I have a question about the use of programmable thermostats for furnaces/air conditioners. What temperature variances are within acceptable limits?
In the winter, I like to have the temperature in the house around 22C. During the day when no one is at home, I let the temperature drop to about 15C, then an hour before I get home the furnace kicks in to 22C. At night, it drops to about 19C and again heats up about 45 minutes before everyone wakes up. So the hour before coming home, the furnace runs steady for that full hour, or so, bringing the temperature up. The other option would be to have it only drop a couple of degrees during the day.
Which scenario would be harder on the furnace? Lots of inactivity during the day with a long continuous period of activity, or a lot of stop/start activity during the day. One furnace inspector said furnaces are made to run so it’s not doing any harm and the actual harm would be more in the constant starting and stopping. Others say that the long period of activity would actually be harder on the furnace motor. In the summer, I let the house get to about 26C and again let the air conditioner start up an hour before coming home.
I want to lower my heating bills but don’t want to get stuck with a large furnace repair bill.
This is a very interesting question about a very common energy efficiency upgrade in many homes. Electronic, programmable thermostats are easy to install, moderately priced and can reduce energy consumption and automatically make homes more comfortable, particularly at night. Good quality thermostats are now available for less than a hundred dollars, making them a smart investment, especially when upgrading a furnace.
The reason that you are getting different opinions on this issue is that the answer to this question will largely be a product of the personal experience and training or each individual you ask, which will vary widely. I am not an expert in heating systems, but have good general knowledge, so take that into account when weighing my comments, as well. Even licensed heating contractors that have been in business for decades and installed hundreds of furnaces may differ in their answers. This is because the technology for furnaces has changed so much in the last 10 years, that issues that may have applied to a standard efficiency furnace manufactured in the 1980’s will have little bearing on a new High Efficiency unit.
This and many other factors or variables may also have bearing on how your questions are answered. The high tech controls in newer furnaces make the use of electronic thermostats more practical, where an older unit may not respond as quickly to frequent changes in your thermostat program. Newer furnaces likely have more efficient and more powerful blowers to improve air circulation within the home. Also, the overall energy efficiency of your home, including insulation levels, windows, and air leakage will also have an effect on how hard your furnace works.
This may not satisfy all your concerns, but to directly address your questions, you should be able to vary the temperatures as much as you wish. I agree with what the “furnace inspector” said about furnaces being designed for continuous operation, provided it is not extremely old or deteriorated. If the blower motor has worn bearings or the heat exchanger has deteriorated welds, continuous operation for an hour at a time may finish it. If the furnace is well maintained and relatively new, changing the frequency and duration of the cycles should have little overall effect on the unit.
The last thing I would like to address is one possible misconception you may have. Just because you drop the thermostat setting to 15C in the daytime, doesn’t mean that the house will automatically reach that low temperature. With no home occupants opening doors or windows on moderate days, if you have a well insulated home, it may take a long time for the house to cool 7 degrees. Conversely, it should not take a full hour for a furnace to raise the air temperature in a home a home 5 or 6 degrees. Your furnace is not likely running this entire time, except on the coldest days where it may run almost continuously. So, excess wear and tear on your furnace should not be excessive even with your variable thermostat settings.
Furnaces should be sized for a home to run almost continuously on the coldest days of the year. Variable temperature settings should have much less effect on a furnace than poor maintenance. Regular annual servicing, which should be a complete cleaning and servicing not just a safety inspection, along with regular maintenance will do more to prevent damage to a furnace than anything you can do with your thermostat settings. Changing filters regularly, cleaning air conditioner condensers, maintaining sufficient combustion air and other regular maintenance will also help avoid unnecessary repair costs.