Heat Pumps 101

About Air Source Heat Pumps

How do air source heat pumps work?

Air source heat pumps (ASHP) are electric appliances that provide heating and cooling by moving heat into a building (for heating) or out of a building (for cooling).

Heat pumps do not create heat through burning fossil fuels like conventional heating systems. Instead, heat pumps transfer heat from one place to another by using a refrigerant that absorbs heat from the air and moves it in or out of a building. This is similar to the way that a refrigerator or air conditioner works—except that it can move heat in either direction to provide both heating and cooling.

Since it takes far less energy to move heat than it does to create heat, air source heat pumps are much more efficient: while a fossil fuel heating system is 80-90% efficient (wasting 10-20% of every dollar you spend on oil, gas, or propane), an air source heat pump can be 200-400% efficient, providing 2-4 units of heat for every unit of electricity you pay for.

Air source heat pumps are considered to be clean heating and cooling technologies because the source of heat used for your home comes from the outdoor air as opposed to burning fossil fuels. The electricity from Hudson Light and Power that will power your heat pump is 90% zero-emissions, allowing you to reduce your carbon footprint by over 90%.

ASHPs are installed as either supplemental systems (displacing some but not all of the heating or cooling from the existing heating or cooling system for select areas in your home) or as primary systems completely replacing the existing system (or keeping the existing system in place but only for backup). Special controls are available to coordinate with your existing system.

Types of Air Source Heat Pumps

There are two types of ASHP systems commonly installed in homes in Massachusetts:

Ductless air source heat pumps can provide heating and air conditioning without the need for central ductwork. Each ductless system includes one outdoor unit connected to one (single-zone) or more (multi-zone) indoor wall, floor, or ceiling air distribution units. Ductless ASHPs are often referred to as ductless mini-splits.

From left to right: (1) a ductless outdoor unit, (2) a standard wall-mounted indoor unit, (3) a floor-mounted indoor unit, and (4) a ceiling cassette indoor unit.

Ductless air source heat pumps are the most efficient air-source systems and can be installed as a primary source of heating and cooling or installed to supplement existing systems. These supplemental applications could include, for example:

Installing indoor units in the most frequently used larger rooms like family rooms or master bedrooms to displace heating or cooling from your existing system
Installing indoor units in new additions as the sole/primary source of heating or cooling
Placing indoor units in hot or cold spots of your home that can’t seem to be fixed with better insulation or more invasive modifications to your home

Ductless indoor units come with remote or wireless controls (or smartphone apps!) that give you control over each unit and allow you to use them for heating, cooling, dehumidification, or as a fan. Because each indoor unit can be controlled individually (forming independent heating/cooling "zones"), you can heat or cool different zones in your home to different temperatures depending on personal comfort preferences—or reduce your energy use even more by turning down the unit in zones that are not being used.

Ducted air source heat pumps have an outdoor unit that is connected to an air handler (similar to a furnace or central air conditioner) in a building's ductwork, which is used to heat or cool the building. Ducted (also known as “central” or “unitary”) ASHPs are not much different from central air conditioners, except that they provide heating and cooling in a single system (and they don't really look any different from afar!). Ducted ASHPs can work with your home’s existing ductwork, though some modifications may be necessary to adapt it from being suited for a furnace or central AC to being suited for a heat pump.

Ducted air source heat pumps can be installed in a “hybrid” or “dual-fuel” configuration, where the heat pump is directly matched with an existing or new furnace. This configuration automatically switches to the lower-cost heating system based on outdoor air temperature and can be a great option for homeowners with existing ductwork that want to keep the existing furnace in place while saving on heating and cooling. When temperatures drop below a certain level, the furnace kicks in, and in the summer, the heat pump provides central air conditioning.

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About Ground Source Heat Pumps

How do ground source heat pumps work?

Ground source (or geothermal) heat pumps (GSHP) are electric appliances that provide heating and cooling by moving heat into a building (for heating) or out of a building (for cooling).

