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Heat Pump Installation Considerations

Heat pumps are one of the most economical ways to heat and cool a home. When properly sized, designed, and installed, they provide low-cost heating, air conditioning, and dehumidification on a room-by-room basis.

Heat pumps are available in a wide range of options. There are multiple indoor unit types, sizes, outdoor unit types, mounts, and design options. Here are some considerations to review with your installer.

Indoor unit location

  1. Heat rises — While an indoor unit might deliver some heat to the floor above, it won’t send any heat to the floor below. Likewise, cool air from a first floor unit in air conditioning mode will not cool floors above it. Heated air is unlikely to get past a stairwell going up and cooled air is unlikely to flow past a stairway going down.
  2. Consider air flow — Air flow is tough to predict and every building is different. In general, open spaces tend to be easier to heat and cool from one indoor unit, while it can be challenging for heat to go through a doorway into other rooms. A room with a door that is typically closed may not benefit from a heat pump located outside the room.
  3. Avoid thermostat conflicts — Combining multiple heating systems in the same space can be tricky. If, for example, a boiler thermostat is in a space heated by a heat pump, then that boiler thermostat may never fall below its setpoint and may never ask for heat. As a result, other areas served by the same zone (such as bedrooms served by the boiler but not by the heat pump) may become cooler than desired. If it’s not possible to install enough heat pumps to heat the whole home, then we recommend installing enough to displace entire boiler zones.
  4. Height —Indoor units work well when installed high on a wall but installing them too high can make them hard to reach when it comes time to clean the filter or service the indoor unit.

Indoor unit types

Here are some points to consider when picking an indoor unit:

  1. Wall units are by far the most popular. They are the most efficient and, because they are mounted high on a wall, they can heat or cool a large area. They are also the most conspicuous.
  2. Floor units are mounted on the wall down by the floor. They are less conspicuous, but are not as efficient. Their airflow can be obstructed by furniture, meaning that they may not be able to heat and cool as large a space.
  3. Ceiling cassettes are mounted above the ceiling and only their vents can be seen. They are typically the size of a suspended ceiling tile and they blow air in four directions from their edges. They are nearly unnoticeable, but tend to be less efficient. These are often installed in attic floors or above suspended ceilings.
  4. “Mini-Ducts” or “Compact Ducts” have an indoor unit located above the ceiling or below the floor that is connected by short runs of ductwork to one or more registers. One advantage is that the indoor unit is out of sight and the registers are inconspicuous. Because one indoor unit can be ducted to multiple registers, they can also be well suited to heating several small rooms like bathrooms and bedrooms. A common configuration is an indoor unit installed in an insulated attic connected to a grill in a hallway ceiling below. Hallway air is returned to the unit, heated or cooled, then supplied to multiple adjacent rooms via ceiling vents. Alternatively, they can be installed beneath a floor (typically in the basement ceiling below). Super-insulated homes with very small heating demands may be good candidates for a small mini-duct indoor unit with ducts throughout the house.
  5. Unlike mini-duct systems, centrally ducted systems can be used to heat an entire home using one indoor unit connected to a network of ducts. In some homes, centrally ducted heat pumps can use pre-existing forced hot air furnace ductwork. While not as efficient as the other indoor unit types listed here, reusing existing ductwork and only installing one unit can be a cost-effective approach.

Ducted system considerations

Here are some points to keep in mind related to ducted systems:

  1. While it is convenient to run ductwork through uninsulated parts of a house (e.g., in an attic above the insulation), it’s best to keep ductwork inside a home’s insulation. If this isn’t feasible, ductwork that runs through unconditioned space should be thoroughly insulated and sealed.
  2. Because the dust filter needs to be rinsed or replaced regularly, it will need to be installed in a place that’s easy to access.  Filter slots that are built into ductwork should have an airtight cover.
  3. All spaces connected by a duct system will get heating/cooling based on one shared thermostat. 
  4. “Balancing” a ducted system is the process of adjusting airflow to match the heating demand of each room. To enable balancing, a damper should be installed for each room.

