7 ms·

Be careful doing that. A smart thermostat isn't exactly trivial, and doing it wrong can be bad.

Gratsby 11y ago
> A smart thermostat isn't exactly trivial, and doing it wrong can be bad. That's sarcasm, right? I automated my HVAC in the '90s and even then there were a ton of roadmaps to follow by people who had done it before. It was an afternoon project using something similar to these: https://www.sparkfun.com/products/245 https://www.sparkfun.com/products/245
click170 11y ago
As Nest demonstrated time and again. Their thermostats had a defect at one point (not sure if it was ever actually fixed) where if your heating or cooling system caused a surge in your wiring, your nest would get stuck with both heating and cooling at full blast, or something equally silly. The Nest thermostat was certainly not designed to fail safely or sanely.
hueving 11y ago
if temperature > max(desired_range): activate_ac() deactivate_heat() elif temperature < min(desired_range): deactivate_ac() activate_heat() else: deactivate_heat() deactivate_ac()
- ryandrake 11y ago
  Code review #1: You're going to want to add debounce logic, or you're going to be erratically turning on and off your heat and ac when the temperature is exactly equal to the ends of desired_range.
  - Grishnakh 11y ago
    Nitpick: it's not "debounce logic" (that's for handling pushbuttons, because the contact physically bounce, meaning you get multiple on/off readings within a few milliseconds), the word you're looking for is "hysteresis". Because there's a giant surge current when you start up a motor (like that in an A/C compressor), and it's just plain bad and even annoying to have your A/C cycle between on and off rapidly, a thermostat will intentionally overshoot the set point when cooling, and then undershoot that same set point when waiting for the house to heat up before it turns the A/C back on (or vice versa for heating). Hysteresis is absolutely necessary to avoid both this problem, and also the problem where your temperature sensor isn't perfectly accurate and the readings from it will vary a bit from reading to reading.
    - ryandrake 11y ago
      Both are necessary. You are absolutely right that you need to account for hysteresis. Debounce is also necessary for sensors that have precision problems, where, in boundary conditions, they bounce back and forth between a measurement that would cause an "on" signal and a measurement that would cause an "off" signal. But, I think we're agreeing with each other :)
      - Grishnakh 11y ago
        Yep, we're just disagreeing over minutiae :-) For the sensors, it seems to me that a debounce routine wouldn't work, and (this is assuming the sensor is giving an analog reading, not an on-off binary reading) what you really want there is some kind of averaging routine. Usually, the way a debounce routine works is that you look for a state transition (off->on), and then wait 30ms (general rule of thumb for human-actuated switch) and look again to make sure that the state is still "on" and then you do your operation based on that. But for analog sensors, you probably want something that's more like a running average so the readings don't vary too much, and one erroneous or too-extreme reading doesn't affect things much. I could be wrong though; this is probably highly application-dependent.
  - rwjwjuwjudf 11y ago
    Also you can decouple the handling of ac and heat, and check the current state to prevent a redundant transition into the current state. if (temp > max + error) if (off (ac)) turn_on (ac); if (temp < max - error) if (on (ac)) turn_off (ac); if (temp < min - error) if (off (heat)) turn_on (heat); if (temp > min + error) if (on (heat)) turn_off (heat);
    - protomyth 11y ago
      I'd prefer a nice fuzzy logic rule set with some variable speed on the heat/fans.
    - will_hughes 11y ago
      If in between samples you go from above max to below min you'll have both heat and ac on. The take-away is that like programming lift controllers[0] it's not quite as simple as it might first seem - many many edge cases. [0] http://play.elevatorsaga.com/ http://play.elevatorsaga.com/ (for example)
      - rwjwjuwjudf 11y ago
        I think that's wrong, because all 4 if-statements are evaluated for each sample. But yes agreed, in general there are lots of edge cases. I actually wrote a finite state machine specification for an elevator in a logic class once. Without having really thought about it much, it looks like the biggest problems with thermostats are dealing with scalars, sampling frequencies, and sampling error.
  - Retric 11y ago
    You can use this logic if the range is wide enough and you don't check that often. Or better yet wait 5 min after changing state to check again. You often have at least high and low heat which adds complexity. Also, overshooting you ideal temp is fairly easy to do as there can be a lot of hot air in the vents even after you turn down the heat. So a little predictive modeling can be useful.
  - voltagex_ 11y ago
    I know what debouncing is, but I don't really grok it. Is there a Khan Academy-style explaination somewhere?
    - ascorbic 11y ago
