Hydronic System Instrumentation

Hydronic system instrumentation is relatively straight-forward and basic in nature. We don't need to know about high tech things like water chemistry. Knowing system flow would be nice, but it's not necessary either.

We simply need to know about system pressure and differential temperature across the heating source. Well, at least that's what I'm interested in knowing because my system is heated with wood and for the time being, manually controlled.

Let's take a look at how my homemade hydronic heating system is configured to see what instruments I'm using, and why. This system diagram will open in a new window so you can switch back and forth between the discussion below and the diagram.

As shown in the diagram, there are only three items that can be considered hydronic system instrumentation - two temperature probes, and a pressure gauge.

The temperature probes are inexpensive units that are available in many locations. They sense inside temperature at the unit and outside temperature via a sensor on the end of a long wire.

The remote sensors are securely attached to the inlet and outlet piping of the stove, and the display units are located in the living room where I can keep an eye on incoming and outgoing water temperature. Using this information, I can:

  • Determine the amount of energy my stove is imparting to the water. This gives me a clue as the efficiency of the wood stove heat exchangers and the amount of heat I'm getting out of my source of fuel.

  • Stay informed about the status of my system, especially if it's getting too hot. In response, I can close off air to the stove or start up the furnace recirculation mode to draw heat off the system with the water-to-air heat exchanger.

  • Know when the temperature of the system is such that the water-to-air heat exchanger can be used to heat the house via furnace ducting. Circulating 80 degree F air throughout the house won't be comfortable, so I let the temperature of the system reach about 135 before I turn on the recirculation mode on the furnace. This puts out air from the ducts that's a comfortable 100 degree F.

  • Understand when my system reaches "heat saturation" and needs to "idle" or "simmer" to avoid overheating. Once inlet and outlet water temperatures start to get within 10 degree F of one another at elevated temperatures, I know that the system isn't pushing out the heat fast enough to warrant a hotter fire, so I let it "coast" for a while before adding more wood.

The pressure gauge is another basic part of hydronic system instrumentation. It shows system pressure while pumping and while the system is shut down. This hydronic system instrumentation is most useful when filling the system. It allows me to charge the system with a reasonable amount of pressure, about 15 PSI, without over pressurizing it.

It also helps me confirm that the system is holding pressure, a confirmation that there are no hidden leaks.

The gauge lets me monitor pressure during system operation to confirm pressure increases associated with pump operation and water expansion due to heating. It also allows me to see the reduction in system pressure associated with air being driven out of the system as it's heated.

Done with Hydronic System Instrumentation, back to Alt. Energy Sources

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