Controlling a dual-heat system

This is the last of three columns describing a combisystem that uses an air-to-water heat pump and a mod/con boiler to supply heating, cooling and domestic hot water. We discussed the concept in the October Hydronics Workshop column (“Dynamic duo”) and the piping in the November column (“Piping a dual-heat system”). This month we’ll look at the control system, and make sure the system is fully documented.

Figure 1 shows the piping schematic as we left it last month.

There are several ways to provide controls for this system. One is to search for an “all-in-one” controller to manage all the necessary control functions such as turning the heat pump on and off, operating the diverting valve, monitoring the buffer tank temperature in both heating and cooling modes, and operating the boiler. Assuming you could find this controller, you would just wire all the necessary connections and sensors to it, program in some settings, and turn it on, confident it would optimize all modes of system operation.

Unfortunately, no company that I’m aware of has an all-in-one controller for a system as unique as the one we’re discussing.

Furthermore, using an all-in-one controller — assuming such a product could be found — might be great initially, but if that controller fails years later, you had better hope the manufacturer is still in business and the now “legacy” controller is still available. If it is, don’t plan to haggle over its price.

Another approach would be a fully programmable control system using analog and digital input/output modules to convert sensor information into a form that a computer can digest. This hardware would be combined with a desktop PC and some custom software to create the necessary logic. The software would operate in a continuous loop, constantly determining what is happening, as well as what should be happening within the system. It would send output signals back through isolation relays to turn drive components such as circulators and valves.

Although I’m confident the necessary hardware and software could be assembled, and that it could perform all the necessary control tasks, I’m not as confident about the reliability of the PC, its operating system and its spinning hard drive. If the disk drive failed, the computer’s operating system “hung up,” or the system was infected by some malicious virus through its Internet connection, the building’s comfort system could be left in limbo.

I prefer control systems that use hard-wired logic in combination with readily available, highly reliable and relatively simple controllers. Hard-wired logic is just a fancy name for how electrical devices such as switches, relays and temperature controllers are connected to each other.

This approach improves the likehood that replacement components will be available as the system ages. These components might even come from different companies or suppliers if necessary. For example, it’s possible to source a one-stage or two-stage temperature setpoint controller from several suppliers. This also is true for basic outdoor reset controllers or three-way motorized mixing valve controllers. A wide range of electrical switches and relays are available from dozens of sources.

It’s likely these basic controllers, which are used in many control applications, will remain available for years to come. As proof, look at the continued availability of electromechanical temperature controllers with capillary tube sensors. Some of these devices originated in the 1940s and yet remain a few clicks away from being ordered through Internet suppliers. They also are available at many over-the-counter wholesalers.

Hold onto the ladder

With the above in mind, I’ve standardized on basic control components and “ladder diagrams” for design and documentation of complete control systems.

If you’re not familiar with ladder diagrams, the idea is simple. Have a look at the basic ladder diagram in Figure 2.

Line voltage loads are treated as “rungs” across two vertical lines in the upper portion of the diagram. The red vertical line on the left represents line voltage and the blue line on the right represents neutral.

About halfway down the ladder is a transformer that creates 24 VAC for low-voltage loads. The red vertical line on the left, and below the transformer, represents 24 volts. The blue vertical line on the right, below the transformer, represents the common side of the transformer.

The control system represented by Figure 2 is very simple. When the main switch is closed, line voltage is present on the upper left vertical line of the ladder. The primary winding of the transformer is connected between this line and neutral, and thus it is energized. This implies that 24 VAC is present across the lower portion of the ladder. When the thermostat contacts closes, 24 VAC is passed on to energize the relay coil. This causes the normally open relay contacts in the upper portion of the ladder to close, passing 120 VAC to the circulator.

Thus, a low-voltage control device (the thermostat) is able to turn on a line-voltage load (the circulator), through use of a relay.

Although the control system represented in Figure 2 works, it doesn’t do much. However, if the concepts shown in Figure 2 are understood, the ladder can be extended to include many more low-voltage and line-voltage devices.

Figure 3 shows a ladder diagram for the control system that will manage the piping system shown in Figure 1. All the electrical devices in the ladder diagram shown in Figure 3 have designations (in this case shown in green text within parentheses). Examples include circulators, such as (P1) and (P2), a flow switch (FS1), and a two-stage setpoint controller (SPH).

Individual relays also get designations such as (RH1), (RH2) and (RB). The poles on each relay should have individual designations. For example: (RH1-1) indicates pole No. 1 on relay (RH1), whereas (RH1-2) indicates pole No. 2 on the same relay. This is the only way to associate the relay contacts with the relay coil to causes them to move.

When a component in the ladder diagram also is shown in the piping diagram, it’s critical it has the same designation.This allows cross-referencing between the two diagrams, which is essential in understanding how the wet-side components are “orchestrated” by the control system components. This is the essence of how a customized system, such as the combisystem being discussed, is synthesized during the design process. Component designation and cross-referencing are extremely helpful when a service tech is trying to troubleshoot a system that is not performing correctly.

