Normal EWT rise/fall during run cycle
Last Post 16 Oct 2015 07:06 PM by jonr. 10 Replies.
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brwallerUser is Offline
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24 Sep 2015 12:28 PM
I have recently installed a horizontal closed loop heat pump. I'm aware that EWT fluctuates through the season, or daily for that matter. I'm curious if there is an acceptable range for EWT rise during a run cycle. For example, if I adjust the thermostat down 2 degrees and A/C operates for 20 minutes, is it acceptable to start with an EWT of 75 and have it creep up to 77.5 during the 20 minute run cycle (LWT temp rises proportionally, with a Delta T around 6 degrees in compressor stage 1). I understand that this is indicative of the heat pump moving heat faster than the ground can absorb it from the loop. I'm just curious if a 2 degree rise in 20 minutes is indicative of insufficient transfer, or if it is an acceptable range. EWT temp seems to drop rather quickly between run cycles.
chrsUser is Offline
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24 Sep 2015 02:23 PM
That sounds very normal to me. You aren't really exceeding the loop capability...you are just seeing what temperature rise you get when you drive it at that level.
Palace GeothermalUser is Offline
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24 Sep 2015 10:47 PM
What really matters is how high the EWT is at the end of the cooling season and how low the EWT is at the end of the heating season. What happens after 20 minutes doesn't really tell you any thing.
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
DickRussellUser is Offline
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25 Sep 2015 07:17 PM
From a heat transfer point of view, after the HP has been off a while, the temperature of the water in the loop will have approached the temperature of the ground surrounding it; the longer the off time, the closer the approach. On restart, the EWT is essentially that of the ground immediately around the loop. In cooling mode, the RWT of course is higher, and that results in heat transfer to the ground immediately around the loop, resulting in heating the ground, more so in the ground around the loop closest to the reentry points. That reheating of the ground must happen, as it is absorbing the rejected heat. Over time, that heat is distributed to the ground further from the loop, and the length of that off time and the size of the loop determine how much the ground around the loop recovers toward the original EWT. As Dewayne hinted at, continued cycling of the HP over the cooling season results in a gradual rise in temperature of the ground around the loop, which must happen in order for that absorbed heat to be transferred toward cooler ground even further from the loop. A temperature difference is required for any heat transfer to occur at any point in the process, from water to the ground or from nearby ground to ground even further away. Clearly, there must be a reasonable balance between size and cost of the loop. Too small a loop results in an EWT rising too much as the cooling season goes on (or falling too much as the heating season goes on), while too small a change in seasonal EWT, relative to what is reasonable from the point of view of COP, means an oversized and thus more costly loop .
Palace GeothermalUser is Offline
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26 Sep 2015 04:10 PM
Well said Dick
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
Bill NeukranzUser is Offline
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07 Oct 2015 03:46 PM
Here's exactly what happens to EWT during a run cycle:
 
http://www.welserver.com/perl/plot/...mpsEWT.png
 
Here you can easily see that it depends on the instantaneous amount of heat being dumped into the earth as to whether or not instantaneous EWT is rising, holding firm, or even falling.
 
Here's the macro view of EWT, over a rolling 13 month period:
 
http://www.welserver.com/perl/plot/...ilyAvg.png
 
Since I'm in a cooling dominant region (Dallas) it's how high the EWT gets in the summer time that's fundamental for me. In my case my loop size is adequate to avoid year over year 'borehole field heating.'
 
Best regards,
 
Bill
Energy reduction & monitoring</br>
American Energy Efficiencies, Inc - Dallas, TX <A
href="http://www.americaneei.com">
(www.americaneei.com)</A></br>
Example monitoring system: <A href="http://www.welserver.com/WEL0043"> www.welserver.com/WEL0043</A>
garyWEL0222User is Offline
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14 Oct 2015 09:55 PM
My WEL data comes close to the same values as posted by Bill (as a0128958) in the above reply -- as you would expect from two similar systems located 30 miles apart in Dallas, Tx. During peak summer loads, my daily EWT will rise 6-7 deg during 14 hours/day runtime, but the change from day to day is minimal. My annual loop temperature ranges from 61F to 79F -- nearly identical to Bill's, and like his, my loop temperature has held the same range over the past five years of operation.
chrsUser is Offline
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14 Oct 2015 10:00 PM
Gary and Bill, if you'd be willing to send some of that balmy water up here to New England, our heating COPs would sure appreciate it. I can offer you some nice cool (40-50 F) water in exchange.
docjenserUser is Offline
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15 Oct 2015 02:41 AM
The loopfield heating up over years is usually only know in larger commercial loops, where the a/c load is significantly higher than the heating load, and the inner bores are surround by other boreholes, so the heat cannot be transported away by the ground.
www.buffalogeothermalheating.com
Bill NeukranzUser is Offline
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16 Oct 2015 03:38 PM
I'm fortunate that my cooling load is not substantially higher than heating load.  See

http://www.welserver.com/perl/plot/...ansfer.png

for a good 13 month picture of the mix of heating and cooling loads.

While my boreholes are all 300' deep (600' of 1" pipe) and at least 20' or more apart, it looks this was overkill.  There was no way of knowing this at the time though.  As has been pointed out before, advance of installation soil capacity testing is only done for commercial structures here in Dallas.

Best regards,

Bill
Energy reduction & monitoring</br>
American Energy Efficiencies, Inc - Dallas, TX <A
href="http://www.americaneei.com">
(www.americaneei.com)</A></br>
Example monitoring system: <A href="http://www.welserver.com/WEL0043"> www.welserver.com/WEL0043</A>
jonrUser is Offline
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16 Oct 2015 07:06 PM
I would think that in some local areas, the soil capacity would vary little. So once someone has good thermal data, everyone in the area can use it.

On the other hand, there are certainly areas where you might be on clay and you neighbor might be on gravel with high ground water flow.
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