5 ms·

> I just want my thermostat to hold whatever temperature I set it at, until I set it at another temperature. If it only takes one of heating or cooling to do t

by pdonis 4y ago

> I just want my thermostat to hold whatever temperature I set it at, until I set it at another temperature. If it only takes one of heating or cooling to do this, not both, a standard "dumb" thermostat can of course do this just fine. Just set it to "cool" or "heat" mode and set a temperature. But of course you will need to change modes as the season changes (unless you live somewhere where only one mode is ever needed). If you want to not have to change modes when the season changes, it gets more complicated, because the simple control system in the standard thermostat is too dumb for that. For example, suppose you set your temperature to 75. The actual temperature rises to 76, above the setpoint, so your A/C comes on. It cools the house to 74 and then the thermostat shuts it off (you have to have some buffer so the A/C doesn't come on again almost immediately). Now the temperature is below the setpoint, so your heat comes on. It heats the house to 76 and then shuts off. Now the temperature is above the setpoint... You can see what's going on. From a pure design perspective, the right way to solve this problem is an outside temperature sensor (and probably a sun sensor as well since sunlight is a significant heat input to your house), so the system can predict what will happen when both cooling and heating are off. Given reliable sensors, the control algorithm is fairly simple (automatic climate control systems in cars already do it). I have never seen this in a house, though. I'm not sure why.

