We are replacing our turn-of-the-century home’s foundation and thought it would be a great opportunity to install radiant in our new day-lit basement’s slab.  The basement walls will be ICFs.  We will also be installing staple-up radiant panels under our existing strip wood flooring (no existing underlayment), as well as stripping our old plaster down to studs, furring the studs with horizontal 2x2s, super-insulating the shell, and installing new windows.  Our house is on a 50’ x 100’ city lot located in Portland, Oregon. As soon as we determine which heating source is best for us, we will be installing the hydronic tubing ourselves with a professional’s design and getting the boiler and manifold system designed and installed by a professional.

We originally thought we would have vertical geothermal loops installed, but after getting a ball-park figure from contractors for both DX and plastic, we decided the extra tens of thousands of dollars for those systems would be better spent on energy conservation. 

We then found out about air-to-water heat pumps, but every homeowner and contractor we’ve talked to had their own bad experience of them to relate, so we’ve decided that until some of the best residential European or Japanese models are sold in the US, we’ll stay away from them. 

What we now want to do is to install a water-to-water heat pump with a DIY horizontal slinky system at about 4.5’ to 5’ below the surface.  Our lot has no trees and very little shrubbery, and, since we are majorly disrupting the landscaping with our foundation replacement anyway, we thought it would be easy and inexpensive to install.  The soil is a clay/silt loam.  Our heating load including domestic hot water is 2.5 tons and we will not be cooling.   Winters are extremely mild here.  The ICF foundation will be a Quad-lock R-32 and the slab and edge of footing will be insulated.

Here’s our question:

Is our lot big enough to provide an adequate heat reservoir for a slinky loop system?  Every professional we’ve talked to has no experience with small lot horizontal loops and has expressed skepticism, and we can’t find an example on the internet, yet every description we’ve read about indicates our yard has more than enough thermal mass to make this work.   

This diagram illustrates the best slinky configuration we’ve come up with.

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Thanks,
Brint

Hi Brint,


Welcome to the forums.  Using the GeoDesigner Program from Climate Master,  I ran a report on your house with the info you provided in your post.

The report is attached below.

According to this report, you need 225 feet of trench with a total of  1800' of pipe.   You now have 440' of trench with 2400' of pipe.

This shoud make your job a lot easier. 

It is not recommeded to place the slinky right next to the house.  Minimum of 5' separation is needed.  Are there utilities ( water, power, sewer ) that you need to avoid?

I would try and do your slinkys similiar to the attached drawing.

If you make each slinky with 1000' of 1" pipe  with a 3' diameter and 18" pitch then each  trench would be about 110 feet long.

Good Luck and let us know how it all comes together.

p.s.  what program did you use to draw those nice looking slinkys

I should have said that I my situation is similar to yours. My lot is about twice as big as yours, my house is bigger as well. I live in Salt Lake City where the winters are a lot colder. I have a 4.5 ton heat pump. I stood my slinkys in a vertical trench. I have 5 trenches 125' long with 1250' feet of 1" pipe per slinky. I used all of my back yard and most of my side yards for trenching.

My system has been in for three years now and it works like a champ. We just finished the heating season and my loop temp bottomed out at 41° .

Hi Dewayne,

Thanks for such an informative reply.

We are limited by some existing shrubbery, but we could accommodate your suggestions.  The one contractor we talked to who seemed to know something about horizontal loops told us the following rules of thumbs:

1.  No pipe lengths over 750' -- too much resistance.
2.  Larger pipe diameters also increase resistance -- don't go above 3/4"
3.  Slinky loops installed in the vertical position trap air and create resistance at the top of each loop.
4.  1000' of pipe per ton max.
5.  Don’t return loops in the same trench -- for obvious reasons. 

<!--[if !supportEmptyParas]--><!--[endif]--> His advice in #2 does not make intuitive sense to me.  I know that smaller diameter pipes increase resistance and that there is an exponential relationship between diameter and surface area, so a 1” pipe has potential advantages in heat absorption. 

<!--[if !supportEmptyParas]--><!--[endif]--> His advice #3 also seems counter-intuitive assuming that the system can purge air (I don’t know whether they can) as long as the dissolved oxygen content was low, the bubbles that did form would be readily reabsorbed. 

