Circulator Sizing

Circulator Sizing

Sizing circulator pumps in a hydronic system

Many people struggle when it comes to figuring what circulator pumps to use in any given hydronic heating system. Oversizing a circulator may cause excessive pressures, noise, damage and premature wear of all components, and a waste of energy. Under sizing one can result in unacceptably high temperature differences between supply and return waters (ΔT), not enough heat (BTU/h) delivered or removed.
Improper sizing of circulators is the  #1 reason for hydronic system failures with #2 being the under sizing of primary and secondary circuit tubing.

To correctly size a circulator we need to know three major criteria of your heating system;

  1. Flow
  2. Head Loss
  3. Temperature Difference

is usually expressed in Gallon Per Minute (GPM) in North America and Liter per hour (l/h) in Europe. For this article we will use GPM. The larger the inside diameter of a pipe, the more liquid will flow through it if all other conditions (temperature, pressure, viscosity) of the liquid remain the same. M Copper and PEX tubing may share their outside diameter for their nominal size, but the inside diameter of PEX tubing is much smaller than that of a corresponding M Copper tubing. For this you will need to play close attention of the tubing material when selecting a circulator. For more info please see our GPM and BTU Capacity of M Copper and PEX Tubing page.

Head loss

pressure loss or friction loss, refers to the frictional resistance of a tubing to the flow of the liquid passing though it.  Its unit of measurement is foot of water column (ft) in North America and meter (m) in Europe. For this article we will use ft.

To size a circulator you will have to know both the GMP requirements and the head loss of your circuits.

Temperature difference

is expressed as ΔT. It refers to the difference between the supply and the return water temperatures in a hydronic circuit.  Contrary to belief it is not always desirable to have a large ΔT.

The formula is BTU = GPM x ( ΔT x 500). If we were to follow this then to deliver more heat we’d need to do is raise the ΔT or raise the GPM or both. There are two opposing forces that should prevent us from overdoing any one of those. The more GPM we push though a tube its resistance to flow will grow exponentially. The higher the ΔT the more uneven the floor temperature will be, the higher the stress on a boiler’s heat exchanger is.  Wile high efficiency stainless steel condensing modulating boilers love cold return waters, medium and low efficiency cast iron boilers and their venting get destroyed if the return waters drop under 132°F.

The goal here is to find the sweet spot between flow and ΔT. Here are a few recommended ΔT targets:

  • Floor Heating 15-20°F ΔT
  • Snow Melt 20-30°F ΔT
  • Pool heat exchanger 30°F ΔT
  • Flat plate heat exchanger 30°F ΔT
  • Air handler, fan coil 30°F ΔT
  • Indirect water heater 30°F ΔT

Let’s do a real life system calculation. The system is  for a 1,500 sq’ basement floor heating. We want 15°F ΔT for the floor to make sure it is evenly heated. The heat loss calculations and loop design calls for a 7 loop manifold with 0.6 GPM  per loop of 300′ 1/2″ PEX tubing. Total heat requirement for the floor is 30,000 BTU. The manifold will be 30′ from the boiler and the pumps.


Step 1

Lets look at the Plastic Pipe Pressure Drop Calculator page and enter the relevant values for the 1/2″ tubing:

  • PEX
  • CTS
  • SDR 9
  • 1/2″
  • 0.6 GPM
  • 300′
  • 100% Water
  • 100°F


Flow: Regime Turbulent
Pressure Drop: 2.2 Psi, 14.9 kPa
Head Loss: 5.0 ft water
Velocity* 1.1 ft/s, 0.3 m/s

head loss 300 foot 0.5 PEX

Looking at the  GPM and BTU Capacity of M Copper and PEX Tubing page we can see that we can deliver up to 34,500 BTU/h in a 3/4″ PEX tubing. Now we need to find out the head loss for the supply and return tubing to the manifold that is 30′ away. 30′ supply plus 30′ return is 60′ total length. To find out the GPM we use this formula:

GPM = BTU / (ΔT x 500) which gives us GPM = 30000 / (15 x 500) = 30000 / 7500 = 4

At the Plastic Pipe Pressure Drop Calculator page and enter the relevant values for the 3/4″ tubing:

  • PEX
  • CTS
  • SDR 9
  • 3/4″
  • 4 GPM
  • 60′
  • 100% Water
  • 100°F


Flow: Regime Turbulent
Pressure Drop: 2.3 Psi, 15.7 kPa
Head Loss: 5.3 ft water
Velocity* 3.6 ft/s, 01.1 m/s

head loss 60 foot 0.75 PEX

Step 2

We now have the values we can use to select a circulator:

Total head loss =  5.0 ft + 5.3 ft = 10.3 ft 

Total GPM = 4

Looking at all the Circulators, we find that on the Rhella T15-17-19 FC circulator’s performance graph, if we mark 4 GPM and 10.3 ft head on their  respective axis, they cross at the point I marked with a red dot. That point is above the #1 but under the #2 and #3 curves. This means the pump will be able to satisfy the demand in #2 speed setting.

T15-17-19FC 4 GPM 10.3 foot

This is all there is to it. A few important rules and codes to remember when sizing circulators.

  1. Do not exceed the maximum developed lengths as per the table below:
    Nominal SizeMaximum Length
    3/8" Floor Heating200'
    1/2" Floor Heating300'
    1/2" Snow Melt150'
    5/8" Snow Melt250'
    3/4" Snow Melt300'
    1" Snow Melt500'
  2. Try to keep the total head loss low by selecting larger diameter supply and return tubing and/or shorter circuit loops. The higher the head loss the more expensive the  circulator that can satisfy the load is.
  3. Equalize the lengths of all heating loops if possible.
  4. Always use the longest circuit loop in the manifold to calculate the head loss