Ductless Mini Split Heating and Cooling – Is it any good?

I’ve had the concept of a Ductless Mini Split HVAC unit brought up recently. I’m out doing a home energy audit, I’ve been asked on Twitter and in other contexts several times.  A discussion on a Professional Linked In group and my follow up comment has resulted in this post.

First – What is a Ductless Mini-Split?  The simple answer is ‘One type of residential HVAC equipment’.  Other common types of residential equipment are Single Package Unit and a Split Package System. An example of a Mini Split on the right by LG Electronics USA Commercial Air Conditioning. The top image is what you see inside. The bottom two views are of the outside unit. LG is one of many mini-split manufacturers.

If you are building to Energy Star, your HVAC contractor must perform various calculations to figure the size of the units, set up the duct work and select the unit. These calculations are specified by the Air Conditioning Contractors of America (ACCA) an industry trade group. Known as Manuals J (size) D (ducts) N (equipment selection).

If the process and calculations, especially Manual N will show a Ductless Mini Split as fitting the need, great! An example of a Single Package Unit on the right. Outside both summer and winter.

If you are not building the Energy Star – the 2012 recommended Energy Code requires the same process.

If your jurisdiction has not adopted the 2009 or 2012 Energy Code, the prior Residential Mechanical Codes require ACCA calculations or similar. The outside condenser of a Split System on the Left.

Heating and Cooling equipment is routinely oversized in existing homes and in new construction. This approach avoids the math and fits the American image of ‘bigger is better’. It also avoids after hours service calls concerned with the home not heating up or cooling down fast enough.

HVAC equipment, just like your car, operate most efficiently traveling at a constant speed. For your car a highway speed without starting or stopping in city traffic is the efficient speed. Note the Fuel Economy Numbers show the best and the worst MPG figures for each model. At the right is an example of the inside unit of a split system.

Due to the variations in climate from South, with little heating and lots of AC; to the North with a lot of heating and no AC; ACCA uses a design temperature in the calculations. Essentially you can figure the design temperature for your area. The National Weather Service publishes the daily highs, lows and average temperatures for each weather station. The report you want is monthly and is referred to as a J6.

How to figure that is a little much for this post.

A properly designed HVAC unit, like your car will run constantly at or in excess of the design temperature. So these hot summer days, most of us are above the design summer temperature; your AC is OK if it runs all the time. Preventive Maintenance is needed for the HVAC equipment, not sleep.

All that aside; a ductless mini-split is a great choice for a smaller space. My experience with specifying these for Homeowners is that HVAC manufacturers and contractors dearly love them. They are priced accordingly. At the left, the outside duct of a Single Package Unit, typically with no insulation.

 

The Mini Split gets away from the use of ductwork to distribute the conditioned air. That is the strength. Most ductwork in our homes is not designed correctly, it leaks and requires too much fan capacity to distribute the conditioned air. In the right sized space, going without ducts has many advantages.

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