Natural Gas Furnaces
Two important considerations in choosing a natural gas furnace are size and efficiency. The size of your furnace is determined by the heat loss of your house. A qualified contractor should be able to supply a heat loss calculation. There is also a calculator available from CHMC to help with this calculation. http://www.cmhc-schl.gc.ca/en/co/renoho/refash/refash_018.cfm.Natural gas furnaces are rated by their Annual Fuel Utilization Efficiency (AFUE), which is the ratio of the heat output to the total energy consumed by the furnace. All new furnaces should have a label on the cabinet indicating the AFUE. In Alberta there are three types of gas furnaces.
- Standard-efficiency (50 to 79 per cent AFUE). These exist in many homes and small commercial buildings, but are no longer available to purchase. They use a standing pilot light, single stage heat exchanger and indoor air for combustion.
- Mid-efficiency (80 to 85 per cent AFUE). These also use a single stage heat exchanger with electronic ignition, indoor air for combustion and a power controlled vent fan that reduces the indoor air lost up the chimney, although some heat can still be lost even when the furnace is off.
- High-efficiency (90 to 97 per cent AFUE). The high efficiency is achieved through the addition of a secondary heat exchanger that recovers an additional 10 to 17 per cent of the heat produced by the burning gas. The cooler exhaust gases are vented through a wall instead of up a chimney.
How high-efficiency furnaces work
There is no pilot light in a high-efficiency furnace as burner ignition is achieved electronically. In the secondary heat exchanger of a high-efficiency furnace, the combustion gases are cooled to a point where the water vapour condenses, releasing additional heat into the building. The condensate then drains to a floor. The exhaust gas is cool enough to be vented through a plastic pipe, which is usually out the side wall of the building.
High-efficiency furnaces have induced draft room sealed burners and take their combustion air through a second plastic pipe from outside the building. These eliminate any loss of heated indoor air through the vent. Depending on the combustion and heat exchange design, fuel savings of up-to-40 per cent relative to standard efficiency gas furnaces can be achieved. Greenhouse gas emissions released into the environment are also reduced. In addition to the natural gas savings, a reduction of 20 to 30 per cent of the electricity used by the furnace fan can be obtained by selecting the variable speed brushless Direct Current Electronically Commutated Motor (DC ECM) blower motor, an alternative to the AC induction motor.
Energy Efficient Tips
- Annually have a qualified contractor inspect the furnace operation for optimum performance.
- Set back the thermostat when your home is not occupied and at night (this can done manually or with a programmable thermostat).
- Clean the furnace filter monthly during the winter and replace every three months.
- Keep all supply and return air vents free from obstacles for unrestricted airflow.
Does setting back the thermostat save energy?
Setting back the thermostat overnight or during unoccupied times will save energy. The amount saved depends on two factors – the duration of the setback and the number of degrees set back. The rule of thumb is you can save up-to-two per cent of your heating bill for every 1°C set back for eight hours duration. Other factors can impact savings and these include:
- efficiency of your heating system
- insulation levels of your home
- condition and orientation of your windows
- current outdoor ambient temperature
Research studies, taking these factors into account, have indicated optimum savings are achieved with a 6°C setback; however, you should be aware that such a setback may create other issues as outlined below.
When is a good time to set back my house’s thermostat?
There are two timeframes to consider when setting back your thermostat: short- and long-term intervals. Short-term setbacks are defined as overnight and unoccupied times during the day. Long term is defined as a time when you are away for more than 24 hours.
Short-term interval
For short-term intervals, you need to think about recovery time and comfort. Recovery is the amount of time it takes your home to warm up to a comfortable level after a temperature setback. This may take up to 1.5 hours. The size of the furnace as well as your home’s insulation levels and air tightness will impact the recovery period. A programmable thermostat can address this problem as it can be set to have your home at the desired temperature when needed.
Long-term interval
For long-term intervals, more energy can be saved with greater setback temperatures; however, other issues may arise.
The first relates to possible damage lower setback points can cause. Houseplants or other personal items may be affected by lower temperatures. You will need to determine what minimum temperatures will affect these items and act accordingly.
The second is more technical in nature. Lower temperatures can create potential issues with humidity and air circulation. Relative humidity increases as air cools causing condensation or ice to form on the inside of windows, especially if there is little air circulation. Air circulation is less when your thermostat is set back because the furnace fan runs less. This can be resolved by simply monitoring your house conditions and making adjustments such as opening drapes, running your furnace fan continuously or adjusting your humidifier. Increased fan operating hours will increase your electricity use.
High-efficiency condensing furnaces
An additional issue arises with high-efficiency condensing furnaces. The temperature of the air returning to these furnaces needs a minimum of 15°C to prevent damage to the heat exchanger and possibly affect the warranty. You need to check with your furnace supplier or a qualified contractor to ensure you meet the manufacturer’s criteria for high-efficiency furnace return air temperatures.
Alberta Government Furnace/Boiler Rebate
All Albertans who purchase and install a qualifying replacement furnace/boiler on or after January 1, 2009 are eligible for a provincial rebate. Visit Climate Change Central for information on the Furnace/Boiler rebates.Read Climate Change Central's article Fall into Efficient Home Heating for cold weather energy-saving tips.
