An Ideal Heating/Cooling System for Your Small Addition

Last week I explained why the use of a ductless mini-split heat pump system as the sole source of heating and cooling is not the right application for the small (less than 1000 sq. ft.) addition. In order to properly size for the heating load on colder days, you’d be left with grossly oversized cooling capacity. And that will cause the heat pump to short-cycle when cooling on any day except for the very warmest. This short-cycling will result in less comfort, less efficiency, increased maintenance and shorter equipment life. Not good. After all, who wants to stress about their heating/cooling system.

You want to be comfortable all year long. If you want to expand your home with a small addition, I recommend ductless mini-split heat pumps for cooling in applications where the primary heat is supplied by another source — like radiant floor, finned-tube baseboard, radiant panel or hydro-air. That way I can size the cooling portion for ideal comfort and the heating portion of the heat pump can be used as a backup heat source if the main heat source is down for maintenance.

But now there’s an exciting new technology coming to the market. It combines the best of variable-speed compressor heat pump technology with hydronics to provide super high-efficiency performance with the awesome comfort of radiant heat and central cooling.

An air-to-water heat pump is the heart of this technology. During heating season it extracts heat from the outdoor air and transfers it to water using an indoor heat exchanger. In the cooling season, the process is reversed — indoor heat is transferred (via the same heat exchanger) to a refrigerant and expelled outdoors by the heat pump.

Since the heat pump has a maximum heating output temperature of approximately 120˚F, it’s a perfect match for low-temperature radiant — thin-slab, above-floor tube and plate, walls, ceilings, panel radiators and some types of finned-tube baseboard. This is the most comfortable heat around.

To cool the air, a pump circulates water (chilled by the heat pump) from the heat exchanger through a cooling coil located in an air handler. This distributes cooled and dehumidified air throughout your addition. If the design of the addition permits it, a standard air handler with the familiar ductwork can be used. But if equipment space is at a premium, a high-velocity mini-duct system with its 2” diameter ducts may be a better fit.

The efficiency of an air-to-water heat pump is rated in terms of its Coefficient of Performance (COP). COP is a ratio of the amount of heating (or cooling) produced to energy consumed. It’s not unusual for a variable-speed air-to-water heat pump to have a published COP of over 4.0 and an average COP of 2.7 to 3.0. In simple terms, for every one unit of energy consumed, the heat pump can produce almost three units (annual average) of heating or cooling. The only system more efficient than that is a geothermal system. (More on that comparison in future installments.)

Even though some of these heat pumps are advertised to operate in outdoor temperatures down to -4˚F, when the outdoor temperature drops to about 20˚F, the cost of energy input increases to the point where an alternate heat source is more efficient. A separate gas- or oil-fired boiler can provide a backup heat source for the coldest days as well as domestic hot water (DHW) production year-round. And if the main part of the house is hydronically heated, the air-to-water heat pump and the existing hydronic system are a match made in heaven!

But even if you need a backup/DHW boiler, the incredible efficiency of the air-to-water heat pump will offset the higher initial equipment cost with fuel savings in just a few years.

If there’s a small addition in your future, consider an air-to-water heat pump with radiant heat and chilled-water cooling as a renewable-energy alternative that pays for itself.

Heidronically yours,

Wayne

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