Fundamentally there are two things every hydronic system you’ll ever design or install “wants” to do — they all want to operate at simultaneous conditions called thermal equilibrium and hydraulic equilibrium. Both conditions represent a balance between energy input and energy release.

Thermal equilibrium means every system seeks to operate where the rate of heat input to the water from the heat source (boiler, heat pump, etc.) is exactly the same as the rate of heat release from the water to the heat emitters. The supply-water temperature and the temperature drop the system stabilizes to are always those necessary to balance these rates of heat flow. In some situations, these temperatures can be significantly higher or lower than what we expect based on control settings.

Imagine a hydronic floor-heating system that has eight parallel 350-ft. circuits of 1/2-in. PEX tubing embedded in a bare concrete slab. The system is directly piped to a 50,000 Btu/hr. boiler as shown in Figure 1. The boiler’s temperature-limiting controller has been set by the installer for 140° F because that’s the temperature he thinks the system should operate.

When the system is fired up, the boiler outlet temperature climbs over a period of four hours and eventually stabilizes at 99°. The burner remains in continuous operation, but the water temperature leaving the boiler refuses to go above 99°. The installer thinks something is wrong because the boiler is not reaching the temperature he “told it to” by setting the limit controller to 140°. He grabs a wrench and taps the limit controller a couple of times to show it who’s in charge.

After a few more hours, the boiler is still purring along at a 99° outlet temperature, so the installer shuts off the system and pulls the limit controller off the boiler. Next stop — his local supplier, where he demands a replacement for the “obviously defective” controller.

Nothing is wrong with that controller. The system was simply operating at thermal equilibrium where the rate at which heat is added to the water by the boiler matches the rate heat is dissipated by the distribution system. The radiant floor-heating subsystem just happened to be capable of releasing 50,000 Btu/hr. of heat input when supplied with 99° water. There was no need for the water temperature to rise any higher.

Because the limit controller is set well above this temperature, it cannot intervene and thus can’t affect the current operating conditions. Its presence is irrelevant as far as controlling this system.

Incidentally, if the boiler supplying this system was not designed to operate with continuous flue gas condensation, the installer will soon have more to contend with. Nature doesn’t care if the boiler is condensing or corroding, it only cares that the heat flow rates are balanced.

You can see it

Ever stand in a mechanical room and watch the temperature indicator on a boiler? Sometimes it slowly climbs as the boiler fires. What you’re seeing is the system trying to find thermal equilibrium. The fact that the temperature is increasing means the boiler is currently injecting more heat into the system water than the distribution system is releasing.

Many of you have seen the opposite effect; a decrease in boiler temperature as the system operated. Perhaps it occurred when a cool slab with embedded tubing came online with a boiler that was already up to an elevated temperature.

If the system did not have a mixing device that monitored boiler inlet temperature, you probably watched a rapid temperature descent to a condition where the boiler was only putting out lukewarm water, even with its burner firing continuously. Once again, the system is simply seeking thermal equilibrium where the heat output rate from the boiler matches the heat absorption rate into the slab. Since the slab is cold, it’s absorbing heat from the circulating water much faster than the boiler can reproduce heat. Hence temperatures are headed downward until a balance is found.

So what can you conclude if you see the temperature indicator on a boiler holding steady, even with the burner running nonstop? Thermal equilibrium has been achieved.

Remember, every hydronic system always tries to operate at thermal equilibrium. It’s only the intervention of limiting controls that sometimes prevents this condition from occurring.

Head in = head out

Another form of energy is constantly being put into and taken out of hydronic systems as they operate. It’s mechanical energy called head. An operating circulator converts electrical energy into head energy and imparts it to the fluid. At the same time, every component through which this flow passes removes head energy due to friction.

All hydronic systems will stabilize at a flow rate where the rate of head energy input exactly matches the rate of head energy dissipation. This “hydraulic equilibrium” is usually achieved within a few seconds of turning on the circulator and remains intact until the system operating condition is altered (i.e., a valve is adjusted, zone turned off or on, etc.). The concept is illustrated in Figure 2 (page 17).

You can determine the hydraulic equilibrium point by finding the intersection of the system head loss curve and pump curve for the circulator (see Figure 3 on page 17). The point where these curves cross is called the operating point and is the only possible condition where head input exactly balances head loss.

The system curve changes every time a zone circuit turns on or off, or when someone adjusts a flow regulating valve. When either of these actions take place, the system quickly and automatically adjusts to a new flow rate where head input from the circulator matches head loss from the currently configured piping system. Think of the operating point sliding up or down the pump curve as necessary to maintain the balanced condition.

As is true with thermal equilibrium, the operating condition at which hydraulic equilibrium is established isn’t necessarily one that will deliver the proper rate of heat flow or allow the system to operate efficiently or reliably. It’s just where the rates of energy input and output are balanced. Our task as designers is to ensure each system we design can deliver comfort, efficiency and reliability while operating under these naturally balanced conditions.