Pressure Difference and Pump Flow

This article is written by Michael Housh, owner of Housh Home Energy and a regular contributor to HVAC School. Thanks Michael!

This is another article in trying to relate the air-side of HVAC with the water-side.  In this article I’m going to talk about how to determine pump flow (GPM) based on the pressure difference across the pump.


I relate checking pump flow to checking air flow with static pressure on the air-side.  Without knowing how many gallons per minute we’re moving the less we can truly tell about what our system is doing.  Most of the equations that revolve around hydronic systems (or air-conditioning systems) revolve around how many pounds of a substance we’re moving through the system.


The trouble with many of the systems out there is that most installers/designers don’t add anyway for us to check this in the field.  I’ve primarily only seen any sort of pressure port or gauge on larger commercial/industrial systems, or sometimes older pump flanges that have a pressure port built-in.  Most people today are installing isolation flanges on their pumps, so I strongly encourage everyone to spend the few extra dollars on isolation flanges with a drain port built-in, this can allow a technician or installer access to checking the pressure difference across the pump.


Measuring pump flow (just like air flow) requires looking at manufacturer’s charts on the pump you are working on.  But instead of checking static pressure across an appliance we use the pressure difference across the pump. In the U.S. the amount of energy produced by a pump is measured in ft. of head, however all of the gauges that are used measure in PSI.  Luckily there is a formula to convert PSI -> Ft. of Head.


H = PSI / (0.433 * SG)




H = feet of head

PSI = pounds per square inch

0.433 = constant

SG = specific gravity (for rule of thumb we can use 1.02 for water)


The constant (divisor) can be simplified by multiplying 0.433 * 1.02 = 0.44:


H = PSI / 0.44


At this point I must give a WARNING about some of the pictures I’m going to show, as they will surely be controversial, but until my mind is changed on this subject Testo will have to remove H2O from their measurable media type.

Before I move on, let me lay some groundwork.  My latest system had a design load of @ 80K BTU.  This system utilized two zones each having a pump.  By using the “rule of thumb” sensible heat rate equation for hydronics, I can layout what my total system GPM should be to deliver the load.  Most hydronic systems are designed around a 20° deltaT on the distribution side, which is what I’ll use in the below equation (for more on the sensible heat rate equations you can read this tech-tip I wrote).


80,000 / (500 * 20) = 8 GPM


So this means my total system needs to move 8 gallons per minute in order to meet demand.  In my scenario I actually have 2 zone pumps (one does a bonus room which needs about 2-3 GPM, and the other the main house needing about 5 GPM).  I utilized the same 3-speed pump on both zones, and I will cover how I set the speed based on the above GPM that I need to deliver. Below is the pump curve (similar to a fan chart) that we need to compare to, each line representing one of the speeds of the pump.

I take the pressure at the inlet and the outlet of the pump to get my deltaP, using the drain port on the isolation flanges.    

DeltaP = 3.5 PSI (as you can see this is very hard to take from an analog style gauge).  I can then convert my deltaP to ft. of head using the formula above.


3.5 / 0.44 = 7.95 ft. of Head


We can then compare to the above pump curve (for low speed, lowest curve), which shows a flow rate of about 2.8-3 GPM, which is where I want to be for that zone.  I can then do the same for the other zone pump (set on medium speed).


This gives me a deltaP of 4.8 PSI and converted ft. of head of 10.9.  When comparing my deltaP to the medium speed pump curve my flow rate is about 5 GPM, giving me a total system flow rate of my desired 8 GPM.


To take it a step further, I then compare my system loop deltaT, which in this case actually worked out perfectly to 20°, so I won’t go through the formula again, but this proves that we’re delivering the desired 80K BTU to the space.

— Michael


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