Making the case for proper water flow

While this is mainly for the pros, I thought it would also help some homeowners so I am posting it here.

A client called today asking about water around his Geo. I stopped out to check it out and found ice on exposed loop line surfaces, along with water dripping from the humidifier because of a clogged drain. I fixed the "water around his Geo" problem but was curious about the ice so I ran a Heat of Extraction...

GeoComfort E036 (With a scroll compressor kit retrofit)
Three 150' vertical closed loop system.
Pressure in 43 PSI out 36 PSI Delta P=7 PSI
EWT 33.2* LWT 27.5* Delta T=5.7*
7 PSI @ 30* EWT = 11GPM
11 GPM X 5.7* X 485(brine)=30,409 Btu/Hr
Heat of Extraction rated at 18,400 Btu/Hr @ 30* EWT

As you can see we were pulling TOO much heat from the ground. I backed the flow off to 9 GPM and ran the numbers again...

Pressure in 41 PSI out 36 PSI Delta P=5 PSI
EWT 32.5* LWT 27.7* Delta T=4.8*
5 PSI @ 30* EWT = 9 GPM
9 GPM X 4.8* X 485 (brine)=20,952 Btu/Hr...much closer to published ratings.

An hour after the adjustments the loop field temperature has come up two degrees. By having just 2 GPM more flow, we were over drawing the loop field design and were driving down the temps.

 

We supply throttling valves and pump options to all of our clients for the sole purpose of "commisioning the system correctly". Sadly it is seldom done unless they suspect a problem and I wind up back there because "it is allways the well guys fault".
This is the poster child for it is either tuned correctly or not exactly.
Eric

 

What I have noticed is that greater GPM allows brine less time in contact with surrounding soil- artificially reducing EWT.
We noticed this on the first 3 speed flow center we were shipped in error. I was alarmed at low EWT until I changed speed.
I'm not clear on 2 things here.
1) Since we know a 3 ton can't pull more than 3 tons out of the ground how could you pull too much from the loop field. In fact wouldn't the numbers you described be great (2.5 tons of heat from 30* EWT)?
2) Why would your Delta T fall with reduced GPM if all other things are recorded correctly?

What I have observed in a recent situation on a 3 ton vertical is that loop temps can swing significantly with a change in speed. My heat of extraction did not due to change in Delta T that coincided with change in Delta P. This was after a thermometer failed reading about 10* low and had me freaked. Hindsight (never there when you are trying to find a problem) may have suggested to me the unit would probably lock out if the temps were as low as displayed.
Test your test gear.
Can DT fall with reduction in DP?
Many here are more scientific than I (Ijust fix things) but throttling never made sense to me. If we take coil erosion and pump cost out of the equation, can there be diminished returns on more GPM from a conventional flow center? Can a 3 ton heat pump extract too much heat from a 3 ton loop field?
You're all smarter than me so I yield the floor.
J

 

I'm with Joe on this one. I can see how the EWT might drop, but not how Heat of Extraction would rise.


If I came to this forum asking if I could increase the output of my heat pump up to 50% by increasing the flow through the loop, the answer would be a resounding NO.

Interested in what others have to say.

 

Thank you Bergy. I like seeing a spelled out example of this calculation for reference.

 

I use a Fluke 16 multimeter with their probe thermometer. It is very accurate. For the pressure gauge I use a simple 0-100PSI gauge with 1 PSI increments. I double checked the numbers when I performed the HE because I thought it odd that the Delta T would go down as the Delta P went down. Maybe I'm missing something here??

Our company did not design, or install, the loop. The home owner knew the owner of a looper we do not use and contracted him. However, I do believe the loop was designed for a minimum 40* EWT.

Bergy

 

I was using the same Fluke arrangement.
Expensive not necessarily accurate.
j

 

Can the temperature be calibrated by the user? In my recent meter research I see that some brands suggest an ice water test and a screw to turn for adjusting the temperature in their manuals.

 

Adjustment not available on mine (that I know of). What's worse is the failure appeared to be in a curve meaning when I compared it to another thermometer at room temp it was close but EWT and LAT were off quite a bit.
I made the mistake of thinking my high end equipment was more accurate than inexpensive testers.
Joe

 

Here is some info on calibrating a Fluke 16:

http://assets.fluke.com/manuals/16______cieng0100.pdf

 

I don't buy the idea that one can overdraw a loop field. If the house needs the heat, configure the system to get all it can from the field. Artificially reducing extracted heat via throttling valves will shift load to COP 1 strips without reducing loop pump power. Why do that?

Is the heat of extraction at 30 EWT an upper limit for the machine? What is the heat of extraction with 70 EWT? (typical on the street corner where I hang out and do business)

I would only reduce extracted heat if it was causing the compressor to over amp and I couldn't fix the over amp by increasing airflow. I've had to wrestle that here, but I doubt that's an issue up north.

