Dual temperature probe to constantly monitor HVAC performance

(Alex) #1

Background: I have had a couple cases in two different homes where my electric bill sky rocketed over $300 in a month due to issues with my HVAC system. The last time this happened I found a leak in both of my coils requiring they both be replaced (Thank you Lennox for the faulty coils!) in warranty. This summer is one year after the coils were replaced. While it appears that my AC systems are both operating normally, they tend to stay on for much longer (ecobee retains detailed data on what the system is doing) making me wonder whether there is an issue.

Goal: Automated monitoring of AC performance by measuring the air temperature on the input and output of the AC unit. I believe proper functionality is when there is a difference of 8-11C (15-20F).


Viable Sensors: Thermocouple or DS18B20 Digital temp sensor

Hardware to measure and share on ST: This is where I am looking for suggestions. I am hoping to find a Zigbee or Zwave device with two temperature probe inputs. All I have found either has an onboard sensor (not useful) or only has a single input (Qubino ZMNHID3)

If this were a stand alone system I would consider using an Arduino with an LCD but I would like something where I can pull in the data in ST easily.

Any suggestions? I am trying to avoid having to purchase Qty 4 of the Qubino device listed above given I have 2 separate systems…


I’m not quite sure I understand the project, but the Fibaro RGBW controller can accept Input from four analog sensors. People quite often use it for pond or hydroponics management for that reason.


(Alex) #3

Interesting device. Trouble is that the input is 0-10V so I would have to find TC amplifiers or a pre-conditioned temperature sensor. I’ll look into it to see if there is anything simple and economic.

One of the many articles that talks about why I would want to measure and compare the two temperatures on my AC units:


(Eric) #4

why bother with bulb sensors? It appears to me you can get your 4 desired points with $100 in wireless contact or motion sensors that have temperature built-in. Drop in the discharge ducts and hang them in the return - done in 190 seconds.


I’d be surprised if you could get signal out from inside the air conditioner, let alone what conditions would do to a battery operated device, but I suppose you could try. Probe sensors are typically used when conditions at the point of data location are unsuitable for Z wave/zigbee radio to operate.

(Alex) #6

@ero4444 - I guess I could use those sensors if I place them inside the house behind the return filter and behind, or stuck on, one of the vents but it is not my preferred solution. I am aiming at taking the measurements just before the evaporator coil and just after the blower. There is an insulated distribution box (plenum?) just before and after this equipment with holes already made by techs who tested the temp difference. I believe that is where I would get the most accurate and reliable measurement.

(Eric) #7

in my experience, the difference between return temp and blower discharge temp is insignificant, less than 1F. (this is my trade for 25 years).

I’d be surprised if you could not read from inside the air handler. If Zigbee doesn’t work then Zwave will.

For reliability I would avoid the Fibaro+DS18B20 . I use a couple of them in cooking, but they have been troublesome to keep logging. I don’t know if the wired alternative would be more reliable, but the Zigbee wireless temp like Centralite/SmartSense door contact, has been perfect for several years.

(Alex) #8

@ero4444 - I do not doubt the validity of your setup and I appreciate your confirmation that the temperature does not vary much between the two possible locations. My projects are often overkill or geekier simply for the cool factor. I also work for a test and measurement company so some things, related to measurements, just got ingrained in my DNA :wink:

(Joel W) #9

What about this. [

Qubino Z-Wave Plus 110-230V or 24-30 VDC On Off Thermostat Module ZMNHID3 with External Temperature Sensor


by Qubino

(Alex) #10

This is the one that I also found which would do the job but I would have to buy 4 costing more than $200.

(Michael) #11

Very old post but here is an Arduino based solution that has 2 temp probes that integrate with ST. This project used the now unavailable ThingShield but a Wi-Fi based Arduino using ST_Anything would work these days.

(Steve Peck) #12

As A 30+ year HVAC contractor (own my own business). The article you quoted about 14* to 20* ATD or air temperature drop, is a rule I have used for most of my career. That measurement is taken as close to the return air coming into the air handler and the supply coming out of the unit. (This measurement should be 20*+ ATD ) Measuring at each vent might tell you if the ducting has failed in some way. that reading would be closer to 14*-20*ATD.
I recently installed this unit in my house to monitor just as you are saying and to control my Humidifier. It also is connected to smartthings hub.

(Jeff H) #13

Hello Mr Peck
I was looking for a way to be alerted if my capacitor for my compressor ever failed again. (My post here) Monitor house A/C condenser for failure

Will this Honeywell kit do that for me? If so that would be great. I currently have the regular Honeywell wifi color thermostat now.

(Alex) #14

Thanks for all the great suggestions! I think the path I will take is Arduino as I believe I can measure all 4 temperatures (2 systems) with a single Arduino. I forgot there already was a way to bring the data into ST and that was why I was looking for alternatives. Well… Another reason was lack of time so a turnkey solution is still appealing. The system suggested by @Speck1962 seems really cool but I would need 2 and would have to replace my existing ecobees so I don’t think it would work for me.

(Steve Peck) #15

Hey @Jeff_H
Sorta… and it’s as good as anything. Except a temp. monitoring app maybe?
When your cap fails it simply won’t “help” the compressor or condenser fan start, so I can’t think of an easy way to alarm for that.
It does have alarms (it will send alert) for house creeping up over temperature (you set the temp for high and low alarm) and if air temp is not within defined settings.
Again there probably is an app that will tell you if temp at a sensor is over or under XX degrees.

(Jeff H) #16

Thank you. I just found this in the install manual

Delta T Alerts are displayed for basic failures including no
heating or no cooling that may be related to bad igniter, bad
flame sensor, bad gas valve, bad draft motor, bad blower, bad
contactor, bad startup capacitor, dead compressor, etc.