A typical West Medford addition
Clement Boston Heat Pumps designs a heat pump for a home addition the same way as a whole house: from a load calculation. In this job, a West Medford family adds a one-story rear family room and mudroom to a 1920s colonial.
The addition is 480 square feet over a sealed, insulated crawlspace. The main house keeps its basement gas furnace. The nearest supply trunk ends 30 feet from the new wall, and the furnace already reads 0.78 inch of static pressure, so a branch from it starves the new room.
| Job detail | This addition |
|---|---|
| Where | West Medford, National Grid territory |
| House | 1920s colonial with a 480 sq ft rear addition |
| Before | Basement gas furnace; no heat or cooling in the addition |
| After | 1.5-ton cold-climate ducted heat pump for the addition |
| Price | $21,900 before rebates |
| Time on site | Four working days, in two visits |
What the measurements decide
The room-by-room load calculation for the addition comes to 11,200 BTU an hour of heat on a design night and 9,400 BTU an hour of cooling. That’s a small load, because the builder insulates the new walls and roof to Medford’s energy code.
Why a small variable-speed unit
A 1.5-ton variable-speed cold-climate unit covers the design load with capacity to spare at 5°F, and it turns down far enough to run steadily on a 45°F day instead of cycling on and off. Our sizing guide explains why tonnage alone never settles this.
Medford has adopted the state’s specialized energy code, which reaches new construction and major renovations. An all-electric heat pump fits the builder’s energy path for the addition without a second fuel line.
The rough-in, while the walls are open
The first visit happens after framing and before insulation. Everything that disappears behind drywall goes in now, and the building inspector sees it before it’s covered.
- Air handler closet. A vertical air handler goes in a 30-inch closet off the mudroom, with a filter rack and a return grille in the closet door frame.
- Supply ducts. Five 6-inch sealed metal runs in the floor framing, inside the insulated crawlspace, to floor registers under the windows.
- Line set. An 18-foot run from the closet through the crawlspace rim joist to a side-yard stand.
- Condensate. A drain line to the crawlspace pump the builder already plans.
Ducts inside the insulated space lose almost nothing; the same runs in a vented crawlspace leak heat all winter. Our heat pump ductwork page covers duct design in more detail.
Setting the equipment and starting it up
The second visit comes after the floors and paint are done. We set the air handler with 3 kW of backup heat and the outdoor unit on an 18-inch snow stand, then wire both.
The existing 200-amp panel has room for two new 240-volt circuits, so no service upgrade is needed. Medford’s electrical and sheet metal permits go through the city’s online portal.
Commissioning, step by step
- Pressure-test the brazed line set with nitrogen.
- Pull a vacuum below 500 microns and hold it.
- Charge by weight for the 18-foot line length.
- Measure airflow and static pressure at the air handler.
- Set the thermostat and lock out backup heat above the recorded temperature.
The addition then heats and cools on its own thermostat, separate from the furnace that still serves the rest of the house.
Price, schedule and rebate
The job costs $21,900 before rebates, inside our typical range of $20,000 to $40,000 and up for a heat pump with new ductwork. It takes four working days on site: two for the rough-in, two to set and start the system.
Because the heat pump serves new space and doesn’t replace an existing fuel, Mass Save’s basic rebate applies: $250 per ton, so $375 here. The home’s electric service is National Grid, a Mass Save sponsor.
Choices that change the number
- Converting the whole house at the same time. Replacing the furnace with a whole-home heat pump opens the larger whole-home rebate path.
- Keeping the furnace as backup for the main house with a dual fuel heat pump.
- An attic instead of a crawlspace. A second-story addition often puts the air handler in an attic, which adds a platform and drain pan.
Wall-mounted ductless units are the other common answer for additions, and our ducted vs ductless guide compares the two. If you’re planning an addition, bring us in while it’s still on paper.