Outdoor unit location

There’s a lot of flexibility as to where outdoor units can be installed. Here are some considerations:

  1. Aesthetics — Outdoor units are most commonly mounted on or next to homes. Because outdoor units can be mounted over fifty feet from their indoor units, they can be installed on an inconspicuous side of the house, under decks, or even far-removed from the home.
  2. Unobstructed airflow — Although it’s tempting to tuck units into tight places for aesthetic reasons, it’s important to remember that they extract heat from the outdoor air. The more airflow they get, the better they’ll work. Avoid shrubs, places prone to snow drifts, and structures that might block airflow. It’s also important to make sure outdoor units aren’t blowing onto one another.
  3. Door, window, and walkway interference — It’s best to avoid installing the outdoor unit where it could interfere with the operation of a door or window. In addition, outdoor units release water when they defrost in the winter, which can form icy patches if installed near a walkway. Be sure to pick a spot where that won’t be an issue.
  4. Roof runoff — If the outdoor unit is going to be installed under a roof drip line, then be sure the unit is equipped with a rain cap to minimize the risk of it icing up.
  5. Serviceability — Keep in mind ease of service when selecting an installation location.

Outdoor unit considerations

  1. Mounting — The primary goal of a mounting system is to keep the outdoor unit above the snow.
    1. Foundation brackets do the best job at minimizing noise and staying out of the way of rakes, shovels, and lawn mowers.
    2. Ground stands minimize noise, but can be susceptible to frost heaves if installed with inadequate drainage.
    3. Wall mounts keep units away from rakes, shovels, and mowers, but can transmit a low hum inside.
  2. Multi-zone vs. single-zone systems
    1. Single-zone heat pumps have one outdoor unit for each indoor unit. The advantages of single-zone systems include:
      1. Lower operating costs
      2. More efficient, especially at lower speeds
      3. Better dehumidification
      4. If one unit fails, others still operate
      5. Each room can be in a different mode simultaneously (e.g., AC, dehumidify, fan-only, and heat)
    2. Multi-zone heat pumps have two or more indoor units for each outdoor unit. The advantages of multi-zone systems include:
      1. Lower upfront costs
      2. Lower maintenance costs
      3. Less outdoor space required
      4. Less visible
      5. Note:  The temperature of each multi-zone indoor unit can be set separately. However, if heat is only turned on in one room, a small amount of heat will still be delivered to the other rooms, potentially making some rooms uncomfortable.
  3. Sizing — When sizing a heat pump indoor or outdoor unit, bigger is not always better. Smaller units tend to be more efficient and can often do a better job of heating and air conditioning than oversized units.
  4. Extreme Cold Weather Performance — Heat pumps can keep a house warm without backup even in extreme cold. Some produce heat down to negative 22 (regardless of wind chill).  However, it may be more practical to install heat pumps to handle all but the most extreme temperatures and rely on a combination of heat pumps and supplemental heaters like a wood stove, electric space heaters, etc. for those rare events. Be sure to ask your installers what they recommend.

    Line set considerations

    1. Indoor — Heat pump indoor units are connected to outdoor units with copper tubes and wiring called “line sets.” Finding creative ways to simplify installations can save money and improve aesthetics. To minimize visible line sets indoors and avoid the cost of patching walls and ceilings, line sets can be run through closets, basement/attic stairways, attached garages, basements, crawlspaces, or attics.
    2. Outdoor — Matching the color of outdoor line set covers to a house’s exterior, grouping them together into larger covers, and making them shorter can minimize their visibility and reduce cost.

    Other considerations

    Here are some other things to consider during installation:

    1. Ease of expansion — Unlike central boilers/furnaces, it’s easy to add heat pumps if your initial installation leaves you with underheated rooms. There’s little to no financial penalty for installing units over time.
    2. Condensate drain line — When heat pumps are air conditioning or dehumidifying, they remove water from the air, which can be drained to a sump hole, plumbing drain, garden, or gutter.
    3. Ensuring comfort throughout the home — Be sure to ask your installers how they recommend ensuring comfort throughout the house. Space heaters, electric baseboard, and wood stoves can be used if there are spaces not fully served by heat pumps. Insulation can also help address cold spots.
    4. Avoiding frozen pipes — Turning off a boiler or furnace in an insufficiently insulated basement can increase the risk of pipes freezing. This risk can be minimized with insulation, heat tape on at-risk pipes, or space heaters set to keep the temperature just above freezing.