I get asked this question often and the answer is typically something along the lines of "that depends". The engineers who design residential comfort cooling and heating products designed the equipment to handle a fairly forgiving spectrum of comfort levels so that people will have choices without damaging the equipment. There are some guidelines that we must follow to protect the equipment so we will start with those.
In the cooling mode or summer time, we really should not be setting the system below 68 degrees generally speaking. Keep in mind that I'm specifically speaking to humid climates in my article as that's where I live, equipment in arid climates such as Nevada, New Mexico, and Colorado are totally different. When we drive the indoor temperature down we are also driving the dew point of the air down and thus reducing the equipment's ability to remove moisture. In a humid climate I don't think I have to stress why this could be an issue. If you think about an average house where we set the thermostat at 67 degrees and our indoor relative humidity is 62%, that means our dew point is 53 degrees. This means that something has to be below 53 degrees to remove moisture from the air. The evaporator coil is what does this and if the return air is 67 degrees moving at roughly 375 cubic feet per minute, our coil is likely to range from 35 degrees in the beginning but closer to 47 degrees before the refrigerant leaves the coil. Since that 47 is so close to the dew point, we may not be removing the right amount of moisture from the air and potentially causing issues within the home not just in terms of comfort.
We likewise don't want to set the temperature too high in the cooling mode. I know that summer is a great time to vacation and many people think to set the temp higher to save on electricity while they are away. This is a good idea but there's a limit to that too. Engineers will say not to set the indoor temp higher than 80 degrees and to make a very long explanation short, that's to protect the compressor as most residential comfort cooling systems are not designed to cool with a return air temp above 80. I'm going to go a step further and say if you have duct work in your attic, I wouldn't recommend setting the thermostat any higher than 78 degrees. Duct work in an attic in the summer time can pick up convection gains as further heating the air before it can return to the evaporator coil. There are a ton of factors here such as length of return duct in the attic, insulation value of the return, how tightly the return is sealed from the attic air. I prefer to air on the side of caution and say set it to 78 and you should be fine.
We're going to shift here and talk about heating mode or winter, but I need to specify that I will be specifically talking about heat pumps. Fuel fired heating appliances are a little different and kind of a dying breed as our current shift towards electrification across the United States is really taking hold. In the heating mode I really don't like to see the temperature set any lower than 60 degrees and rarely do I ever run into this so I'm not going to harp on it. In all my years of doing HVAC, I've only met 3 clients that say they set the heat below that. Engineered data will typically agree amongst the big residential manufacturer's that setting the heat higher than 76 is not really recommended. This primarily has to do with the fact that if we run a heat pump too long in the heating mode at too high of a temp, again we can start to damage the compressor as the indoor coil won't be able to give off enough heat at a certain point. Heating is very forgiving in a heat pump especially since most have auxiliary heating elements to assist with lower ambient conditions.
Hopefully this helps to understand the guidelines and recommendations. Keep in mind this is a very general article specifically geared towards heat pumps installed in humid climates. This is what we mainly see in my area, equipment in your area may vary so I recommend consulting your local HVAC professional if you have any questions. Thank you for taking the time to read.
Kenneth Casebier
Comments