maxerickson 4y ago
I wonder if the quote is referring to balancing 2 systems. I don't have AC, but I use my heat to maintain a minimum acceptable temperature, not to maintain the temperature I find ideal. I don't want it to go a lot lower than the setting.
tshaddox 4y ago
Surely a basic PID controller with reasonable tuning could mostly mitigate the problem you describe, which isn’t really a different problem than a thermostat that would attempt to turn the furnace on and off multiple times per second if the thermometer was fluctuating between 74 and 75 multiple times per second. Granted, tuning the PID controller implies loosening the “maintain the exact target temperature at all costs” spec.
- pdonis 4y ago
  > tuning the PID controller implies loosening the “maintain the exact target temperature at all costs” spec. And if you loosen that spec, then, as others have already pointed out, just having two setpoints, a lower one for heating and a higher one for cooling, will be sufficient as long as the difference between them is large enough (probably 2 degrees or more would be enough for many use cases). That would be simpler than adding a PID controller.
deleted 4y ago
[deleted]
kkielhofner 4y ago
This is solved in thermostats with hysteresis and even many DIY or close to it solutions like esphome[0] implement it. I know on my Nest when heat and cool mode is on the set heat and cool temperatures have to be two degrees away from one another, which is fine with me because for whatever reasons 67 in the summer feels great but I want close to 69/70/71 in the winter. The linked esphome docs are actually great whether you care about esphome or not - they explain all of this very well and you can tell from the configuration values and samples what kind of logic goes into all of this. [0] - https://esphome.io/components/climate/thermostat.html https://esphome.io/components/climate/thermostat.html
- evulhotdog 4y ago
  Your summer and winter preferences may vary due to humidity levels. It makes a world of difference it how humans perceive temperature because of it.
  - hunter2_ 4y ago
    I really wish I could have my thermostat's set point use "heat index" (a straightforward function of temperature and humidity) instead of just temperature. I am perfectly comfortable (and therefore want my system to turn off) after my AC has been running long enough to drop the humidity a bit, even if the temp has only dropped a fraction of a degree. As humid air leaks in, I want the AC back on even if the temp has barely increased a degree. Basically the heat index in my house swings enough to change my comfort and therefore could be a useful trigger, even if the temperature doesn't swing nearly enough to be a useful trigger. The Nest even has a humidity sensor, but it only gets used in very specific (hardly useful) situations. I considered writing a Works With Nest app for this. Then that platform disappeared in favor of Works With Google, but I never found the time for either...
    - anonomousename 4y ago
      If you’re willing to spend the money, I’d recommend getting a ZigBee or z wave thermostat(or an EcoBee, as it has local control through HomeKit) and then you can combine the temperature readings with your own humidity/temperature sensors - as mentioned above, ESPHome/Home Assistant may be overkill, but would work great for this kind of setup
wildzzz 4y ago
You'd just set a low point and a high point far enough away from each other that the thermostat wouldn't cycle back and forth between heat and AC. You could also add a timeout so that the heat cannot turn on for at least two hours after the AC cycle just finished. During November, we always seem to get crazy temperature swings where we run the heat one day but need to run AC the next. I'd love a dumb thermostat that didn't require me to manually switch over to heat or AC mode.
shitlord 4y ago
Is it not possible to forecast temperature changes rather than installing a bunch of extra sensors? If the device knows its location and has an internet connection, it can examine the day's weather forecast, know where the sun will shine, and know how cloudy it will be.
- ilyt 4y ago
  You'd still need correction for how sunny/shady house walls are
  - shitlord 4y ago
    Good point. I wonder if the temperature sensor in the thermostat would be able to help correct that error. E.g., if the thermostat expected a temperature of 75 F at 12 PM but it was actually 78 F, then you know the house received a lot of sunlight and can correct accordingly.
    - KMag 4y ago
      The rate at which your house warms while the heat is on is an indirect measure of the effect of both the temperature of the outside of your walls and the amount of solar radiation coming through your windows. As others have indirectly pointed out, for the range of circumstances a given house is going to encounter, the linearizing assumptions in a standard PID controller are probably sufficient. If tuned properly, the "differential" portion of the PID controller will shut down the heater earlier when it's warmer out and/or there's more solar energy coming through your windows.
ilyt 4y ago
> If it only takes one of heating or cooling to do this, not both, a standard "dumb" thermostat can of course do this just fine. Just set it to "cool" or "heat" mode and set a temperature. But of course you will need to change modes as the season changes (unless you live somewhere where only one mode is ever needed). Why ? What would be so hard in detecting "every time I don't do anything temperature goes up therefore summer and I need to use AC" ? > If you want to not have to change modes when the season changes, it gets more complicated, because the simple control system in the standard thermostat is too dumb for that. For example, suppose you set your temperature to 75. The actual temperature rises to 76, above the setpoint, so your A/C comes on. It cools the house to 74 and then the thermostat shuts it off (you have to have some buffer so the A/C doesn't come on again almost immediately). Now the temperature is below the setpoint, so your heat comes on. It heats the house to 76 and then shuts off. Now the temperature is above the setpoint... You can see what's going on. All would it take is to put a gate "if I used cooling to correct temperature last time, don't use heating for few hours or until it drops to higher difference and vice versa". You're trying to portray it in way more complicated way that it is. > From a pure design perspective, the right way to solve this problem is an outside temperature sensor (and probably a sun sensor as well since sunlight is a significant heat input to your house), so the system can predict what will happen when both cooling and heating are off. Given reliable sensors, the control algorithm is fairly simple (automatic climate control systems in cars already do it). I have never seen this in a house, though. I'm not sure why. Too simplistic. Which side of house you monitor, is it in shade or does it show temperature when sun heats up the wall ? Which room is closest to "hot" wall/window?. It would be a lot of work for very little improvement
deleted 4y ago
[deleted]
anikom15 4y ago
This is solved with two set points, 68° for heat and 78° for cold, and appropriate clothing.
pdonis 4y ago
> If you want to not have to change modes when the season changes, it gets more complicated There's an additional complication for multi-story houses that I didn't mention before. In multi-story houses, when you're cooling, you want most of the ducted airflow to be directed to the upstairs outlets, to take advantage of natural convection (cooler air sinks); similarly, when you're heating, you want most of the ducted airflow to be directed to the downstairs outlets (warmer air rises). That requires adjusting dampers in the ducts, usually just downstream of the output of your HVAC unit. Unless that adjustment can be automated (which would require automatically controlled motors on the dampers, something I haven't seen marketed, though again I'm not sure why), your airflow will be suboptimal for either heating or cooling if the system automatically switches between modes.
- maxerickson 4y ago
  Switch controlled open/close dampers are common enough. A typical suggested application is controlling the airflow to a room using a thermostat to open or close the damper, but they can be hooked up to anything with a switched output.
  - pdonis 4y ago
    Can these dampers stop somewhere short of full open/close? Typically one wants full open for one season and mostly closed but not fully closed for the other.
    - maxerickson 4y ago
      Apparently? I'm not an HVAC guy, just poking away at understanding my current system and thinking about what I might change. As an example, this damper has an adjustable bleed rate, with a 2 wire input for switching the state: https://www.resideo.com/us/en/pro/products/air/forced-air-zoning/zone-dampers/6-in-automatic-round-damper-ard6tz-u/#resources https://www.resideo.com/us/en/pro/products/air/forced-air-zo... (Click the "Install" tab to access installation guides)

