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Archival copy: for current recommendations see http://edis.ifas.ufl.edu or your local extension office.

Fact Sheet EES-75 December 1991

Efficient Control with Heat Pipes1 Roy Johannesen and Michael West2

Building moisture is usually controlled by air moist air is blown through a coil. In the coil, conditioning (AC), but some installed systems cannot the air is cooled below its dew point temperature. control the extreme moisture load encountered in Florida. One efficient approach to removing this The dew point temperature is defined as the excess moisture is the heat pipe. A heat pipe can temperature of the air when the relative humidity is greatly increase the moisture removal ability of an AC 100 percent. Relative humidity is defined as the system and save energy at the same time. Another amount of moisture in the air relative to the most advantage is that heat pipes have no moving parts and moisture the air can hold at the same temperature. are essentially maintenance free. As air is cooled it loses its ability to hold moisture. So, relative humidity is increased by cooling the air, An AC system that doesn’t control humidity can as well as by adding moisture to it. For example, as induce a variety of health and comfort problems. the air cools on a muggy night the relative humidity This fact sheet explains how adding a heat pipe increases. When the relative humidity reaches 100%, system to some air conditioners will control the the air has been cooled to its dew point and dew humidity in a home or business. Information is also forms on surfaces. presented on possible energy savings, and on installation and operating costs. Similarly for the air conditioner, once the air is cooled below the dew point, the air releases moisture The key to understanding what a heat pipe system which collects in a drain pan, and drains out of the will do is to know how an AC system removes system. The cooled and dried air is delivered to the moisture. The next section will explain this and why building. The air now has a lower dew point called some systems are inadequate. the exit dew point.

HUMIDITY CONTROL BY Many air conditioning systems do not remove adequate amounts of moisture for Florida’s climate. A building’s air conditioning system is responsible Most AC systems are designed to handle peak load for removing moisture from the air in order to conditions -- the hottest afternoon of the summer. provide for both human comfort and mold-and- Accordingly, they work best during the hottest times mildew control. Inside the air conditioner, warm of the year but not so well at other times.

1. This document is Fact Sheet EES-75, a series of the Florida Energy Extension Service, Florida Cooperative Extension Service, Institute of Food and Agricultural Sciences, University of Florida. Publication date: December 1991. 2. Roy Johannesen, Former Energy Extension Specialist; Michael West, Assistant Energy Extension Specialist, Dept., Cooperative Extension Service, Institute of Food and Agricultural Sciences, University of Florida, Gainesville FL 32611.

The Florida Energy Extension Service receives funding from the Florida Energy Office, Department of Community Affairs and is operated by the University of Florida’s Institute of Food and Agricultural Sciences through the Cooperative Extension Service. The information contained herein is the product of the Florida Energy Extension Service and does not necessarily reflect the views of the Florida Energy Office.

The Institute of Food and Agricultural Sciences is an equal opportunity/affirmative action employer authorized to provide research, educational information and other services only to individuals and institutions that function without regard to race, color, sex, age, handicap, or national origin. For information on obtaining other extension publications, contact your county Cooperative Extension Service office. Florida Cooperative Extension Service / Institute of Food and Agricultural Sciences / University of Florida / Christine Taylor Stephens, Dean Archival copy: for current recommendations see http://edis.ifas.ufl.edu or your local extension office.

Efficient Humidity Control with Heat Pipes Page 2

AC systems are designed to remove a certain reducing speed, and adding reheat with either hot amount moisture at peak conditions. This is called gas bypass or electrical strip heating. Reheat methods the ratio of the system. is result in substantial increases in electrical heat in terms of degrees one reads on a common consumption, especially electric strip reheat which will thermometer. Latent heat, the other kind of heat, is triple the required energy input. heat in terms of moisture removed. Sensible plus latent is the total heat removed. The latent heat ratio If new AC equipment is being selected, remember of an AC system is the portion of latent heat it can that cooling coils with more tube rows or greater remove out of the total heat it can remove. It is number of fins have a higher latent heat ratio. typically around 30 percent at peak conditions (95°F Conventional AC equipment that can handle high outdoors), and a few percent higher at night latent loads may have a low SEER. Make conditions (75°F outdoors). comparisons between different manufacturers.

