Ductless Mini-Split Heat Pumps and the Small Addition

The popularity of ductless mini-split heat pumps has grown tremendously in recent years. They’re a great way to add cooling to a hydronically heated home because they don’t need bulky ductwork. But like many new things, there’s a tendency to apply them to as many situations as possible, including some that they may not be well suited for. One such misapplication is as the sole heating and cooling source for a small addition.

Ductless mini-split heat pumps are usually an air-to-air heat pump — meaning it takes outside air and strips it of its heat value and transfers that heat to your home to provide space heating. For cooling, the cycle is reversed — it pulls the heat out of your house and expels it to the outdoors. You could think of a heat pump as an air conditioner that’s capable of working in reverse.

Heat pumps are nothing new, but the configuration of the ductless mini-split is. The condensing unit is located outdoors and a refrigeration lineset, small drain and wiring are run into your home through a 3˝ opening in an outside wall. They supply the indoor unit, which is usually mounted high on a wall and contains the blower and indoor controls. Ductless mini-splits are incredibly quiet (inside and out) and efficient.

I’m often asked to design a heating and cooling system for a small addition to an existing home (less than 1000 sq. ft). The first thing I look at is the capacity of the existing system to handle the addition’s extra heat and cooling load. More often than not, especially with forced air, the existing system can’t do the job. The system in a hydronically heated home can almost always handle the additional heating load but it obviously can’t provide cooling.

That usually leads to someone suggesting a ductless mini-split heat pump for heating and cooling the addition. It’s tempting, because it’s a relatively easy, quick and inexpensive installation. And here’s where the misapplication comes in. As an example, let’s apply a ductless mini-split heat pump to a typical 700 sq. ft. master bedroom, bath and laundry addition.

An addition like this would typically have about a 14,000 Btu/hr heat load on the coldest day of the year (considered 0˚F in the Rochester, NY area). It would also require just under 1 Ton (12,000 Btu/hr) of cooling on the warmest day of the year (considered 90˚F in this area). Both design loads would keep the indoor temperature at 70˚F.

Now, when sizing a heat pump, you size for the greatest load (heating or cooling) so you can be sure there’s enough capacity for both seasons. In our example case, as with most applications in this climate, the largest load is the heating load. So wouldn’t logic dictate that we’d need a heat pump rated for 14,000 Btu/hr? Not so fast.

We also need to consider the fact that as the outdoor temperature drops, so does the efficiency of the heat pump. In fact, even though some of the newer models are capable of providing heat down to an outdoor temperature of -4˚F, at those temperatures their heat output drops to near 50% of rated capacity. So now we realize that we need to DOUBLE the capacity of the heat pump to have any chance of maintaining our 70˚F indoor temperature on a 0˚F day.

That means we’re looking at installing a heat pump with a 28,000 Btu/hr minimum capacity, which actually works out fairly well, because heat pumps come in a 2-1/2-Ton size (30,000 Btu/hr). So now that we’ve decided that we need a 30,000 Btu/hr unit for heating, let’s see how that works for the cooling side.

Remember, the cooling load is 12,000 Btu/hr on the warmest days (90˚F). And there’s a 30,000 Btu/hr capacity. Simple math tells us that on even the warmest days, our heat pump is oversized (for cooling) by 250%. And, as you’ve heard me preach before, in cooling (and heating), bigger is not necessarily better.

Most of the better ductless mini-split heat pumps these days use inverter technology to modulate the compressor speed, which tailors the output to the load. With our example, the compressor would modulate down to 40% of capacity on the WARMEST day. That means that on a milder day it may need to be operating in the single-digit-capacity numbers. The problem arises when the compressor is only capable of modulating down to 30% of capacity, meaning that anything less than 75% of the maximum cooling load (in our example) will be asking the heat pump to work below its minimum capacity — which will be the bulk of the cooling season!

When a heat pump is asked to work in a range below its minimum capacity, it will short-cycle and, as a consequence, fail to properly dehumidify. We’ve discussed short-cycling and its consequences before — less comfort, less efficiency, increased maintenance and shorter equipment life.

For this example addition, the ductless mini-split “kind of” does the job. It can either do an acceptable job of heating with not-so-good cooling, or an awesome job of cooling with unacceptable heating performance.

So if a ductless mini-split isn’t the answer to heating and cooling your small addition, what is? Be sure to check next week’s Heidronics blog post for the answer.

Heidronically yours,

Wayne