(e.g why does 70 degrees on “cool” feel colder than 70 degrees on “heat”)?
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Depending on the system, most of them just blast cold or hot air, much colder or hotter then the preset temperature. Once the air return has picked up air flowing at the right temperature it shuts down. Then repeat.
When you’re cooling, the air conditioner is also dehumidifying the house. When you’re heating, the heater does not dehumidify. This is why the same temperature will feel different.
Because today it’s 55 at night and 90 in the day. I open my windows to get it down to low 60s inside in the morning and don’t want it to heat. Then I don’t have to run AC until mid afternoon when my home finally exceeds 73.
Having a single temperature setting would mean the unit would have a hard time knowing what to do, heat or cool. Some thermostats will let you set a high and low temperature, to achieve what you desire. We keep ours on 67-72, so the heat comes on if it gets down to 67 the heat comes on, if it gets warmer than 72, the AC comes on.
In my house we have a unit that heats and cools with the same coil. If the system runs in one direction, it gets cold. When it runs the other direction the coil gets hot. It’s ideal for it to just run in one direction. If it kept switching directions a lot of time and energy would be wasted heating up the cold coil and cooling the hot coil.
There are homes that use separate systems but they most often use the same vents so it works a bit in the same way. Your vent is full of cool air when it’s on cold, switching it to hot you still need to push out that cold air still in the vents and warm them up before you get decently hot air.
The final point is that when you’re cooling your home, it’s most likely hot outside. When the cooling is not active then your house is going to get warmer naturally, you don’t need the heat to activate. Of course vice versa when it’s cold outside.
In addition to all the excellent descriptions of why you don’t want to engage a mechanical system to turn on and off excessively, in many locations there are energy codes requiring a deadband- that is a band of temperatures between heating and cooling to ensure you don’t waste energy by overcooling and then starting your heater to rewarm the same area.
Sometimes you have to heat up to get to a temperature, and other times you have to cool down, and these involved two different systems (one A/C and other Furnace).
As a side, A/C creates such a chill that 23 on A/C feels colder than 20 without it.
Probably the biggest factor is humidity. At any given temperature, the air is capable of holding a certain amount of water vapour in it without it condensing. We measure the humidity commonly in relative humidity, that is the amount in the air as a percentage of the maximum value for the current temperature. Because humans regulate body temperature by having sweat (mostly water) evaporate from our skin, the ability for this to work is highly dependent on relative humidity. If the relative humidity is low, sweat evaporates easily, and we can regulate our body temperature easily, so it feels comfortable. If the humidity is high, it does not.
The actual amount of water that the air can hold as vapour is heavily temperature dependent. 100% relative humidity on a cold day might have the same actual quantity of water vapour in the air as 30% on a hot day. If you take cold air and heat it up, the result is air with a very low relative humidity because no extra water is added. If you take hot air and cool it down, you will probably end up with actual condensation and air that is 100% relative humidity. Air conditioners are designed to compensate for this, by cooling the air down much colder so that the water condenses out, then bringing its temperature back up to the desired value, and adding the right amount of water back into the air to achieve the target humidity. What most people think of as air conditioning, though, is just air cooling, that does not control humidity like this. Heating systems almost never incorporate humidification. This means that if you run the heating on a cold day you end up with warm and dry air, but if you run the cooling system on a hot day, you get warm but very humid air.
The Heat setting controls the furnace and the Cool setting controls the air conditioner. The furnace puts out hot air and the AC puts out cold air. The job of the thermostat is to regulate the temperature in the home, not the temperature of the air coming out of the furnace or the AC.
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