Understanding boiler bypass lines

Key concepts Boiler bypass lines must be installed with care to prevent serious difficulties from developing in the system.

By Mark Hegberg , ITT Industries, Morton Grove, IL August 1, 2000

Key Concepts

  • Boiler bypass lines must be installed with care to prevent serious difficulties from developing in the system.
  • A circulator on the boiler side of the bypass raises the return water temperature.
  • A circulator on the system side of the bypass lowers the supply water temperature.

Boiler manufacturers emphasize bypass lines today—and with good reason. Boilers are relatively small now. Their heat exchangers are much more efficient than are those in older boilers. And because they are smaller, modern boilers have specific flow rates and temperature needs. Consequently, many require a flow-bypass line.

Flow-bypass lines must be installed with care. It pays to read the instructions thoroughly. Locating a bypass in the wrong place can invite trouble. Here are some pointers for placing these devices properly and for pumping a boiler bypass line in the correct way.

First, decide what you want the bypass to do. It can either raise the temperature of water returning to the boiler, or lower the temperature of water heading out to the system. Different systems have different needs.

Next, make a sketch of the piping around the boiler. Use the tip of your pencil to trace the water’s path as it flows through the boiler and bypass. Remember, when water enters a tee, it can move out in two ways. The water can flow either way through the bypass, depending on where the circulator is placed. And when it comes to system performance, that placement makes a world of difference.

Placing the circulator

Figure 1 shows four possible locations for the circulator. Two locations are marked with an “A” and two with a “B.” Naturally, either the “A” location or the “B” location will be used, but not both. In other words, your circulator may be located on the supply side of the system (always our first choice), or on the return side. The decision is yours.

In the “A” position (Fig. 2), the circulator takes hot water out of the boiler and uses it to raise the temperature of the water returning from the system. The water flows from the top of the bypass to the bottom.

However, whether the circulator is located on the supply side or the return side of the boiler, it is always on the boiler side of the bypass (Fig. 3). Note this fact: A circulator on the boiler side of the bypass raises the return water temperature.

Why would you want to raise the temperature of the water returning to the boiler? Consider an old gravity installation. It has a high-volume system and a low-volume boiler. If the returning water is cool (less than 140 F for a cast-iron boiler), flue gases condense inside the boiler and cause corrosion. The possibility of thermal shock also exists, although this condition is usually less of concern than condensation.

Without the bypass, fuel bills are typically much higher than they need to be, because the low-volume boiler finds it difficult to reach its high limit and shut off. Piped this way, a bypass prevents these common problems.

Looking at the other side

In the “B” position for the circulator (Fig. 4), it mixes cooler return water with hot boiler water. In other words, it lowers the temperature of the hot water heading out to the system. This time, the circulator is on the system side of the bypass. Add this fact to our previous note: A circulator on the system side of the bypass lowers the supply temperature.

Why lower the temperature of water leaving the boiler? Although at first glance it seems to be a waste, it is really an inexpensive way to run radiant heating (Fig. 5).

The system operates at, for example, 120 F, while 180 F is maintained in the boiler to satisfy a tankless domestic hot water coil. However, this type of bypass is not recommended as a control, because it doesn’t respond to temperature. For example, if the configuration had more than one zone, the system simply wouldn’t work well.

Nevertheless, this type of arrangement is found in most boiler manufacturers’ operating manuals. It is important to understand clearly the difference between the two bypass piping arrangements. If you were trying to protect a boiler from a potential flue-gas condensation problem, misplacing the circulator on the bypass line would cause serious problems.

Setting up bypass lines

Now consider a copper fin-tube boiler. This type of unit accepts cooler return-water temperatures (typically 105 F), but it is very sensitive to the flow rate across its heat exchanger. If the water moves too slowly across a copper fin-tube boiler, the safety alarm will shut off the boiler.

With copper fin-tube boilers, the circulator always goes on the boiler side of the bypass, whether the pumping takes place on the supply side or the return side. And most manufacturers insist that the bypass line should never be smaller than 1 in. in dia. Check the instructions to be sure.

When a bypass line is set up around a modern system, using flow control valves to send the water back to the boiler can save on a few fittings. Enter on the side of the flow control valve and bypass through the bottom to accomplish this arrangement. And always use a true balancing valve in the bypass line so that the correct temperature and/or flow rate can be achieved. Ball valve manufacturers caution against using their products as balancing valves. Ball valves should be either fully opened or fully closed, but not throttled. A balancing device is a calibrated test and adjustment device designed to let the user know the flow at each valve position.

Finally, do not forget the basics of pumping. This kind of system uses all the pump head available to it. Underflows and overflows may occur in circuits, and the user may not know what is going on except that it is cold in a zone. For example, a two-zone system (Fig. 6) was installed to prevent condensation or shock on the boiler and had separate circulators on each of two zones. Pumps were supplied with the system; they were not selected specifically for the piping geometry.

However, on such a primary-secondary system, pump selection is critical. In this case, the boiler pump was rated for 20 gpm. The pumps for zone 1 and zone 2 were each rated for 20 gpm. As a result, personnel were warm on the second floor (zone 1), but those on the first floor (zone 2) were freezing.

The boiler pump must provide the total flow of the system (in this case, at least 40 gpm) plus whatever flow is necessary to keep the return water temperature as high as desired. If the piping in the zone has less resistance to flow than the pump head rating at the flow design, the pump will provide more flow for the pipe’s given resistance. Flows go to the point of least resistance. In this case, zone 2 may receive only recirculated zone water and no supply from the boiler. Check all pipe sizes and resistances. This type of system typically requires some type of balancing device to make the system perform the way it is intended.

Edited by Jeanine Katzel, Senior Editor, 630-320-7142, jkatzel@cahners.com

More info

Technical questions about this article may be directed to author Mark Hegberg by phone at 847-966-3700 or e-mail at mhegberg@ fluid.ittind.com.

Additional information on this subject is also available from the Fluid Handling University web site at fhs.ittind.com.