(e.g why does 70 degrees on “cool” feel colder than 70 degrees on “heat”)?
In: Technology
Instructing the system to repeatedly cycle between heating and cooling modes to maintain a narrow temperature range would consume A LOT of electricity/fuel as the system would potentially be running nearly constantly.
While on a hot summer day, you really don’t want to run the heater just because the AC ran a little longer than necessary.
The same reason why analogue thermostats have them. “Heat” turns on the furnace and “cool” turns on the AC unit. Which is the same reason why one feels colder than the other. On heat the thermostat is trying to reach its sets temperature so the vents are blowing hot air. On cool the opposite it happens. In order to reguajte the temperature of the whole house the thermostat is place away from vents. So a room may be cooler or warmer than the set temperature when the thermostat turns off.
Depends on the thermostat, but generally cool is connected to an AC unit while heat is connected to a furnace. When heat is on the temperature might go above the selected temperature and when cool is on it might go below the selected temperature. It would be extremely inefficient to make the temperature be exact since a house has a pretty large volume. Thus the heating/cooling system is more of a greater than/less than system.
Your heater and your AC are two different systems. Only one of them operates at a time. If the system is set to 70 the heater or ac isn’t puting out 70 degree air. It’s puting out air as hot or cold as it can. That air then diffuses through out the house.
The temperature on the thermostat just tells your system when you want it to stop blowing out air.
There are two interesting parts to this answer. The first is covered by most of the other comments, in that it would be impractical to maintain the temperature at exactly one degree (e.g. 70). Instead it makes more sense to have a cool setting – “keep it at least as cool as 70 degrees”, and hot setting – “keep it at least as warm as 70 degrees”, to allow a little wiggle room.
The other portion not covered in the other comments (at least so far) is that how cold or hot it feels is based on more than just the absolute temperature. Other interesting factors include things like direct sunlight, air flow, humidity, and air pressure. Besides the seasonal differences you may have for these that make you feel like 70 in the winter feels different than 70 in the summer, AC also serves as a dehumidifier, contributing to an AC induced 70 feeling different than a heat induced 70.
70 is the goal temp, but HVAC systems use much cooler or much hotter air to efficiently get the temp to that level. An AC might blow 40 degree air, while a furnace blows 100 degree air. Each does do until the hot/cold air mixes with the ambient air to hit the desired temp.
Also, an AC turns on and cold until temp drops to 70, while a furnace turns on to warm until it heats up to 70.
None of these answers do a good job of explaining why the switch between heating and cooling isn’t automatic. Put in a buffer of +-5 degrees, and let me never have to worry about messing with the thermostat again.
Some do. The Ecobee thermostats have an “AUTO” setting option, that will compare the temperature comfort range you’ve set against the inside temp sensor and the outside internet based weather rating, and decide to switch between HVAC and heat modes. You still can set different values for ‘heat’ and ‘cool’ simply because people tend to be comfortable at a wide range (say for example 65-75F) so when ‘heating’ no need to go to the max end of the range, and when ‘cooling’ no need to go to the min end of the range.
But I think your parenthetical has been answered already by others. Humidity, other modifiers on absolute temp (which the Ecobee also has some settings adjustments for). Some even have so-called ‘energy savings’ modifiers which try to target use outside of peak cost times. e.g. pre-cool the house a bit more below the desired max set point earlier in the day, so as to not have to work as hard during more expensive times. So while set to “70” it isn’t always truly at 70. The balance of your ducting room to room, and sun load also make a difference.
Knowing that AC in particular gets LESS effective the bigger the difference is between the desired interior setting and exterior ambient temp, that they’d let you set a moving target, e.g. I’d prefer to keep inside at 75deg F max but if outside temp is higher than that by 20 deg F or more, then just keep inside at (exterior – 20).
Most people have covered the whole AC and furnace thing, I wanted to go over how a house HVAC works in comparison to a car HVAC (assuming both as a new-ish model) because I think it’s important to know this to understand the other answers.
Your car HVAC tries it’s best to output air at the temp that you set it to. So if you set it to 70, it tries to output 70 degree air, regardless of what temperature it is outside (or inside).
In your house HVAC, you set the temperature that it starts/stops at. The heat and cool setting determines the min or max of the setting you chose.
Example: if it’s 75 degrees in your house and your thermostat is set to 70-cool, the AC turns on (because it’s higher than the max) and shuts off when the thermostat reaches 70 degrees or less.
Set it to 70-heat and nothing happens because it’s already passed 70. However, when the sun sets and temperature drop to below 70, the heat turns on (because it’s now less than the minimum). Until it reaches 70, then turns off.
Interesting note. I had an uncle who had a system in his house that the thermostat didn’t show a temperature at all. You just tapped out cooler or hotter depending on how comfortable you were. It was supposed to be more energy efficient than seeing a temp.
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