"$aving Money With Your Setback Thermostat Or Creating
Winnipeg, Manitoba - One of the most common questions that we get asked as contractors and service technicians are, "What temperature should we setback (lower) the heat to when the home is unoccupied," or if it's summertime, "What temperature should we set forward (raise) the air conditioning to when the home is unoccupied."
In 2005 the National Research Council (NRC), in partnership with Canada Mortgage and Housing (CMHC) and Natural Resources Canada (NRCan) conducted a research project in the twin houses of the Canadian Center for Housing Technology (CCHT) in Ottawa to evaluate just that, the effect of setback and set forward strategies on energy consumption. This study took place over one winter and one summer season.
The results revealed that in the winter even in a high-performance home there were savings in natural gas consumption from 6.5% to 13% depending on the setback strategy used. The strategies all used a winter daytime benchmark setting of 22ºC (71.6ºF). One used an 18ºC night setback (7 hours) only. Then they used an 18ºC (64.4ºF) day (7 hours) and 18ºC (64.4ºF) night (7 hours) setback. Finally, they dropped the temperatures to 16ºC (60.8ºF) day (7 hours) and 16ºC (60.8ºF) night (7 hours) setback.
Savings can be had, but at what expense? Comfort, damage to the structure and or its components (walls, windows, furnace). The results included the findings made mention of the potential for occupant discomfort from too low a temperature and/or too long recovery times to the desired 22ºC (71.6ºF). In the high-performance test homes the recovery times were 2 hours, so it's very possible to create discomfort from this aggressive setback strategy. In addition, they noted that if the temperature of the indoor surfaces of exterior walls was too low condensation could occur and damage building materials including encouraging mould growth. Window condensation or frost should be expected on the lower parts of the window in the very cold weather.
What about forced-air furnaces? As HVAC Contractors and Technicians we see first hand damage to heating equipment through premature failure of heat exchangers from over aggressive setback temperatures. Many homes have passive outdoor fresh air intake ducts connected to the return air, others have mechanical ventilation equipment providing a continuous minimum ventilation exchange, and finally many have continuous furnace blowers that circulate the air in the home 365 days of the year.
Knowing the Range of Rise
Furnaces are setup by the installer and designed to operate within a certain temperature range. We call this temperature rise. Temperature rise is calculated by measuring the supply (hot) air temperature and subtracting it from the return (cold) air temperature, the difference is the temperature rise. Manufactures place the actual operating temperature rise on the rating plate found within the appliance cabinet. Typical temperature rise might be between 7.2ºC (45ºF) - 23.8ºC (75ºF) or 12.7ºC (55ºF) - 26.6ºC (80ºF) depending on the make, model, type and efficiency of the appliance.
If the temperature rise is less than the minimum stated by the manufacturer, condensation can occur within the appliance in places it is not intended, resulting in premature rusting and corrosion of the heat exchanger and venting system. Conversely if the temperature rise is greater than the maximum temperature rise the results are overheating of components such as operating and safety controls, damage to the venting system and eventual stress cracking and failure of the heat exchanger.
Most furnaces today are approved to operate with minimum return air temperatures between 12.7ºC (55ºF) - 15.5ºC (60ºF). So homeowner's who set their thermostat too low or too high can negatively impact the life of their equipment, in addition to the structural and comfort related problems that could arise. Homeowners need to know if they have an outdoor air duct connected to the return air or if they have a mechanical ventilation system so that they can have a better understanding of the consequences of aggressively setting back the temperature in their home.
The research also looked at and concluded that the daytime temperature set forward strategy was not necessarily the most effective approach for cooling. The main reason for this is that fan and air conditioner electrical consumption depend on solar radiation. If every summer day was sunny and the set forward was raised from 22ºC (71.6ºF) to 25ºC (77ºF) the electrical savings could potentially be 13.2%. However, if everyday were cloudy and overcast that savings would be reduced to only 2.9%. In addition, the recovery times due to moisture loading of the structure during the set forward times turned out to be much longer, up to 7 hours on the hottest days, the same as the set forward itself. This length of recovery would certainly affect occupant comfort.
The best approach to reducing energy use in the summer is to follow good housekeeping practices with regards to the mechanical equipment, keep shades drawn in the hot part of the day, build homes with large overhangs, install or maintain awnings, plant shade trees, etc.
For summer, air conditioning should never be set lower than 23ºC (73.4ºF). On those hot dog days of summer its best adjust the indoor temperature to be within 6.5ºC (12ºF) - 8ºC (15ºF) of the outdoor temperature. At these temperatures the indoor air will still feel much better than outdoors, the air will be drier, more comfortable, and the risk of thermal shocking of occupants minimized. Often homeowner's do not recognize this risk until a family member becomes light headed and faints. Large public buildings and retail spaces must pay close attention to this especially during heat waves.
Caution, is the message that needs to be taken from the research as to how far a homeowner sets back their temperature during the winter months. A good "Rule of Thumb" for set back of not less than 19ºC (66.2ºF). However, remember "Rules of Thumb" can be bent and all homeowners should look for signs of distress to their homes structure and consult with their HVAC Contractor and Service Technician to ensure that the amount of set back that they are using will not cause harm to their heating appliance.
If you still do not have a programmable thermostat, get one, and make sure that you fully understand how to maximize your savings while at the same time ensuring that there will be no negative impact on your heating appliance. Some types of heating appliances are not well suited to setback strategies, so always consult with your HVACR Contractor or Service Technician. This article was written to discuss natural gas or fuel oil forced air furnaces and not to address all system types.
Visit these websites to locate local either an "HRAC Contractor" http://www.hrac.ca or "NATE Certified Contractor" http://www.natex.org/
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