they’re dirty, the coils are probably dirty as well, and everything needs a good cleaning.

Shield outside air intakes for air handlers to tight structures. Wind can affect the building pressurization and fresh air volume intake.

Large buildings have a purge cycle overnight to flush all the high-CO2 air out and replace it with 100 percent fresh air. This technique also sweeps all the hot air from the building. Be sure the outside humidity isn’t higher than the indoor humidity.

Put a clear, locking cover over the thermostat so nobody can adjust it when Mom and Pop aren’t home.

Save fan energy by matching the air handler fan output to the heating and cooling load. This requires a two-speed or variable-speed fan and controls but saves a lot of fan energy.

Short cycling (the AC turning on and off frequently) happens if the system is over-sized for the cooling (heating) load. This can happen if many energy-saving projects have lowered the peak load substantially.

Adjust the “deadband” temperature for the AC—the temperature range where no heating or cooling is called for—and let the space temperature drift. This is an interim fix for short cycling. The deadband should be 4° to 10° F.

Manual switching the thermostat from “heat” to “cool” mode keeps the HVAC from shuttling between heating and cooling on the same day. A wide deadband can also help eliminate unnecessary heating and cooling.

If one area needs special heating or cooling (a grandma suite or a baby’s room), install a dedicated unit, e.g., a window unit or mini-split system.

In very humid climates, a second cooling coil can add dehumidification when needed, while the rest of the system is configured for optimal coolingonly efficiency.

A setback temperature strategy is an adjustment after hours (at night or during the working day) to save on heating and cooling costs.

A proximity switch can turn off a room AC when the windows are open. They’re often used in motel rooms, but also apply in residential guest rooms, exercise areas, or other limited-use spaces.

The efficiency of radiators is not very dependent on maintenance, but regular maintenance keeps the heating capacity high and prevents people from using the equipment in an inefficient manner.

Don’t trap heat from a floor register or radiator against the walls via draperies or window treatments or furniture.

Move radiators away from outside walls to prevent a large heat loss due to the high-temperature difference across the wall. Insert insulation or reflective material between the radiator and the wall.

Through-the-wall heat pumps and air conditioners need regular maintenance, just like the central AC system. They need lubrication, adjustment, refrigerant charging; cleaning of the filter, coils, fan blades, and condensate drain.

Seal around wall AC units and install a seasonal cover over the whole unit during the off-season to eliminate leaks through the unit.

When a fan motor fails in the air handler (inside) or the condensing unit (outside), replace it with the highest efficiency motor available.

When the AC (or refrigerator) compressor fails, replace it with the most efficient compressor available. If the system is old and inefficient, it may behoove you to replace the whole condensing unit (refrigerator) with a more efficient model.

Eliminate outside air ventilation for window AC units.

18.1 THE BIG CHILL.

A heat pump operates most efficiently with a full charge of refrigerant. That’s the first thing the AC technician checks on a house call.

Requiring a top-off every couple of years is okay, but if they’re coming out more often, you may have a not-so-slow leak that needs to be fixed. Freon damages the ozone layer and also contributes to global warming.

 18.2 WEATHER DATA FOR HEATING AND COOLING LOADS.

An insightful measure of how hot your summers are, and how cold your winters, is a metric called degree days. Given an inside winter temperature of 65° F, a degree day is a twenty-four-hour period when the outside temperature is 1° F away from that inside temperature. Sum all the degree days for a year, and you get a number called Heating Degree Days (HDD). Similarly, for Cooling Degree Days (CDD).

Do a google search on “Wikipedia HDD CDD” to find a more rigorous explanation. They also have a great map of the United States that has contour-type lines for CDDs and HDDs.

Once you have a handle on the cooling and heating degree days of your location, you’ll know if you’re main HVAC concern is heating or cooling. This helps to prioritize specific projects. For example, a high-HDD location benefits most from a dark-colored roof—a high-CDD location, a light color.

18.3 THE STANDARD FIX IS TO ADJUST THE THERMOSTAT.

The recommended summer thermostat settings are 78° F in the daytime and 82° F at night (whatever you can handle). The ideal winter settings are 68° F during the day and 62° F at night.

