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OurCoolHouse

Legacy Setup
For early adopters.
Using the 8 channel Thermostat Input board (WEL-HVAC-8S).

Sometimes you just need to know what all your thermostats are calling for, but it's difficult to interface a 24 VAC thermostat signal to a digital bus.  A classic way is to add a small mechanical relay with a rectifying diode and filter capacitor.  You could do this with the WEL, and feed the relay contacts to a standard run monitor input.  However, if you need to sense more than a few inputs, the relay solution gets a bit cumbersome.  This is why I created the WEL-HVAC-8S board, it provides an easy way to monitor 8 thermostat outputs (either AC or DC).  Here is the schematic of the board.

My only BIG WARNING is that the WEL-HVAC-8S is NOT designed to monitor any voltage greater than 24 VAC. 
Go above this and you'll start overheating the series current limit resistors and possibly damage the inputs.
NEVER even consider connecting the inputs to mains 110 VAC or 240 VAC

 

1-Wire Ground BLACK   YES
1-Wire Bus YELLOW   YES
Aux. Power RED   Optional

Now that I've got that out of my system, there are a lot of different ways to use this board.  Let's cover the basics first.

For starters, this board only needs two of the three classic 1-Wire signals: the Black (Gnd) and Yellow (Bus) wires.  The Red (Pwr) may be connected, but it's not used in any way, except to pass through to the other 1-Wire connector. 
The WEL-HVAC-8S is shown here with its two 1-Wire pass through connectors (one on either side) and its strip of eight pairs of sense inputs.

Input channel 1 starts on the top-left and ends with channel 8 on the top-right.

To detect the presence of a control voltage, just connect it across the desired pair of input terminals. The board will detect AC or DC inputs.
When the WEL discovers the WEL-HVAC-8S, it adds it to the device list as a block of 8 devices.  The first device will be type 41 and the rest will be type 14.  The 8 devices correspond to input channels 1 to 8.

Inputs 1,2,3 & 5 are used in this example. Input 5 is inverted.
With the default scale and bias values, if there is a voltage present on a pair of inputs wires, the WEL will report a scaled value of 1.0.  This can be changed to any other value simply be changing the scale or offset for that device.  Here are some examples:

 Direct HVAC monitoring.

If you have access to the thermostat call inputs on the HVAC equipment then connect one of the HVAC-8S input pair wires to the C (Common) terminal and the other wire in the pair to the line you want to monitor. This could be the Y1, Y2, W, O or G.  See inputs 1,2 & 3 in the picture at the left.

Indirect Thermostat monitoring.

In some instances you don't have access to the GSHP controller board, and so you can only connect to the two wires going to a simple thermostat switch.  In this case there are two options for connecting HVAC-8S inputs.  Parallel or series.

For a parallel connection, the two HVAC-8S input wires are connected in parallel with the thermostat.  In this configuration the WEL will see a "1" when the thermostat is not activated, because the 24VAC is present across the open thermostat contacts.  In this situation it is necessary to "invert" the input logic in the WEL.  This is simply done by setting a scale of -1.0 and an offset of 1.0

In some systems, simply connecting the HVAC-8S across a thermostat, may cause the controller to see the thermostat as ON.  This will typically occur if the controller is using a low current DC voltage to sense the thermostat closure.  In this situation, you need to use a series connection (instead of parallel), by breaking the wire going to the thermostat and using the two HVAC-8S input wires to re-make the connection.  Now, then the thermostat switch closes, it will draw current through the HVAC-8S input which will be seen by the WEL as a '1' signal.  No inverting is required.
Voltage Isolation One of my key design decisions for the WEL-HCAV-8S was voltage isolation.  Each pair of input wires is optically isolated from the WEL's 1-Wire bus, AND all the other input Pairs.

This means that you can safely monitor voltages from several different systems without any fear of them interfering with each other.  Just keep all the thermostat wires away from the 1-Wire connectors and you'll be fine.

 

The Web Energy Logger is an Internet Appliance that monitors a 1-wire sensor network, and posts live data to a web site for display and analysis. The WEL was designed for people interested in monitoring the energy distribution of their own homes or businesses. For more background, visit the WEL homepage.
©2008 Phil Malone - - www.OurCoolHouse.com