AEL&P Heat Pump Information

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AEL&P strives to provide information that allows our customers to make well informed decisions related to the efficient use of electricity, which allows us to get the greatest value out of Juneau’s electric system. Many of the energy questions we get at AEL&P relate to space heating. Most space heat in Juneau is provided by heating oil or electric resistance, but recently more and more customers are choosing to install heat pumps.

While the information on this page goes into some detail about heat pumps, and you can always call AEL&P to ask about heat pumps or any other energy-related questions, Alaska Heat Smart is the best resource in Juneau for folks interested in learning more about heat pumps. Alaska Heat Smart is an independent non-profit organization that is funded in part by a CBJ grant to assist Juneau residents who are interested in heat pumps. They offer home assessments and make recommendations about what type of heat pump system to pursue, how much installation might cost, and what type of energy savings the heat pump will provide. They even offer assistance with obtaining quotes from heat pump installers.

Other organizations that provide useful information about heat pumps are:

A heat pump is an electric appliance that uses a refrigeration cycle to provide space heating or cooling. Over the last decade, advances in the technology have made heat pumps more suitable for use in Juneau’s cold, damp climate. The primary benefit of using a heat pump for heating is the ability of a heat pump to generate large amounts of heat with little energy input, meaning you get 2-3 times as much heat per kilowatt-hour of electricity from a heat pump than you do from an electric resistance heater like an electric boiler, baseboard, or plug-in space heater.

A heat pump that you can install at your home or business will extract heat from cold air, water, or soil, and move that heat into a building. Heat pumps work because of the refrigeration cycle. When a refrigerant expands, it cools to very low temperatures, and when the refrigerant is compressed, it reaches high temperatures.

Heat pumps have a “source” and a “load” side. The source is the substrate – air, water, or soil – with which the heat pump uses to exchange heat, either pulling heat from the source or exhausting heat to the source, depending on whether the heat pump is in heating or cooling mode. The load is whatever the heat pump is intending to heat or cool – your home, for example.

Every household refrigerator is a kind of heat pump. The load is the inside of the refrigerator, where evaporator coils inside the fridge allow extremely cold refrigerant to absorb heat. That refrigerant is then compressed to a high temperature, and hot refrigerant passes through condenser coils to exhaust heat into the source, which is usually the kitchen. From the condenser coils, the hot, liquid refrigerant then passes through an expansion valve and evaporates into a gas, lowering the refrigerant temperature and starting the cycle over again.

Figure 1: Schematic of the refrigeration cycle.

 

The heat pump in a refrigerator only operates in one direction, but the types of heat pumps used for buildings have a reversing valve that allows the cycle to move in either direction, meaning the heat pump can provide heating and air conditioning.

Types of Heat Pumps

Heat pumps are classified by the source from which they extract or exhaust heat. If the heat is exchanged with soil or groundwater via pipes installed in ditches or wells, that is called a ground-source heat pump. If the heat is exchanged with seawater, it is a seawater heat pump, and seawater heat pumps are essentially ground source heat pumps with additional corrosion protection. The most common type of heat pump used in Juneau for space heating is an air-source heat pump, where heat is either extracted from or exhausted to the outside air depending on whether the heat pump is in heating or cooling mode.

Ground Source Heat Pumps

Ground source heat pumps (GSHPs) rely on pipes buried in the ground, typically installed either vertically as long pipes moving up and down one or more wells, or they may be installed horizontally as coiled piping in one or more ditches. These pipes connect to the heat pump itself, which looks like a large metal box. Most systems are “closed loop” systems, meaning they simply circulate water and antifreeze through the source piping. Some systems are “open loop” systems, which pull groundwater or seawater directly from one location and reinject it in another location. Whether open or closed, this piping system is referred to as the “source loop,” and it transports heat either from the building to the ground, or from the ground to the building.

Figure 2: Schematic of some different options for installing ground source heat pump source loop piping.

 

Inside the heat pump itself, the source loop connects to a heat exchanger, where refrigerant can either exhaust heat to or extract heat from the fluid in the source loop. This refrigerant then moves through the refrigeration cycle inside of the heat pump, and the GSHP will commonly use either another heat exchanger on the load side to heat water that can circulate through a building to heat or cool it, or it can use a blower motor to blow air over a coil to provide heating or cooling through air ducts. If the GSHP is supplying a hydronic system, it is often called a water-to-water heat pump, and if it is supplying an air system, it is called a water-to-air heat pump.