The most effective (so you never forget) and easiest way to maintain these settings is to get a programmable thermostat. Program in these settings and forget about it!

 18.4 HOT AND BOTHERED.

Some folks have their thermostat catawampus. Comfortable for ordinary folks is miserable for them in the summer. Comfortable in the winter for them is freezingcold for everybody else. They need to sleep in a cold room even in the summer (while you shiver under a blanket).

Instead of keeping the whole house cold in the summer, put a window unit in the bedroom and run it at night. (Sorry, you’re just going to have to wear long underwear or flannel PJs in the winter.)

A more expensive option, but more energy-efficient, is a small mini-split heat pump system. The condensing unit is outside, so split systems are much quieter than window (or wall) units.

 18.5 WHAT IS COMFORT ANYWAY?

Get an inexpensive digital hygrometer (measures humidity) and experiment with your thermostat at home. For example, 72° F at 55 percent humidity feels a lot cooler than 72° F at 75 percent humidity. That means you can raise the thermostat several degrees if the inside humidity is low.

It’s the dew point, not humidity, that governs what feels comfortable. Unlike humidity, which has wide swings during any given twenty-four-hour period, the dew point remains relatively constant. You can find it on most weather websites, but the dew point is typically close to the nighttime low temperature.

A dew point below 65° F feels comfortable, and below 60° F seems dry.

Air movement also makes the temperature feel cooler, as from a ceiling or floor fan. Air forms a thermal layer near your skin if you don’t move a lot. Movement (of you or the air) keeps the air layer dynamic, transferring that heat away.

Sweating (evaporative cooling) is even better.

18.6 WHERE, EXACTLY, IS YOUR THERMOSTAT?

Thermostats are supposed to read the “whole-house” temperature, right? That’s why they’re usually located near the return-air grill (the one with a filter that you change every month or two).

What if the thermostat’s in the down-flow of an air-supply vent? On a wall that gets direct sunlight in the afternoon all winter? If it’s near a radiator or other heat source? Blocked by nearby furniture or curtains?

 18.7 SMART THERMOSTATS.

The experts say a programmable thermostat saves $180 a year in heating and cooling costs. However, will you use it?

I usually start low-tech on items like if I’m comfortable sleeping with the temperature 5° or 10° F warmer in the summer (you set the thermostat to let the temperature rise after you’re asleep); vice-versa in the winter. In the winter, you can pull an extra blanket on the bed, but summer can be a bother if you’re heat-averse.

There’s no use buying an expensive thermostat if you won’t be using it. Try the settings on a regular thermostat first, and if it works for you, purchase the programmable one.

Note: some home security systems offer a programmable thermostat as part of their package. Free tech! Go for it.

18.8 BURNING WOOD TO HEAT A HOME.

When I purchased my house in the Appalachians, it had an incredibly inefficient gas-log fireplace in the living room. All the heat went up the chimney! I replaced it with a high-efficiency wood-burning stove insert with an efficiency north of 90 percent.

I don’t fire the hearth up until it gets frigid outside, so cold the central heat is at its least efficient (close to straight electric heating). A couple of logs keep the whole house so warm all night that the heat pump never even comes on.

Full disclosure: Most of the wood I burn comes from dead trees in my own yard.

In this capacity, the hearth is supplemental heating, used when the heat pump is at its most inefficient. The colder it gets outside, the less efficient a heat pump is: A COP of 3.0 at 32° F, dropping uniformly until 0° F when COP is 1.0, or the equivalent of strip heat.

 18.9 SOLAR VERSUS CLOUDS.

The one drawback of my removable insulation system for the windows is that when the sun is low and to the south in the winter, the blocked-out windows miss out on sunlight streaming into the house for some pleasant passive warming. Since this area is cloudy in the winter, and my house is surrounded by mountains and tall trees, I don’t miss out on that free heating very much.

