Every room warm. Even the ones the radiators never reached.

A heat pump moves heat rather than burning fuel to make it. In winter the outdoor unit draws heat out of the outside air, compresses the refrigerant to raise its temperature, and carries that heat indoors through two small copper lines. There is usable heat in air well below freezing, and cold-climate models keep running below 0°F, at reduced output.

In a 1920s house in Lowell or Haverhill, with plaster walls and no ductwork, those lines run to wall-mounted heads in the rooms that need them. Each head needs one three-inch opening through an exterior wall, and the lines drop down the outside of the house under a painted cover. Every head has its own controls, so the north bedroom and the sunroom stop working against each other. Many owners keep the existing boiler for the coldest week of the year, and we will tell you plainly whether yours should stay. In summer the same system runs in reverse and cools the house, without window units.