I have a horizontal closed loop system. The earth surrounding my loop is of a red clay which is roughly 30 % water. The installers added enough methanol to protect my loop to 25 degrees F. My 2 stage unit exhibits an incoming vs. outgoing delta of approx. 3 degrees on low and 4 degrees on high speed. It's only the middle of Jan. and my outgoing loop is already down to 33 degrees. My understanding of the science of freezing water is that enormous amounts of energy is released as it freezes. Once my outgoing loop temp. reaches 32 degrees and begins to try to freeze the surrounding earth and its water content, will the loop temp. stabilize and cease to cool while absorbing this massive energy release? If not, it would appear to me my loop is substantially undersized, and I'm in for an expensive rest of the heating season using my backup resistance coils.
Mark: Hello and welcome. Take a deep breath. The BTUH exchange you are talking about is called change of state energy. Going from gas to liquid or liquid to solid. Steam, Water, Ice. It is latent heat exchange. The amount of energy to take 212* water at sea level to 2128 steam. You are more worried about freezing your machine than freezing the loop field. So if your LWT is 32* what is your EWT? The earth wins. It is warmer than the freezing point of water most of the winter, if you are deep enough. What is footer depth in your area? I want to be at least a foot below that. I do not know what your cost per KWH/delivered is, but from what you are saying you should be fine. It is OK to run the strip heaters when it gets cold. EWT is the key to if you are short lopped or not. The geo unit will shut down if it does not like what it sees. Mark
I think your loop and system will be fine for the reasons you stated. Most loop designers size the loop and EWT to reach 30° at the lowest point. You are not there yet. As Mark hinted, the coldest point in the loop is when the water exits the heat pump. You just need to make sure that your freeze protection is low enough to protect your heat pump. 25° is marginal. We protect our loops to 20°.
If LWT is 33 EWT should be 38-43. That sounds OK for this time of year in most places. The amount of energy available from freezing liquid water at 32*F to ice at 31.99*F is same as water cooled from 176*F to 32*F, so yes, quite a lot.
So water with antifreeze in it follow the same math above, or its it the actual turning to ice that takes that much energy?
The coldest part is the end of the heat exchanger. Yes, it must change its phase, otherwise the energy is not released. Yes, the loop will stabilize around 30F ( and stay within 2 degrees for the entire heating season, and absorb the phase change energy. Only if grossly undersized it will go beyond the phase change and drop quickly. I have seen badly designed loops undersized by 250% still supporting the geo system through the winter. Your loop seems to perform very good. http://welserver.com/WEL0712/ Second graph from the bottom shows you a typical stabilized loop between 30-32F for 4 month during the winter.
Wow. I really appreciate the rapid and many responses to my query. I have been watching my loop temp. drop from the beginning of the heating season and worrying about it's rate of descent the entire time. I should have asked this question a couple of months ago and saved myself some grey hairs. As I read all of the responses and try to assimilate the information, one thing begins to stand out. My 25F. methanol/water loop mix should probably be revised downward to add protection to at least 20F. If my EWT stabilizes at 30F, and my unit runs on high speed, the LWT wold be around 26F, assuming the delta stays constant, and it has so far down to LWT's of 33F. Not much of a safety margin considering the last thing I want to do is freeze my unit. One member indicated he protects to 20F, this sounds like a better lower protection antifreeze mixture. Maybe I should protect down to 15F as this is my unit's freeze protect shutoff limit?
My horizontal loop field is small compared to the general "rules of thumb" and comparable sizing I've seen, but we had excellent soil and moisture conditions to take advantage of. I was nervous my first heating season with the rapid descent of EWT to 32 degrees, but then watched it stabilize about 30. The next winter even in the midst of long polar vortices, my bare minimum was 28 EWT with LWT around 24. This warmer winter, EWT has stayed up above 30 so far. My freeze protection is around 17*. Once you have extended period of LWT below 30, your pipes in the ground will form a sleeve of ice around the outside of them which should stay fairly close to 32 degrees. The heat will transfer from the warmer ground through the 32 degree ice sleeve rather efficiently. Worst case, you may have a bit of localized frost heaving around your loops. Based on information, it seems that this is your first winter with the system. If so, loop performance should continue to improve in out years. Even the best backfill and compaction can not do as well as time and mother nature.
What is highly recommended? Palace, or letting Mother Nature back fill loops. Trent: Just enough. Not more not less.
Thanks again, for all of the responses and help. I talked with my installer, he was surprised my loop was this cold already. He is going to come back and add antifreeze to protect to 15F. I think this makes the best sense as my units freeze protect would trigger prior to a freeze up in the worst case event.
