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):


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…

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7 Responses to Energy flow in Watt

  1. henk says:

    Hello Robert,

    At 22:14 8 dec. your table shows this;

    Remco 16.06 16.25 -0.19 20.3 72

    I think this is impossible you can not heat a room using cold water,


  2. Hi Henk,
    Haha, I waited for this to happen at the time I made the screenshot 😉
    Just to show what I meant with “unexpected results”.
    This can happen when the boiler stops and the flow temp drops faster than the return temp, resulting in (short) periods where flow temp < return temp. The 72W you see is the last “succesful” calculation based on flow > return. I still have to change something on how to handle these situations in software – just display 0 W seems the best thing to do; but I just haven’t done that yet.

  3. henk says:

    Hi Robert,

    Maybe you only have to calibrate the sensors, the return temperature never can be higher then the flow temperature. Just put them together in a isolated jar (themoskan) and some hot (70 degrees Celcius) water and log the temperature, slowly going down to room temperature if you leave the lid of. If you would like to have lower readings also add some ice and watch the temperature going up.

    • Hi Henk,
      Sorry to disappoint you, but the return temperature can be higher. 🙂
      What these sensors are measuring is not the same object, like in your example of 1 jar with hot water, which can be assumed to be perfectly homogeneous with respect to temperature.

      But what happens under real life conditions is that when the boiler stops burning, the temperature of the water running through the pipes will drop very rapidly. Due to the amount of water inside the radiator, it is very normal that it will take some time before this temperature drop will also show up at the other end of the radiator. It’s the same thing you witness when you turn on the hot water: your boiler starts producing DHW, but it takes some time before you feel that hot water coming out. The cause for this delay is the water inside the pipes from boiler to the tap; and a radiator is nothing more than just a specially crafted piece of pipe in this perspective. That’s why return can be higher.

      Before I started, I calibrated all 12 sensors and all were within 0,0625 degrees of each other, so the sensors are fine. Things that do give errors in the measured temperatures are: bad contact between sensor and radiator, bad isolation (to the surrounding air), difference in height between flow and return sensor (distance of the sensors to the floor are relatively low, so a few centimeters can mean a lot, maybe…).

      I added some isolation to 2 sensors last week, but I didn’t see much difference actually…

  4. Henk says:

    Hi Robert,

    Thanks for the explanation,


  5. henk says:

    One last question, how can the water temperature in Remco’s chamber be so low if the house is around 20 degrees Celcius?

  6. Now (22:03), the flow temperature is 35-36 degrees, not much different from the rest? If you meant the room temperature, well he left his door open (again), which is close to the parents room, where it’s always cold, or should I say less warm. That door is closed now, so the temperature will start rising again.

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