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Heating Ventilating and Air Conditioning

Heating of living spaces has been a preoccupation of mankind all the way back to the dawn of civilization and beyond. Traditionally this has been done by burning plant material (mostly wood) and the methods of extracting the heat while assuring that the combustion byproducts stay outside have improved considerably over time. Today there are many options for both heating and cooling of living spaces, but the energy expenditure remains quite high.

The Insulation of Living Spaces
Let There Be Light
Air Conditioning
Passive Heating and Cooling
Ventilation and Filtration
Combustion Furnaces
Heat Pumps

The Insulation of Living Spaces - Keeping the heat in (or out).

Keeping a living space warmer (or cooler) than the outside ambient temperature requires some form of insulation. Just a single thin sheet, like tent sides, provides a basic level of insulation mostly by stopping air currents that would carry warm air away from a heat source. Heat transfer through this thin sheet is however still extremely high. The traditional insulating material was wood, which has a resistance to heat transfer value of about R1 per inch. That is one inch of wood allows one BTU of heat to transfer through per square foot of area for every degree Fahrenheit of temperature differential. Softwoods typically have significantly higher R values than hardwoods, with as high as R2 insulating values for some dry low density softwoods. Thick wooden planking for siding and thinner interior paneling with a dead air space in between was the standard form of insulation for many centuries. The thickness of the wood is the main contributor to the insulating value of this type of wall, and the dead air space does surprisingly little to keep heat in since the cold air inside the walls tends to just fall out onto the floor through cracks between the boards.

An actual insulating material can do a whole lot better, and in the past century stuffing walls with something has become a universal practice. Glass fiber insulating products have been widely used because they provide a substantial R3 to R4 per inch insulating value for the finished wall and do not mold, decay, melt or rot. There are some concerns with health hazards associated with small quantities of this glass fiber material making it's way into the living spaces, and other insulating materials have been used as well. Stuffing walls with cellulose based insulating material is a widely used alternative, but there are problems with deterioration of this cellulose material as well as significant fire hazard concerns. Foam boards have also been used for insulation, and they also have their own unique problems. The main problem with either polystyrene foam (Styrofoam) or polyurethane foam is that they emit toxic vapors if they are burned. For this reason foam board insulation is usually considered best used as an auxiliary insulating product placed on the outside of a building where danger from the vapors is less severe in the event of a fire. Despite the fire hazard risk foam board insulation is a highly desirable building material because of it's stability and exceptionally high insulating capability. Polystyrene foam board usually has an R value of about R4.5 to R5.5 per inch, and the significantly more expensive polyurethane foam board can attain R6.5 or even R8 per inch.

Floors have traditionally been the most overlooked parts of buildings in terms of providing insulation against heat loss. The argument has been that since heat rises, floors don't really need much in the way of insulation. This is partially true in that an un-insulated floor is not nearly as large of a heat loss as an un-insulated ceiling, but un-insulated floors still represent huge heat losses and make buildings feel cold even with substantial heat input. Concrete slab foundations will eventually heat up, and the rate of heat transfer to the surrounding ground will also slow down over time. In recent decades the practice of laying polyurethane foam board under poured concrete floors has gained popularity, and this really does stop the heat loss to the surrounding ground. The high thermal mass of concrete floors can still make a building feel cold, and tends to waste energy during rapidly changing outside temperatures. Wooden floors over perimeter foundations have also quite often been left un-insulated, and this tends to make for an even colder and less efficient building than a concrete slab. One of the traditional problems with insulating floors was that the glass fibers would come up from between the cracks in the floor. This was a particularly large problem for uncovered wooden board floors. Some sort of a barrier between the insulation and the living space is required, either in the form of polyolefin sheeting under the flooring or a thick rubber pad on top of the flooring with carpet over it. Insulating floors is further complicated because adding foam board insulation is normally not possible, where walls can easily have extra insulation added on the outside in the form of sheets of foam board. Sometimes sheer thickness of wood is used for insulating floors, which although expensive can sort of work. Two inch rough cut redwood subflooring with inch and an eighth plywood on top can provide a substantial R4.5 insulating value, and adding carpet or linoleum slightly increases this as well. Still though heat loss through the R4.5 wooden floor is a whole lot more than through R19 or R22 fiberglass insulation between the floor joists.

