I respectfully disagree that a specific fluid velocity is needed for heat transfer, or that an oversized ground source loop is a probem, except for the circulation pump if the static head gets too high so that volume falls off dramatically. I realize that in some systems it can appear so, but the reality is that it is not. The BTU heat transfer from the pipe to the soil is not affected by the velocity of the water. As the water flow slows, the return water temperature may be have a closer approach temperature to the soil temp, but BTU's are not improved, since water volume is less. It is a matter of physics and conservation of energy. It is a very popular misconception among skilled plumbers, HVAC techs, and some very skilled auto mechanics.
A larger loop will let your approach to the soil temperature be smaller and is not problematic. In Alaskanoob's case, his loop will return will be colder than 32F. What I forgot to consider before is Alaskanoob will always be at least a few degrees below 32F back from his ground loop, and might be getting only 20F (or worse) water back from the loop by January, as the soil around the pipes gets chilled. A larger loop will help that, not hurt, but I suggest he look again at the heat pump performance data for 22F return water temperature. Some data from similar GSHP systems in the area sure would sure help.. The paper I referenced above would help an engineer do the ground loop design since they did measure soil thermal transfer in 32F soil.
Alaskanoob, do you have performance data for 22F incoming water? That may greatly affect the heat output. Sorry I forgot that you will won't see 32F or better at 9-10 foot until late summer or fall. My mistake.
Another paper suggests that modest depth increases will not help in your near frozen soils.
You can’t size the unit based on its listed output it has to be able to absorb the heat from the ground and have enough off time for the ground to recoup.Thank you for those wise words. Somebody else here in Alaska who is more familiar said that I need to see the efficiency of the pump with inlet water temps of 32F so that might be how cold it gets at 10 feet underground here. The design heat load from the calculations I've done using some software, appears to be 17K BTU for our small cabin. The unit is a two ton unit so it can put out 24K BTU. But with 32F inlet temps, the efficiency of the unit drops down. I'm still trying to figure out the chart to see how many BTU it can put out with the colder inlet temps but haven't cracked the code yet. It appears that at 30F and the compressor running full speed, it can put out 29 MBTUH. I'm not sure how that equates to BTU output of the unit though to see if it will meet the 17K BTU design heating load.
On the subject of service and parts availability. We have had a terrible time finding competent techs who can do both the ground loop and compressor side of things. Topping that off, we have been waiting over a month for a simple expansion valve to replace our clogged one.On another note it will not do any good if you go with a piece of equipment that the people in your area are not familiar with and don’t have repair parts available for, especially in Alaska. I am in Wa state and have huge issues getting repair parts so I stock the more common ones for the units I work on.
I appreciate all that information. Can you tell me where you are getting you soil temp data from? Alaska is a huge state with a lot of temperature differential so you may be thinking I'm up north. From the Alaska energy folks I've talked to in Fairbanks (which is much colder than where I am) they told me to make sure the unit can handle 32F temps. They have a horizontal ground loop in Fairbanks and have done some reports on it and they indicate that the temp only nine feet down maintained about 32F during winter. Our location is much warmer in the winter and cooler in the summer. So while our winters are more mild than their winters, we don't get the recharge they get in the summer.I respectfully disagree that a specific fluid velocity is needed for heat transfer, or that an oversized ground source loop is a probem, except for the circulation pump if the static head gets too high so that volume falls off dramatically. I realize that in some systems it can appear so, but the reality is that it is not. The BTU heat transfer from the pipe to the soil is not affected by the velocity of the water. As the water flow slows, the return water temperature may be have a closer approach temperature to the soil temp, but BTU's are not improved, since water volume is less. It is a matter of physics and conservation of energy. It is a very popular misconception among skilled plumbers, HVAC techs, and some very skilled auto mechanics.
A larger loop will let your approach to the soil temperature be smaller and is not problematic. In Alaskanoob's case, his loop will return will be colder than 32F. What I forgot to consider before is Alaskanoob will always be at least a few degrees below 32F back from his ground loop, and might be getting only 20F (or worse) water back from the loop by January, as the soil around the pipes gets chilled. A larger loop will help that, not hurt, but I suggest he look again at the heat pump performance data for 22F return water temperature. Some data from similar GSHP systems in the area sure would sure help.. The paper I referenced above would help an engineer do the ground loop design since they did measure soil thermal transfer in 32F soil.
