The installation, maintenance, and operating efficiency of the heatpump system are like those of no other comfort system. A heat-pump system requires the same air quantity for heating and cooling. Because of this, the air-moving capability of an existing furnace is extremely important. It should be carefully checked before a heat pump is added. Heating and load calculations must be accurate. System design and installation must be precise and according to the manufacturer’s suggestions.
The air-distribution system and diffuser location are equally important. Supply ducts must be properly sized and insulated. Adequate return air is also required. Heating supply air is cooler than with other systems. This is quite noticeable to homeowners accustomed to gas or oil heat. This makes diffuser location and system balancing critical.
On mild temperature heating days, the heat pump handles all heating needs. When the outdoor temperature reaches the balance point of the home, that is, when the heat loss is equal to the heat-pump heating capacity, the two-stage indoor thermostat activates the furnace (a secondary heat source, in most cases electric heating elements). As soon as the furnace is turned on, a heat relay deenergizes the heat pump. When the second-stage (furnace) need is satisfied and the plenum temperature has cooled to below 90 to 100°F (32 to 38°C), the heat-pump relay turns the heat pump back on and controls the conditioned space, until the second-stage operation is required again. Figure 2-12 shows the heat-pump unit. The optional electric heat unit shown in Fig. 2-13 is added in geographic locations where needed. This particular unit can provide 23,000 to 56,000 Btu/h and up to 112,700 Btu/h with the addition of electric heat.
If the outdoor temperature drops below the setting of the lowtemperature compressor monitor, the control shuts off the heat pump completely and the furnace handles all the heating needs.
During the defrost cycle, the heat pump switches from heating to cooling. To prevent cool air from being circulated in the house when heating is needed, the control automatically turns on the furnace to compensate for the heat-pump defrost cycle (see Fig. 2-14). When supply air temperature climbs above 110 to 120°F (43 to 49°C), the defrost limit control turns off one furnace and keeps indoor air from getting too warm.
If, after a defrost cycle, the air downstream of the coil gets above 115°F (65°C), the closing point of the heat-pump relay, the compressor will stop until the heat exchanger has cooled down to 90 to 100°F (32 to 38°C) as it does during normal cycling operation between furnace and heat pump.
During summer cooling, the heat pump works as a normal split system, using the furnace blower as the primary air mover (see Fig. 2-15).
In a straight heat pump/supplementary electric heater application, at least one outdoor thermostat is required to cycle the heaters as the outdoor temperature drops. In the system shown here, the indoor thermostat controls the supplemental heat source (furnace). The outdoor thermostat is not required.
Since the furnace is serving as the secondary heat source, the system does not require the home rewiring usually associated with supplemental electric strip heating.
The heat pump is a heat multiplier. It takes warm air and makes it hot air. This is done by compressing the air and increasing its temperature. Heat pumps received more attention during the fuel embargo of 1974. Energy conservation became a more important concern for everyone at that time. If a device can be made to take heat from the air and heat a home or commercial building, it is very useful.
The heat pump can take the heat generated by a refrigeration unit and use it to heat a house or room. Most of them take the heat from outside the home and move it indoors (see Fig. 2-11). This unit can be used to air condition the house in the summer and heat it in the winter by taking the heat from the outside air and moving it inside.
Newer hot-air furnaces feature printed circuit control. The board shown in Fig. 2-8 is such that it is easy for the technician installing the furnace to hook it up properly for the first time. The markings are designed for making it easy to connect the furnace for accessories, if needed. Figures 2-9 and 2-10 show the factory-furnished schematic. See if you can trace the schematic and locate the various points on the printed circuit boards.
The room thermostat should be located where it will be in the natural circulating path of room air. Avoid locations where the thermostat is exposed to cold-air infiltration, drafts from windows, doors, or other openings leading to the outside, or air currents from warm- or cold-air registers, or to exposure where the natural circulation of the air is cut off, such as behind doors and above or below mantels or shelves. Also keep the thermostat out of direct sunlight.
The thermostat should not be exposed to heat from nearby fireplaces, radios, televisions, lamps, or rays from the sun. Nor should the thermostat be mounted on a wall containing pipes, warm-air ducts, or a flue or vent that could affect its operation and prevent it from properly controlling the room temperature. Any hole in the plaster or panel through which the wires pass from the thermostat should be adequately sealed with suitable material to prevent drafts from affecting the thermostat.
Make the field low-voltage connections at the low-voltage terminal strip shown in Fig. 2-7. Set the thermostat heat anticipator at 0.60 A (or whatever is called for by the manufacturer). If additional controls are connected in the thermostat circuit, their amperage draw must be added to this setting. Failure to make the setting will result in improper operation of the thermostat.
With the addition of an automatic vent damper, the anticipator setting would then be 0.12 A. As you can see from this and the schematic, the anticipator resistor is in series with whatever is in the circuit and is to be controlled by the thermostat. The more devices controlled by the thermostat, the more current will be drawn from the transformer to energize them. As the current demand increases, the current through the anticipator is also increased. As you remember from previous chapters, a series circuit has the same current through each component in the circuit.