Note: I also haul the removable insulation out of the mechanical room for six weeks or so, in the peak heat of the summer, to save on cooling costs: and to block out all that direct passive-heating sunlight!

 18.10 LATENT OR HUMIDITY LOADS IN A HOUSE.

Humidity entering the building envelope is costly in the summer because the HVAC must spend energy extracting latent moisture before it can cool down the air.

Think of the air conditioning process in terms of weather: a cold front moving through the humid Gulf Coast region in the summer typically causes a long line of thunderstorms along the frontal boundary. The air doesn’t cool down until all the moisture leaves the air in the form of heavy rain – with lots of lightning and thunder! (Rain = the result of nature’s dehumidification.)

18.11 SWEAT IT OUT, DUDE.

The human body sweats to cool down. Every gram of sweat extracts water’s heat of vaporization (540 calories) when it evaporates. A typical workout produces 1 liter of sweat, which, when it evaporates, draws 54,000 calories of heat energy from the body. Wow!

18.12 TOWELING DRY.

A loose drying—a pat-down instead of a rub-down—so that some water remains on your skin after bathing cools you down nicely.

Or if you’re at the gym and sweating buckshot, instead of wiping down, stand in front of a big floor fan and feel that 540 calories per gram heat of vaporization sucking all that heat out of your body.

 18.13 ACTIVITY VERSUS MOISTURE LEVELS.

One person adds fifty grams an hour of moisture (breathing) to a space while sleeping, 80 g/h when seated, and 100 g/h doing housework. So, if you remain indoors all day at a minimal activity level, you add almost 2 Kg of moisture to your home.

Other activities will add even more moisture: washing dishes (500 g), taking a shower/bath (1500 g), and cooking (up to 2000 g). The HVAC system must extract all that moisture.

Add more people, and you have a substantial latent load on the HVAC. Whatever you can do to send that moisture outside via exhaust fans is beneficial. A clothes washer can add 500 g of moisture. A vented clothes dryer sends ten times that much moisture (5 liters) outside, so you sure don’t want to dry clothes inside (in the summer).

Moisture loads: grams/hour

• 50 – breathing
• 80 – seated
• 100 – housework
• 500 – clothes dryer
• 1500 – shower/bath
• 2000 – cooking

 18.14 SUMMER VERSUS WINTER ATTIC VENTILATION.

Heat always flows toward the lower temperature. We’ve seen this in the outside walls of the building envelope, and in ceilings, where we also learned that heat not only rises, so much so that attic ventilation in the summer isn’t as important as in the winter.

How significant is this phenomenon?

The air space in double-pane windows is an insulating feature. The R-value of single-pane windows is about one; double-pane windows two; triple-pane windows three. (The cost goes up by about the same ratio. Is it worth it based on energy saved?)

The R-values for each type of window assembly are lower in the summer because heat rises, going with the flow in the summer, but going against the flow in winter (when cold air in the air space wants to fall versus “warm air rises”).

Venetian blinds in the horizontal position impede vertical airflow and therefore have an insulating effect upon the glass surface. Tilt the blinds at 45°, and you increase the U-value of a double pane window to that of a triple-pane window.

 18.15 THE PEAK LOAD AND THERMAL MASS RELATIONSHIP.

To review: The peak load on your AC system occurs on the hottest, sunniest afternoon of the summer. The system is sized to satisfy that maximum demand.

The effect of thermal mass is to move that peak HVAC load forward in time. An empty building reaches the outside temperature hour(s) later (even later still if air is circulating inside the building, to distribute temperature evenly from the envelope/walls/floor).

If you fill that building with housewares and books/walls/ceramic floors/furnishings, all that additional mass moves the peak load forward in time even more. Bricks and masonry add thermal mass to a building, too (especially when located inside the building envelope).

Ideally, construction mass should be placed where shaded in the warmer months (on the north side) but where it receives sun in the winter months (near a window).

Adobe-wall construction in the American southwest absorbs the summer day’s worst heat, and by the time that heat “gets through” (heats uniformly) the walls, it’s already dark. The outside temperature after sunset drops quickly in an arid climate. The temperature difference in the exterior walls reverses, and the heat in the walls radiates toward the exterior.