The building load also has a latent heat ratio: it An undersized AC unit runs longer since it does is the portion of latent heat that needs to be removed not easily satisfy the . Longer run cycles from the building out of the total heat that needs to allow the system to remove more moisture from the be removed. At peak conditions there is much more air, but indoor temperature may rise 4 to 7 degrees sensible heat than latent heat. At night and on cooler during the late afternoon. days the amount of sensible heat shrinks but the amount of latent heat does not. And on wet days, the Reducing the indoor fan speed causes the coil amount of latent heat grows. The latent heat ratio temperature to drop, and also allows the air to may rise to 50 or more. During humid and/or cool remain in the coil longer. This lowers the dew point weather the AC system indeed cools the building, but of the exit air, but system efficiency is reduced. it can’t dehumidify adequately. Reheat methods heat the air after it passes To make things even worse, SOME new air through the cooling coil. This allows the removal of conditioning units have sacrificed latent (moisture moisture without over-cooling the air. Hot gas bypass removal) capacity in order to increase their nameplate uses hot to reheat the air, and electric SEER ratings. (SEER stands for Seasonal Energy strip reheat uses resistance coils. When reheat is Efficiency Ratio and is a measure of energy used, the cooling ability of the AC system goes to efficiency.) One way manufacturers increase SEER waste. Electric reheat uses over twice the electricity is to raise the cooling coil temperature. to reheat the air as it took to cool it! Unfortunately, this means that the air blown through the coil does not reach a low dew point temperature. use a form of reheat. The heat Some of these high efficiency units have a latent heat removed from the air to cool it to its dew point is put ratio of 15 percent or less at design conditions. back before the air is blown out of the unit. Again, potential cooling ability is totally wasted. INCREASING MOISTURE REMOVAL HEAT PIPE HUMIDITY CONTROL Ideally, an AC system is properly sized and designed before it is installed. A complete and Heat pipe technology offers enhanced moisture accurate building heat load analysis, with special removal for very little additional energy input. Since attention paid to internal moisture sources and heat pipes have no moving parts and are sealed units, , allows the selection of the proper they offer reliability equal to the air conditioning equipment. Unfortunately, equipment sizing is often system to which they are fitted. done using time-saving rules-of-thumb. For example, residential moisture load is rarely calculated: it is assumed to be 30 percent of the sensible heat load! Heat pipes are a simple yet elegant way to move Furthermore, building usage and occupancy often heat from one point to another (Figure 1). If one change, and AC system requirements change with end of a small heat pipe is dipped into a cup of hot them. coffee, the other end becomes very hot very rapidly. The sealed pipe is filled with certain amount of a Traditional methods of increasing the moisture refrigerant. The refrigerant is in a saturated state removal capacity of AC units include undersizing, (that is, there is and at the same Archival copy: for current recommendations see http://edis.ifas.ufl.edu or your local extension office.

Efficient Humidity Control with Heat Pipes Page 3

Condensed liquid refrigerant returns to the end of the heat pipe by gravity. The overall effect is that heat is transferred from the flame to the other end of the pipe very quickly.

To help understand how a heat pipe is applied to air conditioning, the process has been divided into three principle steps (Figure 2). In step 1, the air is pre-cooled by the heat pipe system. In step 2, moisture and heat is removed from air by the air- conditioning cooling coil. (The cooling coil has refrigerant or chilled flowing through it.) In step 3, heat is added to the air by the heat pipe system.

There are two fluid streams that are being Figure 1. A simple heat pipe moves heat from candle to air very affected by the process, namely air (which passes over quickly. the cooling coil and heat pipe) and refrigerant (which flows within the heat pipe). Only the effects on the air stream were described in the three-step process mentioned above. A more in-depth heat exchange explanation follows in the next paragraph. An air temperature graph that corresponds to the system drawing shown directly above it is also depicted in Figure 2.

