My thermostat differential is set to 1 degree, can it be changed?

I keep my air on 82F with a 3 degree differential. It kicks on when it hits 85. I just got a programmable thermostat because it says it will save you money, however it will hold my temperature (while I’m home) at 82 within one degree. When I changed my degree differential to 2 degrees last year my usage and cost went up. I changed it back to 3 and it went down. Won’t a programmable thermostat keeping it at 82 cost me more? Is there a way to change the differential on a programmable thermostat? I have a Honeywell 5 day / 2 day thermostat with 4 time settings.

Answer from Green Energy Efficient Homes

First, for readers who aren’t familiar with the intricacies of programmable thermostats, we need to explain the difference between thermostat differential, and multiple programmable thermostat settings.

Every thermostat has a temperature gap or differential between the activation and shutoff of the cooling or heating equipment. For example, if you set your thermostat cooling temperature to 82F, the thermostat might activate the air conditioning when the temperature hits 83F, and shut if off again when the temperature hits 82F. The gap between activation and shutoff is the differential – in this case, (83F-82F = 1F).


Programmable thermostats are a particular type of thermostat, where you can program the thermostat to switch between economical and comfortable temperatures at preset times of the day. Some programmable thermostats let you set a different time program for each day of the week, while others only give you a choice of weekday or weekend settings. You can generally set the economy and comfort temperatures, and the times to switch between them, across a fairly broad range of values.

Back to programmable thermostat differential: on many thermostats you cannot change the differential. On others, it may take the skill of an experienced HVAC technician to do so. This means that generally you are stuck with the differential that is factory preset, whether you have a programmable or a manual thermostat.

What is the effect of differential on efficiency?

The bigger the differential between activation and shutoff, the longer it takes the air conditioner or heater to bring your living space to the comfort temperature, and the longer it takes your living space to go back to the temperature at which the cooling or heating starts up again.

Too small a differential can cause short-cycling, in which the air conditioner or heater kicks in for short periods, then shuts off when the small differential is spanned. Short cycling increases the number of times the system starts up, which causes wear and tear on moving parts; it is a bigger problem with air conditioners than furnaces. Short cycling is not a problem for passive electric heaters, but electric heaters with fans can be affected because of the moving fan parts.

The converse problem of having a really wide thermostat differential relates to the peak operating efficiency of furnaces or air conditioners. Most furnaces have a low-fire and high-fire setting; the low-fire setting is used to maintain current temperature or bring temperature up slightly, while the high-fire setting is used to raise the temperature quickly. When you have a wide programmable thermostat differential, the furnace typically kicks in on low, but after a few minutes if it has not achieved the comfort temperature, it moves to high burn, which is less efficient. This means that a very wide differential on heating increases the percentage of heating accomplished by high burn, which increases overall gas usage.

High efficiency, newer air conditioners also have different efficiency levels for high and low cooling power. When the air conditioner has a small cooling load it typically slows down the compressor so that it operates at a more efficient speed (in terms of BTUs of cooling power per kwh of energy used). This would suggest that a low programmable thermostat differential such as you have, would lead to better efficiency than a high differential, because the unit would cycle more often. However, if the space being cooled is small, or if the thermostat is placed too close to the air conditioning vents and is therefore instantly cooled down as soon as the thermostat turns the air conditioning on, you will experience short cycling and both increase energy usage and wear out the equipment.

With that background in mind, let’s get back to your original question, which I interpret as: If I have a manual thermostat with a differential that can be changed, and past experience suggests that setting it to a 1 degree differential means it takes more energy to operate than setting it with a 3 degree differential, will a 1 degree programmable thermostat differential mean that the reduced efficiency of the lower differntial eat up all the savings that a programmable thermostat could otherwise provide?

To answer that question we need to understand the relative savings obtained by (A) varying the temperature setting between economy and comfort settings, and (B) having a wider or narrower programmable thermostat differential. In general, if you are routinely out of the house for several hours during the day, and allow the air conditioning temperature to climb considerably from its comfort to its economy setting, you can easily save 5-15% on your energy bills (depending on the range of temperatures, your home insulation levels, indoor/outdoor temperature spread, etc.). If you use the economy setting at night when the sleeping human body can tolerate a bit more heat, those savings climb further. If you found that the 3 degree differential on your manual thermostat saved you more than 15% over and above the 1 degree setting, it may be that you are better of sticking with the manual thermostat.

But my hunch is that the programmable thermostat is the correct choice, because there are so many variables that affect your air conditioning energy consumption. It’s impossible to conduct an accurate experiment where you switch the thermostat differential from 1 degree one day (or one week) to 3 degrees the next, and compare energy consumption, because in all likelihood several variables have changed: day length changes through the year (which affects solar heat gain by your home); outdoor temperatures change (which affects temperature gain through walls); humidity levels change, which affects the effort the air conditioner expends removing humidity. And if you pay time of use for your electricity (where the utility charges you different rates for different times of day) your energy costs may change even if your kwh of consumption did not.