I’ve arranged much of the thermal mass in my house (thousands of books) along the outside walls. I live in the southeast United States. Thermal mass works like a charm!

I run the AC at 72°F at night (when it’s colder outside, so the heat pump is more efficient), then raise the thermostat to 78°F at dawn. After that, the unit only runs to dehumidify. The temperature downstairs never gets above 70°F during the day (heat rises!) or above 75°F upstairs, even on the hottest day. Hence, the AC is off all day long.

What this means, in practice, is that I’ve shifted the HVAC demand to late at night when (if my utility had such a rate structure) the electric rates are 50 percent lower. So, I don’t save energy, but I sure could save a lot of money. (And it’s nice being able to sleep in a lower-temperature house.)

18.16 QUALITY FEEDBACK HELPS TO MONITOR YOUR PROGRESS.

Each month, I receive an email from the utility company with a graph of my house’s utility cost versus similar homes in the area. I’m twice as efficient as the “most efficient homes” on their chart, and the average temperature inside my house is 10°F lower than those “efficient houses,” thanks to thermal mass.

18.17 HEAT PUMP STAGES.

The heat pump for my house has several stages. The lower stages (when the load on the compressor is lowest) of the compressor are the most efficient. The maximum heat pump load in my area is the winter heating load (which is a fair bit higher than the peak cooling load). Consequently, the heat pump operates in its most efficient bandwidth during the cooling season.

18.18 CLIMATE CHANGE IS INEVITABLE

Global warming guarantees that today’s summer peak load will be pushed higher. Naturally, when that happens, you’ll know how to reduce the load on your HVAC system: close the blinds and curtains in empty rooms, move thermal mass to the outside walls, extract moisture before it becomes a load on the furnace, wear loose clothing, and turn up the (programmable) thermostat.

Why wait to do all that stuff? Start saving energy now!

18.19 HEATING DRIES OUT THE AIR IN YOUR HOUSE.

Forced-air heating systems (as opposed to radiant heating via old-fashioned radiators) dry out the air. An arid environment dries out your skin and saps moisture from wood furnishings. Consequently, the extra humidity from cooking or bathing is beneficial in the winter.

If in doubt, purchase an inexpensive digital thermometer with a humidity readout. Summer humidity shouldn’t rise above 50 percent, or winter humidity fall below 30 percent. That’s the comfort zone: 30 to 50 percent.

My HVAC kicks on automatically when the summer humidity goes above 55 percent. There’s an add-on device that can increase humidity in the winter, but I just keep a dehumidifier going in the living/dining room/kitchen area. I have dry eye syndrome and must maintain a humidifier going all winter to keep the humidity above 50 percent.

18.20 SUPPLEMENTAL HEAT COMES AT A HIGH COST.

Heat pumps provide heat efficiently down to well below freezing. However, as the temperature drops below 50°F outside, the heat output of the system steadily decreases. As the temperature drops (and the demand for heat increases), the unit must rely increasingly on pure electric supplemental heat (the least efficient). At 0°F, all the heat is pure electric, which is relatively inefficient.

I had my HVAC contractor disable the supplemental heat controls on my unit, so the pure-electric heat never comes on, no matter how cold it gets outside.

Don’t be smart and disconnect the breaker for the heat pump’s 240-volt heat circuit(s). The unit still needs those electric-heat strips for the defrost mode. That’s when ice builds up on the outside coils. The unit needs that electric heat source to melt the ice.

Disabling the unit’s call for supplemental heating must be done by a contractor, inside the programming of the system (which is inside the programmable thermostat for my unit).

18.21 GET SMART AND FIND YOUR COMFORT ZONE.

There are more factors to “comfortable” than just room temperature. Humid air usually feels warmer inside, just as it does outside. Humid air in the winter makes it feel colder. It’s my experience that summer humidity is the defining factor for comfort. In the winter, it’s radiant cooling from too much exposed glass.

