Setback Thermostats | CMHCThis is some information I found at Canadian Mortgage
and Housing.
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Setback Thermostats
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The following fact sheet is part of the About Your House —
General Series
Setback Thermostats
What is a Setback Thermostat?
Thermostats control heating and cooling appliances in houses.
A setback thermostat gives the user the option of changing the temperature
setting automatically during the nights and during the work day when the
occupants have left the house. A setback thermostat can help reduce overall
household energy consumption.
A conventional thermostat simply regulates house heating at
one temperature. For instance, in the winter, if you set the thermostat to 20°C
(68°F), it will activate the heating system when the house temperature drops
below 20°C and will shut the system off when the house air warms up past 20°C.
A setback thermostat contains an electronic clock. It can
automatically turn down the temperature setting at night, when you are asleep,
or midday, when you are at work. It can also return the temperature to a more
comfortable level before you wake up or arrive home from work. That way you can
have the energy savings of a lowered thermostat setting without the discomfort
of having to wait for the house to heat up again.
The setback thermostat can also be used as a set-forward
thermostat for an air conditioning system. It can allow the house to heat up
when it is unoccupied and return it to comfortable temperatures before you
return from daytime activities.
Although this About Your House deals with setback thermostats
and forced-air heating systems generally, you can apply some of the advice to
electric baseboards or to summer usage.
You can use a standard thermostat to set your house
temperatures lower during times when the house is unoccupied. This will lead to
similar energy savings as with a setback thermostat but not the convenience.
What is a Normal House Temperature?
CMHC has randomly surveyed Canadian houses. Thermostat
settings in winter tend to be quite closely grouped around 20°C – 21°C (68°F –
70°F). Summer temperatures range much more widely, depending upon whether the
house has air conditioning.
To What Temperature Should I Set Back the Thermostat?
The more you reduce the thermostat setting, the greater the
possibility for savings. Generally, a drop of two degrees will lead to some
savings and little risk. Some householders reduce temperatures four to six
degrees. However, temperature differences this large create potential comfort
and moisture problems that are discussed below.
Does Setting Back the Temperature Save Energy?
Yes. Research from the Canadian Centre for Housing
Technology, shows that winter setbacks for the houses tested would result in
heating costs savings of five to fifteen per cent. The highest savings came
with a setback of six degrees.
Savings for summer were about the same, although simply
raising the thermostat set point in summer from 22°C (71°F) to 24°C (75°F) led
to more significant savings than the set-forward strategy and offers better
indoor humidity control.
Note that these savings are for two airtight, well-insulated,
unoccupied houses. The savings in your home may vary, but are likely to be in
the same range.
What Can Go Wrong?
There are several potential problems.
The first one is comfort. A cool house can be uncomfortable
in the winter. If you wake up at night before the time the thermostat resets to
20°C or 22°C, the rooms will be cooler. The same applies if you have a setback
daily when you leave for work, but you return home earlier.
A setback schedule works best for people with predictable
work and sleep periods. If your schedule is completely irregular, you might as
well simply turn down a conventional thermostat when you sleep or leave the
house, rather than trying to anticipate it with a setback thermostat.
A second problem is the possibility of high humidity in the
winter.
Cool air can hold less moisture than warm air, so the
relative humidity (RH) rises as the air cools. For instance, house air at a
reasonable 35 per cent RH at 22°C will see an increase to 50 per cent RH when
the same air is allowed to cool to 16°C
(6°F). This can lead to condensation on windows and walls
(for instance, in closets or behind furniture).
Basically, you are creating a more humid environment, all
things considered, when you allow the house temperature to drop significantly.
This may not be a problem in a dry house or one where you can modify the
humidity, for instance by turning off a humidifier. The house humidity should
be monitored, especially in winter. For more information on how to measure
humidity in houses, go to Measuring Humidity in Your Home.
Window condensation will be the first place that you notice
excessive house humidity. Blinds and curtains usually increase the occurrence
of condensation. If condensation is taking place, make sure that you wipe it up
diligently or raise the temperature setback (eg. from 16°C to 17°C) until you
have lowered house humidity levels and condensation is no longer a problem.
A third potential problem is a sustained lack of air
circulation as the house cools down.
If you set back your thermostat in the evening from 20°C to
16°C, the house may not cool to 16°C until 5 a.m. During that time, there will
be no furnace circulation fan operation, unless you set the fan to run it
continuously. For houses where bedroom doors are kept closed, this could lead
to stuffiness in those rooms and a lack of fresh air.
There are ventilation devices and furnace fan cycling
controls that can compensate for this, if you find bedrooms and other rooms
underventilated during setback periods. Setting back to a higher temperature,
such as 18°C (64°F), will reduce the period of furnace inactivity (and your
consequent savings). Setting the furnace fan to run continuously will help but
this will increase electricity consumption.
The final potential problem with setback thermostats is the
time required to regain temperature, or to heat the house up once more.
An oversized furnace will return the house to the higher
temperature quickly, but a properly sized furnace can take a long time — as
much as one hour — to bring the house back to a comfortable temperature. You
have to experiment with this as it will depend on the type and relative size of
your heating equipment and your home’s energy efficiency and other factors.
If it takes an unacceptably long time for your house to heat
up again, consider a smaller setback or start the high temperature setting
earlier so that it is comfortable for you when you get out of bed or return
from work.
Related to this problem is the length of time required for
the room enclosure and furnishings to heat up. If your bathroom has cooled to
16°C overnight and the furnace has kicked in at 6 a.m. to bring up the heat,
your bathroom walls will still be cool when you start a shower at 7 a.m.
Testing in the research house at the Canadian Centre for Housing Technology
house showed that it takes up to several hours for wall temperatures to return
to their highest temperature. Cool wall surface temperatures can lead to
comfort and condensation problems.
What Houses Have the Greatest Risk for Problems?
Many houses will not see significant problems with the use of
setback thermostats, but there will be some houses more at risk.
If you have a house with moisture problems in winter (mold,
condensation on windows, and so on), get those problems fixed before you
consider installing a setback thermostat or set back your conventional
thermostat.
If you have a forced-air furnace and closed bedroom doors,
you will need to ensure adequate ventilation of bedrooms at night, one way or
another. If you already keep your house at an unusually low temperature (such
as 18°C), then a temperature setback is more apt to get you into the high
humidity danger zone. If you are the sort of person who has difficulty
operating a VCR or DVD player, you will find many electronic thermostats
confusing, especially when compared to the simplicity of an older dial
thermostat.
So, How Do You Set These Things Anyhow?
Check your manual or ask a teenager to do it. We can’t help
you with this. Each system is different.
All These Problems — Is It Worth the Trouble?
Yes, it can be worth the trouble. If your house is in good
condition, if you do not have excessive moisture problems, if you have a fairly
regular schedule, then by all means use a setback thermostat. It will save
energy. Just be aware that all energy-saving activities can have unexpected
results, such as those outlined in this article and be sure to take steps to
avoid these problems.
Related Links
a.. Measuring Humidity in Your Home
b.. Replacing Your Furnace
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