      If you hook up an oscilloscope to a physical button, you will see that there's not a nice clean on/off when you press it [1]. The signal bounces on and off for quite some time after. This is a problem in a lot of situations. If, for example, a momentary push button is used as a power button on a device, it might be connected to an interrupt on a microcontroller. Without debouncing the device will rapidly switch on and off dozens of times over a millisecond. While there are easy ways of debouncing in hardware, most don't bother, as it's very simple to do in software. The simplest is just to ignore or disable the interrupt for a short period (a few milliseconds, perhaps). Here's an Arduino sketch showing a simple way of doing it. https://www.arduino.cc/en/Tutorial/Debounce https://www.arduino.cc/en/Tutorial/Debounce [1] http://www.labbookpages.co.uk/electronics/files/debounce/bounce.png http://www.labbookpages.co.uk/electronics/files/debounce/bou...
      - voltagex_ 11y ago
        Many thanks
  - supergeek133 11y ago
    Or just put in a deadband, that's what most thermostats do. Also, you should note that most of them also don't show the exact temperature in the room... it's an algorithm. Your furnace might heat to 68 on the display but the actual temp may be between 67 and 69, but if your display temp constantly bounced it would result in mass consumer disappointment.
- CamperBob2 11y ago
  While it's tiresome to read comments on HN like the GP's that seem to come from a "Look at me, I'm more Catholic than the Pope" mentality, the reality is that a good thermostat algorithm is a fair bit more complicated than that. You need to take hysteresis into account, temperature overshoot/undershoot, your compressor's duty cycle and startup latency (especially as it impacts MTBF), emergency/backup furnace control, failsafes to make sure the house doesn't freeze or overheat if/when any of the above goes wrong... lots of stuff like that. The spec for an HVAC control system isn't overwhelming but it does warrant some thought.
  - TheOtherHobbes 11y ago
    In the UK, many homes and older office buildings still have traditional bi-metal mechanical thermostats. For typical domestic applications they work just fine. For finer control of room temps, people often use mechanical radiator thermostats. They work just fine too. New electronic models are usually just a thermistor with some analogue hysteresis. They work fine as well. I might expect something with the extras you're describing to appear in a new office building, but in the UK at least adding PID and/or some kind of learning system to a domestic thermostat is overkill. Generally, simple solutions are less likely to go badly wrong. That's the tragic irony of Nest. The design is great, but the added value offered by a buggy and overcomplex implementation is negative.
  - schwap 11y ago
    In home systems aren't most of those problems handled by the furnace/ac system itself? Thermostats are pretty dumb.
    - CamperBob2 11y ago
      Depends on the hardware. An old-fashioned bimetallic strip/mercury switch thermostat has mechanical hysteresis built in. (Actually, they can have too much lag -- some of them include heating elements to 'accelerate' their response.) Old-school mechanical devices aren't always as dumb as they look. As far as short-cycling the compressor goes, my current HVAC is smart enough to disallow that, but I've lived in older houses where that wasn't the case. You could potentially damage the compressor by wiggling the thermostat back and forth, forcing it to start up against back pressure. Understanding the requirements is really job #1, and first on the list of reasons why HVAC control isn't just a simple "if too cold then turn on heat else turn on A/C" statement. That doesn't mean people shouldn't jump in and try it, of course. It's the best way to learn, if not always the cheapest.
- draw_down 11y ago
  Pretty sure you just illustrated GP's point
- RichardCA 11y ago
  Throw an anti-short-cycle delay in there. You've never programmed HVAC have you.
  - sk5t 11y ago
    And also PID control... and defrost or heat exchanger cooldown intervals... and further checks to avoid overcycling and damaging anything with a compressor. And probably a dozen or a hundred other things I don't know about because I'm not a process controls engineer.
- terryf 11y ago
  It sounds simple, but this algorithm will very quickly break both your ac and heating units. You want a PID controller in there (in some cases you might get away with just PI, but that needs analysis of the specific case) Ideally of course you'd want to model the exact energy loss curve of the specific building you're in but nobody really does that. The best part is of course that a PID controller sounds complicated, but the simplest one is actually just around 15 lines of code.
cmdrfred 11y ago
I'm really just looking for a thermostat that I can adjust from bed. Maybe add some scheduling in. I dont really want something "smart", just programmable.
- CamperBob2 11y ago
  In my last (rental) house, I actually went to the trouble of moving the thermostat off the wall and mounting it on my nightstand, courtesy of 50' of Cat5 UTP cable. Highly recommended "life hack."
RichardCA 11y ago
What do you mean? The home-hobbyist community has been doing this for years using protocols like X10, Zwave, etc. And we used Usenet groups like comp.home.automation to meet and discuss stuff. Home Automation has a very long tail of history and those of us who came up that way all knew Nest was going to be a joke.
astebbin 11y ago
I don't know why you've been downvoted, as you're absolutely correct. A defective thermostat can easily shorten the lifetime of your HVAC system. If your furnace itself is already defective, a buggy thermostat could even cause it to overheat and start a house fire. EDIT: to those who mention the hobbyist scene, consider that such projects are not necessarily trivial, and that getting it wrong remains potentially catastrophic. I believe the parent's point still stands.

