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All boilers that burn hydrocarbon fuels such as natural gas, propane, oil or wood produce water vapor as a byproduct of combustion.

About half of the boilers currently sold in North America are “conventional” boilers with heat exchangers constructed of cast iron, steel or copper tube. They’re designed with the intent of being operated at conditions that do not allow this water vapor to condense, on a sustained basis, within the boiler’s heat exchanger.  

The two italicized words in the preceding sentence deserve further attention. Just because manufacturers intend their boilers to operate at conditions that prevent sustained flue gas condensation does not mean that these boilers cannot operate under such conditions. As one industry trainer once told an audience of aspiring hydronic professionals: “Any boiler can be a condensing boiler.” That statement drew its share of confused looks, but it’s 100% correct. There is no such thing as a hydrocarbon-fueled “non-condensing” boiler. Given a suitably low incoming water temperature any hydrocarbon-fueled boiler will operate with sustained flue gas condensation. In addition to boiler corrosion, these conditions can quickly destroy galvanized steel vent connector piping and even masonry chimneys.  

The word sustained is also important. All hydrocarbon-fueled boilers experience intermittent flue gas condensation following a cold start.  It happens because the combustion side of the boiler’s heat exchanger is well below the dewpoint of the water vapor in the exhaust gas stream.  However, a properly designed balance-of-system allows a conventional boiler to quickly warm its internal surfaces above dewpoint, and thus evaporate that initial condensation and prevent it from reoccurring over the remainder of the burn cycle. Short-lived, intermittent flue gas condensation is not a problem.

The key to preventing sustained flue gas condensation is keeping the inlet water temperature above the dewpoint of the combustion gases whenever possible. Most “legacy” hydronic systems did this by selecting and sizing low thermal mass heat emitters that required relatively high water temperatures (such as 180° F or even higher) to dissipate the boiler’s rate of heat production. This more or less ensured that the return water temperature would stabilize quickly and well above the dewpoint temperature of the flue gases.

However, many modern hydronic systems are designed around much lower water temperatures. In some cases, modern hydronic systems also contain components having much greater thermal mass compared to their finned-tube predecessors. A heated concrete floor slab is a good example. So is a system with a larger thermal storage tank.

Does this mean that conventional boilers cannot be used with low-temperature heat emitters or high-thermal mass systems? Absolutely not. There are plenty of options for mixing assemblies that reduce water temperature on the supply side of the system. However, as the North American hydronics industry painfully learned when radiant panel heating was resurrected in the 1980s, it’s just as important to pay attention to what’s happening at the boiler inlet.

When a hydronic heating system contains lots of thermal mass, there will be times when the rate of heat absorption by the balance-of-system (e.g., everything in the system other than the heat source) can absorb heat from circulating water at rates significantly higher than the heat output of the heat source.  

A good example is when warm water is circulated through a cold concrete slab during a cold weather start-up. Assume the slab is initially at 45° F, and 110° F water is pumped into the embedded circuits at the same flow rate that would be present at design load. Under such conditions, the water exiting the slab will likely have a temperature of about 55°. That condition represents a temperature drop of 55°, almost three times greater than a typical design load temperature drop of 20°. Three times the ∆T at the same flow rate implies three times the rate of heat absorption. That cold slab is acting like a “black hole” for Btu. It’s sucking them off of the water stream much faster than the boiler can reload them, even when that boiler is running flat out.

If allowed to occur, this condition can return water to the boiler at temperatures well below the dewpoint of the exhaust gases. Copious condensation will occur. If you’re standing close to a gas-fired cast iron boiler operating under such conditions, you might even hear condensate dripping from the bottom of sections onto the burner tubes. And this condition isn’t limited to gas-fired boilers. I’ve known of oil-fired boilers that have dumped multiple quarts of water on the floor when their combustion chamber door was opened after several hours of condensing mode operating. This is not what the boiler manufacturer intended, nor should it be dismissed as inconsequential.

Thermal clutches

 The only way to consistently protect against such a condition is to create a way for the boiler to thermally uncouple itself, when necessary, from that heat-guzzling balance-of-system. You’ll need what I call a “thermal clutch” that can fully regulate the rate of heat transfer from the boiler to the balance-of-system.

 In a vehicle, a mechanical clutch regulates mechanical energy transfer from the engine to the drivetrain. When the clutch pedal is fully depressed the engine and drive train are uncoupled. When the clutch pedal is partially depressed there is some mechanical energy transfer from the engine to the drivetrain. When the clutch pedal is fully out, there is maximum coupling between the engine and drivetrain. The latter condition allows the drivetrain to receive mechanical energy at the same rate the engine is creating it.

 A thermal clutch is analogous to a mechanical clutch. The difference is that it regulates the transfer of thermal energy rather than mechanical energy. To consistently protect a boiler from sustained flue gas condensation a thermal clutch must provide both of the following functions: First, it must sense and react to boiler inlet temperature; and second, it must allow the boiler’s heat output to be fully uncoupled from the load when necessary.