Alaskanoob, do you have performance data for 22F incoming water? That may greatly affect the heat output. Sorry I forgot that you will won't see 32F or better at 9-10 foot until late summer or fall. My mistake.
Another paper suggests that modest depth increases will not help in your near frozen soils.
I definitely will do that. I just want to come up with my best guess and some idea before I go to a contractor.I can tell you are not trained in geothermal, flow rates have everything to do with the heat transfer of a geothermal system, you need turbulent flow for good transfer.
Also more tube will not do anything after a certain point, once you have reached the capacity of the ground it doesn’t help to add more tube you can’t absorb more heat than is there, going over a 48” pitch slinky usually doesn’t gain much unless you are in very wet soil. You need more ditch length and then size the pumps for the extra head pressure.
I run into systems all the time that the contractor or home owner decided installing more tubing was better, most of the time it doesn’t mean more heat just a bigger pump station and less efficiency. You need more ditch, tubing and usually a larger pump station for added capacity.
To the OP get several quotes and get references from past jobs, design is critical on these systems. Take advice over the internet with a grain of salt. I have seen a lot of messed up jobs in my 32 years in HVAC, I have been a HVAC contractor for 20 years and design these systems for a living. I am also a IGSHPA certified designer/installer.
Our soil is thick grass soil for about a foot, then clay for about three feet, then a mix of sand and clay down from there. That's based on digging I've done and some guys driving piles down for a foundation. Ten feet down is sandy clay. We have very very few rocks here. It's rare to find one when I dig.You can’t size the unit based on its listed output it has to be able to absorb the heat from the ground and have enough off time for the ground to recoup.
The unit might be able to out put that BTU rating at 32 degree incoming water temp but if the unit doesn’t have long enough off cycles it will continue to pull the ground temp lower until the unit fails on high head pressure.
People think it is all about the equipment but the loop field is the most important design aspect. I can’t give you advice on the size of equipment or loop field, it is really location specific. We haven’t even talked about the type of soil you have at 10’ of depth, clay, sand, rock it can make a huge difference on your heat transfer.
I had a job that I got called out to try and fix, I pulled up to the house that had a huge circular entry drive with a very large fountain in the middle. The first thing that caught my eye was the fountain tilted to one side, the installing contractor had installed some of the loops under the front drive and the system pulled the ground temp down and caused frost heaving.I appreciate all that information. Can you tell me where you are getting you soil temp data from? Alaska is a huge state with a lot of temperature differential so you may be thinking I'm up north. From the Alaska energy folks I've talked to in Fairbanks (which is much colder than where I am) they told me to make sure the unit can handle 32F temps. They have a horizontal ground loop in Fairbanks and have done some reports on it and they indicate that the temp only nine feet down maintained about 32F during winter. Our location is much warmer in the winter and cooler in the summer. So while our winters are more mild than their winters, we don't get the recharge they get in the summer.
Of course you're talking about the loop removing heat and over time in the winter that will cool the surrounding ground more than the ambient soil temp, so perhaps that is the issue you're discussing. But I would imagine the Fairbanks people up north would have had that issue too.
Lots of neighbors around her have root cellars that don't freeze in winter. Frost depth chart shows five feet for my location.
Also, my understanding is that oversizing the loop should also help keep the loop from having that freeze problem you were discussing. We are planning for a two ton unit, and 1000-1500 feet of loop.
I definitely need all the info I can get on this one because you're right about how important the soil temp is going to be. The Fairbanks test system supports your point. Year to year the loop started freezing the soil because it couldn't recharge enough in the summer. They ended up creating a big ice field over an 8 year span. But in the first couple years it worked great.
Thanks for passing on the learned experience. I think our soil ten feet down is warmer than 32F from all the data I've seen. But the loop over time making that soil colder could be an issue. Our cabin doesn't get hot in the summer so there isn't any reason to cool the cabin and return heat to the loop, but maybe we can open up our windows in the summer and set the cooling to 50F or something just to make the unit transfer heat back into the ground loop. We'll have more energy in the summer than we know what to do with, so perhaps that is a strategy to keep the ground recharged.Nice project, not sure I would try that in that environment but..
