During our technical sales meet with various clients, we often discuss what should be the optimum flow rate and pipe size required for a heat pump system. Many people in the industries have concerns with how efficiently could they maintain the flow rate requirement for the heat pump system with economic pipe size.

Knowing that many of you may also have had a similar question so we decided to give you a set of things to consider before installing one such system.

In piping design, we often discuss how a pipe material performs with respect to corrosion and temperature resistance when interfacing with different fluids is a significant consideration during system design. Nearly equal in importance is how the fluid is moving through the pipe. Flow rate plays a significant role in determining the longevity of a system, as well as its day-to-day energy consumption.

Understanding the efficiency with which a fluid can pass through a piping material is a significant step toward ensuring the long-term reliability and cost savings potential of certain materials. Coming up next are the factors, which influence the flow rate and pipe size in the piping system:

pipe sizing

Pipe Sizing and Pressure Drop:

One regular factor in flow rate design is the calculation of pipe size. To diminish capital cost, some engineers may decrease the pipe size and increase the velocity.

  • Adversely, the higher velocities can decrease the life of metallic pipes because of erosion and possible surge pressures.
  • To optimize the flow rate through a network of pipes, we should look out to minimize how much pressure or head is lost throughout the system.
  • The pipe sizing should always be done on the notion of balancing the cost with efficiency and reliability.
  • The general rule of thumb for the hot water system is to maintain velocity in the range of 1 – 3 m/s.


Proper piping material selection and system design control the flow rate and velocity in a way that minimizes energy costs.

The purpose behind this is as faster fluid translates to greater friction loss, increasing stress on pumps. More energy will be consumed and larger pumps may need to be installed.

In addition, certain materials, such as metals, will corrode more quickly at high speeds. Corroded materials are less smooth and thus create more friction loss.

By selecting an economic pipe diameter for the designed flow rate, the system will have maximum efficiency. As one increases the pipe diameter, the cost of the pipe increases but the pressure drop decreases, so that less power is required to pump the liquid. The net result is that there is a minimum cost as manifested in the net present value. The diameter corresponding to this minimum cost is known as the economic pipe diameter.

Reliability and Service Life

Optimizing the pipe size and velocity of the fluid not only improves the system efficiency but can also increase the service life. Some of the issues faced during increased velocity and undersized piping:

  • High velocity in pipelines promotes corrosion/pitting; piping may be repaired or replaced sooner than expected.
  • Abrasives and Erosion can cause premature failure
  • Surge pressures due to water hammer cause more damages at higher speed.

Upfront Cost vs Service Cost

The main constituents that determine piping cost are piping material, pipe size, pipe running meter, and installation. If the piping system is undersized, the capital cost will be less expensive but in long run, the service cost and energy cost associated with the system will be very high. If the piping system is oversized, the system cost will be expensive to purchase and install.

This is why the relationship between fluid flow, operating costs and upfront cost should be a balancing act.

One should design the flow rate and pipe size considering the above-mentioned points before installation. In general, we try to design our piping system that balance cost with efficient and reliable. In other words, we design a specific system that will

  • fit a budget
  • not inflate energy consumption
  • Not require excessive repairs and maintenance in the future.


To know sustainable heat pump solutions and cost benefits, check out our products to know more.