The Art of Making New Boilers Work with Old Systems

Replacing an old steam boiler with a new, efficient lower water content boiler has become an art form. You, the contractor, are charged with all the responsibility of having to connect this new replacement boiler, which contains less water, has a smaller steam “separating” chamber and makes steam more violently, into a system of piping and radiation that can be 70 to 80 years old!

As the old boiler made steam, its water line would start to drop down as the water was being converted into steam. This steam, which headed out to the system would then condense back into water and gravity drain back to the boiler.

Whether the condensate returned in 5 minutes or 20 minutes, the old boiler held so much water that its water line never dropped to the automatic feeder’s feeding level. But replace that old boiler with one of these modern boilers and see what happens.

The problem originates from the physical size of these replacement boilers. Not only do they hold less water, but the steam separating chamber, which is used to separate the steam from the water in the boiler, is also smaller. This allows some of the water to leave the boiler prematurely. It gets carried out with the steam in a mist like spray. This is why near-boiler piping has become so critical.

The time it takes for the condensate to return to the boiler, also known as the system time lag, is also very important. If it is too slow for the new boiler, it can cause the low water cut-off to either shut off the burner, or activate the automatic feeder and bring in more water. Of course when all the condensate finally returns, the boiler’s water line is too high and the boiler floods.


Boiler Feed Units
Steam systems found in apartment buildings, churches and schools with gravity returns and automatic feeders are especially famous for causing replacement boilers to flood because of this “system time lag” problem. The best way to solve this type of situation is to install a Hoffman boiler feed unit.

Boiler feed units are sized according to the steaming rate of the boiler vs. the system’s time lag (which is different for every system!). Also remember we’re not talking about all the water in the replacement boiler-only the amount found between the normal water line and the low water cut-off level.

These units come with large receivers that act like reservoir tanks which are intended to hold the water needed by the new smaller boiler. These tanks have pumps attached to them which are typically controlled by one of McDonnell & Miller’s series of pump controllers–such as the 150MD–which are piped onto the boiler.

This pump controller monitors the water line in the boiler and activates the pump whenever the water line drops too low. Once the water line is brought back to the right level, the pump controller turns the pump off preventing the boiler from flooding. Modern boiler design has caused many contractors to install these boiler feed units so that the new boiler will work properly with the original system.

Open to the atmosphere!
When you install a boiler feed unit into a system that previously was a closed gravity return system, you’ve changed the operating characteristics of that system. The most significant change is to the return lines. They ALL have to drain into the feed tank. And this tank is vented to the atmosphere with a vent pipe because it can’t withstand any pressure inside itself.

This means you need to install Hoffman F&T traps (float & thermostatic) at the base of every riser that drips into a wet return line and at the end of every steam main. You need these traps because without them, there is nothing to stop the steam from working its way through the piping and eventually showing up at the receiver’s vent pipe.

Before, when the returns drained directly into the boiler, they were exposed to the back pressure of the boiler which created a balance between the pressures on the supply and the return. With; the vented receiver, there is no back pressure on these return lines, only the pressure from the supply. And this steam pressure will be more than happy to blow through the water seals and eventually show up at the receiver’s vent pipe. Of course creating water hammer along the way!

If you are adding a boiler feed unit to a two pipe system that already has radiator traps, it is important that those traps are working properly. If not, steam will pass into the returns, create water hammer and eventually blow out of the vent pipe. That’s because the dry returns also drain into the feed tank.

When you need a boiler feed unit think Hoffman! We have a complete line of boiler feed tanks and condensate handling equipment. If you need more information or have any questions with a particular steam system, call your Hoffman/McDonnell & Miller representative.