One of the irrefutable benefits of hydronic heating is the ease of zoning heat delivery to accommodate different comfort preferences or operating schedules. Hydronic systems can be tailored for owners who, for example, want to sleep in cool bedrooms, but still expect a warm bathroom to greet them in the morning. Or someone who wants to “occasionally” heat a basement workshop. Even a client desiring separate comfort control in every room of a building.

Although there are several methods of zoning, one of the simplest uses a fitting, generically called a diverter tee. Many of you long-time Wet Heads probably know them better as the B&G’s trademark brand Monoflo® tee, which over the years has become synonymous with so-called one pipe hydronic systems.

Creating The Urge: Before water will flow through any piping path there has to be a pressure differential to “push” it along. Diverter tees create this differential between the points where a branch piping path leaves, and eventually reconnects to, the main circuit as shown in Figure 1. Water — like electricity — “thinks” about what path it will follow when given a choice. If two parallel paths exist between two points in a piping circuit a flowing stream of water divides up according to the resistance each path creates between the two points. And like most things in nature, including human beings, water favors the path of least resistance.

Diverter tees create pressure differentials in two different ways depending upon how they are installed. When located at the supply riser connection to a heat emitter, the pressure drop through the orifice of the tee, along with the scoop shape of its internal baffle, tends to push part of the entering flow into the supply riser. When installed on the return riser, the tee creates a “venturi effect” that attempts to pull water out of the return riser and back into the main. If diverter tees are used on both the supply and return risers, they work as a push/pull team to increase the amount of flow through the branch path.

It’s crucial to install diverter tees in the correct orientation depending on whether they will “push” or “pull” fluid through the branch piping. Monoflo® tees have a red ring and/or groove on one side. In all cases the tee should be installed so that this ring/groove is facing the piping between the tees. Look for the red lines on the tees in Figures 1–3.

When heat emitters are connected to a piping circuit using diverter tees, the heat output of each unit can be regulated independently. This requires a valve in the supply riser going to the heat emitter. It might be a simple, manually-operated globe valve, or a non-electric thermostatic radiator valve (TRV). Both are capable of adjusting, or entirely stopping flow through a heat emitter and hence regulating its heat output. Neither, however, are able to signal the boiler and circulator when hot water is needed by the heat emitter they control. Hence, they function as heat limiting devices, rather than completely independent operating controls.

Another control option is an electrically-operated zone valve installed in the supply riser. The end switch of the zone valve can be used to enable boiler firing and circulator operation, and thus allow its heat emitter to operate completely independently of the others.

Is One Never Enough? The classic application guidelines for diverter tees specify that a single tee at the return riser is generally all that’s needed for radiators, convectors or baseboard. However, when high resistance heat emitters are used, or the risers go down from the main, these guidelines recommend two Monoflo® tees to overcome either the higher head loss or the buoyancy effect created when pushing hot water downward. The latter criteria tends to be more of a necessity in older hydronic systems with low flow rate mains, high mass cast-iron radiators and high supply water temperatures, but is not always necessary in modern hydronic systems.

I’ve done some calculations on the differential pressure created by down-fed vertical risers connected to a heat emitter operating at a 20 degree F temperature drop. The difference in pressure depends on the difference in density of the water in the two columns, which in turn is a function of the water’s temperature. This pressure differential is also proportional to the height of the risers. The greater the difference in density, and the greater the vertical height, the greater the pressure differential. The formula to estimate the pressure differential is:


Dhot & Dcool are the densities of the water at the supply and return temperatures respectively (in lb/cubic foot)

h = the vertical height of the risers (in ft.)

144 = a conversion factor to make the answer come out in psi

If we assume a vertical drop of 9 feet (as would be typical from a main pipe installed in the second floor framing down to a baseboard on the first floor level), with 180 degree F supply water and 160 degree F return water, the pressure differential created by the buoyancy effect of the water is:

This is approximately equivalent to the head loss of 9 feet of 3/4–inch copper tubing operating at 1 gpm. Such a small differential, by itself would not necessitate two diverter tees. Keep in mind, however, that this small pressure differential exists only after the heat emitter and risers are up to normal operating temperature. When the heat emitter and return riser are cold the pressure differential is significantly higher because of the greater difference in water densities. For example, if the return riser temperature is 50 degree F and the supply riser is 180 degree F the differential increases to 0.12 psi, which is equivalent to the head loss of 42 feet of 3/4–inch copper tube at 1 gpm. Heat emitters with high thermal mass (such as cast-iron radiators) will take some time to warm up and thus be slow to reduce the initial buoyancy pressure differential. This could lead to sluggish performance if only a single diverter tee is used to induce flow through a long down-fed riser. Low mass heat emitters like fin-tube baseboard should warm up quickly and thus reduce the initial buoyancy pressure differential in short order.

