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Sun, July 21

Pros, cons of mini-split HVAC system

Q: My heating and cooling system is getting old and needs replacement. Currently I have a centrally forced air based heating and cooling system, but have heard that new mini-split systems are a replacement option. What are the advantages and disadvantages of such a system?

A: Ductless mini-split heat pumps are ideally suited for compact, highly energy-efficient homes and where outside ambient temperatures are moderate. They also make good retrofit add-ons to houses with "non-ducted" heating systems and a good choice for room additions where extending or installing distribution ductwork is difficult and expensive, if at all feasible.

They are based on air-sourced heat pump technology powered by electricity and are identical to the air conditioner in your home or car, the only difference is they can reverse direction and heat in winter and cool in summer.

Mini-split systems comprise an outdoor compressor unit and single or multiple indoor evaporator/blower units that mount on interior walls, in ceilings or free stand on floors. Cables and refrigerant pipes have to be plumbed between the units, and a typical installed cost of a single ductless mini-split system is $3,000 to $5,000, although many variables can affect the final cost.

Mini-split systems use a variable speed compressor. As the temperature drops it speeds up and as the temperature rises it slows down. The diagram below shows how the concept works. Most heat pumps work well down to an ambient temperature of between 40 and 45°F. For single-speed units efficiency starts to drop below 45°F as shown in the diagram; however, the variable speed mini-split starts to increase its speed to compensate for the normal loss and increases its speed until it reaches its maximum shown at 120 rpm at 5°F. Below this the variable speed unit also starts to lose efficiency. The limitation is that as the speed increases the unit needs more source power to run the compressor and as the temperature drops the compressor extracts less heat energy from the air, so it takes longer to heat up.

As an example, specifications below from a major mini-split manufacturer show that at a constant 47°F temperature the higher compressor rotational speeds produce more output energy, but also use more input power.

Now comparing the energy generated against a natural gas alternative; where gas costs $1.13 a Therm. The heat pump at 47°F costs $1.12 for an equivalent Therm and less above 47°F, so it's more efficient, but at lower temperatures it becomes a lot more expensive. The same heat pump issue the dual fuel system solves.

Because traditional forced-air heating and cooling systems are custom and assembled on site from many parts, there are a number of ways for installers to make mistakes. Mini-split systems overcome most of the custom systems limitations because they are packaged systems that are ductless, and have factory installed integrated components and controls, all of which result in fewer opportunities for installer error.

Another advantage is their small size and flexibility for zoning individual rooms. Each one of the zones has its own thermostat, so you only need to condition occupied space, and because they don't require ducts they avoid the 30% plus energy losses associated with poorly installed and insulated central ductwork.

However, some people do not like the appearance of the indoor units and since they contain a fan they are noisier than a well designed duct system. If homeowners let filters get dirty, output can drop dramatically as the blowers have a low static pressure capability. There must also be a place to drain condensate water from each indoor unit.

As with any heating and cooling system correct sizing is critical. The installer has to establish the heating and cooling load of the home and then each zone, matching the mini-split internal air handlers to those specifications. Oversized or incorrectly located air-handlers often result in short-cycling, which wastes energy, puts excessive wear and tear on the equipment and does not provide correct humidity control.

A single-zone unit in the main space of a tight energy efficient home may suffice if you have an open plan layout or the doors to other rooms remain open; a possibility with common rooms, but unlikely with bedrooms. In a larger home or one that isn't as well insulated, several units would be required, and in many cases installers would also recommend either an electric-resistance heating unit or a fossil fuel heater as backup for colder nights or rapid heating.

Unlike central systems implementing temperature setbacks during the night or while away during the day doesn't appear to save a lot of energy because mini-splits run at full speed when the temperature is set back up, and because they aren't usually oversized for the house load, it can take a long time to get the house back up to temperature without backup; if the thermostat is close too or even in the indoor unit, distant parts of the conditioned space may have large temperature differences and cold spots.

Another issue is that mini-splits systems do not have a way to recirculate and balance the air as a duct system does. Central air based duct systems are balanced so that the return air capacity matches the supply air capacity preventing indoor air turbulence. Mini-split systems have no return path. This causes inside air pressurization that in turn causes drafts, difficulty maintaining stable indoor temperature, higher cost, and envelop damage through air being pushed through cracks and cavities to the outside.

Mini-splits pass the outdoor air only once through its input filter, where a centrally based duct system recirculates the air from supply to return many times continuously filtering the air. The central systems recirculation process also removes humidity more efficiently.

Based on the above, adding mini-splits as a way to add or extend existing heating and cooling capacity and where ducting is not readily available makes a lot of sense. They can be installed with relative ease, require little to no construction changes, look a lot better than window or wall units and provide a lot of flexibility.

However, for new construction or upgrading an existing central system the advantages are not so clear. If a duct system is in place and just the main unit needs replacing there are a lot of less expensive and less complex choices. However, before installation it would be advisable to have the duct system tested for leaks, sealed and insulated; a small price to pay for the benefits of a fully integrated central system.

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