Tap into furnace to sense "call for heat"?

[The Keeper]

Does anyone know of any "best practices" with regards to trying to tap into a standard forced-air gas furnace to pick up either a dry contact or a 24VAC contact (I can probably scare up a 24VAC relay to produce the dry contact), so I can know when the thermostat calls for heat?

Thanks,
Chris

[langenet]

What are you using as a T-stat? Perhaps you can use a DS10 in your thermostat.

Rob

[Steve Q]

On my furnace (Carrier 8500), the controller board has a terminal for a humidifier. This terminal is +24V when the burner is ON (t-stat calling for heat). I have an x10 powerflash module hooked up to this terminal. I have been using it for 4 years.

Steve Q

[ArmandGirard]

If your controller board doesn't have an output when the furnace is running, then another way to accomplish this is to use a sail switch in the return duct.

A sail switch is a micro switch with a wide flap on it. The "flap" part mounts in the duct work while the switch part mounts on the outside of the duct. When the furnace blower starts up (presumably because it has been asked to by the thermostat), the sail switch moves because of the air movement in the duct, thereby activating the switch.


Armand

[drozwood90]

I do almost the same as Steve. I put a UPB DIO32 module in parallel with the call for Heat on my Furnace. Every time I get a call, my module updates HS.

I find that more accurate than polling my T-stats (Z-Wave). It's not Z-Wave's fault. I just bought Cheap T-stats...so they have to be polled.

--Dan

[Steve Q]

If your controller board doesn't have an output when the furnace is running, then another way to accomplish this is to use a sail switch in the return duct.

A sail switch is a micro switch with a wide flap on it. The "flap" part mounts in the duct work while the switch part mounts on the outside of the duct. When the furnace blower starts up (presumably because it has been asked to by the thermostat), the sail switch moves because of the air movement in the duct, thereby activating the switch.


Armand

This will work but it indicates when the fan is on, not when the burner is on. My furnace fan runs for 5 minutes or more after the burner turns off. I also often run the fan to circulate air throughout the house. So if the intent is to measure the total time per day or month the burner/heat is ON, then I would not use this approach.

Steve Q

[X10joe]

How I do this......

I use two power flash modules. One with a cr magnetics current sensor for when the blower motor comes on. A second with contact closure - when the gas valve gets the 24VAC - I have a 24VAC relay across the gas valve terminals and the dry contacts to the power flash module.
I have a lamp module with a incandescent night light plugged into it. When the power flash for the gas valve fires the the lamp module turns on showing the burner is on. I have a event that then writes to a spreadsheet and also saves it as a html in HomeSeer directory which I can view on my HomeSeer website.

Burner OFF 6:31:15 AM 1/13/2012
Burner ON 7:22:07 AM 1/13/2012
Burner OFF 7:54:48 AM 1/13/2012
Burner ON 8:23:32 AM 1/13/2012
Burner OFF 8:40:03 AM 1/13/2012
Burner ON 9:16:32 AM 1/13/2012
Burner OFF 9:32:31 AM 1/13/2012
Burner ON 10:15:31 AM 1/13/2012
Burner OFF 10:31:03 AM 1/13/2012
Burner ON 11:20:31 AM 1/13/2012
Burner OFF 11:36:03 AM 1/13/2012


Blower ON 7:26:30 AM 1/13/2012
Blower OFF 8:10:54 AM 1/13/2012
Blower ON 8:26:37 AM 1/13/2012
Blower OFF 8:54:05 AM 1/13/2012
Blower ON 9:20:08 AM 1/13/2012
Blower OFF 9:46:24 AM 1/13/2012
Blower ON 10:19:29 AM 1/13/2012
Blower OFF 10:45:00 AM 1/13/2012
Blower ON 11:24:46 AM 1/13/2012
Blower OFF 11:50:07 AM 1/13/2012

[jpape]

I use two power flash modules. One with a cr magnetics current sensor for when the blower motor comes on. A second with contact closure - when the gas valve gets the 24VAC - I have a 24VAC relay across the gas valve terminals and the dry contacts to the power flash module.
I have a lamp module with a incandescent night light plugged into it. When the power flash for the gas valve fires the the lamp module turns on showing the burner is on. I have a event that then writes to a spreadsheet and also saves it as a html in HomeSeer directory which I can view on my HomeSeer website.

