Paying attention to situations that can lead to undesirable gravity flow makes good design sense and negates potential callbacks.

I consider myself lucky to live in a rural dairy-farming valley in upstate New York. About 30 feet from my office is a barbed wire fence that usually keeps the cows in the adjacent pasture out of our lawn and garden.

In October of any given year, those cows can decide the grass really is greener on the other side of the fence. On a crisp fall morning, we may wake to the sound of a dozen or so Holsteins grazing on our lawn and feasting on the garden. Those girls simply found a hole in the fence and took advantage of it.

It's interesting how lots of things in nature have that inherent ability to find a hole in the proverbial fence. One example is how hot water in a boiler can spot a cool piping loop above it. Nature quickly establishes a gentle but persistent "gravity flow" that allows some of those boiler Btus to sneak away from the system in a way the designer never intended.

It's simply nature's way of trying to reduce any imbalance in the amount of heat contained by water in different parts of the system. We might not even realize such pathways exist in the system, but rest assured nature will find them and exploit them to the best of her ability.

The development of circulators during the early 1940s was a turning point for the hydronics industry. No longer were designers bound by the constraints of early gravity-flow systems. Hot water now could be sent in any direction with relative ease.

Unfortunately, it's easy for those of us who missed out on these early hydronic systems to forget that the use of circulators doesn't nullifu the law of gravity.

Mending The Holes

Here's a list of several details you can use to minimize or eliminate gravity flow. The ways are illustrated in Figure 1.

  • Install a flow check valve near the outlet of the heat source. The flow check has a weighted plug that sits over the orifice in the valve. The plug's weight is sufficient to prevent the plug from lifting it off its seat until the circulator starts. This detail prevents hot water in the boiler from using the primary loop as a cooling device during off-cycles.

    Never substitute a swing check valve for a flow check valve in any situation where gravity flow would move in the forward direction of the valve. The flapper disc in a swing check is not heavy enough to prevent gravity flow. Been there, tried it - it doesn't work.

  • Install a thermal trap or check valve on the return side of every branch piping circuit to eliminate gravity flow. Such a trap is created by routing the return pipe approximately 18 inches below the tee it eventually will connect to and then installing a U-bend to route the pipe back up to that tee. Use a baseboard tee with a brass plug to form half of the U-turn of the trap. The plug can be removed to drain the trap if necessary.

    Never use a thermal trap as a substitute for either a flow check or spring-loaded check valve on the supply riser of secondary circuit. The trapping effect cannot stop gravity flow up the supply riser after the secondary circuit shuts off. The lighter (hot) water in the supply riser, in combination with the heavier (cooler) water in the return riser, produces a slow but persistent gravity flow that only stops when the primary circuit turns off and all the piping cools down.

  • In systems that combine space heating with indirect domestic water heating, be sure that all space heating circuits are protected against heat migration. Failing to do so will lead almost certainly to a callback, and not during the heating season, but rather during an oppressively hot summer day after the boiler has been firing for domestic water heating. The complaint probably will sound something like this: "Why is heat coming out of my radiators when my air conditioner is running full tilt?" It's pretty hard to dispute the logic in a statement like that, don't you think?

  • Protect indirect DHW tanks from creating convective loops through their own heat exchanger piping after that piping has cooled off. Remember that a tank full of hot water with an adjacent piping loop is just begging nature to do her thing. Install a flow check (or spring-loaded check) as shown in Figure 1 to thwart nature's plan.

  • In primary/secondary systems, all secondary circuits should have a flow check valve on the supply, and one of the following options on the return: 1) another flow check valve; 2) a swing check valve; or 3) an underslung thermal trap at least 18 inches deep.

  • Whenever possible, install the primary piping loop high on the wall of the mechanical room - high enough that the piping passes over the top of the door. Drop down from this loop when creating the secondary circuits, even if the secondary circuit then does a U-turn and heads back into the building. Doing so builds a thermal trap into the return side of the secondary circuit. Remember though, you still need either a flow check or spring-loaded check on the supply side, even if it drops down from the primary.

  • If you are using zone valves, install them on the supply side of circuits to block upward gravity flow. If you really want to do a first-class job, install a swing check on the return side of each zone to block the return side.

    Why should both sides be protected? Because nature has a way of setting up a two-directional flow in a single pipe. Hot water tends to slowly flow up the center of a vertical pipe while cooler water slides down along the pipe wall.

    Don't believe it? Go feel the return pipes where they rejoin the return header on a multizone system without gravity flow protection. You'll find the return piping on inactive zones will be warm, even hot, several feet back from the return header. A swing check at the return end of each circuit puts a stop to this.

    Call me a control freak, but when I see unnecessary heat migration in a system, I know that system isn't performing as well as it could be. Why leave any "holes in the fence" through which some of those Btus can sneak away to unexpected or undesirable locations? Paying attention to situations that allow heat migration makes good design sense and negates a source of potential callbacks. And who doesn't want that?