Ask Boiler or System Questions. Leave CommentsInformation about Monoflo Systems
>Monoflo systems are unique systems because they have a single pipe around the basement, and both the radiation supply and return pipe come off that same pipe. Water always takes the path of least resistance when flowing. If you tried to install a monoflo system without creating pressure differences with specially designed tees, the water would not flow to the radiation; it would just flow around the main pipe in the basement. All heat flow depends on pressure differences, whether it is warm air, hot water, or steam. High pressure always goes to low pressure; you cannot change that. As long as we create enough pressure change, everything works fine. On the other hand, we do not create enough of a pressure change, so it does not work so well.
The invention of the monoflo tee is what makes this system work. It creates a pressure change large enough to make the water flow up or down into the radiation and back into the main loop pipe around the basement. You may ask what the main difference is between a series loop, which is one pipe around the basement, and a monoflo or diverter tee system. A series loop will start at the boiler and go through all the radiation. It does not have tee-offs from the main loop to each section of radiation; instead, it is a continuous loop from one radiator to the next. The monoflo system has tees to isolate each radiation section. A series loop system, if you want to shut off a section of baseboard with a valve, you shut off the flow to the whole loop. In a monoflo system, if you shut down a valve on a section of baseboard, the rest of the system still works. The flow is slightly affected, but it would still function, albeit less efficiently than the complete system's flow, which relies on the flow through the monoflo tee and radiation.
Series Loop
Monoflo system
The monoflo tee is designed to create a pressure difference between the supply tee and the return tee. Let's look at the monoflo tee design. They will have an arrow or a red ring to show the direction of flow.
B&G Monoflo Tee
Taco Venturi Tee Drawing
Monoflo Tee Flow
The way this system works is that as the water flows through the pipe, it will pass the first tee as the straight pipe is the path of least resistance. When the water hits the Venturi on the second tee, the amount of water that passes through decreases, but the velocity increases. The water being backed up and waiting to pass through the venturi is building pressure because the circulator is still pumping. As the pressure builds, it will increase until the pressure at the first tee is great enough to overcome the pressure drop through the radiation connected to the set of tees. The water will circulate through the radiation, and the gpm flowing through the radiation is also increased by the venturi effect. As the velocity increases through the venturi, it creates a suction at the top of the venturi tee. When combined with the pressure buildup on the supply tee, this creates the proper amount of water flow.
The spacing of the tees is critical to get the pressure high enough to make it work. The minimum spacing is 18" between the tees. It is a general rule that the length of the radiation connected to the tees should be the spacing between the tees. For example, if you are connecting a 6-foot piece of baseboard, the tees should be 6 feet apart. The tees connected to a single radiation source form a set. There should never be any tee for other radiation between tees.
Do not install any fittings between the tees for a piece of radiation
The monoflo, venturi, and diverter tees, sometimes called scoop tees, all depend on resistance to create flow to the connected radiation. Adding any fitting between the venturi tee and the standard tee may affect the flow to both pieces of radiation.
Radiation Below the Main
When you have radiation below the main piping, pay special attention to the piping. Since hot water is hard to push down, you will need two monoflo tees. The supply tee will be installed backward. The arrow will point against the direction of the water flow. The flow will enter the tee toward the small side of the venturi, acting as a scoop and forcing the water down. The return tee will be installed the same as all the other tees on the return side. For any pipes going up to radiation and rated less than 15,000 BTUs, you can use 1/2" pipe. On radiation below the main pipe, never use less than 3/4" pipe, even if the radiation is less than 15,000 BTUs.
Flow downward per drawing above
Adding Kickspace Heater Using 2 Tees
Another time you would want to use two tees installed as shown above, but in the up vertical application would be to get proper flow through high resistance radiation. Some of the newer radiation on the market is of higher resistance or a kick space heater, which mounts under cabinets, normally in kitchens and bathrooms.
Removing Radiation From Monoflo Systems
An issue we can run into with these systems is when we want to remove a section of radiation. We cannot just remove a section of radiation and cap the lines. We must either remove the tees and install a section of pipe the same size as the main pipe or cut off the vertical risers and connect them. If we do not remove the tees or connect the riser, it will affect the flow throughout the radiation system. Remember, as we reduce system flow, we reduce the heat output from the system.
Zone Valves or TRV on Monoflo Systems
I am not a proponent of micro-zoning, but there are people who will want to zone the radiation on a monoflo system. If you do, you will want to use 3-way zone valves on the radiation, which gives us a bypass around the radiation when the zone valve is closed. This serves the same purpose as the pipe added when a piece of radiation is removed, as in the above drawing
There are times when the system is drained for some service, or a boiler replacement, and one or more pieces of radiation do not heat when refilled and bled.
This issue is discussed in the link below.
Disclaimer: The information found on this website is for informational purposes only. All preventive maintenance, service, and installations should be reviewed on a per-job basis. Any work on your heating system should be performed only by qualified, experienced personnel. Comfort-Calc or its personnel accepts no responsibility for improper information, application, damage to property, or bodily injury resulting from the application of information found on this website, as a professional should review it.