Cold climate air-to-water heat pumps—Part 2
This month we’ll look at the technical issues associated with applying these heat pumps.
Last month, in the first part of this article we discussed the rationale and emerging market potential of air-to-water heat pumps designed for cold climate applications. We also took a look at the enhanced vapor injection (EVI) refrigeration cycle that allows these heat pumps to operate at sub-0º F temperatures with reasonable performance.
This month we’ll look at the technical issues associated with applying these heat pumps.
The heating capacity and coefficient of performance of any heat pump are strongly dependent on the temperature of the material from which low-grade heat is being absorbed. For an air-to-water heat pump operating in heating mode, that low-grade heat is absorbed from outside air.
Figures 1 and 2 show the heating capacity and COP ratings vs. outdoor air temperature for one EVI-enabled air-to-water heat pump currently available in North America.
Notice both heating capacity and COP decrease at lower outdoor air temperatures. Still, the EVI-based air-to-air heat pump represented by these graphs maintains a COP of 2.55 when producing a 120º leaving water temperature and operating at an outdoor air temperature of 0º.
At an outdoor temperate of 25º, and the same 120º leaving water temperature, the COP is about 2.8. This compares to a COP of about 2.3 for an air-to-water heat pump with an inverter drive compressor operating under the same conditions. That’s a relative performance gain of about 22%, which implies 22% higher heat output for the same electrical input.
Lower is better
The graphs in Figures 1 and 2 also show the heat pump’s heating capacity and COP depend on the temperature of the water leaving the heat pump for the heating load. The higher this temperature is the lower the heat pump’s capacity and COP.
This implies there is an advantage in combining air-to-water heat pumps with a low temperature heating distribution system. This is especially true for COP. For example, at 20º outdoor temperature a distribution system that can deliver the building’s heating load using 110º water would allow the heat pump to reach a COP of about 3.1, whereas a system requiring 130º water would only allow a COP of about 2.5.
Several types of heat emitters including radiant floor, wall and ceiling panels, low temperature panel radiators, and even low temperature fin-tube convectors could be used as the heat emitters in combination with the air-to-water heat pump. A suggested criterion is to select heat emitters so they can deliver the design load of the spaces they serve while operating at a supply water temperature no higher than 120º.
Add some smarts
The best way to operate the heat pump at the lowest possible water temperature is to incorporate outdoor reset control as the “logic” for maintaining the temperature within a buffer tank.
An outdoor reset controller continuously calculates the “target” water temperature that can just meet the building’s heating load based on the current outdoor temperature. It operates the heat pump to maintain the buffer tank within a narrow range of temperature centered on this target temperature.
For example, if the buffer tank supplied a radiant floor heating system with a target supply water temperature of 110º at 0º outdoor conditions, the heat pump would attempt to maintain the mid-height sensor on the buffer tank between a low of 107º and a high of 113º. These numbers are based on the “target” water temperature of 110º and a differential setting of 6º, which is centered on the target temperature and is user-adjustable. In this scenario, the “turn on” condition for the heat pump is 3º below the target temperature and the “turn off” conditions are 3º above the target temperature.
If the outdoor temperature was 35º, the target supply water temperature would drop to about 90º. Under these conditions the outdoor reset controller would operate the heat pump to maintain the buffer tank between 87° and 93º. These conditions, along with the overall operating range of the outdoor reset control, are shown in Figure 3.
Outdoor reset controllers, also known as boiler reset controllers, are relatively inexpensive and readily available from several suppliers.
Putting it together
Figure 4 shows a system designed to supply space heating, central cooling and most of the domestic hot water load using a low ambient air-to-water heat pump.
When the heat pump is operating in heating mode, a motorized diverter valve (DV1) directs heated antifreeze solution from the heat pump to heat exchanger (HX1), which in turn delivers heat to the buffer tank, the heat emitters or both, depending on the current flow rate to the heat emitters. If none of the heat emitters require flow, all flow from the heat exchanger passes into the thermal storage tank. If the flow to the heat emitters is less than the flow from the heat exchanger, the difference in these flow rates passes into the buffer tank. If the flow required by the heat emitters is greater than the flow from the heat exchanger, the difference in these flows comes out of the upper side connection of the buffer tank.
The brazed plate heat exchanger (HX1) has been sized so the water temperature leaving the heat exchanger is not more than 5º lower than the temperature of the antifreeze solution coming from the heat pump. This minimizes the “thermal penalty” imposed by having a heat exchanger between the heat pump and balance of system.
