Header piping

Noel Murdough

I’d like to talk about header piping on steam boilers. In older boilers, the boilers were designed to have a very large open space inside, which allowed the water to boil and bubble, but not “wet” the top of the boiler, inside. This allowed for space inside the boiler for the steam to form and let the water drop back down. This “dry” steam is the stuff that rises up into the mains and heats the house.

In modern cast iron steam boilers, that space isn’t there. The tappings into the casting are as big as the casting will allow, but they are still marginal, and they are VERY, VERY close to the boiler water line. A tremendous amount of water leaves the boiler with the steam. The risers and header piping complete the boiler’s water/steam separating capability. It’s VERY important to build it so that it can do the job.

Picture yourself putting a pan of water on the stove to cook spaghetti. Fire it up on high. As the water begins to boil, the water starts to jump around. You turn the fire down to keep it in the pan. This is with a WIDE OPEN pan, no cover. Imagine, if you will, what would happen to the water if you left the fire on high. The water would leap out of the pan as the steam formed. Lots of it.

Back to the boiler. The boiler outlet is only two to four inches in diameter, depending on the boiler model. The fire is 50 times what it was under that pan. Water leaves the boiler. The risers are more than half full of water, and the stuff is moving at over thirty miles an hour. No kidding!!!

Remember the old stovetop coffee percolators? They were a mini boiler at the bottom with a riser in the center. Remember how much water it threw up into the basket full of grounds? (All of it.)

It’s very important to control where that water goes. As it rises into the piping, the height of the piping lets some of the water fall back into the boiler, if the risers are big enough. The rest lands in the header. The header should be the largest diameter pipe on the whole system. This is to slow the steam and water down, to get the water to lay down in the bottom of the header. It should be one size bigger than the building’s system connection, if there is only a single connection, because the system supply pipe is only carrying steam. The header is carrying all of that same steam, but it is half full of water as well.

The next most important thing to remember in designing the boiler piping, is the direction of flow in the header. If a boiler has one riser, one take-off to the system, and one equalizer back to the boiler, then the tees will be in the right order. It takes more care when there are multiple risers or multiple system connections.

Picture the header as having a river flying through the bottom half of it, and the steam above it, and the flow has a direction to it. The flow is from the first riser at one end of the header to the equalizer at the other end of the header. Those two items will never change. This defines the “racetrack” that the water will follow to get back into the boiler. If there are more than one riser, the other riser connections must be the next fittings in the header. If a system take-off tee is put between two risers, the flow will no longer be from one end of the header to the other. It will be from two risers and aimed at the middle. This causes the flow from the two risers to smash into each other, and not much water makes it back to the equalizer. Without that 30 MPH flow down the equalizer to blend with the cold return water coming from the system, the water leaves the header with the steam, up into the building. Now, rather than a little water coming in the return blending with a lot of hot water, we have most of the hot water LEAVING THE ROOM via the system. This gets replaced at a MUCH faster rate with cold return water. Most of the cracked steam boilers I have seen have the system take-off between the risers, or have the system takeoff piped in AFTER the equalizer. If the equalizer isn’t LAST on the header, the water just flies by it.

Since the old timers sized the mains pretty carefully, you can figure that they probably aren’t oversized. If you have two 3” mains, and all of the radiation is still connected to them, a 3” header isn’t going to drop much water out of the steam. The velocity in the header will be twice the velocity in each building steam main. Size the header big. You buy the pipe once, and if it is sized right, the fuel savings of not needing to lift water every time the heat comes on pays you back fast.

There are a few different shapes for headers. The most basic are in the boiler installation manual. All will have risers that extend at least 24” above the water line. Some will have you increase in diameter right at the boiler outlet. Read the instruction manual for each different model you install. At the top of each riser, there will be at least two fittings (elbow, tee) which make a swing joint. Iron pipe is used to control expansion and contraction movement of the boiler and header. Copper will come apart in a few years, if it is used instead. The risers connect to the header from the side or the top. If they connect from the top, it is considered a dropped header. A dropped header can be lower than the 24” minimum riser height, but if it is too low, the equalizer might back up into the header. Keep it high, if possible. The risers still must be 24” minimum above the water line.

A dropped header would be used any time more than one source of steam is used, to keep the condensate flowing in the header itself. Multiple and modular boiler systems typically use dropped headers.

Another variation of headers found in the books is a change I make to the old standard method of connecting counterflow one pipe steam or two pipe steam to a boiler. Here is the standard method of connecting to counterflow systems.

In this method, the steam and water drop into the main and the water goes down; the steam goes up. I prefer to keep the water closer to the boiler, in this way

The steam enters the main without it’s load of water, and all other things remain equal. Here is an alternate method for two pipe.

If we can keep the water in the boiler, header, and equalizer, and out of the system, we have a chance at heating the building quietly, and economically. Of course, venting, water quality, insulation, plugged returns, improper pitch, and leaks can still be a problem. Take the system as a whole, when solving problems. The header is only a part of the picture.