How to size a boiler-feed pump
You need to understand two important variables.
When it comes to boiler-feed pumps, the two important variables are the amount of water in the operating range of the boiler and the steam-to-condensate time lag across the system.
The operating range is the vertical distance between the center of the gauge glass and the low-water cutoff point. It’s not something manufacturers usually publish in their literature, but if you ask, they’ll be able to tell you how much water sits between those two points. This is the only water you have to work with. It doesn’t matter how much water is in the entire boiler. All that matters to the feed pump is the water in the operating range.
So, first figure out how much water is leaving the operating range. To know that, you have to know the steaming rate of the boiler — how fast water will leave the boiler as steam. Figure it as 1/2 gal. per min. of water for each 1,000 sq. ft. of equivalent direct radiation. For instance, if you have an 8,000-sq.-ft.-EDR boiler, you’ll be sending 4 gpm out into the system for each minute the boiler steams. This part is very easy to determine.
But how much water do you have to work with? That’s where the amount of water in the operating range comes in. This is the information you’ll get from the manufacturers. Let’s say they tell you there are 20 gal. of water between the center of the gauge glass and the low-water cutoff point. That means this 8,000-sq.-ft.-EDR boiler will be off on low water in 5 min. If they tell you there are 40 gal. of water in the operating range, it will go off on low water in 10 min. This part is also easy.
OK, put that thought on the back burner for a minute. Let’s take a look at the other variable: the steam-to-condensate time lag. This is the amount of time it will take the steam to make a round trip through the system. It leaves as steam and returns as condensate. How long does it take?
Well, the easiest way to find out is to time the cycle by using the old boiler. Fire it up and put your hand on the header. Start timing when you say “Ouch!” Stop timing when you feel the condensate return to the boiler room. The result is the steam-to-condensate time lag across the system. That’s how long it takes for the steam to go out and for the condensate to return.
Now let’s pull both thoughts together. Your feed pump will have to match the steaming rate of the boiler, and the condensate receiver will have to be large enough to accept the amount of condensate that will come back after the steam-to-condensate time lag has elapsed.
Let’s size the receiver first. We’ll work with that same 8,000-sq.-ft.-EDR boiler. Suppose we check out the time lag and find it to be 20 min. How much water will have left the boiler during that time? Well, we know steam turns to water at the rate of 4 gpm in this boiler (1/2 gpm per 1,000 sq. ft. EDR). So in 20 min., 80 gal. of water will have steamed off.
The ballcock at the bottom of the receiver would have supplied this water to the boiler and kept it from going off on low water, but now that the original boiler water has returned from the system, we have to have a place to put it. This means the receiver will have to hold at least 80 gal. of water.
But don’t forget the lower 25% of the receiver. That’s always going to be filled with water, so we have to include it to get the final receiver size. The simplest way to do this is to take the 80 gal. and multiply it by a factor of 1.33, which gives us a total receiver size of 106.4 gal. I’d probably opt for a 100-gal. receiver in this case since that’s a standard size.
Can you see how the size and condition of the system will have an effect on the receiver size? If the returns are old and dirty, condensate will take longer to get back to the boiler. If it’s an older building, they may have used thermostatic traps on their riser drips. Thermostatic traps hold condensate longer than float and thermostatic traps, which will affect the steam-to-condensate time lag, as well as the size of the receiver.
And there are other variables:
The shape of the building. Tall buildings return condensate faster than wide buildings.
The distance from the boiler to the furthest radiator. The longer the distance, the slower the condensate.
The pitch of the piping. If the steam and condensate flow in the same direction, the piping should have a pitch of at least 1 in. in 20 ft. In counter-flow piping, the pitch must be at least 1 in. in 10 ft. If the pitch isn’t there, the condensate is going to return more slowly.
The type of radiators. Convectors and baseboard make condensate faster than big cast-iron radiators because they usually have more surface area.
The near-boiler piping. If it’s not right, water will leave with the steam. If the water’s in the system, it’s not in the boiler.
Condensate-transfer pumps. Do you have them? They certainly speed up the return of condensate from the ends of the mains.
Think like condensate
Those are some of the reasons why it’s always best to measure the time lag with the old boiler. That’s real. Anything else is guesswork. Of course, if the old boiler is broken, you’ll have to make an educated guess, so look the job over carefully and try to “think” like condensate. If you were in the piping, could you make it back to the boiler quickly?
Most boiler-feed pump manufacturers work with a rule of thumb allowing for 1 gal. of receiver capacity for every boiler horsepower (one boiler horsepower is 33,475 Btuh). In other words, if you have a 100-hp boiler, you’d use a 100-gal. receiver. They base this on a steam-to-condensate time lag of about 10 min, which is normal in a well-designed, well-maintained system. But then normal is a very subjective term, isn’t it?
If you undersize the receiver, it will overflow when the condensate returns from the building. That’s bad because you’ll have to make up the lost water with raw feed water, which is never good for the boiler. It’s also difficult to explain to your customer why his basement is always flooded.
If you oversize the receiver, you probably won’t have a system problem, but you might not have a job either because your price will most likely be too high. You’ll never get a chance to find out if it was the right size because someone else will be doing the work.
Manufacturers size the pump flow rate within a range of one-and-a-half to three times the steaming rate. I’ve found that if you work with two times the steaming rate, you’ll usually be fine. So for our 8,000–sq.–ft.-EDR boiler, we’d use a pump capable of moving 8 gpm.
The discharge pressure depends on the operating pressure of the boiler. For steam heating, we should discharge at boiler pressure plus 5 lb. per sq. in. gauge. Here, too, you’ll find most feed pumps come with stock motors capable of discharging at 20 psig.
Because of the larger size of a boiler-feed pump’s receiver, compared to the size of a condensate pump’s receiver, high-temperature water isn’t usually a concern in a steam-heating system. That’s because the return water mixes with the start-up reservoir water and often waits a while before entering the pump. If you find the pump cavitating, check the steam traps and the system pressure.
The last decision you’ll have to make has to do with the materials of construction. As with condensate pumps, feed-pump receivers are made from either steel or cast-iron. Cast-iron will last much longer than steel, but it’s also a lot more expensive to buy and, in these sizes, a lot heavier to move around the basement.
For the long haul, I’d choose cast-iron, but that’s between you and your customer.