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Diverter Tee Tips


Dan Holohan
July 16, 2009
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Bell & Gossett and Taco have sold diverter tees for longer than I’ve been alive, and you will find them in buildings all over North America. Not many contractors use them nowadays, preferring to pipe home runs with baseboard, or to install radiant heating systems, with all that delicious primary-secondary pumping and piping and brilliant controls. But there are so many of those older systems around, and the fun starts when some poor soul drains one of these systems for service and then tries to get it going again. And with that in mind, I offer some tips I’ve collected over the years. I hope it helps.

The rings go between the risers.

If it’s a B & G Monoflo tee, it’s going to have either a red or blue ring on one of the ports along the run of the tee. That ring (blue was for high-pressure-drop branches, and is no longer made) should always be between the risers that lead to the radiator. This means that if you’re using two Monoflo tees, they should be facing in opposite directions. If a radiator is not heating as it should, check the position of the tees. If they’re facing the wrong way, the radiator won’t heat well. This would have been a problem from the first day, but it sometimes only gets mentioned after the poor soul works on the system.

Alternate up and down.

When there are upfeed and downfeed radiators next to each other, the original installer often alternated the diverter tees. The first tee (a standard tee) went to the upfeed radiator. The second tee (a diverter tee) went to the downfeed radiator. The third tee (a diverter) came from the upfeed radiator. And the fourth tee (a diverter) came up from the downfeed radiator. Alternating the up and down connections to the two radiators, gives you more resistance to flow along the main and nudges more hot water into the radiators. It’s an old-timer’s trick, and it works well.

It may be okay to remove a radiator, but don’t seal the branch lines.

If you cap the pipes that used to lead to the radiators, all the water will have to flow through the run of the diverter tee. That increases the overall system pressure drop and slows the flow of water everywhere. If you remove a radiator, remove the diverter tees as well. Or if it’s easier, just connect the two branches from the diverter tee with a short length of copper tubing. That way, the water that used to go to the radiator will still have a place to flow.

Take care when replacing an older radiator with baseboard.

When diverter tees first came out, copper fintube baseboard didn’t exist. Those old free-standing, cast-iron radiators or steel convectors were typically about three feet wide. So the installer would usually space his diverter tees the width of the radiator apart on the main. The water that was flowing through the main now had a choice. It could flow three feet along the main, dealing with the pressure drop of the diverter tee (or tees), or it could flow down into the three-foot-wide radiator or convector. It probably chose the latter because that was the path of least resistance. But nowadays, when an old radiator or convector comes out, the poor soul may install 30 feet of ¾” copper fintube in its place. Why? Because he can. And now when the water makes a choice, my guess is that it will stay in the main and not flow through the baseboard. The reason being the same as before – water follows the path of least resistance. Always. Too much baseboard means there’s too much resistance. Bleeding the baseboard seems to help, but only if the poor soul is willing to sit there and bleed the thing for the rest of the heating season.

On downfeed radiation, keep the temperature low and the pressure high to start.

Cold water is heavier than hot water. If the poor soul drains a downfeed diverter-tee zone (such as you’ll often see in basements), and is having a tough time getting it to circulate again, he can try this old-timer trick. Lower the water temperature and raise the system pressure. This brings the density of the hot water in the main closer to the density of the cold water in the radiators and that helps to get things moving. It also appeals to Henry’s Law, which states that gases dissolve in liquids in relation to pressure and temperature. Meaning that the hotter the water is, the harder it will be for air to stay in solution. By lowering the temperature of the water, you’ll force air into solution. High pressure does the same thing. The higher the pressure, the more the air will stay in solution, and the easier it will be to move it. Of course, once he gets things going, the poor soul will have to put things back the way they were before – lower the pressure and temperature to their proper settings.

Pitch the main and the radiators upward in the direction of flow.

This advice goes back to the original diverter-tee installation instructions that came out in the late-1930s. The pipe’s upward pitch makes it so much easier for air to move along on start-up and not get stuck in a radiator. So the poor soul should check those pipes for upward pitch (one inch in 20 feet is all you need). People hang stuff on heating pipes (laundry, for example), so it always pays to check. Without that pitch, air will be a bigger problem than it should be.

Use the right amount of tees.

Radiators above the main will usually work with one diverter tee if the radiator is on the first floor. But put that radiator on the second floor and feed it from a basement main and you’ll find it won’t work without a second diverter tee. And if that first-floor radiator works with, say, ½” branch lines running to and from it, the radiator on the second floor will probably need ¾” lines running to and from it. Why? Because water follows the path of least resistance (there it is again). Radiators below the main always need two diverter tees, and those tees should be the width of the radiator apart. There’s more to these things than meet the eye. You really have to size them to get it right.

Whenever possible, pump away from the compression tank.

When you pump away from the compression tank, the circulator adds its pressure to the system’s static fill pressure. That drives air bubbles into solution and makes it much easy to get rid of the air that appears when the water comes up to design temperature. If the poor soul is working on a job where he doesn’t have the opportunity to move the circulator, it will help if he can lift the weights inside the flow-control valves by turning those little knobs that raise and lower the weights. Those weights produce a hefty pressure drop, and by temporarily taking them out of the circuit, there will be move pump pressure available to move air.