Air-to-water heat pumps come in a wide range of sizes depending on heating or cooling capacity requirements and what type of electrical service is available.
When 3-phase electrical power is available there are commercial-size air-to-water heat pumps with capacities up to at least 250 tons (3,000,000 Btu/h).
When only single-phase electrical power is available, the maximum capacity offered by most manufacturers is a nominal 5-ton (60,000 Btu/h).
When a large home or commercial building is located where only single-phase power is available, but the heating (or cooling) loads exceed 5 tons, one solution is multiple heat pumps.
We’ve done this before
In some ways, a multiple air-to-water heat pump system can operate like a multiple boiler system.
When single-speed heat pumps are used, each heat pump represents a stage of heat input. When a load calls for heat, a staging controller turns on one heat pump (e.g., stage 1). The controller continues monitoring the trend in supply water temperature and compares it to a calculated “target” value. For space heating, that target temperature is usually determined based on outdoor reset control. If the heat output of stage 1 is sufficient to achieve and maintain the target supply temperature, there is no need to operate additional heat pumps. However, when the required supply water temperature cannot be achieved or maintained, a second heat pump, representing stage 2 heat output, is turned on. The staging controller continues to monitor if the measured supply water temperature is approaching the target temperature. If it isn’t, the controller turns on stage 3, and so forth. Off-the-shelf controllers for this type of staging are widely available.
Most air-to-water heat pumps have a delay of 2-3 minutes between when the heat pump gets the electrical signal to operate, and when the compressor starts. When using a multi-staging controller, it’s important to set interstage time delays to give each heat pump time to contribute heat to the system before starting another stage. A minimum interstage time delay of 5 minutes is suggested.
If the measured supply water temperature is trending above the current target temperature, the controller eventually turns off stages.
Some staging controllers can also be used to maintain a target chilled water temperature for cooling. However, most multi-stage controllers designed for boilers don’t have this ability.
Turndown
Just as boiler technology has progressed from simple on/off control to modulating heat output, many air-to-water heat pumps are now equipped with variable speed (inverter driven) compressors. Such units can vary their heat output from full capacity down to about 1/3 of that capacity. This ability is often expressed as “turndown ratio,” which is defined by Formula
Formula 1:
The turndown ratio for a single variable-speed heat pump is typically about 3:1. However, when multiple (and identical) variable-speed heat pumps are combined into a single “plant,” the turndown ratio of the plant is the heat pump’s turndown ratio multiplied by the number of stages. System turndown ratio can be calculated using Formula 2.
Formula 2:
For example, if each heat pump within a four heat pump “plant” has a 3:1 turndown ratio, the turndown ratio of the system is:
This means that the minimum stable heat output of the system is (1/12th) or 8.3% of its maximum heat output. That’s a wide range of capacity control, and in most situations would eliminate the need for a buffer tank.
Simple setup
It’s possible to set up a multiple air-to-water heat pump system as shown in Figure 1.
This piping topology is often called a “2-pipe” system, referring to the two headers that each heat pump connects to.
Each heat pump has its own circulator — either internal to the heat pump, or external. These circulators are controlled by their associated heat pump. The heat pump typically turns on its circulator for 2-3 minutes before to starting its compressor. This ensures that flow through the heat pump is sufficient and stable before activating the refrigeration system. Some heat pumps also control the speed of the circulator to maintain a specific temperature rise (or temperature drop) as heating or cooling output is varied.
It’s important to have a check valve in the piping leading from the headers to each heat pump. The check valve could be within the circulator, as shown in Figure 1, or a separate external component. It’s there to prevent flow reversal through any inactive heat pump when other heat pumps are operating.
The piping for each heat pump is also equipped with a valve that can be used to help fill and purge the heat pump, or in combination with the isolation flanges on the circulator to isolate the heat pump from the remainder of the system if it needs to be serviced or removed.
Each heat pump branch also includes a pressure relief valve, which is necessary whenever a heat source in a closed loop system is capable of being isolated from the remainder of the system.