As shown in the system drawing, the heat pipe is fitted around an air conditioning cooling coil. One end of the heat pipe is placed in front of the coil and the other end is placed after coil. Oncoming air transfers heat into the heat pipe (consequently dropping air temperature), and causes the refrigerant in the heat pipe to boil. The pre-cooled air next travels across the cooling coil where heat and moisture are removed. The refrigerant vapor in the heat pipe travels to the condensing end of the heat pipe. Finally, air exiting the coil absorbs heat from the heat pipe causing the refrigerant in the heat pipe to condense, completing the heat pipe cycle.

The temperature graph shows how the heat-pipe fitted cooling coil compares with a conventional Figure 2. Top diagram shows heat pipe installed around a conventional cooling coil. Compared to the conventional coil, the AC coil. Bottom graph compares drop in air temperature. air entering the heat-pipe fitted coil is now at a lower temperature and therefore closer to its dew point. In both the heat-pipe fitted and conventional coil, the temperature drop across the coil is nearly the same. temperature and pressure; for example, water boiling That means that the heat-pipe fitted coil will chill the in a pot and the steam above it are both at 212°F and air to a lower temperature than the conventional coil. at atmospheric pressure). When heat is applied to Since the air is cooled further below its initial dew one end of the heat pipe some of the liquid point, the heat-pipe fitted coil removes more moisture refrigerant boils into a vapor. This vapor quickly rises from the air. to the higher end of the pipe and condenses, thereby releasing heat into the air at that end of the pipe. Archival copy: for current recommendations see http://edis.ifas.ufl.edu or your local extension office.

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When the air leaves the heat-pipe fitted cooling (3) The hotter it is outside, the less moisture is removed coil, it is too cold. However, as it passes over the -- abouta2to3percent lower latent heat ratio other (condensing) end of the heat pipe the for each 10 degrees F higher outdoor refrigerant vapor in the heat pipe transfers heat into temperature. the cold air and warms the air to a tolerable temperature. The condensed refrigerant is returned So, the true latent capacity of an installed system to the boiling end of the heat pipe. The air is now is at a balance point between the capacity increase conditioned by the system to be at the right due to the heat pipe, the capacity decrease due to the temperature and humidity to meet a building’s resulting dryer air, and capacity changes due to moisture load. thermostat setting, outdoor conditions, and moisture generation in the building. The bottom line is that The overall effect of this process, theoretically, is humidity will decrease. How much it decreases to almost double the moisture removal capacity of the depends on many interrelated factors. cooling coil (at the same indoor temperature and humidity) while reducing total cooling capacity by ENERGY SAVINGS only a few percent. Because total air conditioning system capacity is not significantly affected, heat pipes There is an additional benefit to heat-pipe mois- are an attractive retrofit solution for a humidity ture-removal systems. It is possible to save energy problem. and money with such a system. Here’s why:

HEAT PIPE PERFORMANCE For humans to remain comfortable, both temperature and humidity must be at tolerable levels. It’s valuable to know how the heat pipe system People cool themselves by evaporating moisture from works and, to help predict the performance of a heat their skin. If the air has too much moisture in it, pipe system, it is also useful to know why it works. evaporation is limited and not enough cooling occurs. The heat pipe system takes advantage of an inherent If the air is dry, like the Arizona desert at 110°F and characteristic of all cooling coils: 3 percent rh, a person can still be comfortable in extreme heat. (1) The cooler the air entering the coil, the dryer it exits. Traditionally, an indoor temperature of 75°F and For example, if the thermostat is lowered from 80 (at 50 percent relative humidity is considered an ideal 50 percent rh) to 75°F, a typical AC’s latent heat ratio state (which is difficult to achieve in Florida). increases from 25 percent to 40 percent. The heat Actually, there is a range of humidity and temperature pipe precools the air, so the coil "thinks" the which is quite comfortable. Within this comfort thermostat is lowered, and it "gives" dryer air. range, the lower the humidity, the higher the tolerable temperature. If one can lower building humidity, Unfortunately, AC coil behavior is more then one can raise the thermostat setting and remain complicated than (1) suggests. There is another coil comfortable. characteristic which limits heat pipe performance: For example, let’s say that the indoor relative (2) The dryer the air entering the coil, the less moisture humidity is 65 percent. According to extensive is removed. studies done on people in rooms at different temperatures and , the average person Due to (1) alone, the increased latent capacity would could be comfortable at a temperature of about 75°F. reduce the humidity to about 50 percent (at 75°F). If we lower the humidity to 40 percent (possible in However, due to (2) the latent capacity of a typical Florida conditions by retro-fitting with heat pipes), AC is actually less at 75°F-50 percent rh than at 80°F- then we can increase indoor temperature to 80°F and 50 percent rh (it decreases from 25 percent to 18 still stay within the comfort zone. This represents a percent ). So the humidity will actually be 5 degree F increase in AC thermostat setting. somewhere between 50 percent and 60 percent . A conservation rule of thumb says that for every To make matters more confusing a third degree increase in set point temperature, a two characteristic comes into play: percent reduction in air conditioning energy consump- tion occurs. After subtracting 4 percent (because of Archival copy: for current recommendations see http://edis.ifas.ufl.edu or your local extension office.