> A smart thermostat isn't exactly trivial, and doing it wrong can be bad. That's sarcasm, right? I automated my HVAC in the '90s and even then there were a ton of roadmaps to follow by people who had done it before. It was an afternoon project using something similar to these: https://www.sparkfun.com/products/245 https://www.sparkfun.com/products/245

click170 11y ago

As Nest demonstrated time and again. Their thermostats had a defect at one point (not sure if it was ever actually fixed) where if your heating or cooling system caused a surge in your wiring, your nest would get stuck with both heating and cooling at full blast, or something equally silly. The Nest thermostat was certainly not designed to fail safely or sanely.

hueving 11y ago

if temperature > max(desired_range): activate_ac() deactivate_heat() elif temperature < min(desired_range): deactivate_ac() activate_heat() else: deactivate_heat() deactivate_ac()

ryandrake 11y ago
Code review #1: You're going to want to add debounce logic, or you're going to be erratically turning on and off your heat and ac when the temperature is exactly equal to the ends of desired_range.
- Grishnakh 11y ago
  Nitpick: it's not "debounce logic" (that's for handling pushbuttons, because the contact physically bounce, meaning you get multiple on/off readings within a few milliseconds), the word you're looking for is "hysteresis". Because there's a giant surge current when you start up a motor (like that in an A/C compressor), and it's just plain bad and even annoying to have your A/C cycle between on and off rapidly, a thermostat will intentionally overshoot the set point when cooling, and then undershoot that same set point when waiting for the house to heat up before it turns the A/C back on (or vice versa for heating). Hysteresis is absolutely necessary to avoid both this problem, and also the problem where your temperature sensor isn't perfectly accurate and the readings from it will vary a bit from reading to reading.
  - ryandrake 11y ago
    Both are necessary. You are absolutely right that you need to account for hysteresis. Debounce is also necessary for sensors that have precision problems, where, in boundary conditions, they bounce back and forth between a measurement that would cause an "on" signal and a measurement that would cause an "off" signal. But, I think we're agreeing with each other :)
    - Grishnakh 11y ago
      Yep, we're just disagreeing over minutiae :-) For the sensors, it seems to me that a debounce routine wouldn't work, and (this is assuming the sensor is giving an analog reading, not an on-off binary reading) what you really want there is some kind of averaging routine. Usually, the way a debounce routine works is that you look for a state transition (off->on), and then wait 30ms (general rule of thumb for human-actuated switch) and look again to make sure that the state is still "on" and then you do your operation based on that. But for analog sensors, you probably want something that's more like a running average so the readings don't vary too much, and one erroneous or too-extreme reading doesn't affect things much. I could be wrong though; this is probably highly application-dependent.
- rwjwjuwjudf 11y ago
  Also you can decouple the handling of ac and heat, and check the current state to prevent a redundant transition into the current state. if (temp > max + error) if (off (ac)) turn_on (ac); if (temp < max - error) if (on (ac)) turn_off (ac); if (temp < min - error) if (off (heat)) turn_on (heat); if (temp > min + error) if (on (heat)) turn_off (heat);
  - protomyth 11y ago
    I'd prefer a nice fuzzy logic rule set with some variable speed on the heat/fans.
  - will_hughes 11y ago
    If in between samples you go from above max to below min you'll have both heat and ac on. The take-away is that like programming lift controllers[0] it's not quite as simple as it might first seem - many many edge cases. [0] http://play.elevatorsaga.com/ http://play.elevatorsaga.com/ (for example)
    - rwjwjuwjudf 11y ago
      I think that's wrong, because all 4 if-statements are evaluated for each sample. But yes agreed, in general there are lots of edge cases. I actually wrote a finite state machine specification for an elevator in a logic class once. Without having really thought about it much, it looks like the biggest problems with thermostats are dealing with scalars, sampling frequencies, and sampling error.
- Retric 11y ago
  You can use this logic if the range is wide enough and you don't check that often. Or better yet wait 5 min after changing state to check again. You often have at least high and low heat which adds complexity. Also, overshooting you ideal temp is fairly easy to do as there can be a lot of hot air in the vents even after you turn down the heat. So a little predictive modeling can be useful.
- voltagex_ 11y ago
  I know what debouncing is, but I don't really grok it. Is there a Khan Academy-style explaination somewhere?
  - ascorbic 11y ago
    If you hook up an oscilloscope to a physical button, you will see that there's not a nice clean on/off when you press it [1]. The signal bounces on and off for quite some time after. This is a problem in a lot of situations. If, for example, a momentary push button is used as a power button on a device, it might be connected to an interrupt on a microcontroller. Without debouncing the device will rapidly switch on and off dozens of times over a millisecond. While there are easy ways of debouncing in hardware, most don't bother, as it's very simple to do in software. The simplest is just to ignore or disable the interrupt for a short period (a few milliseconds, perhaps). Here's an Arduino sketch showing a simple way of doing it. https://www.arduino.cc/en/Tutorial/Debounce https://www.arduino.cc/en/Tutorial/Debounce [1] http://www.labbookpages.co.uk/electronics/files/debounce/bounce.png http://www.labbookpages.co.uk/electronics/files/debounce/bou...
