Heating a basement or home addition with a heat pump

What are the main heat pump options for a small space?

Three approaches cover most cases.

• Ductless unit. A wall or ceiling indoor unit linked to an outdoor unit by a thin refrigerant line, with no ducts to run. It suits additions, finished basements and garage conversions, and gives the room its own thermostat.
• Extend the existing ducts. A basement near the furnace or air handler can often be reached with a few new runs. This is cheap if the system has spare capacity and airflow.
• A separate system for the new space. A small ducted or ductless unit with its own controls. It is common for basement suites, where the occupant wants independent control.

Installation is simplest with a ductless unit, and it is also far more energy efficient than electric baseboard heaters or plug-in space heaters, because a heat pump delivers several units of heat for each unit of electricity. Whichever you choose, the new space adds heat load, and the existing system may not cover it.

Can the existing heat pump or furnace handle the extra space?

Not always. A system is sized for the house as it stood. Adding a room or finishing a basement raises the load, and ducts that were not designed for the extra branch may deliver too little air to the far end. A contractor should redo the load calculation with the new floor area, insulation and windows. The method is explained in the guide to heat pump load calculation, and the consequences of a wrong size are in heat pump sizing for cold climates.

If the main system is already near its limit, a separate unit for the addition can be the cleaner answer, since it leaves the original distribution untouched.

What is different about a basement?

Basements lose heat through the foundation walls and slab, but stay cooler in summer and warmer in winter than the air outside, because the ground moderates the temperature. A finished basement with insulated walls usually needs little heat compared with the floors above. Many people find that a basement is warm enough from the heat that rises from the system, and cold mainly at the floor level, which suggests insulating before adding capacity. The insulation upgrade priorities include foundation and rim joist work.

Ductless indoor units are also an easy way to give a basement suite or a games room its own comfort control. Plan for a condensate drain, which in a basement can be routed to a floor drain or a small pump, and keep humidity in mind, since the moisture needs to be controlled either way. Where a floor is being rebuilt, in-floor heat is another possibility, described in the radiant floor heating guide.

What about additions, sunrooms and garage conversions?

These spaces often have more glass, thinner insulation or a slab on grade, which raises the heat loss for their size. A sunroom that gets hot in summer and cold in winter suits a ductless unit that handles both. Ask for the new envelope to be included in the calculation, including windows, and see energy-efficient windows for what affects loss.

Airtight additions need ventilation as well. A heat recovery ventilator can supply fresh air without throwing away the heat, as described in the heat recovery ventilator guide.

What should you check before you start?

Confirm the heat load, the available electrical capacity and where the outdoor unit can go, using the notes in outdoor unit placement. Check whether the project needs a building permit, and whether a secondary suite triggers extra rules such as separate heating controls, fire separation or ventilation; the municipality can confirm the requirements. Then compare quotes on the same basis, using the heat pump quote guide, and ask each contractor to state the capacity of the proposed unit and the design temperature used.

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