Let There Be Light - Heat Transfer Through Windows

In most well insulated buildings the windows end up accounting for nearly as much heat transfer as the walls, roof and floor. Double pane windows go a long way to stopping this heat loss, but windows remain expensive to heat. Standard vinyl frame windows with a 5/8 inch air gap between the panes of glass have an R value for the entire opening of about R2.5 to R3, which is a lot better than a single pane window. Older aluminum frame double pane windows with just 5/16 of an inch of air gap had an insulating value of more like R1.25 or R1.5, which although better than single pane windows is still a huge heat loss. Five eighths inch air gap double pane windows can be brought up to insulating values of as much as R4 with the use of coatings, pigments and low movement filler gasses. This is however only a slight difference and in the case of the pigments and coatings usually not worth the deterioration in transmitted light quality. Much better insulating values can be attained with wider pane spacing and/or the use of three panes of glass instead of just two. As the pane spacing increases beyond about a half an inch the gains in insulating value diminish, and this is often misinterpreted as meaning that anything more than a half inch of pane spacing does not provide more insulating value. The reality is that substantially higher maximum insulating values can in fact be obtained with wider pane spacing all the way up to several inches air gap. There is some ideal pane spacing for any particular filler gas, but this pane spacing for maximum insulating value is different for different temperature differentials. The larger the difference between inside temperature and outside temperature the wider the ideal pane spacing would be.

Air Conditioning - Refirgeration of Living Spaces.

Active heating of living spaces has been done for ages, but the active cooling of living spaces is a relatively recent phenomenon. An early active cooling system was the use of pools of water and living plants in a central courtyard open to the sky with thick windowless walls all around. In the past century though the use of motive power to cool living spaces has become extremely popular. Most people detest the use of large quantities of energy to cool buildings, but when faced with sweltering summer heat and no way to escape from it the prospect of air conditioning can be extremely compelling. Traditionally air conditioning systems were extremely inefficient, mostly because of their field winding type capacitor start motors without disengaging starting capacitors. For more on motor efficiency see  Electric Motors and Generators.

Permanent magnet motors that do not leave starting capacitors engaged all the time can dramatically reduce the energy requirements of air conditioning. Compressors themselves also vary in efficiency. The classic example of inefficient compressors are belt driven automotive air conditioning compressors. The reason that the automotive air conditioning compressors are so inefficient is that they are required to deliver high capacity when the engine is low idling at 600 to 900RPM even though they are normally spun with the engine operating up at 2000 to 4000RPM. Stationary air conditioning at least does not have this wide speed range problem since synchronous AC motors run at a fixed speed. The efficiency of refrigeration systems is expressed as the COP (Coefficient of Performance). In freezers the COP is normally down at about 1.5 to 2, meaning that a mechanical input of one horsepower removes 1.5 to 2 horsepower of heat from the refrigerated space. For air conditioning of living spaces the temperature differential over which the refrigeration system has to operate is much smaller, so higher coefficients of performance can be obtained. For many decades the very best of air conditioning systems were able to obtain a COP of about three, but a COP of four is considered easily obtainable in many situations. What this means is that air conditioning an insulated living space might require only one quarter as much electrical power as heating the same space in the winter time with electric resistive heating.

Air conditioning loads are however usually quite high in the summer because the sun beats down directly on the building. Massive amounts of ceiling and/or roof insulation are very important if a building in direct sunlight is to be air conditioned. If insulation is used only in the ceiling with no roof insulation then an attic fan can help a lot with reducing air conditioning loads. Particularly in poorly insulated buildings shade trees can be extremely important for reducing air conditioning loads. So much so that a few large shade trees very often makes the difference between a building being comfortable with no air conditioning system versus air conditioning being absolutely mandatory to maintain tolerable temperatures. Windows exposed to direct sunlight are also very significant for air conditioning loads. Specialty windows with pigments and coatings that dramatically reduce solar heat gain are available for use on exposed south facing (in the northern hemisphere) windows. Even better than pigmented windows though are external shades to keep direct sunlight from falling on the windows. Again shade trees can do a great job of keeping direct sunlight off of a building, but awnings and shutters can also be used. Large overhanging awnings can maintain a nice view out a south facing window while keeping direct sunlight from coming through that window. If the view out the window can be dispensed with during the heat of the day an external shutter can do an even better job of keeping the heat out. This external shutter needs either to be insulated or to be spaced out away from the window so that hot air does not build up between the shutter and the window.

