Posted By joe.ami on 27 Jan 2014 09:58 AM

DO NOT "REST" YOUR LOOPS. You will spend at least 2.5 times as much for heat and do little good except have a few good hours of EWT.

I am curious what the basis for this is?  It could make sense to allow time to heat soak the surrounding soil but not sure if you are referring to extra air handler time or what?  Or maybe the system having to play catch up and possibly going into resistive heat plus the big stage on the comp? 


I just don't understand resistive heating on geo HP systems.  seems the system should be sized and designed to never need it, esp with variable speed comps today. 

I have a Tranq 27 to install and plan to lockout the aux heaters but I will also have additional and separate radiant floor system so I should be able to modulate with that. 

OH, OK, the term "resting" would mean kicking over to aux heat. Got it. Yeah, as I said, I intend to design to never need that pocket robbing heater and plan to bypass it totally. I just never understood why people don't design around this issue since you have all the control in the world in designing the loop. A little deeper, bigger diam, longer, etc. All would help ensure the COP stays high, BTUs stay high enough for the heat loss, and still cash in the bank for pizza.

Just my thoughts I guess.

From everything I've read here over the years, unless you're doing the entire job as DIY with your own backhoe you would never get a payback by building the geo without backup heat. The resistive heat backup is only used verrrry little if the system is designed right. The cost of way over sizing to try to get around would be verrrryy expensive.

Yeah, I guess I am on the DIY and equipment owner train and not familiar with what companies typically charge. Obviously the air handling would need to be sized right but the geo loop could higher capacity with no down side other than cost like you say.

I guess I am just designing it the way I want it and want it to perform. Already having the CM 27 and ability to do the whole job means it really comes down to a little more time and expense in HDPE pipe to make it jam.

Would it be objectionable to ask what a typical install usually costs for say a horizontal loop in loam/clay soils, no rock? Are they done by the ton, excavation, etc? Too many variables? I would think most companies have this pretty well down to a science.

However, Looking at ALL the numbers, it really does seem that if you have NG plumbed to your home, a geo HP just makes no sense financially. They cost about the same to operate but the install expense is roughly 10X in my estimation.

However, where I am moving, I only have Propane so the HP makes perfect sense.

Running a larger compressor all the time to avoid aux. use some of the time often costs more to operate as well as more to purchase and build electrical and duct infrastructure for. You will not find an educated pro who supports designing out auxiliary heat. Only old school open loopers and the quasi cyber educated think it is a good strategy.

i said nothing about a larger comp and air handler. Actually I said exactly the opposite. I am ONLY talking about an oversize of the geo loop, NOT the entire HVAC system.

The higher you can keep the EWT, the higher the COP and BTUs. I was just asking why some don't target oversizing the loops a bit so aux is not needed. I got an answer and suppose I would have to better learn what installers charge. I suppose an extra 200/season for aux heat but would take 3-4K to avoid using it, that would not make much financial sense.

Apologies for misunderstanding you.

" I was just asking why some don't target oversizing the loops a bit....."

...."a bit" will yield no significant advantage. One may find that the loop field must be doubled in size to raise EWT more than a few degrees and even then.....
I looked at an old unit that wasn't designed for use with antifreezed and it would have taken a mile (literally) of pipe to stay above freezing.

Re you referring to Methanol against Copper? Why not use PG? Just curious. My CM 27 does not have the nickel plate coil in it. Not sure what bright mind installed that with an open loop config but it will see nothing but the right fluids for the rest of it's life when I install it.

Posted By fastline on 09 Feb 2014 12:56 PM
i said nothing about a larger comp and air handler. Actually I said exactly the opposite. I am ONLY talking about an oversize of the geo loop, NOT the entire HVAC system.

The higher you can keep the EWT, the higher the COP and BTUs. I was just asking why some don't target oversizing the loops a bit so aux is not needed. I got an answer and suppose I would have to better learn what installers charge. I suppose an extra 200/season for aux heat but would take 3-4K to avoid using it, that would not make much financial sense.

My system handles pretty much everything down to 15*, but when an unusual polar vortex arises, I need aux heat to maintain temperature. Had I went with the 4 ton as opposed to 3 ton, it could have done the job on the 4 or 5 days where it got to extreme cold. It would have run less efficiently the other 300 days of the year it was running.

The larger point to Joe's earlier comment is that running aux heat may not cost an "extra" $200 a year. It may indeed be more efficient (less expensive) to run a right-sized 3 ton compressor more within it's designed operating range than to run a larger 4 ton unit at the bottom of its range. The larger unit will cycle on and off more and will perform poorly at dehumidification in cooling mode. 