Because the heat pump circuit is an isolated closed loop, it must be equipped with a pressure relief valve, expansion tank, air separator, fill/purging valves and a suitably-sized circulator.
The buffer tank is piped in a “two-pipe” configuration. This reduces the flow velocity into and out of the tank when a load circuit is operating at the same time as the heat pump. Lower flow velocities improve temperature stratification within the buffer tank. This configuration also allows the tank to provide hydraulic separation between the heat exchanger circulator and the variable-speed distribution circulator. The headers seen on the left side of the buffer tank should be as short as possible and generously sized to minimize head loss and thus encourage hydraulic separation.
Space heating is provided by several panel radiators, each of which is equipped with a thermostatic radiator valve. Each radiator is supplied by a home-run circuit of 1/2” PEX tubing routed from a single manifold station. Flow to all radiators is provided by a variable-speed pressure-regulated circulator, which automatically adjusts its speed based on the flow required by the distribution system. The radiators have been selected so they can supply the design heating load of the spaces they serve when supplied with water at 120º.
The heat pump and circulator (P3) are enabled to operate in heating mode whenever the outdoor temperature is below 50º.
An outdoor reset controller monitors the temperature of the sensor at the mid-height of the buffer tank and operates the heat pump to maintain the tank close to the target temperature determined by the outdoor reset controller. Thus, heated water always is ready to flow to a panel radiator whenever a flow path opens.
The heated water in the buffer tank also stands ready to partially heat domestic water whenever there is a demand. This system uses an external brazed-plate stainless-steel heat exchanger (HX2) to extract heat from the thermal mass of the buffer tank and transfer it to domestic water. The hardware is seen on the right side of the buffer tank. The flow switch closes its contacts whenever there is a domestic hot water demand of 0.6 gpm or higher. This energizes the coil of a relay, which switches 120 VAC to a small circulator (P4). Hot water from the upper portion of the buffer tank is immediately circulated through the primary side of heat exchanger, while cold domestic water passes in the opposite (counter flow) direction through the secondary side of the heat exchanger.
A thermostatically controlled electric tankless water heater provides the temperature boost necessary to bring the domestic hot water to the required delivery temperature. An “anti-scald” thermostatic mixing valve is provided to ensure a safe delivery temperature.
When the heat pump operates as a chiller, the diverter valve (DV1) is energized and chilled fluid is routed directly to the air handler using circulator P1. The air handler has been sized to deliver the cooling capacity of the heat pump when operating at a supply fluid temperature of 45º. It is equipped with a drip pan and drain to collect and dispose of condensate. In cooling mode, the rate of heat transfer between the air handler and heat pump is matched. Thus, there is no need to involve the buffer tank in cooling mode operation. This simplifies the piping and controls. It also eliminates the thermal penalty of the heat exchanger during cooling mode.
In situations where the building cooling load is relatively small compared to the heat pump’s cooling capacity or when multiple chilled water air handlers are used, it is advisable to:
A) Provide a separate chilled water buffer tank;
B) Use a two-stage heat pump that can meet the cooling load using only Stage 1 cooling capacity; and
C) A & B above.
These measures reduce short cycling of the heat pump under partial load conditions.
Because the heat pump in Figure 4 has been sized for the building’s design heating load, it has excess heating capacity much of the year. This especially is true in late spring, summer and early fall when outdoor temperatures are mild or even warm. During these times that capacity can be used for domestic water heating or pool heating (with a suitable heat exchanger).
Controls can be configured so domestic water heating is a priority over cooling. Thus, if the temperature of the buffer tank drops below perhaps 120º while the heat pump is in cooling operation, it stops, changes to heating mode operation, recovers the temperature of the buffer tank up to perhaps 130º and then switches back to cooling. If a chilled-water buffer tank is used in the cooling portion of the system, this temporary load shedding will have very little if even a perceptible effect on comfort.
There also are several variations on the system shown in Figure 4. One is to add a boiler to the system, allowing the heat pump to be sized to less than the design heating load of the building since the boiler could provide additional heating capacity for “peak load” conditions. If a single-stage heat pump is used it might be sized to provide the peak cooling load. The use of a boiler also provides the ability to operate the heat pump on off-peak electrical rates (where available) and use the boiler to shed electrical load during peak rate periods. The latter strategy works best if combined with thermal storage.
I’m very optimistic that air-to-water heat pumps will gain increased acceptance in North America as awareness on how they dovetail with modern hydronics technology grows.