Put into words

Along with a piping schematic and electrical system diagram, a well-documented system also has a description of operation. This is where the designer “narrates” how all the system’s elements work to achieve the desired operation. Each mode in which the system can operate should be described. These descriptions should refer to the component designations that have been shown in the piping schematic and electrical diagram.

The following is a description of operation of the system shown in Figures 1 and 3.

Heat-source operation. When the main switch (MS) is closed, power is available to the line-voltage and low-voltage portions of the electrical system. For space-heating operation, the mode selection switch (MSS) must be set to heat. This passes 24 VAC to the RH terminal of the master thermostat (T1). 24 VAC also is present across the R and C terminals of the two-stage setpoint controller (SPH), which measures the temperature at sensor (S1) in the upper portion of the thermal storage tank.

If the temperature is below the user-set stage 1 setpoint (in this case, 125° F), minus half the user-set differential (in this case, half the differential is 5°), then the stage 1 contacts in (SPH) close. This contact closure passes 24 VAC to energize the coil of relay (RH1). Relay contact (RH1-1) closes to pass 120 VAC to circulator (P2) and diverter valve (DV1). Relay contact (RH1-2) closes to complete a low-voltage circuit between the R and Y terminals of the heat pump, turning it on in heating mode.

An internal relay within the heat pump turns on circulator (P1). With circulators (P1) and (P2) on, and diverter valve (DV1) on, heat is flowing from the heat pump to the buffer tank. The heat pump and these associated devices will continue to operate as described until sensor (S1)in the buffer tank climbs to a temperature of 130°.

If the temperature at sensor (S1) in the upper portion of the buffer tank drops to 115°, the stage 2 contacts in the setpoint controller (SPH) will close. These contacts complete a low-voltage circuit powered through the boiler and enable the boiler to operate in a fixed upper temperature mode. The boiler turns on circulator (P4) through an internal relay. The boiler continues to operate until the buffer tank sensor (S1) reaches a temperature of 130°, at which point the boiler turns off, and so does circulator (P4).

Note that the boiler will operate in this mode regardless of whether the mode selection switch is set to heat or cool. This allows the boiler to maintain a suitable temperature in the buffer tank for domestic water heating, even when the heat pump is operating as a chiller.

Space-heating distribution. If master thermostat (T1) is calling for heat, 24 VAC passes from its W terminal to energize the coil of relay (RH2). Contact (RH2-1) closes to pass 120 VAC to circulator (P3). This circulator is set to operate in constant differential pressure mode to provide the necessary flow to any panel radiator that does not have its thermostatic valve fully closed. Circulator (P3) will automatically vary its speed to maintain approximately constant differential pressure across the manifold station serving the panel radiators. The thermostatic valves on each radiator can be used to limit heat input as desired.
Domestic water heating mode. Whenever there is a demand for domestic hot water of 0.6 gal. per min. or more, flow switch (FS1) closes. This passes 24 VAC to energize the coil of relay (Rdhw). Contact (Rdhw-1) closes to pass 120 VAC to circulator (P5). Heated water from the upper portion of the buffer tank will flow through the primary side of heat exchanger (HX2) and transfer heat to the cold domestic water flow through the secondary side of the heat exchanger (HX2).

When the demand for domestic hot water drops to 0.4 gpm or less, flow switch (FS1) opens. This turns off relay (Rdhw) and circulator (P5). All domestic hot water leaving the system passes through a thermostatic mixing valve to limit the water temperature to the distribution system.

Cooling mode. For cooling operation, the mode selection switch (MSS) must be set to cool. This passes 24 VAC to the RC terminal of the master thermostat. If the master thermostat is set for cooling operation and calls for cooling, 24 VAC is passed to its Y terminal. From the Y terminal, 24 VAC is passed to energize the coil of relay (RB). A normally open set of contacts (RB-1) close to pass 120 VAC to the air handler (AH1), turning it on.

In addition, 24 VAC passes from the Y terminal of the master thermostat to energize cooling setpoint controller (SPC). Once energized, (SPC) monitors the temperature of sensor (S2) on the inlet pipe to the air handler. If the temperature is above 60°, the contacts in (SPC) close. This passes 24 VAC from the R terminal in the heat pump to energize the coil of relay (RA). It also passes 24 VAC from the R terminal of the heat pump to terminal O of the heat pump, which energizes the heat pump’s reversing valve.

Contact (RA-1) closes to complete a circuit from the R terminal to the Y terminal of the heat pump, allowing the heat pump’s compressor to operate. The heat pump turns on circulator (P1) through its internal relay. All necessary devices for cooling operation are now active. The system remains in cooling operation until either the cooling demand is removed at thermostat (T1), or the temperature at sensor (S2) on the air handler inlet drops to 40°, at which point the heat pump, circulator (P1) and air handler (AH1) turn off.

This three-part column has presented the rationale, piping configuration and control system design for a combisystem that provides heating, cooling and domestic hot water. Even if you never build this particular system, I urge you to study the concepts and subsystems used in its design. There’s a good chance they can be of use in one of your future applications.

I’ll leave you with some challenges. After studying the system we’ve just discussed, see if you can modify it to include any of the following variations: