The Pressure-Reducing or "Feed" Valve

The pressure-reducing or "feed" valve’s job is to fill the system with water and to keep that water under a few pounds of pressure at the top floor. Your job is to figure out how much pressure you need in the basement to push the water up to the top of the system and hold it there under pressure.

In the beginning…

We haven’t always had feed valves on hot water systems. No, in the beginning, the old-timers did things differently; they filled the systems from the top instead of from the bottom.

Back in the days of gravity hot water heat, an old-timer would use an open expansion tank up in the attic. Since the tank was the high point of the system, he knew he could fill all the pipes from there.

Filling the system wasn’t always an easy job. In fact, before there were city water mains, the old-timer would have to carry buckets of well water up to the attic tank. Later, when pressurized water mains became available, he most likely used a ball cock in the open attic tank to maintain a water level and keep the system filled.

Water has weight!

Now, water has weight, and the higher you stack it, the more it will weigh at the bottom. It’s like bricks. The higher the pile, the more it weighs, right? Same thing with water.

And since weight exerts pressure downward, you could put a pressure gauge in the boiler and read the weight of the column of water in "pounds per square inch." This, by the way, is the same pressure that affects deep-sea divers. If those folks dive too deep, the weight of the water will crush them.

We call the weight created by the height of the water in a heating system "static head pressure," or static pressure for short. Static pressure is pressure that’s there all the time. The further down in the system you go, the higher the pressure will be. That makes sense, doesn’t it? As you go lower, you’re putting more water on top of yourself. More water means more pressure. If you’ve ever tried to take a wrinkle out of the bottom of a swimming pool liner after you filled the pool with water, you know about static pressure!

So when the old-timer filled the gravity hot water system from the tank in the attic, he created a static pressure down in the boiler. The taller the building, the higher the pressure on the boiler. This means that the height of the system determined the working pressure of the boiler. If the boiler were rated for 30 psi, for instance, the relief valve would pop if the old-time stacked too much water on top of it. If he was working with a very tall building, he’d have to use a boiler with a higher working pressure.

Square inches and round dimes

Today, most of your systems don’t have open tanks in the attic; they have "closed" steel compression tanks or diaphragm-type tanks, and most of the time, these tanks are down in the basement. You don’t pour water into the system from the top any more, do you? No, you force it up from the bottom. And that’s where the pressure reducing valve comes in.

The PRV takes the high pressure from the city water main and lowers it to the amount needed to lift water to the top of the building.

Once you fill this "closed" system with water, you’ll have the same static-pressure effect at the bottom as you did in the "open" gravity system. The only real difference is the point at which you fill. But if you’re filling from the bottom, how do you know how much pressure you’ll need to fill the system to the top? Most standard feed valves come factory-set at 12 psi (they’re adjustable between 10 and 25 psi). Is that enough pressure for most buildings? Why do the people at the factory set them at 12 psi and not some other pressure?

The answer is simple: A column of water 2.31 feet high (that’s about 28 inches) will exert on pound per square inch (psi) of pressure down on the bottom. And it doesn’t matter how wide or narrow that column of water is. It could be a ž" pipe, or it could be a swimming pool. If the water is 2.31 feet deep, there’s going to be one pound per square inch of pressure at the bottom. The reason you can be so sure of this is because you’re measuring a pound per square inch (psi). A square inch will always be a square inch, no matter what.

Here, let’s look at it a different way for a moment. Imagine that instead of working with water, you’re working with a stack of dimes.

We’ll create a new term for this example – "pounds per round dime." Using that term, can you see how the weight at the bottom of the column of dimes will be equal to the amount of dimes you stack on top of each other?

But suppose there were more than one stack of dimes?

Has the term "pounds per round dime" changed? No, it hasn’t. Sure, there are more dimes now, but the height of the columns hasn’t changed.

This is essentially what happens in a larger-diameter pipe. There’s more water, but the weight at the bottom in pounds per square inch remains the same. The height of the column, not the quantity of water, determines the static pressure in the system.

So if you set a feed valve for one pound per square inch, it will lift water into the system exactly 2.31 feet above the feed valve. Any piping lower than the feed valve will, of course, also be filled with water. Gravity takes care of that.

It’s important to mention here that this fill pressure has nothing to do with the number of fittings or valves or the width of the building’s piping network. Those things affect the circulator, and we’ll talk about them later. But for now, just keep in mind that the only thing that determines static head pressure is the height of the water in the system.

Why 12 psi?

So why do most manufacturers set their residential feed valves (such as the B&G FB-38TU) at 12 psi? Well, let’s look at a "typical" house where you might install that valve.

You’ve installed the feed valve in the basement, naturally. You’ve piped it at a convenient height, say about four feet below the basement ceiling. Now, let’s figure out how high the feed valve has to lift water to get to the top of the system in this house.

Well, we already said it’s four feet to the basement ceiling. Let’s add another foot of lift to get through the basement ceiling. That puts you on the floor of the first story. Now this happens to be one of those old Victorian houses with a ten-foot ceiling, so add ten feet to get yourself up to the ceiling of the first story.

Now you’re 15 feet above the feed valve. Throw in another foot for the first-floor ceiling and an additional three feet to get yourself to the top of that old cast-iron, water-tube radiator. That gives you a grand total of 19 feet from the feed valve to the top of the system.

Time for a little math. If you need 1 psi to lift water 2.31 feet, how much pressure to you need to lift water 18 feet? That’s simple! 19/2.31 = 8.23 psi

So you need just a little more than 8 psi at the outlet of the feed valve to fill the system in this "typical" house. That gets the water up to the top floor radiator, but once it’s up there it won’t be under any pressure. Remember, static pressure reflects the weight of the column of water. But if you’re at the top of the column, there’s no weight at all is there? And since no weight means no pressure, you won’t be able to vent much air from that top-floor radiator, will you?

Also, suppose someone should set the high-limit aquastat higher than 212 degrees F in this system. When hot water gets to the top where there’s no pressure, it can flash to steam. That’s not only noisy, it’s also destructive and dangerous. So, to avoid these problems, you should always add three or four more pounds of pressure to the feed valve’s setting to give you a positive pressure up at the top of the system.

That’s why we set standard feed valves such as the FB-38TU at 12 psi. We designed it for the ‘typical’ two-story house.

But suppose the building you’re working in is taller than two stories. You’ll have to increase the fill pressure to reach the top floor, won’t you?

What settings?

Here are some sample buildings. See if you can figure out how much fill pressure you need in each of them.

As you can see, once you get to a certain height, you have to start thinking about the working pressure of your boiler. For instance, putting a boiler with a 30 psi working pressure in the basement of a five-story building wouldn’t be a very good idea. The fill pressure you’d need to get water to the top and pressurized would be much too close to the relief valve’s setting. When the water is heated and expanded, your relief valve would pop open.

Something you may not know…

Here’s another important point. Don’t think of a feed valve as a safety device. It’s not there to protect the boiler against a low-water condition. The only thing that can effectively protect a hot water boiler from low water is a low-water cutoff.

A feed valve’s job is to set the initial system pressure. That’s it. For safety’s sake, you should close the supply valve to the feeder once the system pressure is established. This is important because a feed valve that’s left open can mask a system leak. Systems leaks that go undetected can lead to air problems and boiler corrosion problems.

Remember, the only sure protection against a low-water condition is a properly maintained low-water cutoff.

How we make our feed valves

At Bell & Gossett, we use brass as a standard material in all our feed valves. We do this because brass is highly resistant to corrosion and widely recognized throughout the plumbing and heating industry as the best material to use at the point where cold feed water and hot system water meet. You see, minerals in cold feed water come out of solution as its temperature rises. Over the long run, brass handles this situation much better than iron. That’s why we chose it for our valves.

We designed a flexible, low-inlet-pressure check valve into all the B&G feed valves. The check valve helps prevent the loss of system pressure should the supply pressure drop below the system pressure. This check valve’s design is simple yet extremely effective and is less affected by dirt than, say, ball and flapper-type check valves. Keep in mind, however, that this check valve is not a fail-safe device for backflow prevention.

The lever on the FB-38TU lifts the valve seat so the system can be quickly filled on start-up. That’s a real time saver!

Another feature of this valve is the universal, threaded-union tailpiece which has a ˝" male NPT thread and a ˝" female sweat connection. Thread it or sweat it, the choice is yours!

B&G feed valves all feature a cleanable strainer that keeps dirt and sediment from entering the system. You can clean this strainer without removing the valve from the line.

Now that we have the system filled with water, let’s take a look at another component.

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