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Comment by paiute | Hacker News Reader

I still miss my mercury filled thermostat. None of this constant on/off from the digital contraption.

ecobee has all the options you are complaining about, min compressor/furnace time, temp deviation, thresholds, and lot more settings.

[1] https://support.ecobee.com/s/articles/Threshold-settings-for...

Even a simple digital thermostat will likely have an input for cycles/hour. Maybe in the form of some dip switches on the back.

Wait, what is wrong with a digital thermostat?

There can be such thing as too much precision. I have in-floor radiant heat that benefits from a longer duty cycle. I buy thermostats that are designed not just for a particular temp, but will also run without what I would call thrashing. E.g Constantly on and off because someone walked by and caused a breeze of slightly cooler air.

Not to say a digital thermostat couldn't account for this (you have to find them specifically), and are probably better with good firmware, but murcury will have a wider margin before activation.

> but murcury will have a wider margin before activation.

Well, digital thermostats will have as much of margin as you program them to.

Until your comment, I assumed every digital thermostat had hysteresis since the 90's. I can't imagine why they wouldn't. The more expensive ones have PWM control of the heater or heat exchanger, but that's something extra beyond what the mechanical ones did (metal strip thermostats used to be designed with hysteresis too, what isn't something they automatically have).

It's a bang-bang controller. It's so simple that it's what I came up with for controlling power producers / sinks to fill / drain buffers (load matching) in modded minecraft many years ago. It's the very simplest way to do this and is far from optimal. The only thing it optimizes is simplicity of implementation. A PI loop tuned to either minimize error or transitions is going to perform better for whatever goal or balance of goals a user wants.

A bang-bang controller can have hysteresis or not, it's as simple as turning off in a higher temperature than the one it turns on and putting a low-pass filter (a capacitor if you do it in hardware) after the temperature sensor.

The simplest possible implementation of an electronic thermostat would be able to switch thousands of times per second, and this is completely useless for controlling anything mechanical. When you are making real controllers, you always need some extra complexity, but you don't need a lot of it to make a good one.

Filters add more latency than PI(D) loops. They're not used in practice because they perform worse. Would you trust a 20% tolerance capacitor when setting your poles?

Search for a digital thermostat with hysteresis options to solve this problem.

Or a high quality one designed for in-floor radiant heat, e.g. Tekmar 552. Very simple to set up, very few gimmicks, and pretty much just works.

(The Tekmar smart home interface for it is offensively priced. The thermostat works fine without it, though.)

if true, this is kinda sad. I use a free utility that does this with my pc fans and lets me choose exactly what the margin is. how hard would this have been to add to nest?

What problem does adding hysteresis solve on a digital thermostat, though?

Is it a problem that it stays closer to the asked temperature?

If you've ever had a basic electronics course with a raw button, hysteresis might instead be known as 'debounce' or making the button's single human action a single 'press' result on the other side.

I think a digital thermostat could work well enough if they weren't made with a focus on being stupidly cheap or possibly somehow still absolutely energy efficient.

* Wake up and respond FAST to user inputs.

* Show the numbers it's thinking of big (running average over 5, 10, 15 min?), but also the current reading small (but readable) updated whenever it polls.

* If there's a hold or timeout involved display that too, expose the user to the thought process, don't just be a nearly opaque box.

* Be responsive in other ways. If I switch modes at the very least kill the old mode's 'calling' line right then, even if the new one gets a cooldown to prevent equipment issues.

But none of this seems to need hysteresis for the actual activation/deactivation of heating depending on temperature readings?

One reason I could see is to preserve the life of electromagnetic relays, but modern thermostats should use solid state relays.

These points also seem to be solved for any somewhat modern (less than 15 years old?) run of the mill thermostat. Generally they always show the current reading and give the temperature setting only when you ask for it though, that's the only difference I see with what you want.

The comment was something like 'walking by' disrupting the air at the thermostat and that alone being enough to turn the furnace on at that exact moment.

A running average, but also making it clear to the user (both the value the unit is using for the decision and the current instant input) as well as a 'running average' over a given window address that specific comment. I added my own frustrations on top since they're also part of the larger 'dumb down the UI' and make the device frustrating.

The same thing as all digital to analog device comparisons. There is so much more forgiveness in analog. I really feel for those that have never worked with analog and only know the coldness of digital 0s and 1s. Digital is like living in a world of only black & white where analog ha[d|s] shades of gray.

Take an Arudino with both analog and digital pins. A digital pin is either high (1) or low (0). That's it. On or Off. An analog pin can have a range of values from 0v to 3.3v or 5v depending on configuration. That single pin can be wired to several different items that each have a separate resistance allowing them to be identified by their passed voltage.

At the same time, analog has its own problems with that lack of precision, so everything needs to accept a range. Put a resistance on a 5v line, and read from it multiple times and you get multiple readings, but within a +/- tolerance.

They still exist:

https://www.amazon.com/Honeywell-CT87K1004-Available-RoundHe...

Those don't have a mercury switch.

I should have spent more than 15 seconds Googling. You're right. In fact, after more googling they appear to be banned in most states due to the mercury content.

I got rid of my last mercury thermostat last winter because it occasionally failed silently, heating far beyond the setpoint. Mechanical operation is a significant liability once you come to expect solid-state reliability.

Is the mercury switch the important part of that, or the bimetal strip that's tilting the mercury switch? I think you could probably find mercury-free bimetal thermostats still, though I haven't looked.

The mercury switch behaved kind of like the first hill of a roller coaster. It would linger at the edge of transition until it reached a breaking point then it would drop.

Some modern thermostats try to emulate this behavior. The Honeywell T6 line of thermostats have technician settings for configuring this behavior though no technician takes the time to configure them for a home.

I still have my old one on a shelf in case I ever want to go back.

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