Are there any thermostats that work with variable speed furnaces?

I’m talking true variable speed - NOT three-stage, or similar. I’m having my HVAC system replaced next week with a 96% variable speed system. Ecobee and Nest don’t work with variable speed.

I have a Bryant with variable speed fan. I have the Bryant Housewise thermostat from the dealer. It is an Ecobee that is re-branded. I don’t know why any thermostat wouldn’t work. The fan control is done in the furnace, not the thermostat.

Maybe I’m not fully aware of how variable speed works. With a smart thermostat, it will turn the furnace on or off, based on temperature. It can also use multistages (I think up to 3 with Nest) to turn it to high/med/low (I think).

But with variable speed, correct me if I’m wrong, but the fan is constantly modulating faster or slower depending on what temperature the thermostat tells it. So if a thermostat only tells it on/off, and the variable blower motor is attempting to modulate temperature on its own, wouldn’t it run into some problems when it tries to cycle down (not off) to lower the temp?

I don’t know how it works, but my system works and it works with ST. ST
thinks it’s an Ecobee

I have a 3 speed HVAC system (Carrier) and have found NONE, other than Carrier, that supports the 3 speed.

At least when I run their compatibility checker.

I need one that I can change the fan speed (both manually and “Smart”) as well.

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I was under the impression the Nest (but not Nest E) supported multi stage:

This is what Nest says for my Carrier.
. For this reason, many system manufacturer’s high-end and multi-stage brands are proprietary–such as the Carrier Infinity and Bryant Evolution lines.

This is overall somewhat disconcerting news. I hope Carrier and Bryant are at least looking into making their thermostats connected… Are they?

HVAC is being installed here on Wednesday, so I’ll know more then.

A cursory look reveals yes, both (and probably all) manufacturers have wifi/connected thermostat options.

I see some folks are not aware of what a modulating furnace is so let me explain.

A traditional (1 stage) furnace had heat on or off.
2 Stage furnace has Heat Low (usually 65%) and Heat High (100%)
Modulating furnace has many stages, and the real advantage here is that it can go as low as 30%. This is not just the fan that spins slower but the burners as well. It has a variable gas valve and so on.

To achieve good comfort, you want long heat cycles. To achieve that, the modulated furnace needs a “special” thermostat. The main thing it is looking for is the required temperature rise (current temp vs target). It can also check outside temps, humidity levels etc. The other main advantage of the modulated furnaces is that they can solve the problem of undersized (old) ducts. Units have pressure sensors and can regulate speed/fire if static pressure is too high and so on (regardless of what the thermostat wants).
Same thing with cold and A/C.

The sad part is, these HVAC manufacturers are years if not decades behind and each pushes each own protocol, although there is something called ClimaTalk but that is just not going anywhere. Their “smart thermostat” are smart, but lone woofs - impossible to integrate them with whole house smart systems. There is no open APIs and so on. And there is a reason for that. They want to sell you zoning systems, their sensors and so on.

I am actually negotiating new furnace install these days. Looking at the York Affinity line. Support from York is poor, most don’t understand when we ask them for API. Sniffed some packets and it looks like it is using JSON via Amazon servers. Unfortunately I am not that good of a twicker to be able to reverse engineer it. But even if I was, as all things cloud, what happens in 5 years with that service?

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I have a Ruud Ultra Series modulating gas furnace that requires a special thermostat for most efficient operation. (I think you can also operate it as a two-stage furnace with reduced efficiency.) The modulating thermostat outputs a pulse-width modulated signal on its ‘V’ terminal that controls the gas valve on the furnace over a 40%-100% power range. According to the thermostat manual, “Amplitude of this signal is about 10 VDC, frequency is 1 Hz, and the pulse width is variable 350 to 950 [milliseconds] in steps of 50 msec.” See page 7 of the manual at this link. The controller built into the furnace sets the blower speed as appropriate for the burner heat output level.

Sounds like a simple Arduino or Raspberry Pi project, with the appropriate 24 VAC and 10 VDC interfaces :wink:

“Simple” being relative… :slight_smile: