Wednesday, November 30, 2011

Considerations on the boiler system

The behaviour I mentioned at the end of the last post, does make sense.
That is, for the boiler to overshoot by a few degrees, once it has reached the set temperature, and by those means bringing the return temperature to the set point is the only way of avoiding it to cycle more.
Once the boiler stops burning, the temperature of the outgoing water drops almost immediately to the return temperature, so to avoid it turning on again, the return should be close to the set temperature, and that is what the boiler is doing, thereby keeping the water in the whole system at the set temperature, and not at the return temperature that happen to be there when the set temperature was first reached... (I hope someone can make sense of all this rambling...)

The system , with the DIY OpenTherm master, has now been running for a few days, with no problems, and quite an accurate temperature in the rooms. The PID controller just needs some small adjustments in order to converge a bit quicker, and converge always (I sometimes have 0.2 degrees below the set point, which is no big deal anyway...)

I found a few graphics depicting the boiler efficiency as a function of the return temperature. Here are a few, all in Fahrenheit (didn't find any in C):


http://www.mnshi.umn.edu/kb/scale/boilers.html

http://blog.triangletube.com/blog/hydronic-heating-solutions/outdoor-reset-maximizes-spring-energy-savings
http://www.navitron.org.uk/forum/index.php?topic=9066.0

The last is not the efficiency, but the relation of water temperature to outside temperature that I am using. I use a curve near the "1" curve.

1 comment:

J Silva said...

Hi Alex,
I think that control behavior is correct.
From the control perspective, if the heat transfer is 100% the water returning to the boiler has your "room temperature" (if heat-transfer was perfect). This is the temperature that is subtracted from the set point (giving the error), an PID controller will overshoot the control output (water exiting the boiler) but will strive not to overshoot the water returning.