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.
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.
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.
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.
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.
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.
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.
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.