A new piping concept and control wiring challenge.  

Figure 1.

I recently got a call from a local plumber. He had installed a geothermal water-to-water heat pump at his home to replace an aging oil-fired boiler. This was part of a larger retrofit in which he also would be replacing fin-tube baseboard with three zones of radiant floor heating. He planned to equip each of those zones with chilled-water air handlers for cooling. When he called, the system piping was largely completed, but the control wiring - not so much.

Given the circumstances, the only type of floor heating that was practical was a tube-and-plate system under the floor. Anyone who has used this approach in a retrofit application knows that it’s usually not easy. In most cases you have to maneuver the tubing and heat diffusion plates around existing wiring, plumbing, drier vents, cross bracing, nail points penetrating down through the subfloor and other assorted obstacles. You also have the potential thermal limitations imposed by the existing subfloor, underlayment and finish floor.

To add a further challenge, a typical geothermal water-to-water heat pump can deliver water temperatures only up to about 120º F. This can really limit the heat output from a tube-and-plate system, especially if the floor coverings have much thermal resistance. In this case, installing the tubing and plates at 8-in. spacing, and assuming a finish floor resistance of R-1, would allow a radiant heating circuit to release 15.3 Btu/hr/ft2 when the average water temperature in the circuit is 115º.

Although there are houses that can be heated by this rate of heat output, they tend to be newer, better-insulated houses, rather than houses that are several decades old.

Understanding this limitation, the plumber came up with an interesting idea: Why not supplement the heat output of the floors by also circulating warm water through the air handlers? He understood that the greater the total surface area of the heat emitters, the lower the system water temperature could be for a given rate of heat output.

Figure 2.

Making it happen

So here’s the “logic” that got hard-piped into the system: In cooling mode, chilled water from the buffer tank would be supplied only to the air handlers, which are equipped with drip pans to handle the resulting condensation. However, in heating mode, warm water would be sent to both the floors and the air handlers.

The piping that could allow this is shown (in heating mode) in Figure 1. The upper three zone valves all lead to the radiant floor manifold stations. The lower three zone valves all lead to air handlers.

The heat pump adds heat to the buffer tank based on outdoor reset control. When any of the three zone thermostats call for heat, the zone valve for the radiant floor panel in that zone, as well as the zone valve for the air handler in that zone, need to open. Heated water from the buffer tank can flow through the upper zone valves, as well as around the “loop” to the lower zone valves. The three-way diverter valve is de-energized in this mode and directs flow from the AB port to the B port as shown. The A port of the diverter valve is fully closed.

Figure 2 shows the system as it would operate in cooling mode.

The three-way diverter valve is now energized and directs the flow of chilled water from the AB port to the A port. The B port is closed. This, as well as the check valve, prevents chilled water from flowing (or migrating) to the upper zone valves. Chilled water is only available to the lower zone valves leading to the air handlers.

Figure 3.

Now comes the wiring

The piping to create the desired operating logic is relatively straightforward; the control wiring is a bit more of a challenge.

Let’s start with temperature control in the buffer tank. During heating, the water temperature near the middle of the buffer tank is monitored by a standard outdoor reset controller. This controller operates the heat pump, independently of the distribution system, keeping its temperature within a few degrees of a calculated target temperature, as shown in Figure 3.

When the system is switched into cooling, 24 VAC is sent to energize a setpoint controller. This controller continuously monitors the tank temperature and operates the heat pump to keep that temperature between an upper limit of 60º and a lower limit of 44º.

Next we’ll look at how the thermostats and zone valves can be coordinated with the desired operating logic. To do this you need to look at the overall system wiring schematic shown in Figure 4 (page 16).

The heat/cool selector switch at the upper left of the schematic determines how 24 VAC power is passed to the thermostats. When set to heating, 24 VAC is passed to the RH terminals on each thermostat. When set to cooling, 24 VAC is passed to the RC terminal on each thermostat. Keeping the RH and RC terminals separated prevents the possibility that a zone valve leading to the radiant floor would be turned on should someone decide to flip one of the thermostats from cooling to heating during the summer - just to see what might happen. People do this, you know.

