Energy flow in Watt

Now that I have sensors on all the radiators  I can sit back, relax and watch everything happen. I needed some help of the rest of the family to keep the doors upstairs closed, cause that seems to be very hard for some, but after a few days they all knew that when the doors were closed, daddy was doing one of his experiments again.. yep, this house is one big laboratory 😉

However, doing nothing is not really my style, so I went on reading about radiators, calculations etcetera and found some calculation tools for my RADSON Compact and Jaga Tempo radiators. And although it doesn’t add new data, I thought it would be fun to see if I could use the formulas used in these ‘Installer tools’ (in fact Excel sheets) for myself.

And I was right; based on radiator model, dimensions and specific properties it should be possible to calculate the actual heat output performance of the radiators in other circumstances (temperatures) . If you know the heat output conform the European standard EN442 (dutch) for a specific combination of model & dimensions, the so called n-factor (aka the emission line slope) and the air-temperature, you can calculate the heat output for any combination of flow- and return temperature!

So that’s what I did last evening – preparing my system for these semi-realtime heat output calculations.

First I had to add some more flexibility to the database; creating fields for this purpose only didn’t feel right, so I added a free text field in which I could ‘dump’ anything I want – device specific properties. As an example, here are the properties of one of the radiator devices in my system (it feels like ‘Domotica system’ doesn’t quite cover the capabilities anymore):

TYPE=RC
QN=1407
N=1.3404
TAIR=BATHROOM.TEMP

This means this device:

  • is of type RC (as in Radson Compact), 
  • has a heat output of 1407 Watt, based on its dimensions and EN442 at 75/65/20 °C (that’s flow, return and air temperature);
  • has an n-factor of 1.3403;
  • and that the air temperature can be taken from the device value called BATHROOM.TEMP (a foreign key, so to speak)

So every line defines a device specific property, which are dynamically added to the base device class at run-time.

And I had to write 2 Delphi functions to do the math, primarily based on these formulas:

Q is the variable that needs to be calculated, the rest is all known – piece of cake!

Just make sure you take care of unexpected results, like divisions by zero or raising negative values to a power- working with live data can produce strange results, where a radiator can suddenly start cooling instead of heating 🙂

I added the calculated heat output (the “W” column) in the table with all the other sensor data as can be seen below:

real-time calculated radiator heat output

Cool! Although these heat output numbers are based on pretty accurate formulas and properties provided by the manufacturer, the real-life numbers will probably be lower. Dust, bad airflow caused by windowsills and such will always result in deviations. How much? Dunno…

RF to Zigbee gateway

The last piece of missing hardware is finished. The picture below shows the 2nd RF to Zigbee gateway I had to make to be able to receive all the Hydronic balancing sensors I made earlier this week. One of those sensors just couldn’t make it through 3 walls all day long, so I created  a temporary solution on a breadboard to solve this.

A very simple yet effective way (for me) to get the sensor data where I want it (in my Domotica system) with minimal effort.

The JeeNode acts as a RF receiver and just echoes everything with a valid CRC to the Digi XBee; from there it eventually arrives at my Zigbee Coordinator with which I can communicate over TCP/IP.

The JeeNode runs a slightly modified version of the RF12Demo sketch made by Jean-Claude Wippler. I used the NewSoftSerial library to create an additional Serial port, and added a few print statements for the XBee port, right there where the RF12Demo Serial.println()’s the received RF data to the Serial port. Compile, Upload, setting the RF band,  group- and node ID and I’m done!

This JeeNode is powered by a 5V USB adapter and the XBee gets its power from the 3.3V JeeNode ports. The XBee uses a Zigbee End Device AT firmware (2864) with the Sleep Mode set to Pin Hibernate. But because pin 9 is wired to GND, this means that the XBee is permanently on.  Only 3 wires are needed to connect the XBee to the JeeNode: 3.3V, GND and a JeeNode digital pin to the XBee DOUT.

That’s it – moving on with where this all started with: understanding the flow of  heating energy in our house!