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Table 1. An energy efficiency comparison of three reheat methods. Reheat Options Where Moisture Load is 40% of the Total Cooling Load Energy Consumption Relative Reheat Type to a Conventional System* Heat Pipes 1.06 Hot gas bypass 1.60 Electrical reheat 3.04 The following studies were used in determining relative energy consumption: 1991 air conditioning load and energy consumption calculations performed at the University of Florida Mechanical Engineering Department by Michael West. A case study of a well insulated, block construction, 1300 sq. ft., 3 bedroom, 2 bath home in the Gainesville, FL area was conducted. The calculated moisture load is 4700 Btuh (26% of total at 95 F outdoor temperature). An 18,000 Btuh nominal high efficiency (EER=9.75) without heat pipes had a latent capacity of 2250 Btuh (12% of total capacity). With heat pipes, latent capacity increased to 4300 Btuh (23% of total capacity). Indoor relative humidity decreased from 65% to 50%. Annual electricity cost ($0.078/kWh) for cooling increased from $304 to $347. An increase in thermostat setting from 75 F to 80 F decreased annual cost to $322. Calculations conducted in accordance with standard ASHRAE procedures.

1987 test data of a 1.5 ton heat pipe installation. Test conducted by Applied Research Laboratories of Miami, FL. Results show the heat pipes increased latent capacity by a factor of 1.9, decreased EER by 4.2%, and decreased total capacity by 4.3%.

Cromer, J.C. "Desiccant Moisture Exchange for Dehumidification Enhancement of Air Conditioners." Fifth annual Symposium on Improving Building Energy Efficiency in Hot and Humid Climates. 1988.

*For our purposes, a conventional system is considered to be an air conditioning system which has the same total (sensible + latent) heat removal capacity but no reheat capability. Note that, under the 40% latent cooling load condition stated for this comparison, this conventional system would not be able to remove the required amount of moisture.

loss in total system efficiency due to heat pipe fact, most residential heat pipe installations have installation), this results in a 6 percent savings in resulted in small energy savings. It seems that most energy cost for this simple case. home owners who install heat pipes do experience dryer and more comfortable conditions but are In an actual application with a conventional reluctant to change the thermostat to a higher setting. thermostat, savings will be less. This is because increased latent capacity comes at the expense In commercial applications there is more potential decreased sensible capacity. The system will have to for saving money, especially in any building which run longer before it satisfies the thermostat, and it must employ a reheat system to achieve adequate will run correspondingly more hours over the course moisture removal capacity. Reheat works like this: of the cooling season. One way to maximize savings The air conditioning system is sized to remove the is to install a so that the AC cycles on and required amount of moisture. This results in excess off based on comfort, not simply temperature. capacity to remove sensible heat. If nothing were done, the building would be overcooled and Air conditioning loads are in actually affected by uncomfortable. To compensate for overcooling, heat many interrelated factors so this example should not is added to the conditioned air. be considered definitive, but merely illustrative. In Archival copy: for current recommendations see http://edis.ifas.ufl.edu or your local extension office.