Burner OFF 6:31:15 AM 1/13/2012
Burner ON 7:22:07 AM 1/13/2012
Burner OFF 7:54:48 AM 1/13/2012
Burner ON 8:23:32 AM 1/13/2012
Burner OFF 8:40:03 AM 1/13/2012
Burner ON 9:16:32 AM 1/13/2012
Burner OFF 9:32:31 AM 1/13/2012
Burner ON 10:15:31 AM 1/13/2012
Burner OFF 10:31:03 AM 1/13/2012
Burner ON 11:20:31 AM 1/13/2012
Burner OFF 11:36:03 AM 1/13/2012


Blower ON 7:26:30 AM 1/13/2012
Blower OFF 8:10:54 AM 1/13/2012
Blower ON 8:26:37 AM 1/13/2012
Blower OFF 8:54:05 AM 1/13/2012
Blower ON 9:20:08 AM 1/13/2012
Blower OFF 9:46:24 AM 1/13/2012
Blower ON 10:19:29 AM 1/13/2012
Blower OFF 10:45:00 AM 1/13/2012
Blower ON 11:24:46 AM 1/13/2012
Blower OFF 11:50:07 AM 1/13/2012

Can you share your event that writes your data to a spreadsheet?

[The Keeper]

Thanks for the replies. The furnace is a 30-year-old Carrier, model number 397haw036100, but I can't find any manuals online for it. The thermostats are CT30-type, which don't report whether the thermostat is calling for heat. As Dan noted, even though it's not as accurate to poll than it is to receive an interrupt, with a CT30 you either need to pay $40 for a firmware upgrade or you have to tap into the furnace to know when the thermostat calls for heat.

A DS10 at the thermostat isn't practical, and I was hoping there might be a couple terminals in a typical furnace that I could tap into, but sticking a 24VAC relay inside the furnace would be fine; putting a relay in parallel with the gas valve sounds like it would be a good enough place as any. One just has to make sure not do anything to compromise the reliability of the valve.

Thanks,
Chris

[X10joe]

This is one of the four events is use. 2 for burner (ON)(Off), and two for blower (On)(Off).

BurnerON.txt

sub main(sVal)
Dim fso,f1
Const OpenFileForReading = 1
Const OpenFileForWriting = 2
Const OpenFileForAppending = 8
Dim lineval
lineval = 1

       Set fso = CreateObject("Scripting.FileSystemObject")
       Set file = fso.OpenTextFile("C:\Program Files\HomeSeer HS2\html\HVAC\BurnerStatus.ini", 1)

       'Read file contents
        lineval = file.ReadLine()

       'Close file
       file.Close

        '*****Update HVACstatus.ini*****

        Set fso = CreateObject("Scripting.FileSystemObject")
    Set file = fso.OpenTextFile("C:\Program Files\HomeSeer HS2\html\HVAC\BurnerStatus.ini", 2,True)
        file.WriteLine  (lineval + 1)
    file.Close

        '*****use new line # from ini to write new data to BurnerStatus1.xls****

        Set objExcel = CreateObject("Excel.Application")
        objExcel.Visible = False        
        Set objWorkbook = objExcel.Workbooks.Open("C:\Program Files\HomeSeer HS2\html\HVAC\BurnerStatus1.xls")
        '''objExcel.Workbooks.Add
        objExcel.DisplayAlerts = False
        objExcel.Cells(lineval, 1).Value = "Burner"
        objExcel.Cells(lineval, 2).Value = "ON"
        objExcel.Cells(lineval, 3).Value = time()
        objExcel.Cells(lineval, 4).Value = date()
        objExcel.Workbooks(1).Save 
        objExcel.Workbooks(1).SaveAs "C:\Program Files\HomeSeer HS2\html\HVAC\BurnerStatus.htm", 44
        objExcel.Workbooks.Close 
        'file.Close
        objExcel.Quit

end sub 

[Steve Q]

Joe, Thanks for sharing your script. I also use Excel to analyze my Furnace ON/OFF times. I use an approach similar to yours. I have a script that Homeseer runs to save all furnace ON/OFF data to a .txt file for each month. I append the new data to the file each time ths status of the furnace changes.

To analyze the data, I have created an Excel macro which imports the .txt data file and does the analysis. I run the macro whenever I want to look at the data.

Your approach appears to open and close Excel for each data point. My computer would not like this!

Steve Q

[cadillackid]

hobby boards makes (or use to make) an HVAC monitor sensor device that took the 24 VAC outputs and sent them over a 1 wire bus.. you could then see when the various terminals on the furnace are active or not.. you could also put a pIR sensor inside that would turn on when the flame is present.
-Christopher

[Steve Q]

Christopher, I like the idea of using a PIR to detect the furnace burner. Very clever! Do you think it would turn OFF, or would the furnace have to cool down completely before it turned OFF.

Steve Q

[TechnoJunkie]

I've got the reed switch for a DS10 stuck to the top of the burner relay on the boiler PCB. The magnetism from the coil in the relay brings in the reed switch.

[The Keeper]

So, what I ended up doing was to put an opto input across the C and W to sense when the thermostat calls for heat. This is a building that's not occupied all the time (it's the wife's store), so I also put an opto input across C and pin 5 on the gas valve, to sense when the gas valve itself kicks on.

Thanks,
Chris