Air Source Heat Pumps

Air source heat pumps (ASHPs) operate much like a standard refrigerator with larger components. They have a compressor, fan, coil, and sensors located outside. Refrigerant lines and a power and control cable connect the outside unit to an indoor unit that also houses a fan, coil, and sensors. When in heating mode, air source heat pumps extract heat from the cold air outside. The refrigerant is then compressed, which increases its temperature, and circulated to an indoor unit where, most commonly, a fan blows over a coil that has the hot refrigerant circulating through it, allowing the air moving over the coil to be heated and pushed into the room. This most common ASHP is called an air-to-air heat pump, or a mini-split. Some ASHPs will have a heat exchanger where the refrigerant will transfer heat to a hydronic heating system like in-floor radiant heat, and these are called air-to-water heat pumps.

Figure 3: Illustration of an air-source heat pump outdoor and indoor unit. The unit depicted here is commonly called a "mini-split" heat pump. A single outdoor unit can supply up to five indoor units.

 

Types of Installations

Choosing which type of heat pump to install depends on a variety of factors and involves determining what source will provide heat, how the heat pump will distribute heat, and how much of the building will the heat pump serve. On the source side, because of the expense and space required for a GSHP, most people will choose an ASHP. On the load side, heat pumps cannot produce water that is as hot as a boiler produces, meaning heat pumps cannot replace a boiler directly without upgrading the hot water baseboards in homes that use those, so most people will choose air systems. This means the most common type of heat pump installation in Juneau is a ductless air-to-air system, where the indoor unit of the heat pump operates like a room heater, blowing hot air into the room without relying on air ducts.

Split Systems

All “split system” heat pumps are ASHPs. They use the outside air as their source, and the outdoor unit connects to an indoor unit with refrigerant tubing and power and control wiring. The outdoor unit can connect to as many as five different indoor “heads,” and the most common type of indoor head is similar to the one shown in Figure 3.  These systems are the primary type found in Juneau, and while they can be set up to heat an entire home, they are typically installed to complement a central heating system like a boiler or electric baseboards, where the ASHP provides heat in a primary living space and allows that heat to travel into other spaces. This is much like the way people in Juneau have used Monitor or Toyo oil stoves, and when the heads blow air straight into a room in this fashion, they are part of a “ductless” heat pump.

In addition to the wall-mounted head illustrated in Figure 3, indoor units are available that stand on the floor against a wall, mount in the ceiling, or “slim duct” units can be installed in attics or crawlspaces and connected to insulated air ducts to bring heat to multiple rooms.

Ducted Systems

Some ASHPs and GSHPs can act as a direct replacement for an air furnace, supplying hot or cold air to air ducts that travel throughout a home. The air-source ducted systems typically have a maximum heat output that is too low for large or inefficient homes in Juneau. Ducted air-source systems could be useful for smaller, efficient homes or mobile homes. Ground-source ducted heat pumps, also called “water-to-air” heat pumps, can supply levels of heat that are adequate for most homes in Juneau. In fact, AELP’s office in Lemon Creek uses a series of these types of heat pumps for heating and cooling.

Hydronic Heaters

A hydronic heating system is one that uses water to distribute the heat around a home. The most common hydronic heating system in Juneau is a hot water baseboard, which relies on water heated to around 180F circulating from a hot water boiler fueled by oil, electricity, or propane. Many homes also use in-floor radiant heat, which uses water heated to temperatures ranging from 90-130F. Less commonly, people may have low-temperature baseboards or panel radiators that use water temperatures that are similar to what is used in radiant floor systems.

Commercially available heat pumps cannot supply water temperatures greater than around 130F, which makes heat pumps unsuitable for supplying the most common hot water baseboards in use in Juneau. Homes with hydronic heating systems that only require water temps in the 90-130F range can use either air-to-water or water-to-water heat pumps. However, since few if any air-to-water heat pumps are currently available for purchase, for cost reasons, most people with hydronic heating systems choose to install ductless heat pumps to complement their existing heating system, rather than replacing entirely with a whole-home heat pump system.

GSHPs are available to supply low-temperature hydronic heating systems. These function by heating a hot water storage tank from which water is circulated through the home’s hydronic distribution system.

Hybrid Systems

A unique system that some homes in Juneau currently use is an ASHP and electric boiler hybrid system, which relies on an ASHP for nearly all of a home’s heat, but switches to an electric boiler when the temperatures get too low for the ASHP to keep up on its own. This option makes it easier for someone to convert their whole-home hydronic heating system to be heated primarily with a heat pump while retaining the security of a backup, central heating system.

Install Location

There are many issues to consider that depend on the type of heat pump being installed when choosing where to install various components of a heat pump.

For GSHPs, any buried piping will have a finite service life, so it is wise to choose a location that will remain accessible for the life of the equipment. Building a shed or outbuilding on top of where you install the source loop will make it much more difficult to repair or replace the source loop at its end of life. When it comes to the interior equipment for a GSHP, choosing a location is likely relatively simple – install it where it connects to the heat distribution system, whether air ducts or hydronic piping.

