PM Engineer

More of a Good Thing

by John Siegenthaler , P.E.

April 1, 2010

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The use of multiple ground source heat pumps continues to rise.

Interest in ground source heat pumps continues to grow in North America and Europe. Although many first learn of this technology in the context of a single heat pump used in a residential application, there are several ways to implement multiple ground source heat pumps in commercial systems. The most common is the use of multiple water-to-air heat pumps served by a common “building loop” piping system as shown in Figure 1.

In this system, each water-to-air heat pump can operate in either the heating or cooling mode at any time. The units operating in heating extract low-grade heat from the building loop, upgrade the temperature of that heat and deliver it through a forced-air distribution system that serves a small area of the building, perhaps a single room.

When a heat pump switches to cooling, it dissipates heat into the piping loop and delivers cooled/dehumidified air via the same localized ducting.

As long as the number of heat pumps operating on cooling vs. heating allows the building loop temperature to remain between limits of, say, 50º to 70º F, the earth loop circulator does not need to operate. For the system shown in Figure 1, flow simply passes through the closely spaced tees without inducing flow in the earth loop. If the building loop temperature migrates outside this user-selected range, the earth loop circulator operates to supply the necessary low-grade heat input or heat dissipation.

A lesser-known application involves multiple water-to-water (w/w) heat pumps connected to an earth loop. Water-to-water heat pumps deliver their heat output to a stream of water rather than air. The heated water can be used for loads ranging from domestic water heating, to space heating, to snow melting.

For heating-only situations, think of a multiple w/w heat pump system as similar to a multiple boiler system. Each heat pump represents a stage of heat production. The number of stages operating at any time depends on the current load.

Figure 2 shows how three heating-only w/w heat pumps could be configured into a three-stage heat plant to serve a zoned hydronic distribution system. A hydraulic separator is used to interface the earth loop to the headers serving the left (evaporator) side of the heat pumps. Another hydraulic separator interfaces the right (condenser) side headers to the distribution system. Each heat pump has a circulator with an internal check valve on its evaporator and condenser side. These circulators only operate when their associated heat pump is on.

The distribution system could use individual zone circulators as shown. It also could be designed for a single variable-speed, pressure-regulated circulator in combination with zone valves.

Figure 3 shows a similar concept, but using zone valves and a variable-speed, pressure-regulated circulator to control flow through the heat pumps. The zone valves associated with each heat pump open only when that heat pump is operating. The speed of each pressure-regulated circulator changes as necessary to maintain a constant pressure differential across the headers as the zones’ valves open and close. This configuration will reduce circulator energy use relative to the configuration shown in Figure 2, especially if high-efficiency ECM-based circulators are used.

Load Matching

As is true with a multiple boiler system, the greater the number of stages, the better the match between heat output and the load. Figure 4 shows how a four-stage w/w heat pump system, where each heat pump represents 25% of the design load, would operate to handle a hypothetical load profile.

Most multistage boiler controllers with simple on/off relay outputs can operate a multiple w/w heat pump system. Techniques such as boiler rotation, in which the operating order of the heat sources is rotated to achieve approximately equal run time, are equally applicable to multiple w/w heat pumps.

The staging controller also could use outdoor reset control to adjust the supply water temperature to the distribution system based on outdoor conditions. This allows the condenser side of the heat pumps to operate at lower temperatures during partial load conditions. The overall coefficient of performance (COP) of the system will be improved relative to systems that operate the heat pumps to maintain a set supply water temperature regardless of load.

Multiple heating-only w/w heat pumps are well-suited for “envelope-dominated” buildings that have distribution systems that can operate at low temperatures. Envelope-dominated buildings are those in which the heating load is primarily determined by outside temperature and heat loss through the building’s exterior surfaces (e.g., walls, windows, doors and ceiling). Buildings with heated slab on grade floors in which the maximum tube spacing in the floor slab does not exceed 12 inches are usually excellent candidates. Examples include garages, fire stations, aircraft hangers, vehicle service buildings and small to moderate size retail buildings.

Changeovers

Many w/w heat pumps come with internal reversing valves that allow them to operate as either a hydronic heat source or a chiller. When the internal reversing valve is not energized, the unit operates as a heat source. When the reversing valve is powered on by a 24 VAC signal, the previous functions of the evaporator and condenser are reversed and the unit delivers chilled water. 

Some buildings do not have “envelope-dominated” loads, but instead have internal spaces with significant heat generation from people, appliances or industrial processes. Such spaces often require year-round cooling, while the load at the building’s perimeter changes from heating to cooling depending on outdoor temperature and solar heat gain. The system shown in Figure 1, which uses multiple water-to-air heat pumps, is arguably the standard approach to provide both heating and cooling in such buildings.  However, reversible w/w heat pumps bring some unique opportunities to such applications.

One opportunity is when either radiant panel cooling or chilled beam cooling is used to offset the sensible cooling load, while one or more chilled water air handlers, equipped with drip pans, handle the latent cooling load. Chilled water is required for all of these terminal units. That water supplied to the air handler needs to be low enough in temperature to sustain the necessary rate of moisture removal (e.g., latent cooling load). The chilled water supplied to radiant panels or chilled beams must be a few degrees above the dew-point temperature of the space to avoid condensate formation. The latter is usually accomplished using a mixing valve regulated by a dew-point controller. Figure 5 shows a way to configure multiple w/w heat pumps to simultaneously supply both heated and chilled water.

This system includes two buffer tanks, one for heated water, and the other for chilled water. Either tank can be supplied from any of the upper three reversible w/w heat pumps at any time. Each of these heat pumps uses a single-load side circulator and diverting valve to determine which set of mains its output is directed to. A controller monitors the temperature of each buffer tank and stages on heat pumps in either heating or cooling mode to maintain each tank within an acceptable temperature range. The earth loop circulator is turned on only when the temperature of the fluid in the headers on the left side of the heat pump array drifts outside a preset range.

Twice the Benefit

When there is a simultaneous demand for both heating and cooling, the w/w heat pump shown at the bottom of the schematic operates in “double duty.” It extracts low-grade heat from the chilled water buffer tank and dissipates that heat, along with the heat generated by its compressor, into the heated water buffer tank. This heat pump operates at a very high “effective” COP considering that the electrical energy used by a single compressor produces both heating and cooling capacity. Any supplemental heating or cooling required to maintain the buffer tanks within acceptable temperature limits is handled by the upper group of heat pumps operating in whatever mode is required.

All indications are that ground source heat pump systems will continue to gain market share in both residential and commercial applications. It behooves HVAC designers, especially those dealing with hydronic heating and cooling systems, to know how to apply their unique characteristics.

John Siegenthaler , P.E.
john@hydronicpros.com

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