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Table 2. Heat pipe installation costs based on size and type COST required.

Cost of Various Types of Heat Pipe Installations The total cost for buying and installing a heat pipe system was estimated for a range of sizes (Table Installed 2). The flat heat pipe (the first option in Table 2) is Capacity Cost rectangular in shape and is the cheapest of the heat (tons) Range pipe options. It is installed in ducting in a place Type (dollars) where the supply and return ducts run up and down Flat: 2 400 - 760 (perpendicular to the ground) and are side-by-side. (Installed in supply If the ducts do not have a section like this, consider and return ducting) 3 490 - 920 a split heat pipe installation. 5 590 - 1120 Since heat pipe systems have a high initial cost, it Split: 2 680 - 970 pays to try to reduce the building moisture (and sensible (Heat input remote 3 750 - 1070 cooling) load first, before purchasing additional from heat output) moisture removal capacity. To reduce moisture loads, 5 940 - 1330 take these simple and inexpensive steps: Fitted Coil: 3 920 - 1300 1) Call your local utility and arrange to have (Integrated coil- heat pipe unit) 4 1140 - 1620 them conduct a free energy audit of your home or a no cost/low cost energy audit of your business. Tell 5 1390 - 1970 the auditor that you are especially concerned about : 3 1410 - 2000 identifying AC leaks and other areas where (Fitted coil with moist outside air might be infiltrating into your blower and housing) 4 1530 - 2170 building. 5 1730 - 2460 2) Follow up on the low-cost weather stripping Note: Manufacture’s retail price data suggest and sealing actions recommended in the audit report.

• an 18% average increase in price per additional 3) Increase ventilation to problem areas such as ton of capacity bathrooms and closets. Conditioned air must be allowed to circulate in these areas. Installing • a 40% average increase in price per type up- in doors or increasing the clearance at the bottoms of grade doors are two low cost ways of increasing air flow. Exhaust fans, box fans, and paddle fans also help aid circulation. This heat can be supplied in a number of ways. One is the heat pipe system. Another is called hot 4) Reduce internal moisture generation by covering gas bypass. In a hot gas bypass system, some of the cooking pots and using cooler water for washing. Use high-temperature refrigerant which leaves the an exhaust fan to remove moist air from the is routed through a bypass line to a heat bathroom and kitchen before it dissipates exchanger located at the air exit of the cooling coil. There, this hot gas adds heat to the over-cooled air. The following Extension publication offers addi- The most common method of adding reheat to tional information on low-cost moisture and mildew conditioned air is also the least energy efficient. This control: method is called electrical strip reheat and uses electrical resistance heating elements located in the Say Goodbye to Mildew and Save Energy (EES 65) air exit side of the cooling coil. An energy efficiency comparison of these three reheat methods shows the To obtain this free publication and other energy and benefits of the heat pipe (Table 1). humidity related publications, contact your local county Extension office. Archival copy: for current recommendations see http://edis.ifas.ufl.edu or your local extension office.

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GLOSSARY

Dew Point - The temperature of air when it is saturated with moisture.

Relative Humidity - The ratio of the amount of moisture in the air to the amount in air saturated at the same temperature.

Sensible Heat - Energy that changes the air’s temperature.

Latent Heat - Energy that is released when moisture in the air condenses.

EER - Energy Efficiency Ratio, the cooling capacity of an AC system in Btuh, divided by its power consumption in kW at a standard set of operating conditions.

SEER - Seasonal Energy Efficiency Ratio, the EER averaged over a typical cooling season. This takes changes in efficiency at night and on cooler days into account, giving a more realistic prediction of actual operating cost.

Btuh - British Thermal Units per hour. The Btu is a unit of heat, it takes 1 Btu to raise the temperature of 1 pound of water by 1 degree Fahrenheit.

Ton - A unit of AC cooling capacity, equal to 12,000 Btuh. This unit originated as the amount of refrigerating capacity required to freeze one ton of ice in 24 hours.