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The condensate- or boiler-feed pump doesn't behave as it should

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Published
July 10, 2009 Subscribe to RSS feed

The strainer is clogged.
There should be a wye or a basket strainer on the inlet to the condensate pump. This strainer's job is to collect sediment from the system before it can get into the condensate pump's receiver. Too much sediment in the receiver will cause problems with the condensate pump.
Someone should clean the strainer at the end of each heating season, but unfortunately, most strainers get cleaned only when there's a problem. If the strainer should clog, little or no condensate will return to the receiver. The condensate pump won't run often enough to replace the water in the boiler. If there's an automatic water feeder on the boiler, it will feed to keep the burner firing. But returning condensate, unable to get through the strainer, will back into the mains and cause water hammer.
Clean the strainer.

There's a lot of sediment in the water.
When was the last time anyone cleaned that system? Steam systems corrode because air enters on every down cycle, and the pipes are usually wet. Particles of rust wash down with the condensate and work their way into the condensate receiver and pump.
The strainer's job is to protect the pump from this sediment. However, if the strainer isn't working (someone may have removed the screen, for instance) particles of rust will work their way into pump's mechanical seal and cause a leak.
If the condensate pump has a packing gland instead of a mechanical seal (older pumps do), the sediment can cause the gland to leak too much. If you overtighten the packing nut to slow the leak, you might damage the pump's shaft.
Clean the system with trisodium phosphate, and make sure the strainer is clear.

The condensate is too hot for the pump.
As steam traps fail, the returning condensate will get hotter. If the condensate gets too hot (say, near 190 degrees F.), the pump might cavitate when it runs. Cavitation is what happens when a centrifugal pump tries to pump water and the water flashes into vapor. It sounds like gravel is moving through the pump.
This can happen in an open system (such as a steam system) when the water is close to the boiling point. When the impeller spins, the pressure at its inlet drops. The too-hot water flashes into a vapor and expands tremendously. The vapor bubbles then move quickly toward the edge of the impeller where the pressure is high. The higher pressure collapses the vapor bubbles. When that happens, water surrounding the collapsed bubble rushes into to fill the void. This water moves at an incredible speed. It hits the metal at the edge of the pump's impeller with such force that it quickly erodes the metal and causes the pump to fail.
To solve the problem, repair the defective traps. Don't try to cure the problem with a single "master" trap at the inlet to the condensate pump. Get to the root of the problem by repairing the steam traps.

The pH of the water is affecting the pump's mechanical seal.
When the boiler produces steam, the water releases carbon dioxide. This is a result of the carbonates and bicarbonates that you'll find in fresh water. When it's released, the carbon dioxide moves into the system and, if not properly vented, mixes with the condensate on the return side. That creates carbonic acid. Carbonic acid can eat its way through return lines and create leaks. It can also affect the condensate pump's mechanical seal.
Ideally, the pH of a steam system should range between seven (neutral) and nine (mildly alkaline). If the pH gets too low or too high it can affect the ceramic part of the pump's mechanical seal and cause the pump to leak.
Check the water's pH with litmus paper and adjust it with chemicals if necessary.

The boiler pressure is too high.
The condensate pump's job is to put the returning condensate back into the boiler. To do this, the pump has to produce a pressure that's higher than the boiler's operating pressure. As a rule of thumb, if the boiler operates at 50 psi or less, the pump should discharge at the boiler's operating pressure, plus five psi. For instance, if you have the boiler set to operate at two psi, you'd throttle the condensate pump to discharge at seven psi. If the boiler operates above 50 psi, the pump should discharge at the boiler's operating pressure, plus ten psi. So if the boiler were in a dry cleaner's shop producing, say, 90 psi, the pump would have to discharge at 100 psi.
Most condensate pump manufacturers set their standard pumps to discharge at 20 psi. This is good for a low pressure system that can operate up to 15 psi (15 psi + 5 psi = 20 psi). Some condensate pumps, however, are built to order and may have pumps that discharge at a lower pressure. If the pump's pressure can't overcome the boiler's pressure, the pump can't return the condensate to the boiler.
Lower the boiler pressure (if that makes sense), or increase the head pressure of the condensate pump (by replacing it).

The condensate pump's check valve isn't seating tightly.
Since a condensate pump has an atmospheric vent, you have to use a check valve to keep the water in the boiler from flowing backwards into the receiver tank. The check valve goes in the pump's discharge line.
Unfortunately, sludge will often build up in the return line and clog the check valve, keeping it from closing tightly. Make sure you clean that strainer on the condensate- or boiler-feed pump's inlet.
If the check valve doesn't seat tightly, water from the boiler will back up into the receiver, raise the float switch and start the pump. So if you find your condensate- or boiler-feed pump is starting and stopping all the time, take a close look at that check valve. The easiest way to test it is to close the service valve at the inlet to the condensate pump. That will stop the flow of water into the pump's receiver. If the pump continues to cycle on and off, you know the water is coming from the boiler.
Isolate the check valve and clean it out.