If you find that a programmable thermostat differential of one degree is increasing your actual energy usage versus the old manual thermostat with the three degree differential, by all means switch back – but I think this is an unlikely scenario. Either you will save energy with the new thermostat, or there are other factors than the differential affecting overall consumption. Best of luck, whatever you decide to try!

10 replies
  1. Allison
    Allison says:

    I bought a high efficiency furnace/AC (American Standard, Platinum Series) and it has COST me an average or $40 more per month in electric costs. (Little. Or no change in gas consumption). What is wrong and how may I fix it.

    Reply
    • Robin
      Robin says:

      $40 per month more in electric bills is a lot. Let’s assume a price of $0.20 per KW for argument’s sake – that means your furnace could be contributing 200 kwh of extra consumption a month, or 6.6 kwh a day. The Platinum 95 model I checked on the web has a half horsepower blower motor, which translates into 372 watts. So if your fan is running at full speed continuously that is the likely culprit. Some things to check:
      1) Make sure your thermostat has the fan set to “Auto” mode, not “On” mode.
      2) Turn down the default fan speed. On my Bryant furnace, the manual provides instructions on how to change dip switches to set the fan speed. I did this early on to lower the fan speed and this saved me a fair bit of energy. If you’re not sure how to do this, ask your installer how to do it, or ask them to change it.

      If the fan is not running continuously then something else may be wrong. If you get a whole house electricity monitor you can check for current draw with the furnace running and not running (idle) and disconnected (from the circuit breaker) and that may provide clues as to whether it is really the furnace causing the increased usage, or something unrelated that just happened to get added to your consumption around the time the furnace went in.

      Reply
    • John
      John says:

      Depending where you live (such as Ontario), conserving is relatively useless because they keep upping the price of electricity.

      Reply
      • Robin
        Robin says:

        I disagree. In fact, conserving is even more useful if they raise electricity prices because the less electricity you use, the less an increase in prices affects you. I think perhaps your point is that conserving may feel useless, because the savings you thought you were going to get might be eaten up by increased prices. But the motivation to conserve should become stronger the higher electricity prices get.

        My own view as an Ontario resident is that the current provincial government’s plans to cut electricity costs are a wrong-headed knee-jerk reaction to their fear of being tossed in the next election, instead of sound energy policy. The best way to motivate conservation is to keep increasing prices. I hear of people who are spending $400-$500 a month air conditioning their homes in the summer and griping about electricity costs. That’s because they’re trying to live in luxurious comfort in their poorly insulated monster homes during a heat wave. I live in a small, well insulated hundred year old house and even during heat waves manage to keep my bill below $100 a month because I am modest in my use of AC and have done as much energy retrofitting as I can. I realize some people heat with electricity and you do need to stay warm, but a programmable thermostat, combined with weatherproofing, insulation upgrades, new windows/doors etc. is a great way to cut the impact of high (or rising) electricity costs.

        Reply
  2. Allison
    Allison says:

    Service folks coming next week. Variable fan speed makes it hard to tell how much or when it’s blowing. I’ll follow up after service. Thanks. But where do I get the whole house electric monitor?

    Reply
  3. e
    e says:

    I got a Honeywell FocusPro.
    It only turns the heater on for 5 minutes at a times no matter what. It looks like it’s trying to maintain temperature but the fan part is on more than the heating is. The room feels cool instead of warm on cold nights. Like the Honeywell controller but don’t want to be up all night turning heater up to get warm (have multiple health problems)

    Reply
    • Robin
      Robin says:

      Andy, I am pretty sure the original question was about air conditioning not heating. So 70 or less would significantly INCREASE costs :), rather than save money.

      Reply
  4. Brian
    Brian says:

    I have found when it comes to air conditioning that longer cycles and bigger differentials are more efficient. The reason being that it takes a few minutes for the compressor to get up to peak efficiency. It has to pump up the condenser and lower the evaporator pressure and all that takes time so until the unit has been running for several minutes, it has not reached peak efficiency. Shorter cycles mean more total time spent in the less than peak efficiency range. However back to the original question, with the Honeywell programmable thermostats, you cannot set the differential. They operate off a different principle where they are not looking for a fixed temp on and temp off. Instead they use proportional integral control (PI). They are looking at the setpoint and how far the temp is away from the setpoint. Based on the error, it increases an output percentage. That percentage is used to calculate how long it should run and how long it should be off. You can set the cycles per hour in a Honeywell thermostat which will increase the time that the unit runs and the off time. But you cannot set a differential.

    Reply

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