This selector switch also determines which of the temperature controllers is active. In the heating mode, the outdoor reset controller is powered on. In cooling mode, the setpoint controller is powered on.

When they receive 24 VAC power, each temperature controller boots up, takes a look at the tank temperature and decides if the heat pump needs to operate such that the tank is “standing ready” to deliver either heated or chilled water as soon as one of the zones needs it.

If the tank needs to be warmed up, the outdoor reset controller closes a set of isolated contacts that completes a circuit between the R and Y terminals of the heat pump. This turns on the compressor, allowing the heat pump to transfer heat to the buffer tank.

If the tank needs to be cooled, the heat pump’s compressor and reversing valve are activated.

Putting the selector switch in cooling mode also powers on the three-way diverter valve that now directs chilled water to the air handlers through the lower three zones valves in Figure 2.

The earth loop circulators and the circulator between the heat pump and buffer tank are not shown in Figure 4. They are wired to internal relays within the heat pump and operate whenever the compressor is running.

Figure 4.

Passing the baton

Take a look at the low voltage wiring labelled “zone 1” in Figure 4. When the thermostat calls for heat, 24 VAC is passed from the thermostat’s RH terminal to its W terminal and on to zone valve ZV1. This zone valve controls flow to the zone 1 radiant panel. When ZV1 is fully open, its end switch closes and passes 24 VAC to zone valve ZV2, supplying the air handler associated with that zone. The end switch in ZV2 then closes to provide 24 VAC to the circulator relay, which supplies line voltage to the pressure-regulated, variable-speed distribution circulator.

Using motorized zone valves, this all happens in a few seconds.

In cooling mode, 24 VAC is passed from the thermostat’s RC terminal to its Y terminal and on to ZV2. Twenty-four VAC feedback is prevented by the open end switch in ZV1. The end switch in ZV2 closes to pass 24 VAC to the circulator relay and turn it on.

The G terminal on each thermostat is used to operate the blower in the air handler. If the thermostat’s fan switch is set to “auto,” the blower will operate whenever a call for heating or cooling is made in the zone. If the fan switch is set to “on,” the blower will run continuously.

Design considerations

If you decide to build a system like this, keep the following points in mind:

  • The single variable-speed circulator needs to be sized for the total flow and head when all the floor heating and air handlers are operating. So does the three-way diverter valve. Try to keep the flow resistance of the supply header, diverter valve and other piping leading up to the zone valves as low as possible. Set the variable-speed distribution circulator for constant differential pressure operation. This will minimize flow rate variations, depending on which distribution circuits are operating.

  • Be sure all piping and piping components that carry chilled water are insulated and vapor-sealed. Also be sure to use a buffer tank with foam insulation. Insulate and vapor-seal all fittings, circulator volutes, air separators, etc. Don’t insulate the motor assembly on the circulators or zone valves.

  • Be sure both temperature sensors are making good contact with the inside of the thermal well in the buffer tank. Use thermal grease between the sensors and well body.

  • The air temperature leaving the fan coils is not going to be very warm, during partial load conditions, when the water entering the coil may only be 80° or 90°. Because of this, it’s important to mix the discharge air with room air above the occupied space. High wall registers or ceiling diffusers with good horizontal throw are recommended.

  • Because the buffer tank is being used for both heating and cooling, its piping can only be “coordinated” with temperature stratification inside the tank in one of these modes. In cooler climates, where heating loads dominate, pipe the tank as shown in Figures 1 and 2. In deep Southern climates, where cooling dominates, reverse the up- and low-piping connections on both sides of the tank to optimize stratification for cooling.

    So ends another “crossword puzzle” for hydronic piping and wiring. In 33 years of designing hydronic systems, I had never seen this scenario before. It certainly was creative thinking by our local plumber. It’s always a challenge to synthesize the hard-wired logic needed to make these new variants work as expected. It’s also a testimony to the versatility of hydronics that new solutions are always out there waiting for those special applications.

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