Pump sizing and Flow

J Matthers_2

I have a Unico hydrocoil for AC in my attic (pressure drop 12.8 ft water) connected to my geosystem buffer tank 24’ below in the basement with 50’ of ¾ ‘ kitec (head loss – 3.2). The system is closed with a system pressure of 30 psi. The water is being turned by a Grundfos UP15-42F. For optimal performance I need 8GPM flow at the coil. The performance curve for the pump shows 8 GPM at 9ft of head in a closed system.

On the surface it looks like I have 40’ of head with a pump good for 9 feet @ 8 GPM.

I am getting circulation with a Delta T of about 20* on the water. I don’t know what the Delta T is across the coil for the air.

Heat gain on a 90* day is 34,000 BTUH, coil performance is rated at 33,500 BTUH @8GPM and 40* supply water. Buffer tank water is at 38* and I have 40-42* water at the coil. Airflow is correct yet the system runs 24/7 at an outside temp of 80* to get indoor temp to 76*. House is a SIP constructed house so the insulation is very good. Ducting is in conditioned space and well insulated.

All this leads me to believe there is inadequate flow through the coil and the pump is the culprit. FYI, the pump was never specifically sized for the application, it was attached to the buffer tank that came with the geosystem.

A few questions on pump sizing:

1. In a closed system, what head is counted for pump sizing? Just the pipe, valves and stuff the water flows through or do you include elevation as well? If so, my pump is way small.
2. In a closed system, if the only pressure change the pump sees is on each side of the pump, does it matter how much head you have?

This flow stuff is a little slippery for me.

Alan(CaliforniaRadiant)Forbes

Pump Sizing

To answer your questions:

1) Elevation changes do not figure in to pump sizing in a closed system since the rise will equal the fall.

2) Yes, it matters how much head you have because the pump "sees" that head on the discharge side.

If you've done all the calculations and you're not getting enough flow, check to make sure you've purged all the air out of the system.

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J Matthers_2

Thanks, The combined loss over the coil and the loss through 50' of pipe excluding the fitting adds up to 16 ft plus. That is 7 ft over the pumps performance curve of 8GPM @ 9 ft. Looks like I'm not getting the flow I need.

This stuff is starting to sink in! Thanks for the help.