HPAC Magazine

Heat Pump Plus

How to combine an air-to-water heat pump with a boiler (Part II).

February 2, 2014   By John Siegenthaler

In Part I we discussed the energy concepts involved in combining an air-to-water heat pump (AWHP) with an auxiliary boiler. In Part II we will put hardware together into a complete combi-system for heating and domestic hot water. Since we have the heat pump, we will also include a couple of zones of chilled water cooling. Figure 1 shows the overall piping schematic.

This system uses a two-stage self-contained air-to-water heat pump. The outdoor unit is connected to a generously-sized brazed plate heat exchanger located within the heated space. An antifreeze solution is used in the circuit between the heat pump and heat exchanger. It provides reliable freeze protection during a prolonged power outage in sub freezing weather. The heat exchanger is sized to transfer the full output of the heat pump using an approach temperature difference of 5F between the incoming heated antifreeze and the heated water leaving the heat exchanger. This minimizes the thermal penalty associated with having a heat exchanger between the heat pump and the remaining portions of the system. The circuit between the heat pump and heat exchanger includes an expansion tank, combination air/dirt separator, pressure relief valve and fill/purging valves.

The heating distribution system is an extensively zoned low temperature radiant panel system. The rate at which some zones require heat is substantially lower than the heat pump’s first stage heating capacity. The buffer tank stabilizes the heat pump against short cycling under single stage operation. Notice that the heat exchanger (HX1) is piped so the heat pump works with the slightly lower water temperatures in the lower portion of the buffer tank. This helps maximize the heat pump’s heating capacity and coefficient of performance (COP).

When outdoor temperatures are high enough for the heat pump to meet the heating load, it serves as the sole heat source for the buffer tank. The water temperature in the buffer tank is regulated based on outdoor reset control. This control selection allows the heat pump to operate at lower condenser temperatures during mild weather, increasing both its heating capacity and COP.

When the outdoor temperature drops below a preset value, such as 10F, the heat pump is disabled and the boiler serves as the sole heat source for the buffer tank. The boiler is piped to work with the warmer water in the upper portion of the buffer tank. Even here, the water temperatures are low enough to allow the boiler to operate in condensing mode and thus at high efficiency. Stratification keeps the warmest water in the upper portion of the tank where it can be drawn off for either space heating or domestic water heating. The thermal mass of the buffer tank also protects the mod/con boiler from short cycling.

Domestic water is preheated by the external stainless steel heat exchanger labelled as (HX2) in Figure 1. Whenever the demand for hot water reaches 0.6 gallons per minute (gpm) or more, a flow switch inside the electric tankless water heater closes. This switch closure initiates two simultaneous actions.  First, it turns on circulator (P5) using an isolation relay. This circulator moves water from the upper portion of the buffer tank through the primary side of heat exchanger (HX2), while cold domestic water passes through the other side of the heat exchanger in a counter flow direction.

Second, if the temperature of the domestic water leaving heat exchanger (HX2) is too low for use at the fixtures, the thermostatically controlled tankless water heater elements boost the water temperature as necessary to achieve the required hot water delivery temperature. For more information about “on-demand” subassembly read Only As Needed in HPAC‘s May/June 2013 issue.

ROLE REVERSAL

In cooling mode, the heat pump produces chilled water, which is routed to the air handlers. These air handlers have been sized so that their total cooling capacity at a supply water temperature of 50-55F, closely matches the cooling capacity of the heat pump.  The heat pump operates on stage one cooling when one of the air handlers is running. If the second air handler turns on, the heat pump’s internal controller may turn on stage two to maintain adequately low chilled water temperature. Because the cooling capacity of the heat pump and air handlers is reasonably well matched, there is no need to involve the buffer tank during cooling mode operation.

An on/off zone valve regulates flow to each air handler. An ECM-based pressure regulated circulator varies its speed as necessary to provide proportional differential pressure control in the chilled water cooling distribution subsystem. Each air handler contains a drip pan that is connected to condensate drainage piping.

