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
