Air to Water Heat Pump to Replace Aging Boiler for heat and hot water?

A homeowner in Tacoma, Washington is weighing whether to replace an aging, leaking gas boiler with an air-to-water heat pump, a system that heats water instead of air and can run radiators or in-floor heating. Her 1950s house uses radiators and currently needs water at 140 degrees to keep rooms at 68 when it's 20 degrees outside. The bid she's considering, for a SpacePak Solstice unit that would handle both space heating and hot water, comes to $39,550.

A building science expert who replied pointed out a key problem: heat pump capacity and efficiency both drop sharply as the water gets hotter. At 95 degree water, this unit puts out far more heat and uses less electricity than it does at 140 degrees, where it delivers barely half the heat for almost twice the power use. So the real question isn't whether the heat pump can work, it's whether the house can be heated with cooler water than the old boiler used, since the boiler's 140-degree setting may just reflect its minimum, not what the house actually requires.

For homeowners in a similar spot, the lesson is that swapping a boiler for a heat pump isn't a simple one-for-one trade. It helps to know your actual heating load and the lowest water temperature your radiators or floor heating can work with before signing a contract. The expert also noted that a dedicated heat pump water heater, rather than asking the same unit to do double duty, often costs less to install and run for domestic hot water.