Ladder Logic

The electrical wiring diagram for this system is shown as a ladder diagram in Figure 2.

At first glance, this diagram may look intimidating, But once you get oriented to how a ladder works, it is really straight forward.

The line voltage section at the top of ladder supplies driven loads such as circulators, air handler blowers, and the boiler. Each load is enabled by the closure of a normal open relay contact.

A transformer that reduces 120 VAC line voltage to 24 VAC separates the upper portion of the ladder from the lower portion. The thermostats and other low voltage components are located in the lower portion of the ladder.

The master selection switch located just below the transformer allows the system to operate in either heating or cooling mode, or be switched off.

Two of the thermostats (T1 and T2) control both heating and cooling. The remaining thermostats are heating only devices. To reduce the size of the drawing, only two of the heating only thermostats are shown. The wiring for the other heating only thermostats would be identical to that for thermostats (T3 and T4).

A basic outdoor reset controller turns the heat pump on and off as necessary to regulate the water temperature in the buffer tank. That water temperature depends on the outdoor temperature, as shown in Figure 3.

The warmer it is outside, the lower the water temperature is in the buffer tank. For example, if the outdoor temperature is 20F, the target water temperature is 95F, but if the outdoor temperature is 40F, the target temperature in the buffer tank is only 85F. The outdoor reset controller operates with a differential. In the situation depicted in Figure 3, the differential is set to 5F. Thus, at an outdoor temperature of 40F, the heat pump turns on when the buffer tank temperature (at the sensor location) drops to 85-5/2 = 82.5F, and turns off the heat pump when the sensor temperature reaches 85+5/2=87.5F. Most outdoor reset controllers allow this differential to be adjusted. Wider differentials provide less cycling of the heat source, but also allow greater variation in the water temperature supplied to the load. 

STEP BY STEP

The text that follows is a description of operation of the system. It is a detailed narration that describes each sequence, beginning with a call for heating or cooling, and ending with that call being satisfied. As you read the description, be sure to cross reference the components mentioned in both the piping schematic in Figure 1 and the electrical diagram in Figure 2. This is where the rubber meets the road when it comes
to understanding the details of how this system operates.

1. Space heating mode:

The mode selection switch (MSS) must be set for heating. This supplies 24 VAC to the (RH) terminals of thermostats (T1) and (T2). It also supplies 24 VAC to the (R) terminals of the heating only thermostats shown (T3,T4, etc.). If a thermostat is set for heating mode and it calls for heat, 24 VAC is switched to the thermostat’s (W) terminal. This supplies 24VAC to the associated heating valve actuator (VA). When the end switch in that valve actuator closes, 24 VAC is also sent to the coil of relay (RH1). One set of normally open contacts (RH1-1) closes to energize circulator (P4), which then operates in proportional differential temperature control mode.

Upon a call for heating from any thermostat, 24 VAC is also supplied to the outdoor temperature setpoint controller (SPC1), and the outdoor reset controller (ORC). If the outdoor temperature is above the minimum value set on (SPC1) the heat pump will be the heat source. In this case, 24 VAC passes through the normally closed contact within (SPC1) and on to the normally open contact in the outdoor reset controller (ORC). The (ORC) measures the outdoor temperature using sensor (S1) and calculates the target temperature for the buffer tank. It measure the temperature in the upper portion of the buffer using sensor (S2). If the buffer tank temperature is too low to supply the load, the normally open contact in the (ORC) closes. This energizes relay coil (RH2). One normally open contact (RH2-1) closes to energize circulator (P2). Another normally open contact (RH2-2) closes to enable the heat pump to operate in heating mode. The heat pump then turns on circulator (P1) and operates under it own internal control logic.

