Heat pumps and ductless systems, laid out to suit the house you already have.
Most of the houses we work in were built long before anyone planned for air conditioning or ductwork. The right system depends on how the rooms are arranged, what is above and below them, and what heats the house today. This page sets out the options and what each one asks of the building.
Ductless mini-splits
A ductless system pairs one outdoor unit with one or more indoor units, connected by a refrigerant line set, a condensate drain, and a low-voltage communication cable. Nothing moves through the walls except those lines, which makes it the usual answer for a house heated by radiators or baseboard, with plaster walls and no space for ductwork.
The indoor unit does not have to be the familiar white box high on the wall. There are four types, and a single outdoor unit can often drive a mix of them, so the choice can be made room by room.
| Indoor unit | Where it suits | What installation involves | What to weigh |
|---|---|---|---|
| 1. Wall-mounted head | Most rooms. Bedrooms, living rooms, kitchens, and additions with a usable exterior wall. | A single three-inch core through the wall behind the head; lines run down the outside of the house in a painted cover. | The simplest and least costly per room. It is visible, and it needs clear wall space near the ceiling. |
| 2. Floor console | Rooms where a radiator sat under a window, kneewalls on upper floors, and rooms with little wall space up high. | Mounted low on the wall or recessed into it; lines usually exit straight through the wall behind it. | Delivers heat at floor level, which suits winter. It takes some floor space, and fewer models are made. |
| 3. Ceiling cassette | Rooms with an accessible attic or unfinished space above, where nothing should show on the walls. | Set between ceiling joists with roughly a foot of clearance above; lines and drain run through the attic, usually with a condensate pump. | Only a flush grille shows. It requires cutting the ceiling, and the attic run must be insulated. |
| 4. Concealed ducted | Two or three neighboring rooms, such as a bedroom wing, served from one hidden unit. | An air handler in the attic, a closet, or a soffit, with short insulated ducts to a grille in each room. | Nothing visible but grilles. It needs space for the unit and ducts, and a reachable filter. |
Ducted heat pumps
If the house already has forced-air ductwork, from a furnace or a central air conditioning system, a ducted heat pump can use it. The outdoor unit replaces the air conditioning condenser, and a heat pump air handler or coil replaces or joins the furnace indoors.
Whether the existing ducts will serve depends on details we measure rather than assume. A heat pump delivers air at a lower temperature than a furnace, typically around 90 to 110°F against a furnace’s 120 to 140°F, so it must move more air to deliver the same heat. Ducts sized only for summer cooling, or for a furnace running hot, are often too small or too leaky to do it quietly.
- Static pressure
- We measure the resistance of the duct system with a manometer at the air handler. High readings mean noise, weak airflow at the far rooms, and a shorter life for the blower motor.
- Duct sizing and layout
- We compare each room’s calculated load with the air its supply duct can carry. Undersized runs are enlarged or rerouted; a room the ducts cannot serve well may get its own ductless unit instead.
- Leakage and insulation
- Ducts in an unheated attic or crawlspace lose heat on the way to the rooms. Joints are sealed with mastic and runs in unconditioned space are insulated before the new equipment goes in.
Keeping your existing system
Many owners keep the boiler or furnace in place and let the heat pumps carry most of the season. That is a legitimate design, and in some houses it is the right one, particularly where a few rooms are hard to reach with any indoor unit or where the boiler also makes the hot water.
The two systems need to be wired so they do not work against each other. Integrated controls let the heat pumps run first and bring the existing system on only below a set outdoor temperature, or when a room falls behind. Mass Save requires these controls for the partial-home rebate. Without them, a thermostat on the old system will often run the boiler first out of habit, and the savings never arrive.
When we assess the house, we will tell you plainly whether heat pumps can carry the whole load at your town’s design temperature, or whether keeping the existing system as backup is the sounder plan.
Cold-weather performance
A heat pump’s output falls as the outdoor temperature falls, while the house’s heat loss rises. A system that looks ample on a mild day can come up short in January. Manufacturers publish performance at several standard temperatures, and these are the figures we size from.
| Outdoor temperature | What it represents | What we check |
|---|---|---|
| 47°F | The standard rating point. The capacity printed in large type on most brochures. | Nothing we size from. Useful only to compare the shoulder seasons. |
| 17°F | The second standard rating point, and an ordinary winter night in the Merrimack Valley. | Capacity against the room’s load at that temperature, and efficiency (COP) for estimating operating cost. |
| 5°F | The cold-climate reporting point, close to the winter design temperature for most towns we serve. | Maximum capacity at 5°F must meet each room’s calculated load at design. This is the number the system is sized to. |
| Below 0°F | The coldest nights of a typical winter, a handful of hours a year. | The published low-temperature operating limit, capacity at −5°F and below, and whether backup heat is needed for those hours. |
In freezing weather the outdoor unit periodically runs a short defrost cycle to clear frost from its coil. Heat to the rooms pauses for a few minutes, and meltwater drains from the base of the unit. This is why we set outdoor units on a stand above expected snow depth, and keep the discharge clear of paths and steps.
Service and maintenance
A heat pump needs less attention than a boiler, but not none. Indoor units draw room air through fine filters and across a coil and blower wheel that collect dust over a season. Left alone, they lose airflow, run louder, and can begin to smell. Most of the routine care is yours to do in a few minutes; the rest is an annual visit.
- Owner, monthly in heavy use
- Remove and rinse the indoor unit filters and let them dry before refitting. Keep a clear space around the outdoor unit, and brush snow off the top and sides after a storm.
- Annual service visit
- Clean the indoor coil and blower wheel, flush and test the condensate drain and any pump, check refrigerant operating pressures and temperatures, tighten electrical connections, and confirm the outdoor unit’s stand and drainage.
The first step is an assessment
We measure the house, open the panel, and look at what heats it today. You get a written recommendation naming the equipment, where each unit goes, and why, along with the rebates you appear to qualify for.