by pdonis 4y ago

maxerickson 4y ago

I wonder if the quote is referring to balancing 2 systems. I don't have AC, but I use my heat to maintain a minimum acceptable temperature, not to maintain the temperature I find ideal. I don't want it to go a lot lower than the setting.

tshaddox 4y ago

Surely a basic PID controller with reasonable tuning could mostly mitigate the problem you describe, which isn’t really a different problem than a thermostat that would attempt to turn the furnace on and off multiple times per second if the thermometer was fluctuating between 74 and 75 multiple times per second. Granted, tuning the PID controller implies loosening the “maintain the exact target temperature at all costs” spec.

pdonis 4y ago
> tuning the PID controller implies loosening the “maintain the exact target temperature at all costs” spec. And if you loosen that spec, then, as others have already pointed out, just having two setpoints, a lower one for heating and a higher one for cooling, will be sufficient as long as the difference between them is large enough (probably 2 degrees or more would be enough for many use cases). That would be simpler than adding a PID controller.

deleted 4y ago

[deleted]

kkielhofner 4y ago

This is solved in thermostats with hysteresis and even many DIY or close to it solutions like esphome[0] implement it. I know on my Nest when heat and cool mode is on the set heat and cool temperatures have to be two degrees away from one another, which is fine with me because for whatever reasons 67 in the summer feels great but I want close to 69/70/71 in the winter. The linked esphome docs are actually great whether you care about esphome or not - they explain all of this very well and you can tell from the configuration values and samples what kind of logic goes into all of this. [0] - https://esphome.io/components/climate/thermostat.html https://esphome.io/components/climate/thermostat.html

evulhotdog 4y ago
Your summer and winter preferences may vary due to humidity levels. It makes a world of difference it how humans perceive temperature because of it.
- hunter2_ 4y ago
  I really wish I could have my thermostat's set point use "heat index" (a straightforward function of temperature and humidity) instead of just temperature. I am perfectly comfortable (and therefore want my system to turn off) after my AC has been running long enough to drop the humidity a bit, even if the temp has only dropped a fraction of a degree. As humid air leaks in, I want the AC back on even if the temp has barely increased a degree. Basically the heat index in my house swings enough to change my comfort and therefore could be a useful trigger, even if the temperature doesn't swing nearly enough to be a useful trigger. The Nest even has a humidity sensor, but it only gets used in very specific (hardly useful) situations. I considered writing a Works With Nest app for this. Then that platform disappeared in favor of Works With Google, but I never found the time for either...
  - anonomousename 4y ago
    If you’re willing to spend the money, I’d recommend getting a ZigBee or z wave thermostat(or an EcoBee, as it has local control through HomeKit) and then you can combine the temperature readings with your own humidity/temperature sensors - as mentioned above, ESPHome/Home Assistant may be overkill, but would work great for this kind of setup

wildzzz 4y ago

You'd just set a low point and a high point far enough away from each other that the thermostat wouldn't cycle back and forth between heat and AC. You could also add a timeout so that the heat cannot turn on for at least two hours after the AC cycle just finished. During November, we always seem to get crazy temperature swings where we run the heat one day but need to run AC the next. I'd love a dumb thermostat that didn't require me to manually switch over to heat or AC mode.

shitlord 4y ago

Is it not possible to forecast temperature changes rather than installing a bunch of extra sensors? If the device knows its location and has an internet connection, it can examine the day's weather forecast, know where the sun will shine, and know how cloudy it will be.