The float isn't attached to the float rod.
The float switch in the condensate pump turns the pump's motor on and off. When the receiver fills, the float rises with the water and starts the pump. As the receiver empties, the float senses the falling water and stops the pump's motor.
Now and then, however, the float will work its way off the end of the float rod. When that happens, the pump stops operating because it no longer has a way of knowing where the water is. Usually, the condensate rises up in the receiver's vent or overflow line and floods the boiler room, but don't depend on this. From time to time, a float ball will come off the end of a float rod and work its way into the vent line. If this happens, you might not see the water overflow. Some manufacturers weld a baffle across the vent line inside the tank to keep this from happening.
To check for a missing float ball, operate the float switch by hand. You'll feel the resistance if the float ball is still on the end of the float rod. If it feels too loose, remove the float switch and replace the ball.

There are motorized zone valves near the header.
If a motorized valve closes when there's steam in the boiler, a deep vacuum can form when the steam condenses. Since the condensate pump has an atmospheric vent, the pressure inside the pump's receiver will be higher than the vacuum inside the boiler. Water will flow from the receiver tank into the boiler, and the boiler may flood. If you have a boiler-feed pump, the problem will be very noticeable. That's because a boiler-feed pump has an automatic water feeder in its receiver. The feeder gives the pump an endless supply of fresh water.
You can cure the problem by installing a vacuum breaker anywhere in the near-boiler piping or in the boiler itself. Just make sure the vacuum breaker winds up between the boiler water line and the motorized valves.

There's a "master" trap at the inlet to the condensate pump.
When steam traps fail in the opened position, the returning condensate eventually will get too hot for the condensate pump to handle. Some contractors who are unfamiliar with steam heating systems will try to solve this problem by installing a single "master" trap at the inlet to the pump's receiver. This creates problems because the return lines are now double-trapped. The flow of condensate back to the boiler will slow, and the boiler will begin to cycle erratically or flood. Remember, the steam traps are in the building to create the points of pressure and no pressure. If the system could work with a single "master" trap, the Dead Man who installed the system would have done it that way.
Another problem with a "master" trap is that it will release condensate at steam temperature. Much of that hot condensate will flash back into steam as it enters the receiver, causing further problems with the pump.
Remove the "master" trap, and repair the defective traps throughout the system.

The voltage supplied to the pump isn't correct.
And if it's not, the pump will either kick a circuit breaker or fuse, or run slowly enough to burn its windings. The electrical power in some areas can be questionable.
If you're having a problem with the motor, check the voltage. If necessary, use a recorder to track any changes in voltage over time. Notify the power company of what you've found.

The boiler room is too hot.
If the motor continually shuts off on its overload protector, check its nameplate for an ambient-temperature rating. Boiler rooms can get extremely hot, and will often exceed the ambient-temperature limits of common electric motors. Find a way to cool the room.
If it's that hot in the boiler room, there's also a good chance you're also not getting enough combustion air to the burners. And keep in mind, some modern boilers draw their combustion air from duct work attached directly to the burner. With little fresh air entering the room, the ambient temperature can get very hot, very fast. Bring in fresh air to cool the room.

The condensate pump discharges into the Hartford Loop.
The Hartford Loop does a good job of protecting the boiler in a gravity-return system. Should a return line spring a leak, water can't flow from the wet returns, but not from the boiler because of the Loop.
On a gravity-return system, the wet return connects to the boiler's equalizer at a point about two inches below the lowest operating point (this varies from manufacturer to manufacturer so you should always check their installation instructions).
When you have a condensate pump, you no longer have a gravity-return system. Should a return spring a leak, the boiler water can't back out of the boiler because of the condensate pump's check valve. Should the check valve fail, boiler water will back into the condensate pump. The pump will turn on and pump the water back into the boiler. Should the check valve and the condensate pump fail simultaneously, water will back into the pump's receiver and rise up the vent piping. Since this piping is usually several feet higher than the boiler's water level, the water still can't get out. If there's an overflow pipe in the vent line, however, the condensate can back out of the boiler should both the pump and check valve fail. In this case, a Hartford Loop would help on a pumped return system. But other than that, the Loop may cause problem. Water under pressure from the pump can splash up into the boiler header and create water hammer.
If this is your problem, relocate the pump's discharge line to the bottom of the boiler's equalizer.

