Pipe size impact on GPM and water pressure

mvoltin

Silver Supporter
Aug 1, 2016
58
atlanta, GA
What is the impact of pipe size on the water pressure: even with normal GPM, can larger pipes drop water pressure significantly? and is it possible to measure?

More specifically: I have small 7,500 gallon pool with very small head loss and 2 inch piping. Also, my equipment is about 3' above the water line and, even 2000 rpm (~60gpm) i get very low (virtually zero) pressure on my filter. I have to really crank up the pump to raise pressure on the filter to 5-10.

Just added gas heater (Raypak 156a) and the pressure sensor doesn't register the water flowing through. I have flow sensor on the output of the gas heater to ensure there is a good flow and tried anything between 20 to 60GPM to no avail.

Question: Is it possible that, because of the high location of the equipment (3' above water line) and larger piping, I am achieving good GPM but pressure stays very low for the heater to register? I would think the flow sensor (flowvis) needs a decent pressure for the flap to lift and register flow ; so there must be pressure behind it?

If this is true and, even with the adequate flow, there is little pressure for the heater to register, is there a remedy to increase pressure artificially (short of closing some of the inlets on the return side?)

P.S. once i manually trigger the pressure sensor (electric circuit) then the heater is working, so the problem is the actual physical pressure sensor that is supposed to pop down and push on the electric switch.
 

CrystalRiver

Well-known member
Jun 19, 2020
333
Massachusetts
Pressure and flow are related but not to the extent that you are thinking. Yes, the same flow (gallons per minute) through a larger pipe will register a lower flow, but the size of the "pipe" at the flow sensor is fixed by the apparatus.

In short, it doesn't much matter what pressure you have as long as you have sufficient flow. Closing returns downstream will increase the back pressure on the system, but it will also decrease the total flow.

Do you have any diverter valves upstream of the heater that could be adjusted to send more water to the heater?
 

JamesW

TFP Expert
Mar 2, 2011
21,506
Question: Is it possible that, because of the high location of the equipment (3' above water line) and larger piping, I am achieving good GPM but pressure stays very low for the heater to register?
Yes, that is correct. You should install a flow sensor in the plumbing and remove the wires from the pressure sensor and connect them to the flow sensor.

 
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mvoltin

Silver Supporter
Aug 1, 2016
58
atlanta, GA
Pressure and flow are related but not to the extent that you are thinking. Yes, the same flow (gallons per minute) through a larger pipe will register a lower flow, but the size of the "pipe" at the flow sensor is fixed by the apparatus.

In short, it doesn't much matter what pressure you have as long as you have sufficient flow. Closing returns downstream will increase the back pressure on the system, but it will also decrease the total flow.

Do you have any diverter valves upstream of the heater that could be adjusted to send more water to the heater?
There is an internal diverter (that should be good up to 70GPM) and I also have separate external diverter. The flow meter I was referencing is right outside the heater- so, it only measures the flow through heater. I have a separate flow meter for the entire circulation but the 20-60GPM I mentioned was just based on flow through the heater.

I am starting to think that the pressure sensor on the heater is defective...
 

mvoltin

Silver Supporter
Aug 1, 2016
58
atlanta, GA
You should install a flow sensor in the plumbing and remove the wires from the pressure sensor and connect them to the flow sensor.

I have SWCG that has a flow sensor and could use it instead for both (SWCG and heater). Thanks
 

JamesW

TFP Expert
Mar 2, 2011
21,506
You can't use the same flow switch for two different pieces of equipment. You need a separate switch with the right flow activation.

Flow Switch – Model Q-12. 4S Paddle. 2” T with a 1/2” threaded hole. On 23 gpm. Off 18 gpm.


Email Harwil for the exact model you should use.

It will look like the below picture. The below model is similar but it opens at 10 to 12 gpm, which is too low.


 

JamesW

TFP Expert
Mar 2, 2011
21,506
The pressure switch was fine decades ago when all pumps were 1.5 hp single speed pumps and all plumbing was 1.5 inches. This created plenty of back-pressure.

Today, with variable speed pumps, larger plumbing and more complicated systems, using a pressure switch is no longer an acceptable choice.

All heaters need to come with a flow switch that activates at the correct flow rate.

In addition, in my opinion, all heaters should come with a bypass valve and a valve actuator driven from the heater.

I would also remove the thermal regulator in new heater designs because it is excessively restrictive. It is unnecessary if you install an automatic bypass that only opens on call for heat. The automatic valve is the thermal bypass.

Go to a single pass heat exchanger instead of a multiple pass exchanger. Multiple passes might seem like a good idea, but it’s not.
 
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