decafdrinker
Basic Member
Posts:420
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| 03 Nov 2010 10:54 AM |
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First, let me say that I'm more just curious...I'm not about to start questioning my installer or fiddling with the balancing manifold (yet).
Here's the info I've read so far:
Propylene Glycol is LESS efficient than methanol as an antifreeze (less efficient at transferring heat).
Most manufacturers recommend between 1.5 and 4 gpm per ton, which is quite a range.
Most websites indicate a delta of about 10-12 degrees F in the ground loop is ideal.
You can probably see where this is going. I have 4 300-foot-deep ground loops, so 600 feet per loop x 4. The balancing manifold is inside and easily accessible/ajustable.
I have glycol in the system, too.
If I *don't* have a delta of, say, 10*F, is it a good idea to decrease the gpm slightly, so that the water spends more time underground and thus picks up more heat? Or would that cause the ground immediately around the loop to cool quicker than it can recover?
Is there any way to accurately determine the most efficient gpm setting for a situation? I know every house might be different, but at some point, the electricity consumed and the gpm/heat transfer must intersect on a graph.
On a side note, since I can measure the temperature of each loop (well, measure the temp of each pipe as it comes into the house using an infrared non-contact thermometer), would it be better to try to get each individual loop have roughly the same delta, or better to have each loop with the same gpm flow?
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docjenser
Veteran Member
Posts:1400
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| 03 Nov 2010 02:09 PM |
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PG not as good for heat transfer, and is more viscous as well, requiring more pumping power. You also need higher concentration as you would need for methanol for the same freeze protection.
I can only speak for CM, WF and Hydron, but they all specify design GPM on the source side to be between 2.5-3.0 gpm for closed systems, and 1.5-2.0 gpm for open systems (per ton). Open can be higher, since you do not re-circle the water. Temperature drop (closed loop) in heating mode should be between 5-10 degrees, in cooling mode the raise should 10-15 degrees (second stage, less in first stage. Higher in cooling mode, since now it also has to reject the compressor heat as well.
I do not see any reason for reducing GPM manually, all the manufactures data shows that the heat extraction (heating) and the efficiency increase with the increased source flow. Plus you might get below desirable Reynolds numbers for turbulent flow, and your LWT gets down to low. Keep the flow as high as you can, until variable speed pumps arrive, that is. With constant speed pumps, we shoot for the smallest circulation pump(s) which still give us between 2.5 and 3 gpm of flow, and keep the Reynold numbers in each loop above 2500. Sometimes we have too long of a header, or larger loopfield, so we need to increase pumping power.
You are correct, there is a balance point, but assuming you have a constant speed circulation pump, the rule is to choose the smallest pump which gives you that flow. If you have too much flow, you are "wasting" pumping power, but limiting flow is not gonna help you to run the rest of the system more efficient.
Loops are never totally balanced, I would argue that flow is usually indirectly proportional to the delta T. So by balancing the temperatures, you balance the flow. My loops differ by about a degree, I would not bother unless the too much delta between the loops. |
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| www.buffalogeothermalheating.com |
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Palace Geothermal
Veteran Member
Posts:1609
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| 03 Nov 2010 02:17 PM |
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Just to clarify, the 3 gpm per ton is for the heat pump, not each ground loop. |
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Dewayne Dean
<br>www.PalaceGeothermal.com<br>Why settle for 90% when you can have 400%<br>We heat and cool with dirt!<br>visit- http://welserver.com/WEL0114/- to see my system |
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docjenser
Veteran Member
Posts:1400
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| 03 Nov 2010 04:31 PM |
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Yep, 3 gpm/ton is for the heatpump. Many loops are designed as one loop per ton, especially in horizontal applications, since with 3/4 inch pipe you almost max out at 800 feet individual loop length, however, we do mostly 3 tons per loop in vertical application, which would mean 9 gpm/loop. Vertically, that is with 1.25 inch pipe.
That is when pressure drop calculations come in. Flowcenters are relative standard, one Grundfos 26-99, next step is one 26-116, then 2 26-99 in series, next are 26-116 in series. Anything more than that I usually need bigger flowcenters anyway, and usually it is worth to calculate the power consumption of the circulation pumps, and go with something like a Wilo Stratos, which consumes considerably less energy. Usually anything larger than 6 tons. The point is, don't restrict flow because you think you can increase efficiency.