The hydraulic separator, in combination with generously sized header piping, isolates the pressure dynamics of the heat pump circulators from each other as well as from the system circulator. It also provides air and dirt separation for the system. If equipped with a magnet this separator can also capture iron oxide or other ferrous particles in the system. The latter is especially important with circulators with electronically-commutated motors (ECMs) are used.
From 2-pipe to 4-pipe
Although the piping in Figure 1 allows staging and modulation control of each heat pump, it doesn’t allow the heat pump plant to simultaneously supply heating and cooling. This ability is often needed in large custom homes or buildings that have cooling loads in interior spaces while heating is required in exposed perimeter spaces.
Figure 2 shows one approach to a 4-pipe configuration that allows each heat pump to be independently operated in heating or cooling mode.
Each heat pump is equipped with a set of 2-position motorized valves — two 2-way valves, and one 3-way valve. The latter determines if the inlet to a heat pump is connected to the heating headers or the cooling headers.
The two 2-way valves also determine which set of headers heat pump flow is routed to. One valve opens when the heat pump turns on in heating mode. The other opens when the heat pump operates in cooling mode. When the heat pump is off, both 2-way valves are closed. This mode prevents any flow reversal or heat migration through an inactive heat pump.
If all three valves are equipped with spring-return actuators, they can be operated by simple 24VAC on/off signals depending on the heat pump’s operating mode (e.g., heating, cooling, or off).
Only two heat pumps are shown in Figure 2, but the 4-pipe header arrangement can be extended to accommodate more heat pumps.
The heating headers and cooling headers lead back to two hydraulic separators. The hottest flow coming from the heating headers goes into the upper left connection on its associated hydraulic separator. The colder fluid from the heat pumps goes into the lower left connection on its associated hydraulic separator.
Each heat pump piping assembly is equipped with four valves allowing the heat pump to be isolated from the remainder of the system. These valves are also helpful for purging or drainage. Each heat pump also has a pressure relief valve and its own circulator.
The heating and cooling sides of the system each have their own an expansion tank, with fluid make-up supplied by a single fluid feeder. The make-up fluid tubes from the fluid feeder each have a check valve to prevent any possibility of cross flow from the heating to the cooling side of the system or vice versa. The expansion tank connectors go to the headers that lead to the inlets of the diverter valves. This ensures that there is no mode where a heat pump is isolated from an expansion tank.
It’s likely that the fluid expansion volume in heating mode will be larger than that required in cooling mode. However, as heat pumps change between heating and cooling mode, fluid expansion will be shared between the tanks. Conservative design would be to size both tanks for the maximum fluid expansion volume of the system. Also be sure that the static pressure on both tanks is adjusted so that the diaphragms within the tanks cannot “bottom out” against the tank shell when the system is operating at its lowest fluid temperature in cooling mode.
The header piping connecting the heat pumps to the hydraulic separators should be sized for a maximum flow velocity of 2 feet per second. This creates minimal head loss, which, in combination with the hydraulic separators, minimizes any interaction between the heat pump circulators.
Adding a boiler
The heat output of all air-to-water heat pumps decreases as outdoor temperature drops. To ensure adequate heat delivery at design load, it’s common to include some type of boiler as both a supplemental heat source, and, if needed, backup for one or more heat pumps that are down for service. Figure 3 shows a simple way to interface a mod/con boiler with the system shown in Figure 2.
The boiler is connected in parallel with the heating headers supplying the heat pumps. A check valve in each parallel source circuit prevents flow reversal.
The boiler typically operates as the last stage of heat input in situations where all operating heat pumps cannot maintain the required supply temperature to the load. It could also be operated during times when “on-peak” electrical rates make it less expensive to supply heat from the boiler rather than the heat pumps.
There are more ways to expand the concept of a 4-pipe heat pump system. We’ll get into those in next month’s Heating with Renewable Energy column.