For ASHPs, the outdoor units need to have good airflow around them, be able to drain water freely, and they should be in a place that will not get buried in snow. Many homes place outdoor units under eaves, near an exterior wall with their own small roof structure over them, or under decks where there is ample room around the unit to ensure adequate airflow. Placement of indoor units varies with the type of indoor unit, but they need to sit as close as possible to the outdoor unit to lower installation costs, reduce losses from refrigerant piping, and ensure proper operation. This may complicate installation of multi-head ASHPs, where serving multiple spaces in the home will limit where the outdoor unit may be installed while still operating within maximum distance to all indoor units.

When installing an ASHP in conjunction with an existing, central heating source, the placement of the indoor unit may also affect how well the two systems complement one another. For example, in a home with a central oil or electric boiler and a single ASHP indoor unit serving the main living area, it is important to consider how the indoor unit will direct hot air into the space. Will it direct heat efficiently across a wide area? Will it point at the thermostat for the central heating system? Will it direct heat into ancillary spaces, allowing the heat pump to serve as much of the home as possible? Asking questions like these when considering where to locate the indoor unit will help ensure the heat pump provides as much comfort and value as possible.

Electrical Requirements

Heat pumps are electrically powered heating systems. While they do not require the same type of large circuits that an electric boiler does, each home’s electric system needs to be evaluated to ensure it has capacity to install a heat pump. In Juneau, most heat pump installers do not include the cost of installing the electric circuit to power the heat pump in their bids, and a homeowner must solicit separate bids directly from an electrician for that work. Electrical costs can vary substantially based on site-specific conditions such as the distance between the electric panel and the equipment, the potential need for an upgraded electric service, and the obstacles between the electric panel and the equipment. When installing ASHP outdoor units, increasing the distance of the heat pump from the electric panel can increase the cost to install the electric circuit, but that cost impact is typically less than increasing the distance between the outdoor unit and indoor unit of the heat pump. Most installers will recommend prioritizing the location of the outdoor unit relative to the indoor units ahead of its location relative to the electric panel.

Heat Pump Sizing

No homeowner wants to wake up on a cold day in Juneau to find their heating system can’t adequately heat their house. When thinking about a heating system, it’s important to choose a system that can provide enough heat on the coldest days to keep the house warm. Determining how much heat is required to keep your house sufficiently warm on the coldest day depends on the size of the home, how well insulated the home is, and how air-tight the house is. Engineers and many heating contractors can help determine the rate of heat output required to keep each room in the house at room temperature during the coldest temperatures we expect in Juneau. Adding these numbers together provides the total rate of heat output the heating system will need to supply.

Once you know the total heating requirement, the next step is to determine where that heat will come from. With a GSHP, source loop will need to be shown to exchange heat with the source at the maximum rate required by the home after accounting for some efficiency losses and a safety factor. That will determine how many wells and how deep to drill them, or how many coiled loops of piping in a ditch, etc. Things like the anticipated temperature of the groundwater on the coldest expected day will play a role in the calculation, too. The central GSHP itself will also need to have an adequate rating to transfer heat from the source loop into the home.

With an ASHP, the equipment manufacturer publishes engineering manuals that include a list of heating and cooling output levels at a few different outside temperatures. Most ASHP data sheets list the max heating output of the outdoor units at around 40F by default, and the engineering manual includes rated heat output at lower temperatures, as low as 0F or -5F. These manuals also have long tables that list the heat output of indoor units when connected by themselves or as combinations of indoor units to the different outdoor units. If planning to heat an entire home with ASHPs, it’s important to verify that the heat output at low temperatures of both the outdoor units and the exact combination of indoor units that will be connected to the outdoor unit are adequate to supply sufficient heat not only to the whole home, but also to each area to be served by the indoor units.

Heat pumps offer very low operating costs. Instead of converting electric energy directly into heat like electric resistance heaters do, a heat pump uses electricity to drive pumps, compressors, sensors, fans, etc., and heat is supplied by the source, whether that is groundwater, air, seawater, etc. This means that the heat pump can supply anywhere from 1.5 to 5 kilowatt-hours (kWh) of heat for every 1 kWh of electricity consumed by the heat pump. This measurement of the amount of heat energy the heat pump supplies divided by the electric energy consumed is called the Coefficient of Performance (COP). On average in Juneau, GSHPs tend to have a COP around 3.5 and ASHPs tend to have a COP around 2.5, and this means that electric resistance heat has a cost that is 3.5 times greater than a GSHP and 2.5 times greater than an ASHP.

