Section 7 - Heating and Cooling with a Heat Pump
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.
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 continue to operate regardless of the outdoor temperature. The supplementary heating system only provides heat that is beyond the rated capacity of the EES.
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.
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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