18.22 CHANGE THE HVAC FILTER REGULARLY.

Any dust, pet fur, pollen, or smoke particles that get through the air filter ends up on the cooling coils inside the air handler. These coils are where heat is transferred to and from the refrigerant and the airflow.

Dirty coils equal lower heat-transfer rate, lower overall efficiency, and higher utility bills. If your AC contractor doesn’t clean off those coils inside the unit during regular maintenance visits, ask them to do so. It’s part of the service.

 18.23 SWAMP COOLERS USE 100 PERCENT EVAPORATIVE COOLING.

In hot, dry climates, it’s possible to fan-cool air in a heat exchanger and then use this colder air to air-condition a building. However, the cooled air is much more humid than the ambient air. (Hopefully, you realize now that dumping a lot of humidity into a closed space is asking for trouble.)

Given a dry climate, some extra humidity might be okay. However, even if the humidity inside and outside is 20 percent, and your swamp cooler increases the inside humidity to a comfortable 55 percent, remember vapor pressure. Humidity always equalizes, and if the interior humidity is higher, that moisture will try to migrate outside and get trapped inside the walls. Problems!

Swamp coolers are best suited for open buildings like a mechanics shop or garage, or perhaps a semi-enclosed pool or exercise area.

The vital idea here is to recognize the value of water’s high heat of vaporization, to see how this powerful thermodynamic property can be leveraged for air conditioning and cooling.

 18.24 OUTSIDE DINING WITH THE BIG CHILL.

A common practice in hot, dry climates is to enhance outdoor activities with a fine water spray. The system periodically sprinkles the environs with a very fine mist that quickly evaporates and cools down the ambient temperature. (Kind of like a rooftop sprinkler but for people.)

A fine water mist is a much cheaper way to cool a gathering on an outside patio than a portable AC unit blasting out cold air in a noisy torrent of conspicuous consumption.

Or fans blowing air past giant blocks of ice as they do for football players on the sidelines. (You don’t suppose 95 percent of the cooling effect isn’t from the cold in the ice but from evaporative cooling?)

 18.25 FRESH AIR BALANCE VERSUS INDOOR AIR QUALITY.

All human activity requires oxygen to breathe, while carbon dioxide (CO2) is exhausted. An electronic sensor in the air stream can monitor the CO2 levels in the HVAC’s supply air stream.

A high CO2 reading triggers an outside air damper to open at the air handler, increasing the supply of fresh air until the CO2 level drops to within acceptable means.

 18.26 THE STACK EFFECT INCREASES INFILTRATION AND EX FILTRATION. 

Wind blowing against, around, and over a building creates a positive pressure on the windward side (facing the wind) and a negative pressure on the leeward side. The net result of these pressures is to force air into (and out of) the building.

This stack effect is especially pronounced in multi-story buildings, increasing the higher you get. (Can you see that “shrinkwrap” is useful in this situation, especially for the upper floors?)

 18.27 WHOLE HOUSE FANS AREN’T FOR EVERYBODY.

Fans designed to draw outside air through the whole house don’t work so well in a humid Gulf Coast-type of climate. Why? Because that cool air is full of humidity. What you might be saving in cooling costs is minuscule compared to the extra electricity used by the HVAC to wring all that humidity out of the air once you close the house.

So, if you insist on using a whole-house fan, be sure the outside humidity is lower than the inside air. Purchase an inexpensive digital temperature-humidity meter on the electronics aisle. Or check your local weather online. Anytime the outside dew point is below 60-65° F, you’re good to go.

18.28 SOLAR–POWERED AIR CONDITIONING.

Some HVAC manufacturers offer systems with supplemental solar electric panels. If I didn’t live in such a cloudy place (plus tall trees shading the roof), I’d have been tempted to try that system when I replaced my heat pump a few years ago. You usually need to install an expensive inverter (which converts the solar electricity to alternating current), extra writing, storage batteries, altogether a daunting undertaking for even an engineer savvy in all that stuff.

Having the panels and all the electrical lagniappe as part of the whole HVAC package; well, I think it’s just a fantastic idea. If you use solar electricity when it’s available, and the panels have a standard payback: there’s no downside.

