After using Mitsubishi mini-split heat pumps in two houses for two winters I am finally prepared to share my conclusions on their operational cost and ability to maintain comfortable temperature. It will take a number of posts to unfold the details and nuances, and to compare heating costs with fossil fuel alternatives, but here are my main conclusions.

First, these heat pumps, using electricity purchased from Maine’s electric grid, have a lower carbon footprint than other forms of residential heat owing to the relatively low carbon footprint of Maine’s electric grid. This is the main reason electric heat pumps are being heavily promoted in New England, and from a public policy perspective, are achieving the important goal of lowering carbon emission.

Second, my Mitsubishi heat pumps were able to maintain reasonably comfortable inside temperatures (65-70^oF) with the outside temperature as low as -5^oF. During one extremely cold weekend (-15^oF) the heat pumps struggled to keep my cottage above 50^oF inside — but we did not fear freezing pipes and the extreme temperatures lasted just 1-2 days.

Third, on average, my mini-split heat pumps use about half as much energy as would electric baseboard heaters to accomplish the same task. That means they cost half as much to operate as electric resistive heat. In tech language, averaged over my winter conditions, the heat pumps demonstrated an effective heating COP = 2. These energy savings are meaningful but considerably lower than those advertised for these heat pumps which are said to have heating COP’s as high as 3.5. That may be true under certain conditions, but averaged through my heating season they are far less efficient.

The fourth conclusion is that temperature regulation and comfort delivered by my heat pumps, while acceptable, is inferior to that delivered by either my electric heat or propane through-the-wall heater. Even though the set points on the heat pumps remain constant, the indoor air temperature, measured by independent temperature sensors, show fluctuations in temperature in the range of plus/minus 1.5^oF. These fluctuations are significantly larger than those experienced with other heaters and display an irregularity that is difficult to understand.

The fifth conclusion is that the cooling and dehumidification provided by these heat pumps is welcome in the summer. However, while providing a degree of comfort not afforded by other heating systems, it comes at the cost of additional electric use and greenhouse gas emission.

In the next few months I will be posting details to justify the above conclusions. In addition, I will discuss the capital investment required to install these heat pumps, and look at the cost-effectiveness of the electric savings delivered. An electric heat pump can be a cost effective way to both heat and cool a residential space. But it is not always the cost-effective solution, particularly if cooling is not required.