I'm hoping the experts here on closed water loops for GSHPs can answer a question I have:

Is it possible to have TOO much water (GPM) flowing through a GSHP's coaxial heat exchanger?  Too much defined as: possibly damaging to the heat exchanger and/or lowering its life expectancy.

The reason I ask is that my flow is 'off the chart.'  What I don't know is if an 'off the chart' flow rate necessarily says that the high flow is not recommended and/or damaging to my exchanger.

Here are some details.

WaterFurnace Envision NDV064 model, running part load (chart stops at 8.5 DeltaP psi):
EWP = 64.0 psi
LWP = 54.4 psi
DeltaP = 9.6 psi
EWT = 69.5°
Water Flow = 21.4 gpm

I got this water flow by first building a log/log chart that matched up to the WF supplied 'Pressure Drop' chart, and then extending the chart out to about 10 psi pressure drop.

Here's another example, this time for my WF Envision NDV038, again running part load (chart stops at 3.6 DeltaP psi):
EWP = 65.8 psi
LWP = 72.1 psi
DeltaP = 6.3 psi
EWT = 77°
Water Flow = 13.5 gpm

I'm pretty confident of my pressure readings because:

(1) I have a 3.5" highly stabilized and accurate gage

(2) The heat exchanger's pressure drop 'fits' nicely into the overall loop pressure drops - i.e., for the 3 ton system:

One Grundfos UP26-116 pump, @ 13.5 gpm, produces 27 feet (from pump chart)
27 feet / 2.31 feet/psi = 11.7 psi (I don't have antifreeze)

28' of 1" pipe @ 13.5 gpm = 0.89 psi (1.11" ID, C=150)
152' of 2" pipe @ 13.5 gpm = 0.34 psi (1.92" ID, C=150)
1200' of 1" pipe @ 3.375 gpm = 2.92 psi
coax heat exchanger @ 13.5 gpm = 6.3 psi (from above)
other friction losses @ 13.5 gpm = 1.2 psi (i.e., fittings, check valves)
==============================
Total friction loss @ 13.5 gpm = 11.7 psi

The 11.7 psi numbers match.


Thank you very much!

Best regards,

Bill

Good question

How much power does the pump use?

Each pump consumes 385 watts - 1/16th hp. Grundfos UP26-116F.

One pump for 3 ton unit. 2 pumps for 5 ton unit, in push-pull configuration.

Data in earlier posting above is for each GSHP unit running alone (separately), into common closed loop.

Best regards,

Bill

Your system does appear to be overflowing. Normally this is not an issue. Are your pump packs set up with Grundfos 26-116 pumps? It sounds like they are. Typically the more flow the better, and you will see better COP's and EER's with additional flow, however, we did replace a heat pump that was overflowing 2 year ago. The unit was 17 years old and had a shell and tube design, but the waterfurnace is probably coaxial, probably not as big of an issue. Your loop field pressure drop must have been extremely low. It would be good to hear others, but I don't think you should have an issue. If you do, let me know and I would recommend a Flow Center 3 speed B&G pump pack. This is what we are using on all installations and can dial the GPM in +/- .5 gpm for our systems.

Dan
Colorado Geothermal Drilling

Dan, many thanks for the comments - very helpful.

Yes, my pumps are Grundfos 26-116.  The pumps are 'ala cart' - see image below (5 ton unit example - 3 ton unit only has one pump).

Yes, my WaterFurnace heat exchangers are coaxial.  Didn't realize this was a factor, and a positive one at that.

My loop field presents 2.9 psi friction loss at 13.5 gpm flow rate, assuming the 1" pipe is divided into 4 parallel circuits.

Connecting pipe between the GSHP units and the loop field manifold is 2" (0.34 psi).  Add in a little 1" pipe to actually connect to the GSHPs (0.89 psi), and some friction loss due to fittings and one-way valves (1.2 psi).  Total here is 2.4 psi friction loss.

With the loop field at 2.9 psi, the remaining piping at 2.4 psi, and the coaxial heat exchanger at 6.3 psi, total adds up to 12 psi friction loss (at 13.5 gpm).

I thought about closing my ball joint isolation valves (see below) to reduce the flow, which would accomplish the same as your ability to adjust flow rate in +/- 0.5 gpm increments.  But, I have concern that this type of valve would be difficult to incrementally adjust.  I believe to reduce the flow via partially closing valves I need a different type of valve.  I sure would like to someday adjust (reduce) my flow to at least have it 'on the chart.'

Many thanks!

Best regards,

Bill

I wouldn't use a ball valve to modulate flow.

I would suggest getting an answer from WF or a knowledgeable dealer about any risk from being 'off the chart'

engineer, thanks. I've added to my to-do-list-someday replacement of the ball valves with something more suitable for modulating flow.

Best regards,

Bill

I would agree with engineer on this one, we work with alot of different manufacturers, but I'd ask Waterfurnace directly and see what they have to say. I would not recommend balancing with the ball valves either, the pumps are still going to use the same energy, and the system will reduce some in efficency. As far as the Flow Center Pumps, they actually have 3 speeds on each pump, so you get 6 different pump curves to match your system. Hope this helps.
Dan

Given the fairly high power consumption of the pumps (385 Watts) I'd say it would be a worthwhile system optimization to more closely align load with pumping rate via multispeed pumps. If a unit is running in low stage, run pump slowly, etc. Better to do that than throttle the pumps.

I don't THINK being a few GPM off the chart is bad for heat exchangers, particularly coaxial types found in WF, but I'm not sure. There is the possibility that high water velocities could cause premature wear and failure, and such a failure would be messy and expensive to fix so I'd want to be sure.

Thanks. I'll check w/ WaterFurnace on my higher flow situation.

I agree it would be better to change out my pumps than to try to modulate them. 385 watts x 2 is what it's costing me for my 5 ton unit, and one 385 watt pump for my 3 ton.

Best regards,

Bill