We are in Pa, completely different animal and we use a sanden CO2 air to water heatpump , normally for domestic hot water but we use it for our radiant heating... 3/4 pex under two layers of hardwood floor.
We have a 52 x36 shop behind the house that is build slab on sand.. floor heating as well . ,4" of insulation below slab that extends 3 feet beyond the perimeter.. this keeps the frost away from the slab to avoid heaving and cracking. Our local building code had big problems with that theory and design..
It is there now for 15 years and no cracking or heaving.. so far for experts..
Just an idea to explore.. to prevent the frost getting into the ground from above you could lay insulation over your field like at 2 to 4 feet deep and potential along the outer edge of the field , maybe 8ft deep.. probably expensive in material but a diy solution.
Coming back on the sanden CO2 heatpump, maybe with your low ground temp you could use a sanden CO2 equipped with a liquid heat exchanger on the cold side and an antifreeze coolant in your loop. Not that sanden will support you, even my simple floor heating made them raise their eyebrows, haha.
I would imagine that the heat capacity of wet ground is hugely less when it actually becomes frozen, the phase change delivers a large amount of heat.
If you are able to insulate your field then you will be able to dump heat into it during the summer and use it a storage...several projects like that have been done in Canada ( village on solar collectors ) and Germany.
I doubt that you can find an expert with 100 plus installs in you area so better trust yourself .
Good luck, hope you pull it off.
Johan
Thanks for the recommendation about multiple loops, I will give that a think.Slinky has more surface area per cubic yard of soil.. so if it sucks more heat out of the soil then can be replenished it will freeze up.. that is why I mean with experts.. I am sure they had a whole bunch of experts that did not see that coming...
But alas, we learn from other people's mistakes, experts or not .
The turbulence versus laminar in these application is about 8 percent more efficient but that does not mean anything if the soil can not replenish the heat. Also above 1.1 cfm in 3/4" tube around 32f you will almost always have turbulence.
The buty of the internet is that with a day of studying you will have " more " expertise then the experts themselves
How big of a square area do you have to work with? Maybe a large trencher would be nice and efficient to use.
Go multiple parallel loops to keep pumping losses down, lower liquid speeds allow more time to absorb,
If you are going solar then likely you have a lot of overcapacity during the summer, a second air to air heatpump heat exchanger outside the house would allow you to cool the outside of your house and start heating your soil..
Maybe you can add a super heater to your heatpump to make hot water.. give floor heat a thought , pex is cheap, ( if you diy) and warm feet has a psychological effect... very efficient...
How much solar do you have installed and how many kWh do you think you get out of it during the winter?
Climate Master has an excel tool which does exactly what you are requesting:I definitely will do that. I just want to come up with my best guess and some idea before I go to a contractor.
I have seen rules of thumb for minimum horizontal loop length but nothing about maximum. Any idea what the maximum length is or how to calculate it for our two-ton unit? I don't know how much it matters, but the elevation the loop is going is about the same as the cabin, within a few feet. Mostly a flat run.
HVAC contractor here. Insulate very well, build smaller. I have 2 ea 16 SEER 2 Ton AS HP’s and 1 ea 20 SEER 2 Ton Inverter AS HP systems. If you want simple, cheap to repair, easy to find an HVAC tech, buy 14 SEER then add soft start kits. You might need more solar, but you’ll have the extra money and solar doesn’t break as often. If you have lots of money to spend, go Geo.After living with a ground source heat pump as our primary heating source for ten years, we have seldom been so disappointed in an expensive, over-hyped piece of kit. It never delivered on energy savings. It has never been reliable (as I type this, we are again without heat and my feet are freezing). It cost three times what a 'regular' hvac cost.
Did we get a lemon? Is this simply the normal life of a ground source heat pumps ten years ago? Have things changed since then? Don't know, but I do know it would take a ton of first hand factual data before even remotely considering another gshp.
YMMV
Sound advice. Spend less on the best locally available equipment then use that money on more solar gets my vote as well.HVAC contractor here. Insulate very well, build smaller. I have 2 es 16 SEER 2 Ton AS HP’s and 1 ea 20 SEER 2 Ton Inverter AS HP systems. If you want simple, cheap to repair, easy to find an HVAC tech, buy 14 SEER then add soft start kits. You might need more solar, but you’ll have the extra money and solar doesn’t break as often. If you have lots of money to spend, go Geo.