Unlike air conditioners and air source heat pumps, ground source heat pumps move heat in and out of the ground as opposed to the outdoor air by using piping buried underground. This "ground loop" uses water to absorb heat from the ground and an indoor heat pump unit to extract heat from the water and distribute it throughout your building.

Since it takes far less energy to move heat than it does to create heat, ground source heat pumps are much more efficient: while a fossil fuel heating system is 80-90% efficient (wasting 10-20% of every dollar you spend on oil, gas, or propane), a ground source heat pump can be 350-500% efficient, providing 3.5-5 units of heat for every unit of electricity you pay for. Because the ground is more stable in temperature year-round than the outdoor air, ground source heat pumps are more efficient than air source heat pumps and air conditioners at heating and cooling, making ground source heat pumps the most efficient heating and cooling technology available.

Ground source heat pumps are considered to be clean heating and cooling technologies because the source of heat used for your home comes from the ground (by way of the sun) as opposed to burning fossil fuels. The electricity from Hudson Light and Power that will power your heat pump is 90% zero-emissions, allowing you to reduce your carbon footprint by over 90%.

Ground source heat pumps serve as a whole-home heating and cooling solution, replacing your existing central heating system (and air conditioning system) and integrating with the existing distribution system. If you're interested in eliminating fossil fuel combustion from your home entirely, ground source can offer the cleanest, most efficient solution available.

Types of Ground Source Heat Pumps

There are many models and configurations of ground source heat pump systems that could meet the needs of your building. The most common variations are related to the outdoor (ground loop) and indoor components.

Ground Loop

The ground loop serves as a heat exchange surface for a ground source heat pump system. Water running through the ground loop absorbs heat from the ground in the winter and transfers heat into the ground in the summer. Most homes in Hudson and Stow will typically need several hundred to a thousand feet of ground loop piping to be able to provide sufficient heat during the coldest days of the year.

Ground loops are typically divided into "closed" and "open" loop systems:

Closed loop systems circulate a mixture of water and environmentally-friendly antifreeze throughout the length of piping to transfer heat into and out of the ground. Since the system is "closed" off, none of this heat transfer fluid is exposed to the environment. The piping is made out of high-density polyethylene (HDPE) that is warrantied against leaks for decades. Closed loop systems are most commonly installed vertically, where piping is run through a few boreholes drilled several hundred feet into the ground, or horizontally, where piping is run through trenches approximately 6 feet deep and several hundred feet long. Vertical closed loop systems account for the majority of systems installed in Massachusetts.
Open loop systems can be installed where there is an available source of groundwater accessible from the property. An open loop system will pump groundwater into the piping for heat extraction before being safely returned into the ground. Open loop systems are typically cheaper to install and more efficient (due to the more consistent year-round temperature of groundwater), though they require additional pumping energy that can reduce efficiency gains and a readily-available source of clean groundwater. Additional permitting or environmental review may also be required.

From left to right: (1) a vertical closed loop diagram, (2) a horizontal closed loop diagram, and (3) an open loop (standing column well) diagram.

Indoor Unit

Geothermal systems also have an indoor heat pump unit that transfers heat out of or into the fluid in the ground loop. The most common options are "water-to-air" or "water-to-water," which are selected based on your building's existing heating distribution system:

Water-to-air heat pumps are connected to a central air handler and use your home’s existing ductwork to distribute heating and cooling throughout the home. Some changes to your building's ductwork may be necessary to make it suitable for a geothermal system. Water-to-air systems are more common in Massachusetts, as heating and cooling can be provided through ductwork.
Water-to-water heat pumps are connected to your existing hot water distribution system to provide heating. Not all existing hot water distribution systems will be able to work with a ground source heat pump: heat pumps do not efficiently heat water to the same temperature as hot water boilers, so a ground source heat pump will work well with radiant floor heating but will not be able to integrate with an existing radiator and many hot water baseboard distribution systems. An air handler/ductwork or a chilled water system would be needed to provide cooling.