If the outdoor temperature detected by (SPC1) is below the setpoint value, the normally open contact in (SPC1) closes, and the normally open contact opens. This turns off relay (RH2) and disables operation of the heat pump as well as circulator (P2). It also applies 24VAC to the coil of relay (RH3). Normally open contact (RH3-1) closes as a dry contact across the (T T) terminals in the boiler, enabling it to operate. The boiler turns on circulator (P3) and begins operating under its own internal outdoor reset controller settings. It uses these settings, in combination with the outdoor temperature measure by sensor (S4) to calculate the target temperature in the buffer tank. It measures the temperature in the upper portion of the buffer tank using sensor (S3). When necessary, the boiler fires to raise the temperature of the buffer tank sufficiently high to meet the heating load.

2. Space cooling mode:

The mode selection switch (MSS) must be set for cooling. This supplies 24 VAC to the (RC) terminals of thermostats (T1) and (T2). If either of these thermostats is set for cooling mode, and call for cooling, 24 VAC is switched to the thermostat’s (Y) terminal. This supplies 24VAC to the associated cooling relay (RC1) or (RC2). These relays each have 3 sets of normal open contacts. One set of contacts (RC1-1) or (RC2-1) closes to provide line voltage to cooling circulator (P6), which then operates in proportional differential pressure mode.  Another set of contacts (RC1-2) or (RC2-2) closes to provide 24 VAC to the associated cooling zone valves (ZVC1) or (ZVC2). The third set of contacts (RC1-3) or (RC2-3) close to provide line voltage to the associated air handler blowers (AH1) or (AH2). The end switches in the cooling zone valves close when they reach their fully open position. This signals the heat pump to operate in cooling mode. The heat pump then operates based on its own internal control system.

3. Domestic water heating mode:

Whenever there is a demand for domestic hot water of 0.6 gpm or higher, the flow switch inside the tankless electric
water heater closes. This closure applies 240 VAC to the coil of relay (R1). The normally open contacts (R1-1) closes to turn on circulator (P5), which circulates heated water from the upper portion of the buffer tank through the primary side of the
domestic water heat exchanger (HX2). The domestic water leaving (HX2) is preheated to a temperature a few degrees less than the current buffer tank temperature. The domestic water leaving (HX2) passes into the thermostatically controlled tankless water heater, which measures its inlet temperature. The electronics within this heater control electrical power flow to the heat elements based on the necessary temperature rise.  All heated water leaving the tankless heater flows into an ASSE 1017 rated mixing valve to ensure a safe delivery temperature to the fixtures. Whenever the demand for domestic hot water drops below 0.4 gpm, circulator (P5) and the tankless electric water heater are turned off.

Keep in mind that this is just one of several ways in which an AWHP and boiler can be combined. That said, this design contains several synergistic details. It leverages a single thermal mass (the buffer tank) to protect both heat sources against short cycling, and stabilize DHW delivery temperature. The buffer tank also provides hydraulic separation between the various circulators that have piping leading to and from the tank. By using outdoor reset control, this design also keeps the supply water temperature as low as possible to maximize performance of both the heat pump and boiler.

HAVE IT YOUR WAY

This system does a lot and you may not need all the functionality that it provides. Or maybe you prefer to use other equipment such as a geothermal heat pump or zone circulators rather than zone valves. I will leave it as a challenge to readers to sketch out how this system could be modified for the following:

1. Provide only space heating and domestic water heating.

2. Use small circulators rather than valves for zone flow zoning

3. Use a geothermal water-to-water heat pump rather than the AWHP shown.

All of these modifications are possible. It is just a matter of adding or deleting hardware from the piping and electrical diagrams, and then writing a detailed description of operation for the selected hardware. This flexibility is a testament to what is possible using modern hydronics technology. <>

John Siegenthaler, P.E., is a mechanical engineering graduate of Rensselaer Polytechnic Institute and a licensed professional engineer. He has over 34 years experience in designing modern hydronic heating systems. He is also an associate professor emeritus of engineering technology at Mohawk Valley Community College in Utica, NY.

See John at CMPX in Toronto where he is presenting two workshops on March 19 and 20: Unique Hydronic Details For Domestic Water Heating, and Piping and Control Strategies For High Performance Wood-Fired Heating Systems. www.cmpxshow.com/education.cfm

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