<!--[if !supportEmptyParas]--><!--[endif]--> His general reasoning behind limiting resistance was that a larger pump required to move more water or a faster water speed to purge air bubbles takes more electricity and, thus, diminishes significantly the coefficient of performance. 

<!--[if !supportEmptyParas]--><!--[endif]--> Did you install a ClimateMaster heat pump at your house, and, if so, do you prefer these over WaterFurnace?

<!--[if !supportEmptyParas]--><!--[endif]--> Have you done many vertical slinky loop installations and have you had any problems?

<!--[if !supportEmptyParas]--><!--[endif]-->While I know you are in a different heating climate from ours so direct comparisons are difficult, are you able to determine what % of your electric use goes for heating, and can you determine the actual COP of your system?  Was it different from what you calculated in the planning stage?

I will revise my diagram to reflect you suggestions and repost it soon.

Brint

P.S., The diagram was drawn in Autocad and the slinky is drawn using the measure tool which places a block (a circle in this case) at a set equal distance along a polyline. 

. 

Hey Brint,

Pipe size and length need to be determined by the gpm your system needs.  Your 2.5 ton heat pump will need 7-8 gpm.  If you split that between to two loops, then you are pushing 4gpm per loop.  The maximum recommended flow for 1" pipe is 8 gpm.

I also do not agree with #1 and #2 . #2 is just plain wrong! 

#3 is true but the issue can be solved by purging the air prior to using the system.  I know this works because of my system .

Do not go with a self purging, non pressurized pump module!


"His general reasoning behind limiting resistance was that a larger pump required to move more water or a faster water speed to purge air bubbles takes more electricity and, thus, diminishes significantly the coefficient of performance. "

This is a true statement.  However, the best way to lower pumping resistance is to divide the flow with more circuits.   If you have to use 3/4" pipe, then go with four  500' loops instead of two 1000' loops of 1"



If you have the room to lay the slinky horizontal it does make purging easier.

We have done two vertical systems and they both work.  We always do horizontal  when we can.

I actually  have a Florida Heat Pump in my house.  I would go with what ever brand you can get serviced by local contractors.

I have a submeter on my heat pump.  I spent  $190 on heating for the past heating season.   My COP changes as my loop drops.  It varies from 2.9 to 3.5.

This was the first system I installed and I didn't do any calculations prior to installation.

Keep in touch

I absolutely agree, the longer the runs, the more resistance... When you talking about electricity, Not water on a basically level surface.

And number 2 baffles me, the larger the pipe, the completely oppose is true, the less the resistance. 
 

Hi Geodean,

Just wondering how you calculated COP's?  Does the submeter give you kwh and you go from there?  If so, how?

Thanks,

BM99

GeoDean,

<!--[if !supportEmptyParas]--><!--[endif]--> The attachments show two possible slinky layouts.

<!--[if !supportEmptyParas]--><!--[endif]--> Avoiding shrubbery accounts for its asymmetry.

<!--[if !supportEmptyParas]--><!--[endif]--> Questions:

<!--[if !supportEmptyParas]--><!--[endif]--> 1.  So, if I follow your reasoning on pipe size and length: pipe diameter (within reason) does not influence pump size for a horizontal loop, but there is a relationship between smaller diameter pipe and how many individual loop lengths optimally create the total.  If we went with ¾” pipe with four 500’ loops instead, we would be pumping a similar volume of water at the same speed as a 1” pipe with two 1000’ pipes.  However, the surface area of the pipe and, hence, the heat transfer would be greater for the 1” pipe.  Would both pipe sizes with lengths as described here require the same pump and use approximately the same amount of electricity?

<!--[if !supportEmptyParas]--><!--[endif]--> 2.  If the slinky trench is approximately level, can we run the straight runs from the slinky trench to the header 2.5’ below the slinky elevation?  This would put both the supply and return about 2’ below the slab drainage gravel.  Does this influence the accumulation of air if the center portion of the pipe is taller than the ends?  It has the benefit of adding to the heat sink the loop is drawing on. 

<!--[if !supportEmptyParas]--><!--[endif]--> 3.  A possible alternative is to run the supply and return pipe level around perimeter.  This is attractive because it requires no additional excavation – we will already have between 30” to 36” inches clear around our well insulated foundation.  Is this a better placement?

