When Ductless Minisplits Make Sense
Easy-install, high-efficiency HVAC is ideal for retrofits and well-insulated new homes
Synopsis: In this article, contributing writer Scott Gibson explains how this unique type of air-source heat pump works, how it differs from a conventional air-source heat pump, and in what conditions it performs at its best and its worst. A sidebar written by senior editor Andy Engel explains how heat pumps work.
A minisplit consists of an exterior condenser and one to eight interior heat exchangers. Instead of sending conditioned air through a system of ducts, a ductless minisplit sends refrigerant through small-diameter supply lines in the walls to each heat exchanger, where a fan blows air over an evaporator to either heat or cool the space. Not only is it easier to retrofit most houses with a ductless system than a ducted one, but minisplit distribution losses are generally much less than those of a standard air-source heat pump. Although minisplits perform better in cold weather than standard air-source heat pumps, in a leaky, under-insulated house, they may not be able to produce enough heat in very cold temperatures. This can require supplemental heating and/or be an incentive for the homeowner to tighten up the envelope.
Advantages of going ductless
Eliminating ductwork simplifies installation, especially in a retrofit, where putting in ducts could be invasive and expensive. In a ductless system, refrigerant piping, control wires, and the condensate drain fit together through holes small enough to be drilled in a 2×4 wall plate. A simple system that includes a single outdoor compressor and a few indoor heads can be installed quickly, with few of the construction headaches associated with conventional heating and cooling. Ducts also are inherently wasteful, either because they’re leaky or because they lose energy as they pass through unconditioned areas such as attics or crawlspaces. The u.S. Department of Energy estimates that as much as 30% of the heating or cooling energy in a conventional duct system is waste.
Because the small-diameter refrigerant lines used in minisplit systems are insulated and don’t have as much surface area as air ducts, they lose much less energy. Energy loss with minisplit refrigerant lines is as low as 1% to 5%, according to the National Association of Home Builders Research Center.
Minisplits can heat in cold temperatures
The inability of most conventional air-source heat pumps to operate efficiently in cold weather is one reason they are generally limited to moderate climates. As temperatures fall into the 40s, supplemental heating is usually needed to keep indoor-air temperatures comfortable. The supplement might take the form of electrical-resistance coils in the air handler, or a gas- or oil-fired furnace that kicks on when it’s too cold for the heat pump to work. Many minisplit models, however, continue to produce heat even when the outdoor temperature drops below zero. Mitsubishi says its Hyper-Heat models work at 13 below zero, although at reduced efficiency (view sidebar in PDF below on p. 55).
Jim Godbot of Jim Godbot Plumbing and Heating in Biddeford, Maine, had one customer call him up on a windy, 20-below day to say how comfortable his minisplit-heated house was. One potential disadvantage is that unlike most conventional airsource heat pumps, minisplits don’t have any supplemental heating capacity. In superinsulated houses with low air leakage, this isn’t a problem. In drafty, underinsulated houses in snow country, more powerful supplemental heat is needed. If the house is in an area where temperatures fall below the minisplit’s capacity for extended periods, that secondary heat source would have to be able to handle the entire heating load.
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