Making hydronic controls easy

Are you or someone you work with a bit confused about all the hydronic controls available — thermostats, controllers, circulators, zone valves, zoning panels, outdoor reset, energy-saving pumps, snow melt? Don’t be! Hydronic controls, or any other controls, for that matter, are easier than you think.

No matter how new or old, mechanical or electronic, there are only three basic types of controls. Every control is either a power supply, a load or a switch. No exceptions. Every control circuit has one power supply, one load, and one or more (often lots more) switches. There can be nothing else.

Electricity in a circuit is like a bug on a rope. The power supply, the switch, and the load must be wired together in a “circle.” The electricity must go through each control, and return to where it started. If there is a break in the “rope” the “bug” is stopped, and the circuit doesn’t work.

Whether you are installing or troubleshooting, the fact that there are only three types of controls takes away the need to know every individual control that was ever made. What you need to know about drops from thousands of possibilities to a mere three. You have only to figure out if the control is a power supply, a switch or a load. And then you’ll know what to expect of it. Here’s how you can tell which it is.

What is it?

Power supply
Every control circuit must have a power supply. That’s the only place the electricity comes from. Most control circuits are low voltage (24V). That means that the power supply is a transformer. Period. No other possibility. Line voltage (common house current) is wired into one “side” of the transformer. Low voltage, that is 24V, electricity comes out the other side.

If the control circuit is line voltage (120V or 240V) the power supply is the circuit breaker that supplier power to the house or building. The circuit breaker isn’t technically a control, but it is an important part of the control circuit.

The load changes electricity into another form of energy: heat, light, sound, motion or magnetism. A load is commonly the coil of a motor. The motor could be in a zone valve, zone pump or circulator. The purpose of the control circuit is to turn the load on and off at the right time. For hydronics, we are always trying to get hot water to flow to where the heat is needed. That flow is controlled by either a zone valve or a circulating pump. Either device has a motor, which changes electricity into motion.

Note that each load must be in its own electrical circuit. Specifically, each zone valve, pump, or circulator has its own circuit. Each has its own switch. (Some of them often share a power supply, which we will deal with at another time.)

A switch turns the load on at the right time by letting electricity flow though the circuit from the power supply to the load. A thermostat is a switch. When there is a call for heat in the living space, the thermostat allows electricity to flow to the zone valve. The motor (load) of the zone valve opens the valve and allows water to flow through the valve.

An aquastat is another switch. It is dedicated to controlling boiler water temperature rather than room temperature.

A switch is like a drawbridge. When a switch is closed, electricity flows like traffic. When the drawbridge opens, there is no flow. Closed is “on.” Open is “off.” Don’t make the mistake of imagining a switch to be like a valve, where open means “on” and closed means “off.”

Combination controls
Some controls are a combination of more than one of these three types. A relay is a combination of a load (the coil, which changes electricity into magnetism), and some switches. Magnetism from the coil causes the switches to change position. If the relay switches are normally open (off), they close (turn on). When electricity is removed, the magnetism stops, and the switches return to their normal position.

A zone valve is a combination control when it has an end switch. (A two-wire zone valve has just a motor. A four-wire zone valve has a motor and an end switch.) As soon as the motor opens the valve, a little arm mechanically closes the end switch. The closed end switch allows electricity to flow to any other device (typically a circulator, but silly examples could be a light bulb or a door bell).

Each part of a combination control is in a separate electrical circuit. A relay coil is usually in a low voltage circuit. Each of the switches is in a separate circuit. That means that you can switch line voltage loads, even though the coil is low voltage.

The same is true for a zone valve. The motor (a load) is low voltage, but the end switch (a switch) can control either a low voltage or line voltage load.

Panels, programmed controls
Hydronic control panels are combination controls. Whether the panel is electronic or electromechanical, it is made of power supplies, switches and loads. A zoning panel contains a transformer (power supply). It may have various switches, and relays, which are a combination themselves of load and switches. There are a number of terminals where you wire in other switches (thermostats) and loads (valves, circulators). A primary purpose of a panel, no matter how simple or complex, is to organize things for you. Instead of figuring out which wires connect the Zone 1 thermostat to the Zone 1 zone valve, to the transformer, you just wire them to the neatly labeled terminals in the panel. This makes it easier for both installation and troubleshooting. More complex panels have more switching options.

Programmed electronic controls are also are made up of a power supply, switches and loads. The program is computer talk that among other things tells switches when to open and close. An example is night setback. This allows you to heat a room to a certain temperature only at certain time, rather than keeping only one temperature.

No matter how simple or complex the control, it must be one of these three things, or a combination: power supply, switch or load.

Carol Fey is a technical trainer who has been in the HVAC industry for over 25 years. You can find her books and DVD at To see her adventures while a heating mechanic in Antarctica, go to

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