Thanks,
Brint

<!--[if !supportEmptyParas]--> <!--[endif]-->

Answer to #1:

with 8 gpm,

the pressure drop through two 1" loops by 1000'  would be 14 ft of head.

the pressure drop through four 3/4" loops by 500" would be 6.1 ft of head.

so four 3/4" loops will only require 1/2 the pump capacity of two 1" loops.

Answer to #2:


As long as you have a big enough pump to properly purge the system before using,  there is not a problem with elevation changes.   Purging is an important step that often gets overlooked.  Any air left in the system can block a loop so you might only have flow through 3 out 4 loops.  Since you manifold is going to be inside,  you can purge the loops one at a time.  This is a big help.


If #3 is the easiest,  I would probably go that way.

If you take the  temp of the water going into the heat pump and subtract the temp of the water leaving the heat pump then multiply by the gpm through the heat then multiply by 500 this will give you the BTUs extracted from the water.

The assumption is that heat is going into your  house.  


Convert how many watts your heat pump uses to BTUs  and divide the BTUs in by the BTUs consumed and you will have COP.

Hope this make sense.

I found some better info on how to figure COP:

Calculations

 

Heat rejected or absorbed to/from ground loop:

 

GPM x ΔT x 500 = Btuh (water only loop)

GPM x ΔT x 485 = Btuh (water/antifreeze loop)

 

Heat from electrical energy (pump+compressor+blower):

 

1. Determined wattage load:  Volts x Amps = Watts
2. Convert watts to heat: (Watts x 0.85) x 3.412 = Btuh

 

Net Cooling Capacity:

 

Heat rejected to loop - Heat from electrical energy measured = Net Cooling Capacity

 

Total Cooling Capacity to Ground Loop:

 

Heat rejected to loop + Heat from electrical energy measured = Total Cooling Capacity to Ground Loop

 

Energy Efficient Ratio (EER):

 

Net cooling capacity / watts = EER

 

Total Heating Capacity:

 

Heat absorbed from loop + Heat from electrical energy measured = Total Heating Capacity

 

Coefficient of Performance (COP):

 

Total heating capacity / heat from electrical input = COP

Are these calculation based on a certain material and pipe diameter? I would think something like copper would have a better reject / absorb ratio and the diameter of the material in question as well.

The formula has nothing to do with pipe type or diameter. The purpose is to figure the COP of the heat pump. You measure the temp of the water going into the heat pump and coming out of the heat pump.

Pipe type and diameter would come into play when computing how much pipe you need in the ground. Certainly if your ground loop is not constructed properly, it will not be able to transfer the heat to and from the heat pump. This would have impact on the COP of the heat pump.

We were referred to a contractor who has done a lot of horizontal closed loop installations and have gotten a new perspective on slinky loops.  Of the three local contractors I’ve now talked with, this is the first who expressed willingness to give us pointers on our DIY pipe trenching, sell us the best pipe at a reasonable price, fuse our pipes to the collection heads, and pressure test before and after backfill. 

This contractor does not do slinky loops and advises against them for four reasons. 

1. There is an inherent conflict in sending a pipe that is slowly absorbing heat backwards onto its previous path where it could lose heat to the colder pipe/soil behind. 
2. Backfilling and soil compaction is easier with less risk of one pipe crushing the one below.
3. Multiple pipes can be placed in multiple lifts increasing the volume of soil contacted. 
4. Straight run pipe installation is all-around easier than loops. 

They are not suggesting decreasing the total linear feet of pipe, just running more single pipes per trench. 

Again, not knowing the physics of how the number of pipes affects head pressure and resulting pump size, I’m assuming it takes no more energy to run twice as many same diameter pipes a shorter distance?

As to the concerns about freezing soil I posed in another post (see my post “Best loop tubing?” and Cnygeo’s informative reply):  The contractor has never designed a water based system that had loop temperatures that dropped below freezing and thinks that such low temperatures indicate poor design – the efficiency of the system depends on water temperature, and in our mild climate, at the point of below freezing loops, you’re better off with an air-source heat pump.  They saw no real problem running a portion of the piping around the foundation perimeter, especially with our well insulated foundation. 

Our modified design would have (8) 350’ loops – 340’ of individual trenches away from the house, and an additional 175’ of trench around the foundation perimeter.  No one has done any calculations of this layout, but our thinking is that it is the equivalent of 280’ of slinky trenches, just with some of the pipes spread out over a greater volume of the available soil.  Since we are replacing the existing house’s foundation, the loops near the foundation are just a bonus taking very little extra effort.   Soil placement during loop trench excavation is our only real logistical challenge, and that is why some of the trenches are only 32” wide.

