I’m looking for thoughts on a trigger for an automation for an alert.
HA was doing a great job of managing my house during an extended absence until I had an HVAC issue. It wasn’t HA’s fault, but I’d like HA to alert me if it happens again so that I can arrange service.
I have two high efficiency natural gas furnaces. One for my basement/ main floor and one for my 2nd floor. The exhaust from these is minimal, but goes inside 2" plastic up from the basement (where the furnaces are) 2 stories up and out the roof. This has worked without issue for about 10 years.
This year, following an significant cold period (-40 for about a week), the exhaust pipes from one furnace froze solid. When the furnace cannot exhaust properly it detects this and shuts down. It then tries again about 1 minute later. Before too long the house could start to cool off. But - if its the 2nd floor furnace, then heat from the lower floors will drift up and the house is ok - maybe a little cooler, but not much usually. If the other furnace was to then also freeze up though - I’d have no heat at all and at -40 outside, things would get cold fast. When the city had these kind of temps, getting a furnace repair guy out fast can be a challenge. I can’t just wait until the house is cold before alerting me.
So how can I detect this before it gets too far?
I have the following sensors available:
- Lots of indoor and outdoor temp & humidity sensors
- Emporia power draw sensors on each furnace
- Honeywell redlink thermostats (if you aren’t familiar with these think “nest on steroids” but the HA integration only provides temperature and humidity (indoor and outdoor) and heat/ cool running state information). This thermostat may also turn on the furnace fan and humidifier if the humidity is too low (or turn it off if the temperature / humidity differential between outdoors and indoors is such that the humidity should be reduced to stop windows from fogging - so I might see a power draw from the furnace when it isn’t calling for heat.
I’m not currently using the Honeywell thermostat air floor sensors, but could add these.
Potential approaches:
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I can monitor the temperature dropping below the set point - but that’s not perfect as I may change the temp at different times of the day and it might take a while to catch up - causing false alarms.
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I could look for patterns - like the furnace is calling for heat for a long time but it’s not reaching the set point. But if the lower furnace is generating enough heat then this might not trigger. Also, detecting call
-for-heat would be a combination of temperature / set point data and power draw data - with lots of complications due to all the stuff this thermostat is capable of. -
Have someone build an air pressure sensor on an esp32 and install this inside my exhaust pipe. It would need to react quickly though as the furnace will only run for about 5-10 seconds when plugged before it turns off in this situation. The nice thing about this approach is that I could possibly predict that the pipe is starting to plug and tjen take corrective action before it became completely plugged - maybe have HA push the house temperature up higher to create increased exhaust heat / air flow until the partial blockage was relieved - but that might be a bit too fancy for the first attempt at this.
Just an FYI - I’m think this is happening this year because of a combination of the low temperatures and my extended absence - so the furnace doesn’t run as much and isn’t pushing the exhaust air out as much. With high efficiency furnaces that exhaust air isn’t particularly warm anyway.
In the future I hope to mount a camera on my roof to see these pipes - so that if this alert goes off I can visually confirm it.