"$aving Money With Your Setback Thermostat Or Creating
More Problems"
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.
Sensible Savings
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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