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Comment by andylei | Hacker News Reader

your furnace doesn't know what temperature your house is if the furnace isn't connected to a thermostat

The connection between the furnace and the thermostat probably shouldn't go through the internet.

So it's perfectly reasonable for the furnace to turn off when it is disconnected, because disconnection would be a very strong signal for an error state instead of regular intermittent network/service issues.

Are there any furnaces that do that?

Certainly, the standard smart thermostat set up is that your ecobee is connected to the Internet, but controls the furnace using good old-fashioned signal wires

Which is only extremely tangentially related to "if I pull my thermostat off the wall"

The overwhelmingly most common connection between a thermostat and furnace is a contact closure when calling for heat, with no ability to differentiate between “thermostat is present but not calling for heat” and “thermostat is not present” as both present as "these T-T contacts are not closed/shorted together".

I have a nest and it's wired directly to your furnace via the thermostat control wires.

While it is controllable via the cloud, even without wifi it continues to function.

the connection to my thermostat is via a cable, if I pull it out of the wall it won't be connected to anything at all. the whole furnace is not connected to anything but mains power.

They're not, "smart" thermostats have a WiFi frontend that allows devices to connect to it from the network but the thermostat itself is hardwired to the furnace/HVAC.

You could in theory put one next to the furnace in your machine closet but that would be dumb and expensive

yeah the default in this case has to be “off” to prevent damage from running blind, on that note other things in the house should be certified to be able to handle being frozen perhaps

Exactly...

Fail "safe"...not Fail "keep running"

Typically when people are concerned about their house freezing in cold climates, they are primarily worried about water freezing, expanding, and cracking pipes and fittings.

It is extraordinarily hard to design something that can withstand that pressure and still be fit for purpose. The item needs to be able to withstand pressures in excess of ~10k psi for -10c, with the pressure rising as temp decreases.

The standard solution for people that need to winterize a building that will not be heated is to drain as much water as possible from the lines, and then fill them with a liquid with a lower freezing point.

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