Complementary elements

The popularity of geothermal heat pumps continues to grow. These days, you can read about them in just about every issue of every HVAC trade publication.

One type, known specifically as a water-to-water geothermal heat pump, is well suited for hydronic heating and cooling applications. My firm has designed several systems around such heat pumps. Many of the inquiries we now receive from potential new clients state that they are interested in combining the benefits offered by a geothermal heat pump with a hydronic delivery system.

Although it is possible to size a geothermal heat pump to provide the full design heating requirement of a house or small commercial building, there are other options. One that lends itself well to modern hydronics technology is to combine the heat pump with a boiler. This is often called a dual fuel approach and there are several benefits associated with it.

First, a dual-fuel system provides the security that one heat source can cover some or all of the load if the other heat source is down for maintenance.

Second, this approach allows the heat pump to be sized to less than the design heating load of the building. This may be necessary due to limited land area for installation of the earth loop. It may also be necessary in situations where earth loop installation costs are high.

Next, having a heat pump and boiler that can operate simultaneously allows for high heat delivery rates that can speed recovery from thermostat setbacks, as well as provide higher domestic water heating recovery rates when necessary.

Finally, in some locations, utilities offer steeply discounted time-of-use electrical energy prices during periods of low demand. These rates can significantly reduce the operating cost of a heat pump during off-peak hours, while allowing a gas-fired boiler to meet demand during “on-peak” periods when electrical rates are significantly higher. Controls can be configured to operate the heat pump as the first stage of heat generation, or the only stage of heat generation, during these off-peak periods.

WATCH THOSE TEMPERATURES

Most geothermal water-to-water heat pumps can only supply water at temperatures in the range of 120 to 125F without experiencing significant decreases in their coefficient of performance (COP), as well as their heating capacity. Because of this, I recommend that any hydronic distribution system supplied by a heat pump be designed so that it can provide the design heating requirement of the building using a supply water temperature no higher than 120F. This allows for good performance from the heat pump. It also allows for good performance from the mod/con boiler in a dual-fuel system.

One type of hydronic distribution system that meets this requirement is a radiant ceiling panel. When properly designed, a low mass radiant ceiling panel, such as the one shown in Figure 1, can provide an output of 28 Btu/hr/ft² when operated at an average water temperature of only 110F. This could correspond to a supply water temperature of 120F and a return water temperature of 100F, under design load conditions.

The downward heat output of the radiant ceiling panel shown in Figure 1 can also be estimated for other operating conditions using formula 1.

Where:

q= rate of heat output to lower side of panel (Btu/hr/ft²)

T_w= average water temperature in panel (F)

T_R = room air temperature (F)

BOILERS DO NOT CHILL

One of the major benefits of having a heat pump in the system is that it can supply chilled water for cooling. Indeed, the requirement for building cooling is often what sways the design toward a heat pump, rather than a heating-only heat source such as a boiler.

The classic approach to chilled water cooling uses one or more air handlers equipped with condensate drip pans. These air handlers provide both sensible and latent cooling. The condensate that forms on the air handler’s coil as a result of latent cooling falls into a drip pan under the coil and is drained away.

Figure 2 shows the piping schematic for a dual-fuel system that provides space heating using radiant ceiling panels, cooling using chilled water air handlers, and year round domestic water heating. The DHW load is sourced from both the desuperheater in the heat pump and the boiler.

During heating mode, the heat pump and mod/con boiler can supply heat to the buffer tank, individually, or together, depending on the load requirement. Given that the unit cost of heat from the heat pump is usually lower than that from the boiler, the heat pump provides first stage heat input, with the boiler taking up the slack, as necessary, to ensure that the water temperature in the tank is adequate to supply the space heating load. This is all coordinated by a two-stage outdoor reset controller, which monitors the water temperature in the tank. The warmer it is outside, the lower the necessary buffer tank temperature. This outdoor reset logic maximizes the performance of both the heat pump and boiler by only heating water to the temperature necessary to satisfy the current space heating load.

The heat pump’s desuperheater adds heat to the domestic water in the lower portion of the indirect water heater whenever the heat pump is running. The boiler provides supplemental heat to the indirect water heater when necessary. In cooling mode operation, any heat supplied to domestic water by the desuperheater is heat that would otherwise dissipate into the earth loop. This is truly “free heat,” given the alternative of dumping those BTUs into the ground.

The distribution system consists of zoned manifold stations that supply radiant ceiling circuits. Two manifold stations are shown, but more can be added if necessary. Flow to each manifold station is allowed or prevented by a standard zone valve. The pressure-regulated circulator automatically adjusts its speed as the zone valves open and close. The air handlers are shown “grayed out” because they do not operate in space heating mode. Figure 3 shows the same system during cooling mode operation.

The heat pump supplies chilled water to the buffer tank and to the chilled water air handlers. The piping details shown near the buffer tank allow some of the chilled water from the heat pump to go directly to the air handlers when needed. If the chilled water flow rate from the heat pump exceeds the flow rate to the air handlers, the difference in flow rates enters the buffer tank. The “short & fat” header piping that connects to the tank allows it to provide hydraulic separation between the heat pump circulator (P2), and the variable speed distribution circulator (P3).

During cooling mode, the heat pump is operated based on the chilled water temperature in the buffer tank. A typical control criteria is to operate the heat pump as necessary to maintain this temperature between 45 and 60F. This is done using a simple temperature setpoint controller.

Heat from the heat pump’s desuperheater, as well as supplemental heat from the boiler, are again routed to the indirect domestic water heater.

COOL CEILINGS

It is also possible to use the radiant ceiling panels for cooling. A possible subassembly for doing this is shown in Figure 4.

This subassembly could be fit into the distribution system shown in Figure 3. It splits the cooling load between the radiant ceiling panels and chilled water air handler. The objective is to satisfy as much of the sensible cooling load as possible using the radiant ceiling, and handle any remaining sensible cooling load, along with the full latent cooling load (that is moisture removal), using the air handler.

The chilled water temperature supplied to the radiant ceiling panel must be separately controlled based on the dewpoint temperature of the space the panel is cooling. Specifically, the three-way motorized mixing valve must keep the water temperature supplied to the panel circuits not less than 3F above the room’s current dewpoint temperature. This provides a safety margin against condensation forming on the ceiling.

The need for mixing during cooling mode operation requires another circulator to circulate water between the manifold station and mixing valve. During heating mode operation this mixing valve would be fully open (that is no flow in the bypass pipe).

The chilled water flow through the coil of the air handler is regulated by a modulating two-way valve that responds to a controller measuring relative humidity (RH) in the space being cooled. If the RH starts to rise above a target value, such as 50 per cent, chilled water flow through the coil is increased, which lowers the coil’s average temperature and increases the rate of condensation.

The coil in the air handler should be specified with multiple tube rows. The “deeper” the coil, the lower the air flow rate required to provide adequate moisture removal. In some systems, primarily those used in non-residential buildings, the air flow rate is dictated by the ventilation requirements of the space.

Currently, there are not many choices for “off-the-shelf” controllers that can monitor interior dewpoint and drive three-way motorized mixing valves as described above. I for one would welcome additional controllers with such capabilities to the market. Ideally, that new controller would also be able to drive the two-way modulating valve that controls chilled water flow through the air handler.

In the meantime, the system shown in Figures 2 and 3 can be assembled from readily available hardware and controls. Perhaps you have an upcoming project in which it can be used. <>