If anyone has any advice on our latest scheme, we welcome all opinions.  In the attached PDF, I've included the slinky loop scheme GeoDean commented on previously.   Also, is anyone aware of documented cases of water based geoloops freezing the soil and damaging foundations?

<!--[if !supportEmptyParas]--> Thanks for all the advice so far,
Brint

Interesting - just goes to show the difference that region and climate can make. I assume if the contractor is confident that the loops never drop below freezing that he doesn't use antifreeze? That would be a small savings in itself and if a design was "on the edge" a good reason to add length to push the temps well above freezing. There's nothing magic about loop temps dropping below freezing that hurts efficiency. If you pull specs from any manufacturer you'll see that while loop temp certainly has an effect on COP, it is mostly linear with temperature and fairly gradual. I'm pulling numbers out of thin air here, but for example a heat pump might have a COP of 4.5 at 40F loop temp, 4.3 at 35F, and 4.1 at 30F, on down. Higher loop temps are always better for efficiency (heating) but at some point you are into diminishing returns with installation costs outweighing any reasonable payback in performance. Obviously the economics are very different in the Northwest than they are here due to a milder climate and presumably higher mean ground temps.

I don't quite understand the comment about an air-source heat pump being as good at below freezing temps - Even at 25F and lower loop temps, most GSHPs will beat the pants off an air source heat pump at below freezing air temps since the air-source pumps have to defrost the coil periodically. If he's talking about operating cost vs installation cost and payback he may have a point if you have cheap electricity, although the GSHP numbers won't change much between a loop temp of 30F and 35F. It's not like increasing the average loop temp by 5 degrees will suddenly make a geo system cost-effective.

Nothing necessarily wrong with more shorter loops vs fewer longer ones, actually it will decrease pumping work for the same flow. However, you need to make sure that your flow will be sufficient to maintain turbulence in the pipe for good heat transfer. using 8 loops on a smaller system may require you to increase the total flow, which could lead to high pumping work, especially if it is more flow than is speced for the heat pump - the pressure drop across the heat exchanger will get very high. Make sure you run the numbers so that with the flow your heat pump requires you have enough per loop to keep the Reynolds #s well into the turbulent region.

I guess I still cringe at the thought of running loops close to the foundation, but if it is common practice out there, go for it. I can't say that I've ever actually seen a documented case of a freezing loop damaging a foundation, so maybe the risk is blown out of proportion.

It's good you're thinking about where to put the soil ahead of time - my lot is larger than yours but we still dug ourselves into a corner a few times because of poor planning on my part. Not a big deal, but it wasted some time and money moving soil twice unnecessarily. By the time we were done we had a pretty good system down, but I bet an experienced loop digger could have knocked 30-40% off the time it took us.

Thanks Cnygeo, Geodan, and others for your previous feedback.

Since my last posting, I visited a job site were the local contractor who had looked at our plans and given us feedback was installing a horizontal system in a 5’x5’ trench 500’ long with (6) ¾” pipes at the bottom.  He invited the public out to see an installation first hand and give advice and answer questions.  I had told him right from our first interaction that we were looking to install the system DIY and that we would not be a potential client (except for the heat pump which we would only have a professional install), and this did not concern him.  Seems like a great guy who has a keen interest in promoting geothermal.  He advised us on how to make the system better, told us to base it on 3-tons of heating (not 2.5), and said our system would never get below freezing as designed. We revised our design and emailed the plans back to him but have not heard from him either responding to email or phone messages for two weeks – very busy guy, so I don’t begrudge him one bit, the advice was free after all.  The system will now have (6) 375’ ¾” pipes at 5’ deep.  The 5’ wide trenches are 295’ in length, and the 3’ wide sections next to the foundation are 80’ – see attached diagram. 

We are scheduled to have the excavator come out on Wednesday and will lay the pipe perimeter pipe in one day.  The sections next to foundation and under the addition will be done later when we replace the foundation.  We will bury the unsed portions of coils under some plywood to keep them safe until we are get to the foundation excavation. 

