Digital Limit Switch +

My furnace has a limit switch in the plenum. I had the Honeywell L4064. I HATED the controls to adjust the upper and lower limit temperatures on it. They were so touchy. Sometimes if the fan wasn't shutting off and should have, I'd have to go down and tap it. It would click and then shut off.

I searched online for a digital replacement and never really liked what I found, so I built my own. At the heart of it is an Arduino, some temperature probes and a couple of relays. One temperature probe goes in the plenum, right where the old Honeywell L4064 was. A second probe hangs in front of my cold air intake. I added an LCD to display information, and an SD card slot so I could log some data.

It works great! I actually built it in Feb of last year, so I've had some time to use and tweak it. Just like the old Honeywell, I can turn the fan on all the time or put it in auto mode where the fan turns on and off based on the plenum temp. Only now I can adjust the upper and lower control limits so much easier. Unlike the Honeywell though I have multiple auto modes. The second auto mode sets the lower (off) temp based on a delta to the temp of the air going into the cold air intake. So instead of specifying a fixed off temperature, I specify a number of degrees above the incoming air that the fan should turn off. I find that with my setup the temperature of the air going into the cold air intake isn't the same the whole heating season. That led to me adjusting the old Honeywell and my digital replacement as the season went on. I've programmed this second auto mode as a new addition for this season. Hopefully it will keep me from having to adjust the off temp as the season progresses.

Also just like the old analog control, there is an over temp safety switch that will kill the power to the combustion blower and the auger motor if the plenum temperature ever goes above 200.

I've also replaced my old auger rotation counter* with a new optical sensor incorporated into this system. This allows me to track the number of auger revolutions and therefore coal consumption automatically.

All of the data gets logged to CSV file on an SD card so that I can analyze it to my heart's content. I display the two temperature measurements, yesterdays number of revolutions and the number of revolutions for the current day on the LCD.
The grey box on the left houses the Arduino, LCD and SD card. The yellow buttons on it allow me to adjust the limit temperatures, change the mode and reset the controller. The box in the center is a junction box. The Arduino, temperature probes, revolution counter sensor, and relays plug into it. The box on the right has two relays in it. One for the fan and one for the over limit kill switch. There are four cords hanging out of the bottom. One set switches the fan and one set switches the power.

*Automatic Coal Consumption Tracking

So C'mon.

You've got to have a chart or something to show us with these new mods.

Larry is probably chomping at the bit to analyze the data.....

Rick

No problem. Should have thought of that! Here's a chart showing the plenum temps, the cold air temp (I called it 'other' temp) and the state of the blower - either on or off. This is from March 27.