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Heating, Air Conditioning, Fridge, HVAC/frequently varying thermostat settings


I have a question about heating efficiency. To preface, I have read a lot about programmable thermostats and in general varying set points. I keep reading that changing thermostat set points too often will cause you to use more energy, but I would like to know why (from an expert).

My question is regarding forced air, resistive or any other Full-on/Full-off systems. I understand that boilers and heat pumps might add a lot of complexity to this idea, but it is more relevant for me looking at forced air which is prevalent where I live.

So with a system that is either fully on or fully off, why would it lower efficiency to only change the temperature for a couple of hours? What inefficiencies are there in the raising and lowering of temperature? The furnace always operates at it's set intensity, it simply varies the amount of time it stays on. Shouldn't any time that the ambient temperature spends below a "baseline" represent energy savings?

I've been really hoping for a professional opinion on this matter and your effort is very appreciated. Thank you.

The efficiency of the system and the setpoint on a thermostat are not the same.  Efficiency is how much it costs for the unit you have to heat and cool.  The unit is going to use a certain amount of fuel, per a period of time.  The fuel can be electricity, oil, gas etc.  The efficiency is really a comparison to how much energy is wasted to heat and cool.  An example would be a furnace.  An 80% afue furnace has 80% of the heat energy going in the home and 20% going out the stack.  The higher the number the less that is wasted out the stack.

The way it is managed (thermostat), sizing of the system are a whole other issue.  Granted when systems are not installed properly some efficiency can be lost.

Raising and lower temp setpoints is not cut and dry.  There are too many factors that go into wether or not to raise and lower setpoints.  In some cases it might not make sense to or even practical.  IE, before I knew the trade I lived in an apartment complex.  I would turn up my stat up during the day.  My neighbor was home all day.  I found out that our power bills were the same.  My place was hot when I got home and took forever to cool down.  It had to do with the size of the unit and other factors.  I soon discovered I could leave my unit at the same temp all day and my power bill did not change. I had a cool home when I got home from work.   

In your case think about this.  You leave for two hours.  The outdoor temp does not change for a period of six hours.  The in two hours before you left the unit ran for 60 minutes total during the on cycle.  You leave and set the temp up or off.  The unit does not run.  You get home and set the temp back to where it was.  The unit might now have to run for nearly the next two hours. It might take 60 mins of run to catch up, then the other 60 mins to maintain.  It might not even catch up until the outdoor temp changes if the change was drastic.  In that case, you did not gain anything.

You are going to gain more as the time frame increase.  The run time is going to change with outdoor temps, among other factors.  Though there are times when the outdoor temp has no bearing on run time.  What you will see is a change of a couple of degrees one way or the other over the period of a week or month, not hours.  Heating costs are killers at home.  We are normally home as it gets colder outside.  Cooling is different, we are normally home as it cools down outside.


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Craig HVAC Expert


I have been in the HVAC field for the past 18 years. I can help with most HVAC questions. I work on commerical buildings for the most part, and have yet to find anything I could not troubleshoot and repair, when repairable. I work on small 1 ton units to a 2500 ton chiller. Troubleshoot air flow, elect, and control problems. I attend regular classes to keep up with the latest and greatest.


I work on commerical sites, hospitals, gov't buildings. I can troubleshoot just about anything in the HVAC business.

5 years trade school, VFD training classes, Liebert factory training, some York and Trane factory training.

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