But Dewayne, since the ground loop is matching the capacity of the heatpump, 3 gpm of heatpump capacity for heatpumps (source) also means 3 gpm /ton capacity for closed loopfields.Yes, we design our loopfields for 2.5 to 3 gpm per ton loopfield capacity. Flow through the source side of the HP is he same as through the loopfield. |
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Looby
Basic Member
Posts:401
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| 04 Nov 2010 12:08 PM |
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Posted By stuart.wyss on 03 Nov 2010 10:54 AM
... is it a good idea to decrease the gpm slightly, so that the water
spends more time underground and thus picks up more heat?
More time underground would allow the water will pick up
more heat per gallon -- but less heat per minute.
The delta-T between the loop and surrounding rock/soil is
the force that motivates BTUs to migrate toward the pipe...
...delta-T is (sometimes) your friend,
Looby |
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| One measurement is worth a thousand expert opinions. |
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geome
Advanced Member
Posts:987
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| 04 Nov 2010 12:29 PM |
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How would someone that is motivated (not me surprisingly) know when they reached the optimum setup for their system? If the answer is the best heat of extraction/rejection figures, wouldn't this be a moving target? I'm happy with these figures within +/- 10% of spec. |
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| Homeowner with WF Envision NDV038 (packaged) & NDZ026 (split), one 3000' 4 pipe closed horizontal ground loop, Prestige thermostats, desuperheaters, 85 gal. Marathon. |
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docjenser
Veteran Member
Posts:1400
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| 04 Nov 2010 05:42 PM |
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I guess the current specifications are exactly what you are asking for, Genome.
The 3 gpm/ton for the heatpump and the loopfield is the common figure where you have a Reynolds number above 2500, which is close to the the minimum to achieve enough turbulent flow in a 0.75 PE pipe for optimal heat exchange. More flow is not worse but slightly better, but then you paying a price with the circulation pump consumption. This is assuming you use one 0.75 inch PE loop per ton.
With open systems you do not need heat exchange through the pipes, so the reynolds number does not matter, so you can afford to slow it down and allow the heatpump to take more heat. But again, there is balance point, namely the amount of water pumped up (not circulated) from the well, the lesser the better here. If it gets down to more than 15 degrees delta, you pay too much of a penalty for efficiency since the source water gets too cold the further down the water gets inside the heat exchanger.
The answer is that you can increase the heat extraction slightly by increasing the flow, but the costs for the pumping power are increasing much quicker, not worth it. |
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joe.ami
Veteran Member
Posts:4377
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| 09 Nov 2010 08:06 AM |
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This is a "more than one way to skin the ......" thing.
Manufacturers have standard operating parameters, as the industry has certain standards, but many other things work fine as well.
Laminar flow for instance if you have enough field/flow/delta T. As well as more or less GPM with enough field/delta T......
It is an area where some efficiency can be gained (or very little energy conserved), but if a heat pump is working, I suggest folks don't tinker with loop field balance or flow.
Joe |
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Joe Hardin
www.amicontracting.com
We Dig Comfort!
www.doityourselfgeothermal.com
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engineer
Veteran Member
Posts:2749
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| 09 Nov 2010 11:11 PM |
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Geo systems work fine with a broad range of GPM per nominal ton. You could theoretically determine the absolute lowest cost combination of GPM and CFM per ton for your particular system, loop conditions, thermostat setpoint, sunspot activity, etc, but the cost of the data acquisition suite of instruments to reliably make those determinations to a high level of accuracy far exceeds the potential savings. |
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Curt Kinder <br><br>
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joe.ami
Veteran Member
Posts:4377
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| 11 Nov 2010 08:12 AM |
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Posted By engineer on 09 Nov 2010 11:11 PM
Geo systems work fine with a broad range of GPM per nominal ton. You could theoretically determine the absolute lowest cost combination of GPM and CFM per ton for your particular system, loop conditions, thermostat setpoint, sunspot activity, etc, but the cost of the data acquisition suite of instruments to reliably make those determinations to a high level of accuracy far exceeds the potential savings. As an added bonus the "sweet spot" will be a curve not a "spot". Different demands/conditions (ie changing weather) will create different ideal settings. j |
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Joe Hardin
www.amicontracting.com
We Dig Comfort!
