Heat Pumps for Hydronics

Until recently, hydronic contractors have managed to avoid heat pumps with few professional consequences. But heat pumps are an important part of the future of hydronic heating and cooling (yes, cooling). Extracting heat from the ground is a coming technology, and it falls right into the laps of hydronic contractors. It's a business made up of familiar pumps and tubing, yet the technology depends upon ground source heat pumps.

Heat pumps have a reputation with some for being difficult to understand. But they're actually quite simple. As the name suggests, a heat pump moves heat from one place to another. And so does a boiler, when you think about it. So let’s look at boiler heat transfer for a moment, because in many ways it's the same.

Water is pumped through the boiler, where it absorbs heat. A law of science says that heat naturally moves to cool until everything is the same temperature. The water carries heat to the living space. There, the water is hotter than the space, so again the principle that heat moves to cool applies. The now-cool water then returns to the boiler and picks up more heat.

A heat pump is also like an air conditioner. A conventional air conditioner has two heat transfer coils. The indoor coil absorbs heat from the living space into refrigerant. The living space cools as the refrigerant warms.

The refrigerant then moves to an outdoor coil to give off the heat to the outside air. But how can that happen in the summer when the outside air is 90ºF and the indoors is 75ºF? What about the rule that heat moves to cool?

Here's the trick. The air conditioner changes the refrigerant to a gas, and then compresses the gas. Another law of science says that as pressure increases, so does temperature. The compressed gas is hotter than any summer day. The old principle of heat moves to cool still applies. Hot compressed gas cools when it gives off heat to the air. The now-cooler refrigerant is decompressed, returns to its liquid form, and moves indoors to absorb more heat.

A special heat pump feature is that it can reverse the air conditioning process and become a heating unit. In heating mode, refrigerant in the outdoor coil absorbs heat from the outside air, even if that air is freezing or colder. The temperature is increased by compressing the gas refrigerant, still the principle that increasing pressure increases temperature. Inside, the refrigerant gives off its heat to the cooler inside air. And so on. This is a conventional air-source heat pump.

A ground-source heat pump simply uses the earth below instead of outside air as the heat source. This includes soil, rocks and bodies of water. The temperature deeper than about seven feet is constant, 50°F to 60ºF, no matter the season or location. As with the air source heat pump, refrigerant picks up (relative) heat from the ground, and concentrates it to heat the occupied space or living space. The process can be reversed for cooling.

The resultant heat or cooling can be distributed throughout the building with fan coils and ductwork. Or the heat can be stored in a water tank and used hydronically.

Although we're talking about free energy, contractors should be cautious about promising overall energy cost savings. Sometimes system operation costs can be high due to the number of controls, pumps and air handlers. These can eat up savings even though the ground source heat pumps may be operating at 250% to 300% efficiency.

Yet a selling point is that we are using renewable energy rather than fossil fuels. The best cost savings of course come when making comparisons to the more expensive fuels in a geographical area.

And now for some terms:

• Geoexchange means using the earth as a source of heat and cooling. It can be used interchangeably with "ground source."
• Geothermal is also used interchangeably with ground source. But it more precisely means tapping into unusually hot earth temperatures.
• The refrigerant systems described above are called closed loop. Water, too, can be circulated through a closed loop system. There are also open loop systems, where fresh water is pumped from the ground through the heat pump, and then allowed to return to the ground. Air-to-air means heat is taken from the air in one place and put into the air in another place. There are also water-to-air and water-to-water systems. The best energy savings may be water-to-air. On the other hand, with water-to-water, the same system that provides radiant heat in the winter can feed chilled water to a small fan coil in the summer.
• There are three terms that come up commonly for air-source heat pumps, where the outside temperature can drop low enough that heat pump alone cannot provide enough heat. This is around freezing for ordinary heat pumps, and around zero for new technology. A big advantage of ground source heat pumps is that there is a constant source temperature!

Auxiliary heat is electric strip heat that automatically comes on to supplement the heat pump. Emergency heat is the same electric strip heat as auxiliary, but it is enabled manually in place of the heat pump for a quick temperature increase of more than several degrees, or when the heat pump isn’t working.

Emergency heat is known for being expensive. Dual fuel means that there is a conventional furnace in addition to the heat pump. The more efficient heat pump does most of the heating. It accounts for enough fuel savings that having both a heat pump and a furnace makes sense. The thermostat automatically changes equipment based on heating demand.

Ground source heat pumps are the coming thing for energy efficiency. And they are a natural step for expanding the hydronics industry.

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