Photo courtesy of Taco

When I think hydronics, I think traffic. I can’t help it. The two are so similar.

Here, imagine you’re the water in a main heating pipe. You’re driving down the pipe when you notice an accident up ahead. There’s a lane or two closed up there, so you hop on the service road. But you’ll only do that if you’re a local and you know that the service road doesn’t involve a 100-mile detour. If you knew that, you’d probably stay on the highway, wouldn’t you?

Sure you would, and so would I.

This is why there sometimes is no heat in that radiator up on the second floor - the one that the diverter tee and its partner, the standard tee down in the basement, are supposed to take care of. Sure, it looks like an air problem, but it’s a flow-balance problem. You prove this by bleeding the radiator. You don’t get any air, which means it ain’t an air problem.

Time to stop bleeding.

Flow in a closed system is always about traffic. These diverter tees, which are on so many older systems, don’t “scoop” water. They just direct the traffic. If it’s too congested on this main road, the water will get off and take that other road, but only if the detour isn’t that long. The difference in pressure between the different ports of the tees decides where the traffic goes.

Can you see it in your mind’s eye? Can you feel it?


Most diverter tees come with a ring that’s cast into one side of the tee’s run. When you see these tees on a job, the rings should always be between the two pipes that run out to the radiator that the tees serve. Sometimes we use just one tee, but often it takes two tees to get the job done.

Preventing a traffic jam

Here are the rules of thumb:

1.If the radiator is on the floor directly above the main, one tee should do the trick. That tee should be on the return pipe (coming from the radiator), with the ring on the inboard side of the pipes that feed the radiator.

2.If the radiator is on the second floor, you’ll need two tees, and the pipe leading to the radiator should be one size larger than what you would normally use. For instance, 1/2” for the first floor becomes 3/4” if the radiator is on the second floor. This is to keep the pressure drop to and from the radiator to a minimum. Think like water.

3.The tees should ideally be the width of the radiator apart. When these tees were popular, so were freestanding, cast-iron radiators and convectors. These radiators and convectors were typically about three feet wide, and that’s why you’ll often see the tees placed at that distance apart on the main.

4.If you remove a freestanding radiator or convector and replace it with lots of linear feet of baseboard radiation, don’t be surprised if the water decides to stay in the main. You just increased the resistance to flow by increasing the length of the detour. You made the side road longer.

5.If the radiators are below the main, use a diverter tee on both the supply and the return, be certain that the rings on the tees are between the pipes that go to the radiator (meaning that the tees will face in opposite directions), and make sure the tees are as wide apart as the radiator is long.

6.If you remove a radiator and you’re not going to replace it, connect the bulls of the two tees with a 1/2” pipe so that water has a place to go. Otherwise, you’re leaving two major accidents on the main road, and closing the service road at the same time. That’s going to slow the flow of traffic to the whole system.

7.One more thing: Staggering the tees (supply/return/supply/return) will increase the resistance to flow along the main and encourage more water to flow to the radiators. That’s an old-timer’s trick.

Thermostatic radiator valves

While I have you thinking about the difference in pressure and how it can make or break a job with diverter tees, let’s consider thermostatic radiator valves.

These nonelectric zone valves seem like naturals for zoning a one-pipe, diverter-tee system, and I like them a lot for that, but you have to be very careful when you choose them. These valves, even when they’re wide open, offer a resistance to the flow of water through the radiator, and that resistance might be enough to stop the water altogether.

The TRV has two parts. The part that attaches to the pipe is a normally open, spring-loaded valve. You’ll attach to this the other part of the TRV, which is an operator that contains either a fluid or a wax that is very sensitive to changes in room-air temperature. As the temperature rises or falls, the fluid or the wax inside the operator will expand and contract, moving the spring-loaded valve open or closed. Control the flow and you’ll control the heat. You can adjust a TRV to whatever temperature you’d like in the room it serves, typically between 50° and 90° F.

What you need to watch out for, though, is the TRV’s pressure drop. You can see this in the valve manufacturer’s literature. They show it as Cv. That’s an engineering term that always appears as a number.

You’ll see something like this: Cv = 2.5. That 2.5 is gallons per minute. Any number that appears after the = in the Cv equation will always be gpm, and what the equation is saying is that when, in this case, 2.5 gpm flows across that particular valve, there will be a corresponding drop in pressure from one side of the valve to the other of 1 psi.

Cv always relates to a difference in pressure of 1 psi. If you look at two valves, one where the Cv = 2.5, and the other where the Cv = 3.0, the latter valve will have less of a pressure drop.

Think it through. With the first valve, you get a 1-psi drop in pressure with just 2.5 gpm flowing. The second valve can flow a full 3 gpm before the water suffers that same 1-psi pressure drop.

So, if I were choosing between those two TRVs for my diverter-tee system, I’d probably choose the second valve because it has a higher Cv number, which means it offers less resistance to flow. I don’t want the valve, when fully open and just sitting there, to present my flow with so much resistance that flow just stops because, where there is no flow, there is no heat.

That would be onemajortraffic jam.