I would reduce waterflow only if I suspected that it was so high so as to endanger the heat exchangers by erosion. (Been there too)

Their is an enduring myth that too fast waterflows reduce heat of extraction since the water isn't in contact with the loop pipe for long enough. It is a myth. Higher flows always increase heat transfer, though the effect decreases with increasing velocity.

 

I don't see overdrawing a- properly sized- loop field. A 3 ton unit for instance can draw about 36,000 btu's max right? So if it's a 3 ton field.......

I know that water flowing too quickly can lower the EWT as I found with the first 3 speed flow center we had. I didn't initially concern myself with questions of heat extraction, just "why is my EWT lower than anticipated?" Once a lower speed was selected EWT rose and of course pumping power consumption was lower. BTU's extracted were about the same.

 

I've been wondering about this post as well. If you decrease your flow, your Reynolds number will decrease. So then this leads into the whole discussion about how important is the Reynolds number. (I wish I knew ) The research I've done says to keep the Reynolds number above 2500. What happens if it's only 2000? Could slowing down the flow only temporary raise the EWT and over the long haul you could actually lower the average EWT due to the decrease in heat transfer efficiently due to a lower Reynolds number?

 

Fair question.
Short answer is Reynolds number matters less as loopfields grow. Laminar flow can get the job done if you have enough pump and loop.
J

 

I can't figure out the drop in EWT owing to higher flow, unless LWT also dropped as HE rose.

 

Regardless of gpm, I'd expect both EWT and LWT to drop
whenever HE increases.

To a first approximation, average loop temp = ( EWT + LWT ) / 2

More HE requires more BTUs flowing from soil to loop, and
that requires (creates) a larger average soil-to-loop delta-T.

I also agree with Curt that "overdrawing" the loop should
not be a concern. Grab as many cheap earth-source BTUs
as possible -- COPs typically remain very attractive right
down to the HP's low LWT cutoff.

 

I wish my snuffled Ohio

head could get into this thread. Just got back from "finishing" our instalation for the Gayson's Christmas eve, eve installation. I could have left Randy my Cooper SH66, but what if I needed it. He is going to get the tropical fish thermometers soon.

We are running a pair of 26-99 240VAC grundfos pumps to his loops. He has never seen LATs in the high 90s before. We added a tight return air duct work modification and swapped out a big orange Honeywell for a White-Rodgers 1F95-1275, then ran out of day.

I was not happy with the start up Delta T on a loop system idled for four hours. With a two hour plus drive home we went west to home. As it turns out the nearby state park was open and I could have camped if we brought the motor home. So I could have had today to play with the loops.

I was very pleased with the delta P shown by the "cheap" PSIG gauges we installed. Using the ClimateMaster I/O manual we found that we had great flow at 3 gpm per ton for 5 tons or 15gpm total as shown by the delta p to the manufacturer's specs. I know this is correct for R-22, but the flow could be less using R-410A depending on the heat exchanger size

In hind sight I would have idled one of the two 26-99's and watched for a few hours. I could have learned a lot by doing that. We have no idea what is in the ground as far as loops, manifolds, reverse returns, size of pipes, or what ever. We do know that the system does not leak. I left Randy my hygrometer to check the specific gravity of the loop fluid when he gets a chance. Randy is a very handy guy, an electrical engineer by training and I may offer him a job or go to work for him when I retire next year.

My issue with the heat of extraction method is I do not believe in renolds numbers. So I guess I am of the "show me" State's mentality here. Using an ECM drive pump or pumps I believe it is possible to "fine tune" a loop field to its machine for the max COP including watts to water.

 

My most recent system involved 2 WF 026 units sharing a single loopfield. Flow center has two pumps. Well contractor supposedly wired flow center and compressor units so that if either compressor came on it would fire both pumps. Didn't work, don't know why, Well guys gonzo.

I wired each unit to run one of the pumps. I've read that a non-powered pump in a push pull flow center imposes considerable restriction. Perhaps it does...but each unit runs very well using just "its own" pump. Bags of heat, 5-6 Deg F delta T on water side, flow 3-4 GPM / ton. Indeed I had to bump up CFM per ton to get heat off the coils...LAT was exceeding 110 and compressor amps bumping up against FLA while in stage 2.

Running both pumps increases delta-P across the unit heat exchangers to where it is literally off the charts. That leads me to wonder about potentially damaging heat exchangers by erosion from too high water velocity.

I readily grant that crowing about heating performance in Florida comes across as small beer, and I plan to revisit the system in July. Meanwhile it is saving ~250 Watts whenever either unit runs.

 

Not sure what you mean by "heat of extraction method", but
the HE equation (i.e., HE = gpm x delta-T x brine_constant)
has nothing to do with Reynolds numbers and "doesn't care"
whether the flow is laminar, transitional, or turbulent.

.

 

I think you two are saying essentially the same thing.