Does anyone out there see any problems with our system or think we should change pipe size or number.  We where originally thinking we would place eight pipes in at least two lifts, but after talking with the above mentioned contractor and our excavator (and seeing an actual installation in the pouring rain!), we decided that placing all the pipe just at the bottom of a wide trench was the best way to go.  The excavator will start by digging a short (30’ to 40’ to start with, shorter 20’ lengths after that) length of the trench with a backhoe, we will lay that section of pipe, he will use a bobcat to take the excavated soil all the way around the house and begin backfilling behind us.  We’ll work our way gradually around the house to keep temporary soil placement to minimum.  

<!--[if !supportEmptyParas]--> Brint<!--[endif]-->

Given the small lot that you have to work with, I would say that you have a pretty good plan in place.

Be very careful about the loop temp. The water leaving the heat pump can be as much as 10 degrees colder than the water coming in. So if the water coming from the ground gets down to 42 F you are in danger of freezing the coil inside your heat pump. This pretty much ruins your heat pump.

Methanol is not that expensive. There is no way I would install a system with out antifreeze. Since you don't have a lot ground area to work with, your loop temps might get colder than you think.

Please keep us informed as your project gets going.

Good Luck

Second what Geodean says about antifreeze - if you're going without make sure you have at least a few degrees safety factor and MONITOR the water temp going out to the loop. You could always start without antifreeze and if the outgoing temps start dropping too much for comfort you could add it. Of course the cost and difficulty of doing this in the middle of the winter could be great depending on your system! Open loop systems use various freeze-protection devices (refrigerant pressure and/or outgoing water temp switches) so you could look into adding these as well.

In terms of your loop design, my gut says that it is pretty dense - lots of pipe without a lot of surface area. But, you're on the other side of the country from me in a different climate and with different soil, so I'll defer to local experience here. It doesn't look like you have a lot of options to expand anyway!

Good luck with the project!

We have finished installing the outer perimeter of piping of our horizontal loops.  Most of the job went well though it took twice as long as the excavator first said it might take – 20+ hours for the excavating.

<!--[if !supportEmptyParas]--> <!--[endif]-->Two pipe questions:

1.  The pipe would get twisted when rolling it out, and that twist would sort of curl up and poke out of the coil – I started calling these hernias.  The solution was simple, just flip the entire coil until the twist unwound, and the problem would fix itself.  One time during the job the excavator was moving the coils when he perceived things weren’t going fast enough, and I had to insist that he leave the pipe laying to me.  At the very end of the job, once again the excavator decided to “help” with the coils again and one of the coils developed one of the hernias projecting from the coil and the twist developed a crimp while he was pulling on it. The pipe now has a slight indent at the crease line and is lighter in color.  All of the pipes are under 80-lbs of pressure, and that pipe appears to have no leak when soaked at the crimp. This last section is not yet buried. 

The question is, does this compromise the pipe -- should it be repaired or protected in any way?   

I have told the excavator that if the pipe needs to be repaired, he must pay for it. The contractor insist that kinks do not hurt the pipe and that he is in no way responsible for damage to my piping under any circumstances (he did not see that he had crimped the pipe until I saw it later.)

2.  The excavator was pulling dirt back along the bottom of the trench with the track hoe after the pipe had been laid, and he was allowing the bucket blade to ride along the tops of the pipes -- pulling dirt off the pipes. The bucked had a rounded blade edge and no teeth.  When I saw this, I insisted he not do it any more.  He says he does it all the time with water piping, and all these pipes are so strong that he could never hurt them with his bucket.  The pipe had obvious scratch marks running parallel with the pipe.  None of them looked very deep.  What I couldn’t see was any potential damage at places that had already been buried where he might have been doing this.  One of the pipes has lost about 3-lbs of pressure in 16hrs.  Not sure wether it is coming from the manifold or plug at the ends yet, but I’m now a little worried.  My question is, would even a small leak make the pipe unusable?

Brint

esger.

Ref question 1:  What kind of pipe did you use?  Was it HDPE (High Density Polyethylene) or cross-link polyethylene pex-a, pex-b, or pex-c?  Whether permanent damage is done when the pipe is kinked oftentimes depends upon the type or material in the pipe.  Sometimes damage will not show up for several years.  It is best not to kink any type of pipe.

Ref question 2:  In my humble opinion, the bucket blade should never touch any type of plastic pipe.  Why take the chance of damaging pipe that will be covered that deep?  I think you should monitor the pressure in the line that shows a loss of pressure.  If the pressure keeps dropping then try to check the easiest things first.