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