Passive Heating and Cooling

Shade trees and shutters on exposed south facing windows are closely related to passive cooling since they are extremely useful even on buildings without vapor compression refrigeration based air conditioning systems. The basic idea of passive heating and cooling is to use the temperature differential between day and night to heat or cool a building in moderate climates. Since these heating and cooling sources are available only once a day good insulation is required to maintain comfortable temperatures throughout the day and night.

In hot summertime weather the windows of a building are opened in the evening when outside temperatures begin to drop. The cool of the night fills the building with cold air, and also cools off all of the building materials and the contents of the building. The windows are then closed in the morning sometime shortly after sunrise to keep the heat of the day out. This works particularly well for a well insulated house that is unoccupied throughout the middle part of the day. With no doors opening and nobody inside to heat the house with their heat of respiration the house stays quite cool until late afternoon when the outside temperature peaks. A good trick to maintaining this comfortably cool house throughout the heat of the early evening is to cook diner outside instead of inside. Once the sun has set the house is normally opened up even if the outside temperature has not dropped off lower than the inside temperature. If there is a slight breeze the flow of air through the house makes it feel cooler than it would if it were left closed up. This passive cooling can also be augmented with air conditioning if the outside temperatures stay so hot into the late afternoon and evening that the house becomes uncomfortable. For buildings that are in constant heavy use throughout the day passive cooling does not work as well.

Passive heating tends to work quite well because there are more opportunities for heating. If the peak afternoon outside temperature climbs higher than the minimum acceptable inside temperature then the windows can be opened to let this heat in. Even if it never heats up outside though sunlight falling directly on a building will heat it. The best way to take advantage of direct heating is with a large area of south facing windows exposed to the low winter sun. This at first seems in conflict with the requirements of keeping the house cool in the summer time, but there are solutions to this apparent conflict. The most widely known and widely used form of passive heating in cold climates is the solarium. The basic idea of a solarium is a long skinny room on the south side of a building with a large number of opening windows . In the winter time the windows are closed and the sun falling into this room heats it up. When the temperature in the solarium climbs above the temperature in the main part of the building the doors to the solarium are opened to let the heat in. At the end of the day the heat loss through the large area of windows means that the solarium will cool off rapidly, so the insulated doors to the main part of the house must be closed. In the summer time the solarium is prevented from overheating the building by opening the solarium windows and keeping the insulated doors closed. A solarium can be made to work in temperate climates with just single pane windows, but obviously well insulated double pane windows would work a lot better and would allow the solarium to provide useful heating during colder outside temperatures.

The other way that passive heating can be used is with a large area of south facing windows with external shutters. In the winter time the shutters are left open, and anytime that sunlight falls directly on the building substantial heating is provided. In the summertime the shutters are kept closed either all the time or at least throughout the part of the day when direct sunlight would fall on the windows. Deciduous trees on the south side of a building are a form of automatic opening shutters that work quite well for this type of passive heating. In the winter the leaves fall, allowing sunlight to filter through the bare branches and heat the house. In the summertime the thick foliage of the leaves all but totally blocks out the direct sunlight, keeping the house cool.