ilyt 4y ago
You'd still need correction for how sunny/shady house walls are
- shitlord 4y ago
  Good point. I wonder if the temperature sensor in the thermostat would be able to help correct that error. E.g., if the thermostat expected a temperature of 75 F at 12 PM but it was actually 78 F, then you know the house received a lot of sunlight and can correct accordingly.
  - KMag 4y ago
    The rate at which your house warms while the heat is on is an indirect measure of the effect of both the temperature of the outside of your walls and the amount of solar radiation coming through your windows. As others have indirectly pointed out, for the range of circumstances a given house is going to encounter, the linearizing assumptions in a standard PID controller are probably sufficient. If tuned properly, the "differential" portion of the PID controller will shut down the heater earlier when it's warmer out and/or there's more solar energy coming through your windows.

ilyt 4y ago

> If it only takes one of heating or cooling to do this, not both, a standard "dumb" thermostat can of course do this just fine. Just set it to "cool" or "heat" mode and set a temperature. But of course you will need to change modes as the season changes (unless you live somewhere where only one mode is ever needed). Why ? What would be so hard in detecting "every time I don't do anything temperature goes up therefore summer and I need to use AC" ? > If you want to not have to change modes when the season changes, it gets more complicated, because the simple control system in the standard thermostat is too dumb for that. For example, suppose you set your temperature to 75. The actual temperature rises to 76, above the setpoint, so your A/C comes on. It cools the house to 74 and then the thermostat shuts it off (you have to have some buffer so the A/C doesn't come on again almost immediately). Now the temperature is below the setpoint, so your heat comes on. It heats the house to 76 and then shuts off. Now the temperature is above the setpoint... You can see what's going on. All would it take is to put a gate "if I used cooling to correct temperature last time, don't use heating for few hours or until it drops to higher difference and vice versa". You're trying to portray it in way more complicated way that it is. > From a pure design perspective, the right way to solve this problem is an outside temperature sensor (and probably a sun sensor as well since sunlight is a significant heat input to your house), so the system can predict what will happen when both cooling and heating are off. Given reliable sensors, the control algorithm is fairly simple (automatic climate control systems in cars already do it). I have never seen this in a house, though. I'm not sure why. Too simplistic. Which side of house you monitor, is it in shade or does it show temperature when sun heats up the wall ? Which room is closest to "hot" wall/window?. It would be a lot of work for very little improvement

deleted 4y ago

[deleted]

anikom15 4y ago

This is solved with two set points, 68° for heat and 78° for cold, and appropriate clothing.

pdonis 4y ago

> If you want to not have to change modes when the season changes, it gets more complicated There's an additional complication for multi-story houses that I didn't mention before. In multi-story houses, when you're cooling, you want most of the ducted airflow to be directed to the upstairs outlets, to take advantage of natural convection (cooler air sinks); similarly, when you're heating, you want most of the ducted airflow to be directed to the downstairs outlets (warmer air rises). That requires adjusting dampers in the ducts, usually just downstream of the output of your HVAC unit. Unless that adjustment can be automated (which would require automatically controlled motors on the dampers, something I haven't seen marketed, though again I'm not sure why), your airflow will be suboptimal for either heating or cooling if the system automatically switches between modes.

maxerickson 4y ago
Switch controlled open/close dampers are common enough. A typical suggested application is controlling the airflow to a room using a thermostat to open or close the damper, but they can be hooked up to anything with a switched output.
- pdonis 4y ago
  Can these dampers stop somewhere short of full open/close? Typically one wants full open for one season and mostly closed but not fully closed for the other.
  - maxerickson 4y ago
    Apparently? I'm not an HVAC guy, just poking away at understanding my current system and thinking about what I might change. As an example, this damper has an adjustable bleed rate, with a 2 wire input for switching the state: https://www.resideo.com/us/en/pro/products/air/forced-air-zoning/zone-dampers/6-in-automatic-round-damper-ard6tz-u/#resources https://www.resideo.com/us/en/pro/products/air/forced-air-zo... (Click the "Install" tab to access installation guides)