The impeller is clogged.
Steam systems are dirty, and condensate vent lines are open to the atmosphere. If sludge works its way into the condensate pump's receiver it can clog the pump's impeller. The pump will cavitate and not move the returning condensate back to the boiler. Water will back up in the system and overflow through the receiver's vent line.
Check the impeller for sludge or other debris by removing the pump from the volute.

The receiver's vent line is plugged.
The vent on a condensate receiver often acts as the air vent for all the system piping. As steam traps fail, the water entering the receiver will get hotter and hotter. Some contractors deal with this by adding a "master" trap at the inlet to the receiver. This is never a good solution.
Other people try to deal with the problem by plugging the receiver's air vent. They figure what they don't see can't hurt them. They're wrong because a condensate pump's receiver can't take much pressure. If you plug the vent, the receiver can explode. That's right, explode.
Another problem is that with a plugged vent line, air can't escape from the system, and condensate will have a tough time making its way back to the boiler room. Now and then, a float ball will come off the end of a float rod and work its way into the vent line. Some manufacturers weld a baffle across the vent line inside the tank to keep this from happening.
If you find a plugged vent line, clear it immediately.

The boiler's water line is priming or surging.
Dirt is usually the culprit here. When you see droplets of water in the part of the gauge glass above the water line, it's time to clean the boiler. If the boiler is priming and surging, it can turn the pump controller on and off. That will make the boiler-feed pump add water when it shouldn't.
Try raising the water line to within an inch of the top of the gauge glass. If the water in the boiler is clean it will not surge over the top of the gauge glass. If it does, clean the boiler with trisodium phosphate.
Also, make sure you're not overfiring the boiler. If you are, correct it by firing only to the connected load.

The pump controller isn't level.
If you have a boiler-feed pump, it's taking orders from the pump controller, which is mounted on the boiler. The pump controller has two factory-adjusted mercury switches. One switch starts and stops the pump in response to the water level in the boiler. The second switch, set to operate at a lower boiler water level, usually serves as a low-water cutoff switch, although you can also use it as an alarm switch. If the pump controller isn't level, the switches may trip too soon or too late, causing the boiler-feed pump to operate erratically.

The wrong pump controller is on the boiler.
McDonnell & Miller supplies many of the pump controllers used in North America. Their most popular pump controller is the #150, and you'll find it on most steam boilers. Since you can use the #150 on boilers rated up to 150 psi, the people at the factory set the control under high-pressure conditions. The high pressure compresses the controller's bellows and affects the vertical distance between the "pump-on" and the "low-water cutoff" points.
If you use the #150 on a low-pressure boiler, however, the bellows will lengthen, and by doing so, shorten the vertical distance between the "pump-on" and "low-water cutoff" points. Working at low pressure, your burner may shut off on low-water before the boiler-feed pump has a chance to bring the level up to where it should be.
To avoid (or solve) this problem, order McDonnell & Miller's #150MD controller instead of their #150 if you have a low-pressure boiler. The "MD" stands for "maximum differential." It's the same control, but McDonnell & Miller sets the "MD" while they have it filled with low-pressure steam. Low-pressure steam doesn't compress the bellows as much, so you wind up with a wider, vertical distance between the "pump-on" and "low-water cutoff" points.
Don't try to adjust the factory settings of the mercury switches. If you do, you may void the manufacturer's warranty.

There's no pressure-reducing valve on the feed line to the boiler-feed pump's receiver.
Boiler-feed pumps take their orders from the pump controller, which you'll find mounted on the boiler. The feed pump has a larger receiver than the one you'll find on a condensate pump. The oversized receiver holds enough water to keep the boiler operating during the time it takes for the condensate to return from the system. When the condensate finally does return from the system, it enters the oversized receiver, rather than the boiler. Once there, it waits until the pump controller tells it to enter the boiler.
You'll find a float-operated automatic feed valve in the receiver of most boiler-feed pumps. The feeder's job is to make sure water always fills the lower quarter of the receiver. Typically, these float-operated valves can close off against a city water pressure of about 30 psi. If the city water pressure is higher than that, the valve will pass water, although the valve is supposed to be shut at that point. This can lead to a flooded boiler if the water rises up into the receiver's vent line (static pressure pushes water into the boiler). If there's an overflow line on the vent, you'll spill water either on the floor or down a drain.
If you're having this problem, install a pressure-reducing valve on the supply line going to the automatic feeder. Use the same sort of PRV that you would use to fill a hot water heating system.