www.doityourselfgeothermal.com
Dig Your Own Comfort! |
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decafdrinker
Basic Member
Posts:420
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| 11 Nov 2010 08:25 AM |
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Thanks everyone. I was curious because I had a new manifold installed the other day for the loops, and all the gpms are indicating 4+ gpm per loop. I didn't want to tamper with it, but at the same time, "3" always seemed to be the number bandied about. I guess what's most important is the delta. If you have a good delta, either because of flow rate, borehole recharge efficiency, transfer fluid used, etc., then you'll basically be in good shape. Trying to tweak every circuit would be near to impossible. |
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docjenser
Veteran Member
Posts:1400
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| 11 Nov 2010 09:31 AM |
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No, not entirely.....having a "good" delta can just mean that you do not have enough flow. What is really important is the delta x flow, which is the way to get your heat extraction calculated. That way you know how much heat the loop actually extracts (or rejects).
That is one of the reasons why you should not obstruct flow artificially, for example to increase the delta. |
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decafdrinker
Basic Member
Posts:420
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| 20 Nov 2010 04:31 PM |
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so, 4 to 5 gpm per ton would still be ok? For me, 4 tons, would be 16-20 gallons per minute total going through the system. The flow gauges go from 0 to 4. Right now, they are higher, and one even got stuck in the "5" position when the system switched off. Later, it freed itself and went back to normal operation. |
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docjenser
Veteran Member
Posts:1400
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| 20 Nov 2010 06:04 PM |
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Yep,
Heat extraction is actually slightly better the higher the flow, I would not change a thing. |
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decafdrinker
Basic Member
Posts:420
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| 21 Nov 2010 11:11 AM |
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Ok. thanks. The manual for the system says that flow rates above 3 gpm per ton can actually cause water velocities in the exchanger that can erode the walls of the exchanger and cause leaks!
I didn't know if I should/could throttle down the flow using the manifold in order to get about 3 to 3.5 gpm per ton. I guess there's no way to slow the Geo-Flo pumps instead (safely and properly?). There are 2 of them, but I don't think I want to mess with them. |
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joe.ami
Veteran Member
Posts:4377
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| 21 Nov 2010 11:21 AM |
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Any gpm causes some wear. If that were a huge concern then manufacturer's would want everyone in open loop.
Joe |
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Joe Hardin
www.amicontracting.com
We Dig Comfort!
www.doityourselfgeothermal.com
Dig Your Own Comfort! |
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Palace Geothermal
Veteran Member
Posts:1609
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| 21 Nov 2010 12:28 PM |
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Posted By stuart.wyss on 21 Nov 2010 11:11 AM
I guess there's no way to slow the Geo-Flo pumps instead (safely and properly?). There are 2 of them, but I don't think I want to mess with them.
You could unhook one and see what kind of flow get |
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Dewayne Dean
<br>www.PalaceGeothermal.com<br>Why settle for 90% when you can have 400%<br>We heat and cool with dirt!<br>visit- http://welserver.com/WEL0114/- to see my system |
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decafdrinker
Basic Member
Posts:420
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| 21 Nov 2010 01:36 PM |
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That would be safe to try? If I could get 2-3 gpm flow per loop with only one pump, that would be awesome?
It would be cool to hook it up that in stage 2 both pumps come on, but in stage 1, only 1 comes on. A couple relays...
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Palace Geothermal
Veteran Member
Posts:1609
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| 21 Nov 2010 02:03 PM |
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Posted By stuart.wyss on 21 Nov 2010 01:36 PM
That would be safe to try?
Just turn of the breaker to the heat pump, then disconnect power to one of the pumps, cover any bare wires and turn back on. Then check the flow. |
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Dewayne Dean
<br>www.PalaceGeothermal.com<br>Why settle for 90% when you can have 400%<br>We heat and cool with dirt!<br>visit- http://welserver.com/WEL0114/- to see my system |
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docjenser
Veteran Member
Posts:1400
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| 21 Nov 2010 03:31 PM |
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Posted By stuart.wyss on 21 Nov 2010 01:36 PM
That would be safe to try? If I could get 2-3 gpm flow per loop with only one pump, that would be awesome?
It would be cool to hook it up that in stage 2 both pumps come on, but in stage 1, only 1 comes on. A couple relays...
I would shoot for a minimum of 2.5 gpm/outside loop (assuming 3/4 pipe outside) everything below that and you pay a penalty for loosing turbulent flow outside, and your delta T will go up too much. |
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