6. Related Equipment
Upgrading the Electrical
Service
Generally speaking, it is not necessary to upgrade the electrical
service when installing an air-source add-on heat pump. However, the age
of the service and the total electrical load of the house may make it
necessary to upgrade.
A 200 ampere electrical service is normally required for the
installation of either an all-electric air-source heat pump or a
ground-source heat pump.
Supplementary Heating
Systems
Air-Source Heat Pump Systems
Most heat pump installations require a supplementary heating system.
Air-source heat pumps are usually set to shut off at either the thermal or
economic balance point. In the case of an air-source heat pump,
supplementary heat (also called backup or auxiliary heat) may also be
required during the defrost cycle.
Supplementary heat can be supplied by any type of heating system,
provided that it can be activated by the thermostat that controls the heat
pump. However, most supplementary heating systems are central furnaces
that use oil, gas or electricity. Many new EES installations use duct
heaters to supply auxiliary heat.
Figure
16 shows the thermal balance point for a typical air-source heat pump.
To the right of the thermal balance point, the heat pump is capable of
satisfying all of the home's heating requirements. To the left of the
thermal balance point, the house heat loss is greater than the heat pump's
capacity; this is when supplementary heat is required in addition to the
heat pump's capacity.
Figure 16: Balance Point for a
Typical Air-Source Heat Pump
In the shaded area of the graph, the heat pump can operate in two ways.
If heat pump operation is unrestricted by outdoor temperature, it will
operate to satisfy first stage heating requirements each time heat is
called for by the thermostat (see the upcoming sections on thermostats.
When second stage heat is called for, the heat pump shuts off if it is an
add-on unit, or continues to operate if it is an all-electric heat pump
system, and the supplementary heating system provides heat until all
heating requirements have been satisfied.
If heat pump operation is restricted, an outdoor temperature sensor
shuts the heat pump off when the temperature falls below a preset limit.
Below this temperature, only the supplementary heating system operates.
The sensor is usually set to shut off at the temperature corresponding to
the economic balance point, or at the outdoor temperature below which it
is cheaper to heat with the supplementary heating system instead of the
heat pump.
Earth-Energy Systems
Earth-energy systems continue to operate regardless of the outdoor
temperature. The supplementary heating system only provides heat that is
beyond the rated capacity of the EES.
Conventional
Thermostats
Most residential heat pump systems are installed with a "two-stage
heat/one-stage cool" indoor thermostat. Stage one calls for heat from the
heat pump if the temperature falls below the preset level. Stage two calls
for heat from the supplementary heating system if the indoor temperature
continues to fall below the desired temperature.
The most common type of thermostat used is the "set and forget" type.
The installer consults with you prior to setting the desired temperature.
Once this is done, you can forget about the thermostat; it will
automatically switch the system from heating to cooling mode or vice
versa.
There are two types of outdoor thermostats used with these systems. The
first type controls the operation of the electric resistance supplementary
heating system. This is the same type of thermostat that is used with an
electric furnace. It turns on various stages of heaters as the outdoor
temperature drops progressively lower. This ensures that the correct
amount of supplementary heat is provided in response to outdoor
conditions, which maximizes efficiency and saves you money. The second
type simply shuts off the air-source heat pump when the outdoor
temperature falls below a specified level.
Thermostat setback may not yield the same kind of benefits with heat
pump systems as with more conventional heating systems. Depending upon the
amount of the setback and temperature drop, the heat pump may not be able
to supply all of the heat required to bring the temperature back up to the
desired level on short notice. This may mean that the supplementary
heating system operates until the heat pump "catches up." This will reduce
the savings that you might have expected to achieve by installing the heat
pump.
Electronic
Thermostats
Programmable heat pump thermostats are available today from most heat
pump manufacturers and their representatives. Unlike conventional
thermostats, these thermostats achieve savings from temperature setback
during unoccupied periods, or overnight. Although this is accomplished in
different ways by different manufacturers, the heat pump brings the house
back to the desired temperature level with or without minimal
supplementary heating. For those accustomed to thermostat setback and
programmable thermostats, this may be a worthwhile investment. Other
features available with some of these electronic thermostats include the
following:
- Programmable control to allow for user selection of automatic heat
pump or fan-only operation, by time of day and day of the week.
- Improved temperature control, as compared to conventional
thermostats.
- No need for outdoor thermostats, as the electronic thermostat calls
for supplementary heat only when needed.
- No need for an outdoor thermostat control on add-on heat pumps.
Setback savings of 10 percent are possible, with one setback period of
eight hours each day in most Canadian locations. Two such periods per day
can result in savings of 15 to 20 percent.
Heat Distribution Systems
Heat pumps require distribution systems that handle airflow rates of 50
to 60 litres per second (L/s) per kW, or 400 to 450 cubic feet per minute
(cfm) per ton of cooling capacity. This is approximately 20 to 30 percent
higher than the flow rates required by central, forced-air furnaces.
Restricting airflow rates decreases efficiency, and damage to the
compressor can result if they are severely reduced for extended periods of
time. Keep air filters clean and have the air coil cleaned if filter
maintenance has been neglected.
New heat pump systems should be designed according to established
practice. If the installation is an add-on, or a conversion, the existing
duct system should be carefully examined to ensure that it is
adequate.
Previous
| Table of Contents | Next
Source: Natural Resources Canada
(NRCan) Office of Energy Efficiency