Given the number of variables involved it’s impossible to make a general statement as to when two diverter tees must be used. But I do support the opinion that there will be exceptions to the long-standing Two Tee Rule on all down-fed connections. We’ll get into this more next month.

Give ’em Some Room: When installing diverter tees, it’s important to have at least 1 foot of straight pipe between the tees where the risers connect to the main. This allows turbulence created in the upstream tee to dissipate and not affect the flow characteristics of the downstream tee. Turbulence issues aside, the greater the length of main pipe between the tees, the greater the pressure differential between the tees and the greater the induced flow through the branch piping.

Here are some piping ideas for diverter tees that may prove helpful as you design hydronic systems:

In some cases it doesn’t make sense to connect every heat emitter to a piping main using diverter tees. One example is when a room is large enough to require two or more heat emitters. In such cases the heat emitters within the room can be piped together to form a reverse return “sub-assembly,” as shown in Figure 2. Flow in this sub-assembly is induced by a pair of diverter tees in the main loop. The reverse return parallel piping of the sub-assembly ensures all heat emitters connected to it will be supplied with the same temperature water. It also reduces the head loss of the sub-assembly compared to a series piping arrangement. This in turn increases flow through the heat emitters and boosts heat output. Note that only a single control valve is needed to regulate the sub-assembly.

Sometimes it makes sense to connect one or two heat emitters into an otherwise series piping circuit using diverter tees. An example being a series baseboard circuit that also serves a high flow resistance heat emitter, such as a fan-coil convector. The diverter tee induces enough hot water flow through the heat emitter for proper performance without “bottlenecking” the entire piping circuit as might be the case if all heat emitters were piped in series. Obviously, this becomes even more important when several heat emitters with high flow resistance need to be connected to the same distribution circuit.

A nice benefit of using diverter tees in combination with thermostatic radiator valves (TRV’s) is that heat emitters so connected can be purposely oversized to allow for fast warm up, or higher than normal air temperatures in certain rooms at certain times. For example, suppose your client wants to quickly bring a bathroom up to 80 degrees F for a shower, but otherwise keep the room at normal comfort temperatures. You could, for example, install a generously oversized panel radiator in the room and connect it to the main with a diverter tee/TRV combination. Even though the panel radiator is oversized, the TRV will prevent overheating when its setting is reduced. Remember, however, that this heat emitter can only operate when heated water is flowing through the main pipe serving its diverter tees. When a room has to be heated completely independently of other rooms, it’s better to use an electric thermostat/zone valve com- bination instead of the TRV. The end switch in the zone valve is used to initiate boiler firing and circulator operation.

Still another design concept uses diverter tees and TRV’s to connect each of several heat emitters to a common distribution loop. The loop operates with continuous water circulation and outdoor reset control. The colder it gets outside, the warmer the water temperature in the loop becomes. When any given room requires heat the TRV on its heat emitter opens, and the diverter tee(s) induce flow through it. If the reset control is properly set, the TRV’s on the heat emitter tend to remain open a high percentage of the heating season. Their function essentially becomes protecting their rooms from overheating due to internal gains from sunlight, fireplaces, etc. Because of the constant circulation it’s important to insulate the piping loop to limit heat loss from the pipe. Obviously reset control reduces this heat loss during mild weather, but still the idea is to release heat at the heat emitters rather than the distribution piping. Another benefit of continuous circulation is the elimination of the previously discussed start-up lag in high mass heat emitters due to density differences in down-fed risers.

Devices other than diverter tees can be used to create the pressure differential that induces flow through heat emitters in branch piping circuits. A globe valve installed between a pair of standard tees would work, as would a more sophisticated circuit setter valve. Believe it or not I’ve even heard of “installers,” (note I didn’t say heating professionals), who partially crush the copper tubing between the tees with a hefty pair of pliers. Not enough heat in one room you say — no problem, just clamp down on that pipe a little harder. This certainly lends a new meaning to the term “value engineering.” Please don’t even think about doing this.

We’ll get into the numbers needed to properly design diverter tee systems next month. In the meantime, have a happy and blessed Christmas.