How to Set a Pressuretrol

If you have a steam boiler you’re also going to have a pressuretrol. I’ve always liked that word because it’s a marriage of the words pressure and control, both of which have much to do with our older steam-heating systems.

Pressuretrols don’t come with screwdrivers, but most contractors already have their own screwdrivers, so that’s okay. The problem, however, is that most of those screwdrivers turn in only one direction: clockwise. And that’s why so many steam-heating systems are operating at a pressure that’s much too high. My advice is to get a counterclockwise screw-driver, which allows you to turn down the pressure. It costs the same as the other type.

Here’s what to consider before you do that, though, and you won’t find this in the pressuretrol manufacturer’s instructions.

First, know that the pressure the steam-heating system needs depends on the size and condition of the steam pipes and the amount of steam flowing though those pipes. Unless you’re sizing the piping from scratch (and few do that these days), you can assume the guy who did size them based his calculations on a pressure drop of 1 ounce of pressure loss per 100 feet of run for one-pipe-steam systems, and 2 ounces of pressure loss per 100 feet of run for two-pipe systems. That’s been the rule since December 1899. It means that in a one-pipe system, if you start with 16 ounces of pressure at the boiler, you will have about 15 ounces of pressure 100 feet down the line, and so on.

Suppose we have a tenement building with one-pipe steam. It’s 200 feet wide, 100 feet deep and five stories tall. We’ll set the boiler right in the middle of that building with two mains, one going to the left, and the other to the right.

Allowing for turns in the basement, let’s say we have to travel 150 feet to get to the base of the riser that’s furthest from the boiler. Add the height of that riser and we wind up with a longest run of about 200 feet. Our job is to get steam from the boiler to the end of that run.

Since this is a one-pipe job, let’s assume the designer based his pipe sizes on a pressure drop of 1 ounce per 100 feet. That means we should be able to get steam to the end of the 200-foot run with just two ounces of pressure. But that doesn’t allow for the fittings and valves, does it? There’s pressure drop through all of those things as well, so he had to consider it or he would have been in trouble.

Old-timers knew how many fittings they were dealing with because they were installing them. They also had plans and specs. It’s hard for you to tell what’s there now because most everything is buried in the walls and ceilings. So to figure the correct pressuretrol cut-in setting, we’ll use an old-timer’s trick.

Measure the run from the boiler to the furthest radiator and double it. Base your pressure drop on that corrected “equivalent” figure. Hey, it worked for the old-timers and it will work for you.

So in our example, we’d treat that longest run as 400 feet instead of 200 feet. That means the pressure drop from the boiler to the furthest radiator will be 4 ounces. In other words, if we have 1 psig pressure at the boiler, we’ll have 12 ounces at the radiator.

Now here’s the key to setting the cut-in pressure: For best system operation, the cut-in setting of the pressuretrol should never be less than twice the system pressure drop. This is a safety factor and it means that in our example, we only have to have 1/2 psi (or 8 ounces) of pressure at the boiler to get the steam to the furthest radiator. That, by the way, is why standard pressuretrols have a minimum cut-in pressure of 1/2 psi.

Now stop for a minute and ask yourself why you have Mrs. Murphy’s three-bedroom house set to cut in at 3 psi? Could this be part of the problem you’re having on that job? Bet on it.

Okay, let’s move on to the cut-out pressure because we want the burner to shut off when we have as much pressure as we need and no more. How much would that be? Well, let’s consider the square foot of equivalent direct radiation (EDR). That’s the term we use to size radiators nowadays. The old-timers who installed those radiators also used that term. They made sure those radiators would be big enough to heat the rooms on the coldest days.

Next, consider what defines the term EDR. In steam work, 1 square foot EDR is the amount of surface area on a radiator that will put out 240 Btu when the air temperature around the radiator is 70° F and the steam temperature inside the radiator is 215°. That last part is key to understanding this because a pressure of 1 psig steam is equal to 216°. That means that we’re basing steam EDR on a pressure slightly less than 1 psig at the radiator. If you put more pressure than that into the radiator, you’re wasting money and probably going to screw up the system.

In our example building, it took 1/2 psig to get the steam to the radiator. And now we know we need about 1 psig at the radiator. If we add those two numbers together, we’ll have our pressuretrol cut-out setting, which in this case will be 1 1/2 psig.

That’s all you’ll ever need in most one-pipe, steam-heated buildings. So if you have a Pressuretrol with a scale that reads “Cut In,” make that setting 1/2 psig. Then set the “differential” (or “diff”) wheel at 1 psig. The system will now operate between 1/2 psig and 1 1/2 psig whenever the thermostat is calling for heat.

If you have a “subtractive” pressuretrol, you’ll have to concentrate a bit harder because these confuse a lot of people. The “main” scale on a subtractive pressuretrol is your cut-out pressure. Set that scale at 1 1/2 psig. To get to “Cut In” you have to subtract the “differential” from the “main.” So set the differential for 1 psig. This will cycle the system between 1/2 psig and 1 1/2 psig as long as the thermostat is calling.

If you have a vaporstat, consider yourself lucky. A vaporstat is a very accurate (and more expensive) control that senses ounces of pressure rather than pounds. A vaporstat is the nicest present you can give many smaller steam systems.

Set the vaporstat to “Cut In” at about 4 ounces and “Cut Out” at about 10 ounces after making sure that all the air vents are the right size, in the right places and working properly. A few ounces will be all that you need.

And that’s how you set a pressuretrol. Pretty easy, right? Just remember that steam pressure is a function of pipe size and condition, and the steam load. And some long-gone heating professional figured that out already. Your job is to build just enough pressure at the boiler to overcome the frictional losses in the pipes the old pro specified.

So get yourself one of those counterclockwise screwdrivers and starting cranking down those pressuretrols. You’re going to look like a hero every time.


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