Reply To: hot water radiant heat with hot water heater, trouble with return
Harold Kestenholz

It is becoming obvious to we dense people that you are not going to read the instructions for designing a system, so I will copy it to here from the free education site at

“Select a Circulator
Some pumps do not move enough water to carry the required btu through the heat distributing units. A pump must develop enough pressure to overcome the resistance (friction) created in the boiler, piping, fittings, heat distributing units and other parts of the system. Each pump manufacturer publishes charts that show how much pressure his pump will overcome. The charts are usually stated in terms of feet of head pressure, because it is possible to see how strong a pump is by observing how high the pump (even though it is used as a cirulator) will be able to raise water in open pipes.

The height of the heating system above the boiler does not change the head pressure rating required by the circulator. A sealed, filled system has all the interior parts under water. It is already filled to the top. A heating circulator does not have to raise water from one height to another. This is why a pump used in a sealed hydronic system is called a circulator. When making your selection, do not add the static head pressure (due to the height of the system) to the head pressure rating of the circulator.

Rating pump head Pressure
It is possible to test and rate a circulator by the flow it produces in a standardized circuit that includes typical pipe sizes. Most residential circuits incorporate 3/4 inch tubing and baseboard and are less than 250 feet in length. Ordinarily, such circuits have no more than 22 elbows and 2 valves, altogether producing a head pressure rating of less than 12 feet.

Some confusion exists because the circulators are rated for the purpose of raising water above the pump, even though they will be used for circulating water in a system that is already filled. Although circulators can be rated by using a laboratory circuit, there is not an economic benefit in doing so; hot water systems have been in use for a century without a demand for changing the current rating system.

Engineers pump water up lengths of open pipe to observe the gpm pouring over the top. Then they mark their observations on a graph.

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