The installation of a new furnace requires you to follow a factory diagram furnished in a booklet that accompanies the unit. The wiring to be done in the field is represented by the dotted lines in Fig. 2-7. All electrical connections should be made in accordance with the National Electrical Code and any local codes or ordinances that might apply.
Figure 2-4 shows how the manufacturer represents the location of the various furnace devices. The solid lines indicate the line voltage to be installed. The dotted lines are the low voltage to be installed when the furnace is put into service.
The motor is four speed. It has different colored leads to represent the speeds. You may have to change the speed of the motor to move the air to a given location. Most motors come from the factory with a mediumhigh speed selected. The speed is usually easily changed by removing a lead from one point and placing it on another, where the proper color is located. In the schematic of Fig. 2-5, the fan motor has white lead connected to one side of the 120-V line (neutral) and the red and black are switched by the indoor blower relay to black for the cooling speed and red for the heating speed. It takes a faster fan motor to push the cold air than for hot air because cold air is heavier than hot air.
In Fig. 2-5, the contacts on the thermostat are labeled R, W, Y, and G. R and W are used to place the thermostat in the circuit. It can be switched from W to Y manually by moving the heat-cool switch on the thermostat to the cool position.
Notice in Fig. 2-5 that the indoor blower relay coil is in the circuit all the time when the auto-on switch on the thermostat is located at the on position. The schematic also shows the cool position has been selected manually, and the thermostat contacts will complete the circuit when it moves from W1 to Y1.
In Fig. 2-4, note that the low-voltage terminal strip has a T on it. This is the common side of the low voltage from the transformer. In Fig. 2-5, the T is the common side of the low-voltage transformer secondary. In Fig. 2-4, the T terminal is connected to the compressor contactor by a wire run from the terminal to the contactor. Note that the other wire to the contactor runs from Y on the terminal strip. Now go back to Fig. 2-5, where the Y and T terminals are shown as connection points for the compressor contactor. Are you able to relate the schematic to the actual device? The gas valve is wired by having wire T of the terminal strip attached to one side of the solenoid and a wire run from the limit switch to the other side of the solenoid.
Figure 2-6 shows how the wiring diagram comes from the factory. It is usually located inside the cover for the cold-air return. In most instances, it is glued to the cover so that it is handy for the person working on the furnace whenever there is a problem after installation.
Electrical schematics are used to make it simple to trace the circuits of various devices. Some of these can appear complicated, but they are usually very simple when you start at the beginning and wind up at the end. The beginning is one side of the power line and the end is the other side of the line. What happens in between is that a number of switches are used to make sure the device turns on or off when it is supposed to cool, freeze, or heat.
The ladder diagram makes it easier to see how these devices are wired. It consists of two wires drawn parallel and representing the main power source. Along each side you find connections. By simply looking from left to right, you are able to trace the required power for the device. Symbols are used to represent the devices. There is usually a legend on the side of the diagram to tell you, for example, that CC means compressor contactor, EFR means evaporator fan relay, and HR means heating relay (see Fig. 2-3).
Take a look at the thermostat in Fig. 2-3. The location of the switch determines whether the evaporator fan relay coil is energized, the compressor contactor coil is energized, or the heating relay coil is energized. Once the coil of the EFR is energized by having the thermostat turned to make contact with the desired point G, it closes the points in the relay and the evaporator fan motor starts to move. This means that the low voltage (24 V) has energized the relay. The relay energizes and closes the EFR contacts located in the high-voltage (240 V) circuit. If the thermostat is turned to W or the heating position, it will cause the heating relay coil to be energized when the thermostat switch closes and demands heat. The energized heating relay coil causes the HR contacts to close, which in turn places the heating element across the 240-V line and it begins to heat up. Note that the HR contacts are in parallel with the evaporator fan relay contacts. Thus, the evaporator fan will operate when either the heating relay or the evaporator fan relay is energized.
In Fig. 2-2, the electrical heating system has a few more controls than the basic gas-fired furnace. The low-resistance element used for heating draws a lot of current, so the main contacts have to be of sufficient size to handle the current.
The thermostat closes and completes the circuit to the heating sequencer coil. The sequencer coil heats the bimetal strip that causes the main contacts to close. Once the main contacts are closed, the heating element is in the circuit and across the 240-V line. The auxiliary contacts will also close at the same time as the main contacts. When the auxiliary contacts close, they complete the low-voltage circuit to the fan relay. The furnace fan will be turned on at this time.
Once the thermostat has been satisfied, it opens. This allows the heating sequencer coil to cool down slowly. Thus, the main contacts do not open immediately to remove the heating element from the line. So the furnace continues to produce heat after the thermostat has been satisfied. The bimetal cools down in about 2 min. Once it cools, it opens the main and auxiliary contacts, which removes the heating element from the line and also stops the fan motor. After the room cools down below the thermostat setting, the thermostat closes and starts the sequence all over again.