There are several piping assemblies and associated controls that can provide both of these functions. There are also piping assemblies that some practitioners assume provide these functions, but, in reality, fall short.


Two piping assemblies that have been used in North American hydronics for decades, and sometimes for the wrong reasons, are shown in Figures 1 and 2.

Two piping assemblies

FIGURE 1 [click to enlarge]

Two piping assemblies  

FIGURE 2 [click image to enlarge]

These piping assemblies are called “boiler bypasses.” The one in Figure 1 bleeds what is assumed to be hot water from the boiler outlet through the bypass pipe, forcing it to mix with cooler water returning from the distribution system. A balancing valve is used to adjust the bypass flow rate. The assembly in Figure 2 uses a dedicated circulator to create the bypass flow.  

For decades, these piping assemblies have often been viewed as capable of passing sufficient hot water from the boiler outlet into the lower tee, where it mixes with water returning from the load to boost boiler inlet temperature above the dewpoint of the flue gases. In some systems — those with high-temperature/low-mass heat emitters and generously oversized boilers, these piping assemblies usually worked acceptably well to limit flue gas condensation. They also reduced thermal shock to cast iron boilers.

However, as hydronic systems evolved toward low-temperature/high-mass characteristics, neither of these piping assemblies was able to satisfy the two previously stated and required functions.  Neither could react to boiler inlet temperature, and neither could fully uncouple the boiler from the load when necessary.

 Consider what would happen with either of these piping assemblies when the water temperature returning from the load was 95°. Assuming a typical boiler flow rate, the temperature rise across the boiler would be about 20°. That makes the “hot” water leaving the boiler 115°. Question: How do you mix 115° water with 95° water at the lower tee to produce the 130° water needed to prevent flue gas condensation? Answer: You can’t. It’s impossible. The bottom line: Boiler bypass piping, as shown in Figures 1 and 2, cannot consistently protect conventional boilers from operating with sustained flue gas condensation.

A simple rearrangement

Given this deficiency, one might assume that making the boiler bypass circulator in Figure 2 a variable speed circulator that responds to boiler inlet temperature would remedy the situation. It would satisfy function No. 1, but not function No. 2.

Fortunately, a simple modification of the piping in combination with a temperature-regulated variable speed “shuttle” circulator can provide both functions needed to prevent sustained flue gas condensation. That modification is shown in Figure 3.

Two piping assemblies

FIGURE 3 [click to enlarge]

The load in Figure 3 is a large thermal storage tank. An example would be a multi-hundred-gallon tank heated by a pellet boiler or cordwood gasification boiler. When such a tank has not received heat for an extended period, it represents a very significant cool thermal mass.

When the boiler in this system is called to operate, circulators (P1) and (P2) are both powered on. Circulator (P1) establishes a fixed flow rate through the boiler. The variable speed shuttle circulator (P2) monitors boiler inlet temperature. When that temperature is below a preset value (typically around 130° for gas-fired boilers or 140° for cordwood gasification boilers), the variable speed shuttle circulator is powered, but its motor remains off. Under this condition, the flow created by circulator (P1) does a U-turn at the closely spaced tees and returns to the boiler. This prevents all but a trickle of heat transfer between the boiler and load, effectively uncoupling the two.

As the boiler inlet temperature rises above the “anti-condensation” setpoint, the shuttle circulator speed increases, allowing increased heat transfer to the load. When the boiler inlet temperature climbs to 5° or more above the setpoint, the shuttle circulator is operating at full speed.  If some condition then causes the boiler inlet temperature to drop toward the setpoint, the shuttle circulator slows to reduce the rate of heat transfer. This action allows all the heat produced by the boiler to be shuttled to the load but doesn’t allow the load to “dominate” the process by creating a rate of heat absorption greater than what the boiler can sustain. The “thermal clutch” is perfectly controlled.

Not so fast

The above description assumes that the shuttle circulator is completely off when the boiler inlet temperature is below the preset temperature threshold.  There are some circulators with integral temperature controllers, as well as circulators that can be regulated by external controllers, that will operate this way.   However, some variable speed circulators must operate at a minimum speed whenever they are powered, which would be anytime the boiler is called to operate.  In this case it’s important to have a device that provides some forward opening resistance in the load circuit to prevent flow when the circulator is at minimal speed.  A differential pressure valve set to a forward cracking pressure of 1 to 1.5 psi can usually provide this requirement.  That valve, shown in figure 3, will also fully close to prevent reverse thermosiphoning from the tank through the boiler piping when the boiler and circulators (P1) and (P2) are off.

There are several other piping assemblies and associated controls that can protect conventional boilers from sustained flue gas condensation. They include systems using motorized 2-way, 3-way, and 4-way mixing valves, and systems with specifically configured thermostatic mixing valves.  Designers contemplating use of such assemblies should remember that consistent boiler protection requires these assemblies to provide both of the following functions:

Function No. 1: It must sense and react to boiler inlet temperature.

Function No. 2: It must allow the boiler’s heat output to be fully uncoupled from the load when necessary.