The Hidden Costs of Air Source Heat Pumps

For the past 22 years, I have dedicated my career to decarbonizing heating systems.

Writing this post was challenging because air-source heat pumps (ASHPs) have their place in decarbonization of our building stock. They work well for heating in warmer climates, and I do not want to dissuade people from using them for heating in cold climates if they are unable to use ground-source heat pump (GSHP) technology.

ASHPs have many benefits, which I am sure you have read about because they are in the news quite frequently. I want to provide you with some information that I feel is missing from the dialogue about them.

It is important to have realistic expectations of ASHPs. Lifecycle costing is critical and should be based on an hourly energy analysis that factors in defrost and capacity de-rating throughout the year. The operating and maintenance costs, along with realistic first cost estimates can really change the outcomes of the lifecycle costing models for ASHP and GSHP system types.

I have been using direct digital controls to monitor the operating efficiencies and capacities of ASHPs and GSHPs over the past several years. I have kept close tabs on the installation costs between the two system types. I have repaired and re-commissioned broken and underperforming heat pump systems of all types.

Here are some crucial factors when considering ASHPs:

Efficiency - Defrost mode needs to be included to accurately calculate the efficiency of an ASHP, and not all efficiency listings factor in defrost. When an ASHP is in defrost mode, it is no longer providing heat, but the compressor is still running and consuming electricity. In addition to consuming power for the compressor, the heat pump takes heat from the building when it is in defrost mode. If defrost is required prior to completing a heating cycle, additional energy will be consumed by the back-up heat source to make up for the heat being taken from the building to melt the frost off the outdoor unit. Colder temperatures will mean more downtime to defrost and increased reliance on backup heat.

Moisture - Defrost = ice melt, ice melt = water. Proper drainage for outdoor ASHPs is necessary to ensure that ice does not build up and cause ice-jacking of the unit and/or a slip-and-fall hazard.

Capacity - Comparing apples-to-apples capacity in cold climates, an ASHP will be far larger and more expensive to provide the same capacity as a GSHP. This cost difference may be a factor when comparing system costs.

Cost - The cost of an ASHP outdoor unit is not just the hardware. A professional will need to be paid to install the unit, a concrete pad is required to support it, and some locations may require noise reduction, which can be challenging and expensive. If appearance is a factor, there may be an additional cost to hide the heat pump from view, because they do not improve curb appeal.

ASHPs are touted as having low upfront costs, while GSHPs are assumed to be more costly to install. The capital cost and operating cost assumptions that many people make during the comparison of the two system types have some hidden variables, particularly in colder climates.

If cost is your primary driver in selecting an ASHP, you may find that GSHPs can be very economical and extremely efficient. You will also have a longer lifespan with a GSHP, and they require less annual maintenance than an ASHP.

A heat pump of any type is a wonderful choice to decarbonize your heating system.

A reliable comparison of ASHP and GSHP system finances should factor in holistic capital costs and realistic operating costs so that you can make a fully educated decision for your building.