Last month we discussed the merits of combining an air-to-water heat pump with a mod/con boiler. This combination provides the following benefits:

• The mod/con boiler can serve as a “peaking tool” to meet peak loads when necessary.
• The heat pump can be smaller in capacity relative to design load. This is helpful in applications where the cooling load is significantly smaller than the design heating load.
• Having a mod/con boiler as a secondary heat source provides full or partial backup if the heat pump is not operating.
• The boiler’s electrical power demand is relatively small compared to the heat pump. This makes it more feasible to operate the heating system from a modestly sized backup generator during power outages.
• In areas where time-of-use electrical rates are available, it would be possible to operate the heat pump when off-peak rates are in effect, and avoid high on-peak rates by using the boiler as the heat source.

This month we’ll look at one way to pipe a dual heat source system to provide space heating, space cooling and domestic water heating. That configuration is shown in Figure 1.

The air-to-water heat pump operates with a solution of inhibited propylene glycol to protect it from freezing under all conditions. The antifreeze portion of the system includes the heat exchanger (HX1) and the single air handler for cooling. Because it is an isolated circuit, the antifreeze-protected portion of the system also includes a combined air/dirt separator, expansion tank, purging valves, pressure gauge and a pressure-relief valve.

When the heat pump is operating in heating mode, a motorized diverter valve is energized to direct heated fluid from the heat pump to heat exchanger (HX1), which in turn delivers it to the buffer tank via circulator (P2).

When the heat pump operates as a chiller, the diverter valve is off and chilled fluid is routed directly to the air handler using circulator (P1). The air handler has been sized to deliver the full cooling capacity of the heat pump when operating at a supply fluid temperature of 45° F. As such, there is no need to involve the buffer tank in cooling mode operation. This simplifies the piping and controls. The air handler is equipped with a drip pan and drain to collect and dispose of condensate.

There are ways to configure controls so the heat pump could swing back and forth between serving as a chiller for cooling, as well as a heat source for domestic hot water. Since both loads could, and likely will, occur simultaneously, there must be a priority control scheme that would not short-cycle the chiller during a relatively short duration DHW draw. When the heat pump is supplying heat to the buffer tank,  the cooling load would not be served.

Simple solution

Given the complications, cost and potential comfort complaints associated with this approach, I’ve elected to keep the system simple by using the mod/con boiler to maintain a suitable buffer tank temperature for domestic water heating when the cooling mode is active. Heat input to the buffer tank is handled by the second stage of the two-stage setpoint controller, which fires the boiler.

In most climates there are times during late spring, early fall and perhaps even mid-summer when cooling is not needed. Just flip the mode switch back to heating and the heat pump takes over maintaining the temperature in the buffer tank suitable for domestic hot water production.

Speaking of domestic hot water … This system uses a subassembly we’ve discussed in several past columns for extracting heat from the thermal mass of the buffer tank and transferring it to domestic water. The hardware is seen on the right side of the buffer tank. The flow switch (FS1) closes its contacts whenever there is a domestic hot water demand of 0.6 gal. per min. or higher.

This energizes the coil of a relay, which switches 120 VAC to circulator (P5). Hot water from the upper portion of the buffer tank is circulated through the primary side of heat exchanger (HX2), while cold domestic water passes in the opposite (counterflow) direction through the secondary side of the heat exchanger.

Because that heat exchanger has been sized for a 5° approach temperature difference under the design domestic hot water flow rate, the DHW leaving the heat exchanger should be no more than 5° cooler than the water at the top of the buffer tank. Thus, for a DHW supply temperature of 115°, the water at the top of the tank only needs to be maintained at 120°. This is within the operating range of the heat pump. It also allows the mod/con boiler to operate in condensing mode, and thus high efficiency, when it is used to maintain temperature in the buffer tank.

Space heating is delivered by a homerun distribution system supplying panel radiators. Each radiator has its own thermostatic valve, and thus operates as an independent zone. The panel radiators have all been sized to deliver design load output at a supply water temperature of 120°. This makes them a good match for both the heat pump and the mod/con boiler. A pressure-regulated, variable-speed circulator operates in constant differential pressure mode to modulate flow through the distribution system as various thermostatic valves on the radiators open and close.

In my opinion, the combination of this relatively low supply water temperature, and the very low thermal mass of modern panel radiators, eliminates the need for a mixing valve to lower the supply water temperature to the radiators under partial load conditions. This simplified the design and reduces cost.

The water temperature in the buffer tank is monitored by a two-stage setpoint controller. The first-stage contacts of this controller turn on the heat pump when the water temperature at the tank sensor (S1) drops to 120°. The heat pump turns off when this temperature increases to 130°. The second-stage contacts turn on the mod/con boiler if the tank sensor temperature drops to 115°.

The boiler is turned off when the sensor reaches 130°. If the tank temperature drops a bit more than normal due to high loads, both the heat pump and the mod/con boiler fire up to recover the tank temperature in short order. This control arrangement also allows the mod/con boiler to automatically assume the full loading if the heat pump fails to operate when the first-stage contacts call for heat — albeit at a slightly lower starting temperature (e.g., 115° rather than 120°).

That’s a description of the “wet” side of the system. All the different components, when properly selected and sized, stand ready to perform together, just like the musicians in an orchestra stand ready to follow the conductor. In hydronic systems, the wire side of the system is the conductor. Next month we’ll discuss how basic off-the-shelf controllers can make all the details integrated into the wet side of this system play well together.