    - voltagex_ 11y ago
      Many thanks
- supergeek133 11y ago
  Or just put in a deadband, that's what most thermostats do. Also, you should note that most of them also don't show the exact temperature in the room... it's an algorithm. Your furnace might heat to 68 on the display but the actual temp may be between 67 and 69, but if your display temp constantly bounced it would result in mass consumer disappointment.
CamperBob2 11y ago
While it's tiresome to read comments on HN like the GP's that seem to come from a "Look at me, I'm more Catholic than the Pope" mentality, the reality is that a good thermostat algorithm is a fair bit more complicated than that. You need to take hysteresis into account, temperature overshoot/undershoot, your compressor's duty cycle and startup latency (especially as it impacts MTBF), emergency/backup furnace control, failsafes to make sure the house doesn't freeze or overheat if/when any of the above goes wrong... lots of stuff like that. The spec for an HVAC control system isn't overwhelming but it does warrant some thought.
- TheOtherHobbes 11y ago
  In the UK, many homes and older office buildings still have traditional bi-metal mechanical thermostats. For typical domestic applications they work just fine. For finer control of room temps, people often use mechanical radiator thermostats. They work just fine too. New electronic models are usually just a thermistor with some analogue hysteresis. They work fine as well. I might expect something with the extras you're describing to appear in a new office building, but in the UK at least adding PID and/or some kind of learning system to a domestic thermostat is overkill. Generally, simple solutions are less likely to go badly wrong. That's the tragic irony of Nest. The design is great, but the added value offered by a buggy and overcomplex implementation is negative.
- schwap 11y ago
  In home systems aren't most of those problems handled by the furnace/ac system itself? Thermostats are pretty dumb.
  - CamperBob2 11y ago
    Depends on the hardware. An old-fashioned bimetallic strip/mercury switch thermostat has mechanical hysteresis built in. (Actually, they can have too much lag -- some of them include heating elements to 'accelerate' their response.) Old-school mechanical devices aren't always as dumb as they look. As far as short-cycling the compressor goes, my current HVAC is smart enough to disallow that, but I've lived in older houses where that wasn't the case. You could potentially damage the compressor by wiggling the thermostat back and forth, forcing it to start up against back pressure. Understanding the requirements is really job #1, and first on the list of reasons why HVAC control isn't just a simple "if too cold then turn on heat else turn on A/C" statement. That doesn't mean people shouldn't jump in and try it, of course. It's the best way to learn, if not always the cheapest.
draw_down 11y ago
Pretty sure you just illustrated GP's point
RichardCA 11y ago
Throw an anti-short-cycle delay in there. You've never programmed HVAC have you.
- sk5t 11y ago
  And also PID control... and defrost or heat exchanger cooldown intervals... and further checks to avoid overcycling and damaging anything with a compressor. And probably a dozen or a hundred other things I don't know about because I'm not a process controls engineer.
terryf 11y ago
It sounds simple, but this algorithm will very quickly break both your ac and heating units. You want a PID controller in there (in some cases you might get away with just PI, but that needs analysis of the specific case) Ideally of course you'd want to model the exact energy loss curve of the specific building you're in but nobody really does that. The best part is of course that a PID controller sounds complicated, but the simplest one is actually just around 15 lines of code.

cmdrfred 11y ago

I'm really just looking for a thermostat that I can adjust from bed. Maybe add some scheduling in. I dont really want something "smart", just programmable.

CamperBob2 11y ago
In my last (rental) house, I actually went to the trouble of moving the thermostat off the wall and mounting it on my nightstand, courtesy of 50' of Cat5 UTP cable. Highly recommended "life hack."

RichardCA 11y ago

What do you mean? The home-hobbyist community has been doing this for years using protocols like X10, Zwave, etc. And we used Usenet groups like comp.home.automation to meet and discuss stuff. Home Automation has a very long tail of history and those of us who came up that way all knew Nest was going to be a joke.

astebbin 11y ago

I don't know why you've been downvoted, as you're absolutely correct. A defective thermostat can easily shorten the lifetime of your HVAC system. If your furnace itself is already defective, a buggy thermostat could even cause it to overheat and start a house fire. EDIT: to those who mention the hobbyist scene, consider that such projects are not necessarily trivial, and that getting it wrong remains potentially catastrophic. I believe the parent's point still stands.