While operating costs for heat pumps in Juneau are comparatively low, installation costs can range from around $5,000 to as much as $50,000 depending on the type of heat pump installed and whether it provides only part of a home’s heat or is the sole heating source. Since most people install ASHPs that serve only part of the home, installation costs tend to be on the low end of the range listed, which leads to very short payback periods that average around 7 years.

When operating a heat pump as the sole heating source for a home, it will behave similarly to other common heating systems with the added benefit that it allows for cooling. However, when operating a heat pump in conjunction with a different, central heating system, the settings on the heat pump and all other heating systems that are available need to work together to ensure comfort at the lowest possible cost.

Many readily available thermostats are compatible with central heat pumps like a GSHP, and this makes their operation and programming familiar to anyone who uses a hot water boiler or furnace system. Heat pumps have more complex components to manage their operation, so there are more wires involved to provide different signals to the heat pump, but the thermostat’s functions look similar.

Mini-split ASHPs, which are the most common heat pumps used in Juneau, usually come with a control that looks a bit like a remote control with a digital display. These controls have complex options and instruction booklets that can be difficult to understand, but they offer the ability to program a schedule of operation. It is wise to ask the heat pump installer for a primer on how to use the control, or perhaps look for videos from the manufacturer or on YouTube that demonstrate how to use the heat pump control.

One of the biggest differences between the thermostat control for a heat pump versus a standard heating system is the ability for the heat pump to offer cooling in addition to heating. Most heat pump thermostats will include an “auto” mode that allows the heat pump to automatically switch between heating and cooling mode depending on the room temperature. Operating in this mode presents the risk that the heat pump may oscillate between heating and cooling, essentially battling itself to maintain temperature. For this reason, it is wise to either operate only in heating or cooling mode at any given time, or to be sure that there is a wide gap between the heating setpoint and the cooling setpoint. For example, if the heating setpoint is 65F, the cooling setpoint should not be lower than 68F to ensure that heat pump, when in a heating cycle, does not unintentionally heat the space to 68F and cause the heat pump to move directly from a heating cycle to a cooling cycle.

Coordinating the thermostats for different heating systems is an important consideration, also. Since the heat pump is typically the lowest cost heating source in a home, it is best to leave the heat pump control setpoint at a level that is reasonably higher than the thermostat setpoint for alternate heating systems. For example, if the heat pump is set to keep a room at 68F, set the alternate heating system at 58-62F to ensure it does not come on unless the heat pump were to have an issue that prevents it from maintaining 68F.

One caveat to this is when the central heating system for the home is a hydronic system. It may be wise in the wintertime to allow the hydronic heating system to operate periodically to ensure water in the hydronic piping does not freeze. One way to do this would be to use a setback temperature overnight for both the heat pump and hydronic heating system, and then program the hydronic heating system to raise its heating setpoint 30-60 minutes earlier than the heat pump. For example, if the overnight temperature for the heat pump is 60F and the hydronic system overnight temperature is 50F, the hydronic system can be programmed to come up to 62F at 6AM in the morning, with the heat pump programmed to come up to 68F at 630AM. This will allow the hydronic system to operate for 30 minutes or more before the heat pump takes over again.

Defrost Mode

When an ASHP operates in heating mode, cold refrigerant circulates through evaporator coils in the outdoor unit. Because Juneau is a humid place, moisture in the air will condense and freeze to the evaporator coils, reducing the efficiency of the heat pump. This also happens in a refrigerator, where the fridge controls often pause the refrigeration cycle to allow a resistance heating element to turn on and melt ice that accumulates on the evaporator coils located in the fridge and/or freezer. Rather than use a heating element, most ASHPs will periodically reverse the refrigerant cycle – essentially putting the heat pump into cooling mode – for a period to melt the frost that has accumulated on the coils of the outdoor unit. The melt water needs to drain well, so many local installers recommend a heated condensate pan to ensure ice does not accumulate.

Defrost issues can occur in other situations, also. For instance, when a water-to-air GSHP runs in cooling mode, the heat pump relies on the movement of warm air over the evaporator coils to prevent frost buildup. If the fan or blower motor fails on the heat pump for some reason, frost will likely begin to build up on the evaporator coil. Ice may accumulate to the extent that, even when repairs are made to the fan or blower, air will no longer flow over the coils, so the unit will need to be shut off and allowed to melt before the fan or blower can prevent the issue from reoccurring.

Maintenance

Fans and blower motors play a major role in the successful operation of heat pumps, and many maintenance issues can be avoided by ensuring they are kept clean from dirt and debris. Homeowners should regularly clean or replace air filters and remove any dirt and debris that accumulates near any fans or blowers, including on the fins of blower motors. Keeping condensate drains clear is another important maintenance issue. ASHPs outdoor units need to drain properly to prevent ice buildup when the heat pump operates in heating mode, and indoor units need to be able to drain properly when operating in cooling mode.