Combined Btus

I recently designed a system for a new home with a water-to-water heat geothermal pump as its main heat source. The intent was to provide space heating and domestic hot water (DHW) with heat from the heat pump.

The space heating distribution system was extensively zoned. There are seven individual zones with a combined design load of only 29,700 Btu/hr. Some of these zones are single panel radiators with design outputs in the range of 1,400 Btu/hr.

To avoid short cycling the heat pump we planned the system around a 119 gallon buffer tank. A portion of the system’s piping is shown in Figure 1.

Figure 1

The heat pump is equipped with a desuperheater, which is a small heat exchanger that receives hot refrigerant gas directly from the compressor. The other side of the desuperheater receives “cool” domestic water from the lower portion of a domestic hot water tank.

Heat moves from the hot refrigerant to the domestic water as they pass through the desuperheater. After exiting the desuperheater the refrigerant flows on to the heat pump’s condenser, where it transfers heat to “system” water.

Warm water leaving the desuperheater flows back to a 38-gallon hot water tank. A small stainless steel circulator inside the heat pump creates the domestic water flow.

Grabbing Heat

When hot refrigerant gas leaves the heat pump’s compressor its temperature is several degrees above the saturation temperature (e.g., the temperature at which the refrigerant gas begins to condense). This “extra” temperature above saturation condition is called superheat, and it has to be dissipated from the refrigerant before condensation can occur.

A typical desuperheater transfers about 10% of the available heat in the refrigerant gas leaving the compressor to the domestic water stream. The remaining heat is transferred to the system water within the heat pump’s condenser.

In heat pumps equipped with a desuperheater domestic water heating occurs whenever the heat pump’s compressor is running, in heating or cooling mode.

Some heat pumps turn off the domestic water circulator if the water temperature leaving the desuperheater climbs to 130F.
In cooling mode, heat removed from the refrigerant by the desuperheater is heat that doesn’t have to be dissipated from the earth loop. This is beneficial in two respects:

1. It’s “free” heat for domestic water, and
2. Lower ground loop temperatures improve the Energy Efficiency Ratio (EER) of the heat pump, lowering operating cost.

In heating mode the energy transferred to domestic water by the desuperheater offsets what would otherwise by required from the electric resistance heating element(s) in the water heater.

The COP associated with heat from the desuperheater might be in the range of 3.0 to 4.0. The COP of electric resistance heating is 1.0.
A given amount of domestic water can be heated through a given temperature rise using a fraction of the electrical energy required by a standard electric water heater. Furthermore, the majority of this heat is coming from the earth, not being absorbed from the air inside the building, as would be true if a standalone heat pump water heater was used.

The percentage of the total DHW load that is covered by the desuperheater depends on many variables:

• What percentage of elapsed time is the heat pump running?
• How much domestic water passes through the electric water heater when the heat pump is running?
• What is the volume of the electric water heater?
• What is the thermostat setting of the electric water heater?
• What is the heating capacity of the heat pump?

Systems with relatively large electric water heaters, relatively high DHW demands, lower temperature settings, and heat pumps not grossly oversized for design heating loads will tend to allow a higher percentage of the total DHW load to be met by the desuperheater.

However, short of monitoring a specific installation, or setting up detailed (minute-by-minute) software simulations with assumed loads and mechanical equipment performance, it’s hard to know what that percentage is.

When the Heat Pump is Off

One thing is certain. When the heat pump is not running, there is no heating contribution from the desuperheater. This is where another complementary strategy comes in.

The system shown in Figure 1 includes an external stainless steel heat exchanger, DHW flow switch, and circulator arranged into what I call an “on demand” water heating assembly. These components can be seen on the right side of the buffer tank.

This assembly allows heat stored in the buffer tank to be added to the coldest domestic water. This transfer occurs whenever there’s a demand for DHW of 0.7 gpm or higher.

As soon as the cold water flow rate entering the heat exchanger reaches 0.7 gpm the flow switch closes its contacts. This turns on the coil of a relay which in turn applies line voltage to a small circulator (labelled as PDHW in Figure 1).

Heated water from the top of the storage tank is routed through the primary side of the heat exchanger and back into the lower portion of the tank.
With proper sizing, the heat exchanger can lift the domestic water temperature to within 5F of the temperature entering the primary side of the heat exchanger.

We used a brazed plate heat exchanger with 40, 5-in. x 12-in. plates. It was sized to raise 50F domestic water to 95F, at a secondary side flow rate of 4 gpm, and with water entering its primary side at an average temperature of 100F, and a flow rate of 10 gpm. This “pre-heated” water then flows into the cold water port of the electric water heater.

The load reduction on the electric element in the tank or the desuperheater depends on the entering temperature of the preheated water. For example, if the heat exchanger raised cold domestic water from 50 to 95 F, and the final desired water temperature leaving the tank was 120F, the heat exchanger would have imparted (95-50)/(120-50) = 0.64,or 64% of the total energy required to heat DHW.

It would be possible to increase this percentage by increasing the water temperature in the buffer tank. However, the warmer the buffer tank temperature, the lower the heat pump’s COP.

Again, the “optimal” compromise between DHW preheating fraction and heat pump COP are hard to determine theoretically. Small incremental adjustments of the settings, with some form of heat pump performance monitoring will hopefully be possible in the future to help determine optimal settings.

This on-demand domestic water preheating assembly could be used on hydronic heat pump systems where no desuperheater is present. One example is an air-to-water heat pump. It could also be used in combination with other renewable heat sources with associated thermal storage tanks. Examples include solar thermal collectors, cordwood gasification boilers and pellet boilers.

The Last Resort

The “backup” heat source in this system is a 4,500-watt electric element in the 38 gallon water heater. That tank is rated to provide a first hour DHW delivery of 54 gallon per hour, and a recovery of 27 gallons per hour assuming a 50 to 120F temperature rise, and thus absent any preheating effect.

This is sufficient to assume the full domestic water heating load if the heat pump is off and awaiting servicing.

Domestic water heating takes advantage of the higher efficiency of contemporary heat sources operating at lower temperatures. If you’re working with hydronic heat pumps for space heating and cooling, and the building needs domestic hot water, you should leverage this opportunity.

John Siegenthaler, P.E., is a mechanical engineering graduate of Rensselaer Polytechnic Institute and a licensed professional engineer. He has more than 40 years experience in designing modern hydronic heating systems. Siegenthaler’s latest book is Heating with Renewable Energy (see www.hydronicpros.com for more information).