With the use of pumping and thermal storage, solar heating can be made to work even better down to much colder outside temperatures. These types of systems are usually known as active solar heating systems, and they most usually rely on a large insulated tank of water for thermal storage. Air pumped through an insulated bin of rocks can also be used for thermal storage, but the insulated water tank tends to be much more convenient. The solar heat is harvested with evacuated tube collectors, usually mounted on a roof. These glass tube within tube collectors are best used with parabolic trough reflectors, but they are often used without the reflectors for a simpler installation. A temperature switch on the collectors controls a pump which circulates water from the insulated tank through insulated pipe up to the collectors. Once the water in the collectors has come up to a high temperature (usually about 190 degrees Fahrenheit) the pump comes on and continues to run until the temperature at the collectors is no longer substantialy higher than the temperature in the storage tank. Providing freeze protection usually means running a solution of 20% ethylene glycol in the system, and to reduce the amount of ethylene glycol required the insulated tank is normally then heated with a heat exchanger inside the tank. Drain back systems have also been widely used to eliminate the heat exchanger as well as to allow the system to easily be shut down when heating is not required. In a drain back system water is pumped up into the collectors, but when the pump stops running the water all flows back to the storage tank. The main difficulty with the drain back systems is that they require a somewhat more sophisticated control system to guess when the pump should be started. Shutting down an ethylene glycol collector system usually means covering the collectors in some way, which makes them either more complex or less user friendly.

In the end the main obstacle to hot water heating systems is the sheer volume of water required. Finding space inside for a five hundred gallon insulated tank is usually considered prohibitively inconvenient. The insulated tank can also be located outside, but then it requires even better insulation to work well. Locating the insulated tank inside the heated space means that a small amount of heat loss from the tank does little or no harm to the overall efficiency and effectiveness of the system. A five hundred gallon tank operated over a 110 degree Fahrenheit temperature differential provides enough storage to heat a 1500 square foot house over one moderately cold night. A smaller or better insulated house in a temperate climate could work well with an even smaller storage tank. Sometimes much larger tanks are used to attempt to store heat over several cloudy days, but this then tends to also require more insulation around the tank and a much larger array of collectors. Normally hot water heating systems work best when they are designed and installed to augment some other form of heating. Relying entirely on solar heat requires a very large system that is used at a small fraction of it's capacity most of the time.

Ventilation and Filtration - Keeping the living space livable.

Any living space needs a certain amount of air turnover just to assure that the proportions of oxygen and carbon dioxide remain close to what is in the outside air. This is usually a rather small ventilation requirement unless the living space is packed with a wild party. Since even the best plastics, paints and glues outgas slightly when they are new it is usually considered mandatory to have considerably higher air turnover rates. Traditional construction techniques left considerable cracks and chinks where outside air could make it's way into the living space. The tighter the construction the more likely it is that some additional ventilation will have to be provided. For heating systems that draw their combustion air from inside there is a certain amount of forced ventilation. As long as the building is not so tight that a significant vacuum develops inside good ventilation is assured. Since many heating systems draw their combustion air from outside very tight buildings often require a calculated opening area to allow sufficient air turnover.

Once sufficient air turnover is provided for it is often considered highly desirable to provide some means of removing particulate material from the inside air as well. This has long been done with large open mesh furnace filters that collected dust over a period of months or years. This basic level of indoor air filtration is beneficial, but finer filtration of PM10 material (particles less than 0.01" across) is also widely considered highly desirable. A more effective pleated filter material can be installed in the same large furnace filter locations, or a separate filtration system can be used. Because the finest particles tend to become evenly distributed throughout a building simply running a particulate filter type air purifier in a high traffic area is highly effective at keeping the air breathable.

Combustion Furnaces - Burning fuel for heat.

Traditionally natural gas, LPG and oil furnaces attained efficiencies of about 76 to 82%, although some older models from before the 1980's operated at lower efficiencies. Traditional furnaces not intended for use in brick chimneys can attain efficiencies of up to about 86%, but getting the combustion byproducts reliably out a big brick stack requires efficiencies of not more than about 80%. These efficiency numbers are for the furnace itself, and the efficiency of the entire heating system depends on where and how the duct work is run. Un-insulated duct work running through an un-insulated space wastes huge amounts of the heat from the furnace. Even thin layers of insulation on duct work running through un-insulated basement or attic spaces only goes so far towards getting the efficiency of the entire heating system up close to the efficiency of the furnace itself.

