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
![Balance Point for a Typical Air-Source Heat Pump](/web/20061103103724im_/http://www.oee.nrcan.gc.ca/publications/infosource/pub/home/heating-heat-pump/images/fig16e.gif)
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.
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