Furnace Monitoring with Honeywell T5 Thermostats


There have been many Furnace Monitoring threads, but none cover my needs very well.

I use two Honeywell Lyric T5 thermostats, one the main part of our house and one for a rear addition. The rear area is supplied by an auxiliary furnace because the main furnace didn’t have enough capacity to cover the addition. When we are away, the thermostats are set to 55F. I have the thermostats set to send me alerts if the room temperature drops to 53F. If either furnace fails to ignite, I’d like to get an earlier notice of the failure instead of waiting for the room temperature drops.

I understand the Ecobee thermostats have this feature built in. See Huy Nguyen’s comment on “How Can I Monitor My Furnace”:

This was the example email notification I received from ecobee:
"Huy Nguyen, you have an alert message from ecobee for thermostat: XXXXXXX6886:

There maybe a problem with the Furnace. For the past 2 hours the thermostat has been calling for heat, but the room temperature has decreased by 1.6C. Contact your service contractor JD Swallow Heating Contractors INC at 613 822 7974"

I want to create similar functionality for my Honeywell Lyric T5. I’ve been experimenting with a Wireless Sensor Tag thermocouple in my furnace flue and get reliable measurements of flue gas temperature. This tells me if the burner has fired.

I can integrate Wireless Sensor Tags to ST. Now I need to get the "Call for Heat
which should be simple to do with an auxiliary 24 VAC relay on the “W” terminal on the thermostats. The relay dry contacts can feed a ST window/door sensor with dry contact inputs, so I would have a ST on/off signal for the Call for Heat.

Here’s the problem: How can I correlate the “Call for Heat” to the rise in flue gas temperature? If I get a “Call for Heat,” I expect to see a significant rise in flue gas temperature a minute or so later. If the furnace flue gas temperature does NOT rise above 200F one minute after the Call for Heat, I’d like to get an alert message.

  • Would it be possible to do this entirely in ST?
  • Or ST supplemented with webCore?
  • Or do I need Arduino as a simple automation controller?
  • Or a Raspberry Pi as a computer?
  • Or some other solution?

Unfortunately, Wireless Sensor Tags do not support a simple binary on/off input for the “Call for Heat” so doing this in WST seems to be out of the question.

There are so many possible ways to go that I’m not sure how to start.

Of course, I could simply change to the Ecobee thermostats, but my Honeywell T5 thermostats are fairly new (6 months old) and they integrate nicely with my Honeywell Lyric security controller, so I’d hate to give them up.

Thoughts on architectural direction? Or other ideas?