Furnaces generally require a blower to force the air to be heated across the heat exchanger and out through the duct work. Some older hot air heating systems did however rely entirely on upward sloping duct work to deliver the hot air from the basement located furnace to rooms above. Hot water circulation and steam circulation have also been widely used to distribute heat from a furnace, but these systems tended to work better in large buildings where a more constant heat requirement existed. Hot water heating systems generally have used a circulating pump, but can be made to work on a gravity feed as well. Steam distribution systems use a pressure pump to force the condensate back into the boiler, and have not generally been considered appropriate for residential use.

If a hot air furnace is used without a blower then the heat exchanger needs to be large for the rated capacity and generally needs to be quite tall as well. The best example of this is the wall furnace, which usually uses no blower. The smaller 25,000BTU per hour rated wall furnaces with tall full size heat exchangers can attain efficiencies on par with other gas furnaces, about 78%. Often these high 78% efficiency ratings of wall furnaces can only be attained with the front grill removed. With the grill in place the efficiency of wall furnaces is often quite a bit lower. Larger capacity 35,000 and 50,000BTU/hr wall furnaces generally do not attain as high of efficiencies because the heat exchangers are not large enough.

In recent decades condensing furnaces have come on the market that offer substantially increased efficiencies of 93 to 97%. The condensing of the water out of the exhaust is said to extract the last seven percent of the energy from the combustion process (actually 7.3% for propane and 9.1% for natural gas). The condensation process is done in a separate heat exchanger that is used to pre-heat the return air before it goes into the main heat exchanger (essentially the flow of air through the heat exchanger is reversed, so that the air gets hotter as it gets closer to the burner). Condensing furnaces always use a blower on the combustion air, and are plumbed with plastic pipe instead of a metal chimney. The lack of a chimney is a significant efficiency advantage for two separate reasons. One is that a condensing furnace can be plumbed to draw it's combustion air from inside the building, providing all the advantages of forced air turnover and dehumidification, without warm inside air escaping up the chimney when the furnace is not running. Even when a traditional furnace with a chimney is plumbed to draw it's combustion air from the outside the large un-insulated chimney acts as a heat exchanger when the furnace is not running, drawing cold outside air in through the burner, heating it and then exhausting that heated air out the chiming cap. When any type of furnace is running there is no efficiency difference between drawing the combustion air from inside or outside. In either case the amount of air consumed does ultimately come from outside, whether it is drawn directly into the furnace or drawn first into the building through cracks and chinks. Condensing furnaces are available for gas or oil, and the efficiency ratings are similar for all models.

The main problem with the traditional boiler type hot water or steam heating systems is that considerable time is required to bring the boiler up to temperature. When the burner is shut off all of the heat in the hot water quickly escapes up the chimney, and restarting the system requires re-heating the water. There are however ways to get hot water heat to work well for variable heat requirement systems.

The simplest, and usually most effective, hot water heating system uses a tankless heater that heats up quickly as soon as the burner is lit. These heaters have a flow sensor that lights the burner as soon as the circulating pump comes on. For a single loop heating system just one circulating pump is used, and it is controlled directly by a thermostat. For multiple zone heating multiple pumps are required, and usually some type of insulated storage tank is used. A heater with a variable output burner can supply multiple circulating pumps, but heaters are usually considerably less efficient at reduced capacity. Fixed output heaters typically have efficiencies similar to hot air furnaces, making an insulated tank the best option for multiple zone heating. When used with an insulated storage tank the tankless heater is on it's own circulating pump which is controlled by a temperature switch on the tank. The individual circulating pumps come on with their own thermostats in the area of the building heated by that circuit. Modern hot water heating is usually of the in-floor type, but heat registers can also be used. Heat registers are better for a fast responding heating system, and the slow response time of heated concrete floors can waste considerable energy in some circumstances.

In recent decades condensing type boilers have also become available. The condensing boilers function much like condensing furnaces, and also attain 93 to 97% efficiencies. The really big advantage of these condensing boilers over traditional boilers is that they incorporate an insulated storage tank around the burner. The insulated tank provides smooth delivery for multiple circulating pumps, but the lack of a chimney means that the water stays hot long after the burner shuts down. For intermittent heating loads there is however still considerable advantage to the large well insulated storage tanks used with tankless heaters.