On a similar note, there are maybe 1 or 2 times a year where I need to haul a ton or more of stuff from the hardware or garden store. I could have bought a 2500 HD pickup for those few instances. Instead, I saved a few bucks on the 1500 and use my aux (trailer) for those very few instances.  The other 300 days a year, I enjoy the better efficiency of the smaller truck.

"On a similar note, there are maybe 1 or 2 times a year where I need to haul a ton or more of stuff from the hardware or garden store. I could have bought a 2500 HD pickup for those few instances. Instead, I saved a few bucks on the 1500 and use my aux (trailer) for those very few instances. The other 300 days a year, I enjoy the better efficiency of the smaller truck."

LOL!

mtretw, you missed my point and question completely, even after trying to explain again and joe.ami picked up what I was saying. I said nothing about oversizing the WHOLE system. we all know there is no benefit in oversizing the air handler. However, a geo system just does not work the same as a typical air exchange split system. This can be compared very closely to the size of the condenser coil which is thus directly tied to the EER and COP.

In short, 3T "system" with 5T geo loop capacity. I have not run all the numbers yet and don't claim to have all the answers here thus my reason for asking but it certainly makes "some" sense that this would make the 3T system not only crank out more BTUs in cold temps, it would operate at a higher COP and EER possibly making it offset the costs of the install "eventually".

The higher you can keep that EWT, the more BTUs and higher COP.

there is no question that an over sized loop will help your heat pump be more efficient. However the gains will be modest at best more likely marginal. If you DIY, it might pay back in your lifetime. I am not disparaging the idea. I have an oversized loop. My EWT doesn't get much below 45° all winter.

I just want to help you be realistic about the benefits.

You bet Dean and I agree. I will DIY with only the cost of my time, diesel, and pipe, as my costs but I will also have to look closer at the performance graphs because I am sure CM did variable COP testing with EWT, flowrates, etc to determine the "sweet spot" for the exchange rate. Probably is a point where the dT does not change much more and having a higher loop temp really does not net any additional COP. Just not sure yet.


Again, system is being designed as somewhat supplemental anyway as I am installing radiant tubing for the whole structure. That HP might never run which would not really upset me :-)

EDIT:  before someone flames me on the radiant, it will designed with zone heating so I can decide what fuel, how much heat, and where I want it.  It will also provide heat for outdoor concrete deice, shop space, and living area.  I may wish to switch zoning to focus BTUs on deice and kick up the HP to improve comfort in the living area while that is happening. 

Fastline,

 I am not trying to pick it apart. I am wholeheartedly on board with the idea of saving costs and optimizing. I did a partial DIY, and if I had access to my own digging equipment and time, I'd have added more loop for gains on margin COP and loop performance.

The constraining factor comes back to heat load on the house. My load for the house is 3 tons for the 95% design day. The only way for me to make ends meet on the half dozen to dozen days of "Polar Vortex" this year is with 4 Tons of heating capacity. To do that, I need either a way oversized plant or a right sized plant with aux.  FYI, I do not have electric resistance, and my only aux is my pellet stove or plug in electric.

I have experience with some catywampus sizing issues. I have a house with in-law apartment. The load for the house is right at 3 ton, and the in-law apartment is not quite a ton and a half. I installed a 3 ton and 2 ton respectively that share a 5 ton loop field. The 3 ton is right sized (aux needed at the margins). The 2 ton is oversized. The attached file show 2 graphs. The first is the power use at the in-law apt (which has the 2 ton included, with the spikes demonstrating frequent GHP cycles). The second is my 3 ton geo on the main house. The oversized 2 ton with more frequent cycles contributes to less comfort than the right sized 3 ton with less cycles. I am quite confident which one is more likely to have problems sooner.

Keep in mind the CM is a dual stage comp so my 5T will operate at 3T or 5T. Well, I guess technically we should uncouple the tonnage which is air flow, from the thermo transfer. The compressor will run at 66% or 100%. IMO, this should be quite helpful in stopping short cycling.


As said, I have some less than typical plans for my system so I am not sure how this might all work out with the HP as the sole means of heat.

We are probably getting closer to a lot of systems going to the way of VFD controlled compressors so you have nearly infinite capacity to match load.

"Keep in mind the CM is a dual stage comp".....everyone is referring to 2 stage equipment. Some "revolutionaries" have even suggested sizing equipment large enough so that they never have to go into second stage (since 1st is soooo efficient). We call that technology "single stage heat pumps".

Do not forget pumping cost is an issue. If you wish to use glycol and significantly oversized loops you are tanking ROI for COP. In trying to reinvent the wheel some often break it.