Programmable thermostat temperature spread

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 85F, and shut if off again when the temperature hits 82F. The gap between activation and shutoff is the differential – in this case, (85F-82F = 3F).

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 most programmable thermostats you cannot change the differential. So generally you are stuck with the differential that is factory preset, whether you have a programmable or a manual thermostat.

I checked through several Honeywell programmable thermostat specifications and all of them seem to have the same temperature differential of +/- 1F, which I interpret as not a 1 degree spread but a 2 degree spread (1 degree C though).

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 and heat pumps 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. A ‘smart’ furnace coupled with the appropriate programmable thermostat shouldn’t have this problem however; the only time the furnace would go into high-fire setting is when it’s very cold out and the furnace is having to work hard to close the spread, or when the temperature setting changes significantly (for instance when the morning cycle kicks in).

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). I haven’t verified this, but in theory the same behavior follows for a heat pump (which is really just an air conditioner that can also run in reverse): the heat pump tends to operate more efficiently on a low constant heating load than when being frequently switched on and off due to thermostat differential cycling. One additional challenge for some heat pumps in the heating season is the fast-heating mode. If the gap between desired temperature and actual indoor temperature is too high, it may kick in the auxilliary heat, which is typicaly a gas furnace or an electric heating element, both of which are much less efficient than the heat pump itself. This means you wind up using more energy to heat your home than you would if the heating was more constant.

The above 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 of 1 degree (or 2 degrees if we assume your thermostat is the typical “+/- 1F”), and past experience suggests that setting the old non-programmable thermostat to a 1 degree differential means it takes more energy to operate than setting it with a 3 degree differential, will the energy savings of using a programmable thermostat be countered by the reduced efficiency from the 1 degree differential this programmable thermostat temperature spread seems to be set at?

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 off sticking with the manual thermostat.

There are also factors external to the one you changed (temperature spread from 1F to 3F) that may have affected your savings without you realizing it. For example, when you saved money with the 3F spread, was the outdoor temperature warmer (winter) or cooler (summer)? Was there some other use of energy in the home (water heater, stove for fossil gas, or any appliance / lights for electricity) that might account for the difference in energy?

I think a programmable thermostat is the correct choice, because there are so many variables that affect your air conditioning energy consumption. The fact that so many Honeywell programmable thermostats have the same +/- 1F spread suggests that modern furnaces and air conditioners can operate efficiently with this spread. The key is to make sure you avoid short cycling by putting the thermostat far enough from a direct heat/cooling source, and putting it in a room that doesn’t heat or cool quickly. For example, if the room where the thermostat is present is near a heating/cooling vent, damping the vent louvers or the actual ductwork to reduce airflow will help reduce short cycling.