Almost every hydronic contractor who reads this column probably has been doing his heating system controls the same way since he began doing hot water heating. I know I did. I did it the way I was shown by my mentors and, until recently, I never questioned the methods I was using.
When I went to Germany for the last ISH conference, I visited many of the control company booths to see if I could figure out how the Europeans do it, and it was an eye-opening experience.
Let's take a look at the different ways of controlling these wonderful systems and compare the differing techniques.
The bang-bang method This is the most common system used here in the United States. When there is a call for heat from a room thermostat, bang, the system comes on full bore. When the room thermostat is satisfied, bang, the system shuts down.
If the load is only one zone out of, say, five zones, and the load being served is a small zone, the boiler will probably hit its onboard high-limit before the room thermostat's call for heat is satisfied. Bang, again, the burner shuts down.
This system is pretty typical. It's the way I was taught to do it, and I'm positive it was the way my mentors were taught to do it, and, obviously, it's the way their mentors were taught to do it.
It works. Why change what you're doing if it works? Some of the knowledgeable dead men I used to work with and for used to say things like, "Don't mess up a good thing." When I was their apprentice, I would take these sayings as gospel. Now that I'm older and wiser, I now see that they were basically afraid to do things differently.
I, on the other hand, believe that a person should look at all his options and decide which option works best for a given situation. Not all control logic works for every job, but the bang-bang system has a lot of alternatives.
The bang-bang system assumes that every time there is a call for heat that the outside temperature is at design condition. This system has no way of referencing either the outdoor temperature or the indoor temperature. It is relatively blind, dumb and deaf.
Modified bang-bang method
If you start talking about proportional, integral and derivative to them, they're going to show you the door.
This system is slightly more intelligent than its predecessor is. Some of the older control systems incorporated a capillary tube controller with two bulbs. One bulb was placed outside, preferably on the north side of the house. The other bulb was placed on the hot water supply, or boiler outlet. Upon a call for heat, this device had the ability to "look" outside and see what the outdoor temperature was, thereby changing the boiler's outlet temperature based on a preset ratio.
It's much better than the usual bang-bang method, but it still doesn't have the ability to look at the indoor temperatures to readjust the given reset ratio that was initially plugged into the basic control system. Its eyes and ears in the house is the thermostat, which is still a bang-bang control. It's getting closer to the ideal logic, but it still leaves a lot to be desired.
Proportional method This logic is about as close to the ultimate control logic as you can get. It is borderline artificial intelligence. Upon a call for heat, which comes from an intelligent two-way thermostat, the controller looks outside to see what the outdoor temperature is. It then looks inside to see the difference between what the room temperature is vs. what it's supposed to be. It then references an adjustable preset reset ratio and dictates what the supply water temperature is supposed to be.
It now sends a blast of energy to the calling zone and "watches" to see what happens. After a period of time, it looks back in at the calling thermostat to see what has happened to the deviation. If the room temperature has risen, for example, 0.5°F, and its deviation between the setpoint and actual room temperature is 1.5°F, the controller then sends a triple blast of energy to the calling zone and then sits back to see how the room reacts.
If the room hasn't reacted as the controller thinks the room should have reacted, the controller will raise the supply temperature by a few degrees and start the trial-and-error process over. Sending blasts of energy, watching for reactions and establishing recovery profiles.
So the next time a given room calls for heat, the controller does all its looking around, makes some intelligent decisions, sends out the required energy, watches for reactions and adjusts as needed. Pretty cool, eh?
It's kind of like having three or four guys who are in constant radio contact with each other controlling your heating system.
One guy in the room sits there watching the room temperature. We'll call him the thermostat watcher. When he decides the room's too cool, he makes a call to the boiler room and tells the boiler operator he's cold.
The boiler operator calls his buddy the weather man who's outside and asks him what the temperature is, and then the boiler operator decides what temperature of water is needed and sends a shot of hot water to the room where the thermostat watcher is sitting. The boiler operator waits for a while, calls the thermostat watcher up to see what's happening. The thermostat watcher reports back to the boiler operator guy and the boiler operator guy makes the necessary adjustments to satisfy the thermostat watcher's call.
All the while that this is going on, there's a data analyst sitting there recording all the boiler operator's moves and reactions and thethermostat watcher's responses. The next time thethermostat watcher calls for heat, the boiler operator talks to the data analyst, who talks to the weather man and the thermostat watcher and figures out how much energy is needed to make the thermostat watcher a happy man. He communicates this information to the boiler operator and the process continues. Pretty cool, eh?
Now, I know that this is a much simpler explanation than it needs to be for your edification, but what about explaining this whole operation to your customer? If you start talking about proportional, integral and derivative to them, they're going to show you the door.
Next month we'll look at continuous circulation with outdoor reset.
Until then, Happy Hydronicing!