The tankless heaters are very similar in appearance to tankless hot water heaters for domestic hot water, but the tankless heaters for space heating have higher capacity heat exchangers since the input water temperature returning from the storage tank may be as high as 160 degrees Fahrenheit.

Heat Pumps - Refrigerative Heating.

Because vapor compression refrigeration systems can move several times more energy over small temperature gradients than they consume they have also been used for heating. A vapor compression refrigeration system used to heat a living space is known as a heat pump, and most heat pump systems are reversible (with valves) so that they can be used as air conditioning systems during summer heat. In temperate climates where the outside temperature is only ten or fifteen degrees colder than the desired inside temperature heat pumps can attain high coefficients of performance and do a good job of heating a building. When the outside temperature drops though the capacity of the vapor compression refrigeration system falls off and coefficients of performance plummet. Most heat pump systems have come with built in resistive heating elements so that they will continue to work in very cold conditions. The key to getting heat pumps to work in colder outside temperatures is efficiency and flexibility. Obviously a more efficient electric motor will allow a heat pump to work down to lower temperatures, but the loss of capacity over wider temperature differentials is also a significant problem. Getting a vapor compression refrigeration system to work well over a wide range of temperature differentials would require a variable speed compressor so that the capacity could be boosted in cold conditions. Underground heat exchangers have also been used to boost heat pump performance, the idea being that slightly warmer temperatures can be found down below the frost line even during extreme cold snaps. This is in fact a form of geothermal heat, but with a heat pump just a shallow six foot deep heat exchanger is able to extract useful energy at temperatures that would be far from sufficient to directly heat a living space.

Because heat pumps can easily attain high coefficients of performance of up around three to four in temperate climates they can actually be used to attain greater than 100% efficiency when burning fuel for heat. Since a heat engine can easily operate at 45% efficiency even just a COP of 3 yields 135% efficiency even without any use of the waste heat from the engine. A heat engine running at 45% efficiency does not produce nearly as much waste heat as an inefficient gasoline engine running at 25% efficiency, but a significant portion of that other 55% of the energy released by combustion is available for use in heating a building. In poorly running gasoline engines approximately half of the waste heat goes into the cooling jacket of the engine and half of the waste heat goes out the exhaust. In a diesel engine attaining 45% efficiency a much larger portion of the waste heat goes out the exhaust, and less waste heat is available from the coolant. In any case it is easy to see how with a coefficient of performance of four a heat pump and a diesel engine could easily provide significantly more than twice as much heating on the same amount of fuel as compared to burning that fuel for direct heating.

This is understandably a somewhat confusing concept. How is it that twice as much heating can be done than the heat content of the fuel? The answer is that the fuel has a much higher temperature of combustion potential than is being made use of in direct combustion heating. For more on the temperature of combustion potential of fuel see  Combustion Properties of Fuel. When the fuel is burned in a heat engine the piston speed can be set to make the best possible use of the high temperature of combustion potential of the fuel. When the fuel is simply burned for direct heat the fact that it can release it's 15,000 or 20,000 BTU per pound of heat energy at thousands of degrees Fahrenheit does absolutely no good for heating a building up to 60 or 65 degrees Fahrenheit. For some idea of how much more heating could be provided with the use of a heat engine and a vapor compression refrigeration system the relative absolute temperatures can be considered. Room temperature of 65 degrees Fahrenheit is 525 degrees Rankin. If the temperature of combustion potential of some fuel is 2000 degrees Fahrenheit ( 2460 degrees Rankin) then it appears that the fuel is capable of doing almost five times as much work as it does being burned directly to heat a building to 65 degrees Fahrenheit. Since a heat engine has only the difference between ambient temperature and the temperature potential of the fuel to operate over the amount of work that can be done by 2000 degree Fahrenheit fuel is in practice no more than three times as much as for burning the fuel for direct heating.

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