Most manufacturers rate their condensate pumps in square feet EDR. You can easily convert whatever boiler rating you have to square feet EDR by using these conversion factors:
- Multiply boiler horsepower (BHP) by 140 to get square feet EDR
- Divide Btu/h by 240 to get square feet EDR; and
- Multiply pounds of condensate per hour by 4 to get square feet EDR.
Once you have the rating in square feet EDR, you can select a condensate pump for that (or a slightly higher) rating from the manufacturer’s catalog. Most stock units will come with their pumps set to discharge at 20 psig pressure. That’s because the pump has to discharge at 5 psi higher than the pressure at which the boiler is operating. Low-pressure steam boilers operate up to 15 psi, so that’s why the condensate pump is capable of discharging at 20 psi. If you’re running the boiler at 2 psig, which is typical for heating, you’ll need to throttle the pump to 7 psi. If you don’t, the check valve at the pump’s discharge will rattle.
Manufacturers size the flow rate of the condensate pump to three times the system’s steaming rate to get the condensate back in the boiler as quickly as possible. That’s easy to figure because for every 1,000 square feet EDR, a boiler will lose 1/2 gpm of water to steam. That goes for all steam boilers, regardless of size or who makes them.
Here’s an example: Let’s say you have a boiler rated for 8,000 square feet EDR (that’s about 60 BHP). The steaming rate of that boiler is 4 gpm. In other words, for each minute it’s firing, 4 gallons of water will turn to steam and head out into the system (1/2 gpm per 1,000 square feet EDR).
We’d size a condensate pump for this system at three times the steaming rate (12 gpm) to make sure when the condensate returns from the system, it goes back into the boiler as quickly as possible. If we can help it, we don’t want this boiler to go off on low-water (or call on the automatic feeder). That’s also why manufacturers size the receiver to have a net storage capacity of about one minute’s worth of condensate. For this boiler, that would be 4 gallons.
The receiver will have to hold more than 4 gallons of water, though. We don’t ever want to get to a point where the pump runs out of water and starts sucking wind. If we did that, the pump’s mechanical seal would fail and the pump would leak. So to be sure the pump’s suction is always flooded, condensate-pump manufacturers build in an allowance. They call this the gross storage capacity of the receiver.
For that 8,000 square feet EDR system, you’d have a receiver with a gross capacity of about 9 gallons. The 4-gallon “net” storage capacity is the water between the pump-on and pump-off range of the float switch. In many cases, you can adjust the “net” storage capacity of the tank by adjusting the float lever’s swing.
If you’re looking at a rectangular condensate tank and you’re not sure what its “gross” storage capacity is, do this: Reduce all the dimensions to inches. Then multiply the length by the width by the height and divide your answer by 231. That will give you the total amount of water the receiver can hold (in gallons). The net storage capacity will be anywhere from one-third (in the larger units) to one-half (in the smaller units) of the gross storage capacity.
If you use a condensate pump as a transfer unit to return condensate from a remote part of the building back to a boiler-feed pump in the boiler room, you’ll have to take a few more things into consideration.
For instance, let’s say you’re looking to replace a boiler in a building that has an old, buried wet return picking up the end of the steam main. You realize that those old, wet returns are leaking, and you know it’s a bad idea to replace the boiler without doing something about those leaking return lines. If they keep leaking, too much fresh water will come into the system, and that’s going to kill your new boiler.
But what if it’s not practical or possible for the new return lines to run at floor level? Maybe there are too many doorways or hallways to cross over. Maybe that’s the reason they buried those return lines in the first place.
Or, let’s say you plan to install a boiler-feed pump for that new low-water content steam boiler. By opening up the system (with the boiler-feed pump’s vented receiver), you’ll be changing wet returns into dry returns. That means you’ll need steam traps at the ends of the mains. This might be a good time to use a condensate transfer pump. You can get rid of those buried wet returns once and for all by picking up the end of the main through an F&T trap, which dumps the condensate into the new transfer pump. That will let you run a smaller overhead line back to the new boiler-feed pump. Your buried return problems are solved.
But now that you’re pumping the condensate overhead, you’ll have to use more of that available pump pressure. How high up are you going with that line? 8 feet? 10 feet? You’ll have to allow about 1 psi of pump pressure for each 2 feet of vertical lift. If you’re going up 10 feet, you’ll need about 5 psig. Keep in mind, this isn’t like a closed hot-water heating system where the water is circulating around a closed, pressurized loop. There, we don’t have to worry about the lift because the weight of the water going down balances the weight of the water going up. But a condensate return system is open to the atmosphere. We have to lift that condensate up “over the hump.” Gravity will take care of the downhill part of the trip, sure, but we still have to get it up there.
Allow 1 psi of pump pressure for each two feet of vertical lift. This is in addition to the pressure you need to overcome the resistance in the return line. How much you’ll need depends on the size and length of the line.
For instance, let’s say you have to run 100 feet from your transfer pump back to your boiler-feed pump. I’m going to assume the discharge tapping on the transfer pump is 3/4-inch. If you run that line back to the feed pump in 3/4-inch copper, the pressure drop will be a lot greater than it would be through, say, 1-inch copper. That’s because the 3/4-inch pipe is smaller than a 1-inch pipe. It can’t carry as much flow.
The smaller pipe will slow the flow rate considerably, but this might not be important in this case. That’s because the condensate is returning to the boiler-feed pump’s receiver. The boiler won’t go off on low water; the feed pump will see to that.
You don’t have to worry about boiler pressure here because you’re pumping back into the feed pump, not the boiler. The lift, pipe diameter and the length of run are the important considerations here. For instance, can you see how you’d have a problem with a “stock” pump if you had to lift condensate 40 feet and then move it horizontally a few hundred feet? That standard 20-psig discharge pressure probably wouldn’t be enough.
Of course, you’re not likely to run into this sort of situation on a steam heating job, but watch those commercial applications.
One last word of caution: Never add a condensate pump without thinking about how it’s going to change the system. Are you turning wet returns into steam lines by adding that pump? Did you think about steam traps? You’ll need them now. Where are you going to put them?
And if you’re using a transfer pump, are you going to discharge into an existing gravity return? If you are, have you given any thought to how this will affect those gravity returns? Will the condensate from the gravity-return portion of the system be able to flow when the pump is on?
Give it some thought. Always try to see the system, not just the pump.