You might even be able to double up on your rebates for a high-efficiency HVAC system plus solar panels.

18.29 WINDOW AC UNITS.

Wall and window AC units allow movement of air in and out of a room in the winter, especially if there’s an outside air damper. Close off all the louvers and, on a breezy day outside, hold a match in front of the unit—around the edges—to check for leaks.

Or just remove the unit from the window during the offseason and cover the opening with plywood (plus maybe a layer of solid-foam insulation).

 18.30 COPPER REFRIGERANT LINES.

Copper tubes go from the air handler in your house to the condensing unit outside, and back again. Those are the refrigerant lines, and they get very cold. (One is for liquid refrigerant, the other for gas after it absorbs heat and boils into gas.) Those copper lines should be very well insulated, not just for thermal reasons, but because water condenses out of the air on the cold tubing.

Even a small condensation stream can do a lot of damage to the floor, carpet, framing, even household items stored nearby.

 18.31 HEAT PUMPS IN ALASKA?

They used to say that heat pumps aren’t economical anywhere north of 40° latitude. (Natural gas companies still say that.) Modern advances in technology have extended the operational range of heat pumps down to below zero. They’re pure electric heat by that point, COP = 1, but they do work.

Generally, if the average low is in the twenties, you’re okay with a heat pump. Colder than that, and you need a small natural gas furnace as a supplemental heat source. Or a wood-burning stove—anything dependable—because when it gets that cold outside, you want to be 100 percent sure you have heat.

Ground source heat pumps operate at a COP of 3.0 down to zero, and then some. GSHP systems have performed exceptionally well in Alaska, especially in combination with solar panels (as a hybrid system). Just be sure you bury the ground loop well below the frost depth (the depth to which the ground freezes solid in the winter), which can be three to six feet.

 18.32 HOT AND HUMID AIR CONDITIONING.

If you live along the Gulf Coast, or any other location where it’s hot and humid, you need an air conditioning system that’s good at removing moisture from the air.

Some manufacturers boost the rated efficiency of their systems by keeping the condensing coils warmer. Those systems still cool the air okay, but they’re not good at condensing moisture out of the air (which happens only when the air temperature drops below the dew point).

Supply air is typically 60° F, coming off 40° F cooling coils. If the dew point of the house air moving through the air handler is 65° F, you get a lot of dehumidification. However, if the cooling coil is only 55° F (which saves a lot of energy and boosts rated efficiency), you don’t get much dehumidification.

A heat pump matched with a multi-speed blower gives you the most dehumidification (at lower speeds) and good cooling efficiency at higher speeds.

Future Tech! 

18.33

Two-speed condenser-fan motors don’t add much to the cost of the system but increase the efficiency by leaps and bounds.

18.34 

A portable dehumidifier extracts water from the air and adds heat to the environs.

18.35

If you want to humidify the house in the summer, a portable swamp cooler (a fan with a water tank on top that trickles down the face of the fan assembly) is just the thing—free cooling!

18.36

HVAC systems can be programmed to go into “fan mode” during peak hours (4-6 P.M.), moving air around the house but not cooling. In order for the air temperature in the house to increase, the entire thermal mass of the house must be raised—a very slow process.

 

Figure 18.7 The outside condensing unit extracts heat from the refrigerant, causing the freon to transition from a gas to a liquid.

18.37

Likewise, for “emergency heat” in the winter, to avoid operating the compressor at it’s most inefficient (the lower the outside temperature, the lower the efficiency of the system).

18.38

 The AC condensing unit has (1) a fan, (2) metal heat-exchange coils, and (3) refrigerant inside those coils that needs cooling. Add a water spray, and you have the power of evaporative cooling. Can’t manufacturers make that work – to increase efficiency by a whole magnitude? (In the meanwhile: Your AC is much more efficient when it’s raining outside.)

18.39

Cool a whole house down to 70° F at night (when the electric rates are lower), then run the fan all day, using the thermal mass of the house for cooling.