I am wondering how much better Geo is going to work than a very High Efficiency conventional heat pump.

One contractor tried to talk me into something like a 28 or 30 EER, I believe it was a Trane.  This is for NC, where it gets down to maybe 10 or 20 F, and rarely snows.

I think a system which draws heat from the air is going to bottom out and turn on the heat strips often at night up there, and something working off 50-60 deg water (or at least mountainside) is going to work much better, and not turn on heat strips.

However, one Geo contractor bidding just told me I can expect the heat strips to come on at times.  This is a 5 Ton Waterfurnace Synergy 3D.  The reason given is at full load, the water returning from the wells will be colder due to the BTU load being delivered on very cold times.  ( I will have 3, 330 ft deep wells).

This is confusing me a bit.  I suppose Geo is still more efficient, but I am dissappointed to think heat strips may come on.  It makes me think the wells may be undersized.   I have asked at what temperature this will occur, and the jury is out.

So I guess the quesiton is, does this sound right, and is a Geo system really worth the difference.

My guess is Yes.  Largely due to the tax rebate, and a benefit I have not mentioned.  I can also do some radiant floor with this unit.

This sytem installed is quoted in the range of 45-46K, from a couple different contractors, and Waterfurnace from one, and FHP for the other.

Havent decided.  Thats a lot of money for HVAC.  Im still getting used to that, but that includes full installation (less radiant floor) for about 5400 sq ft, ductiing, new construciton, 3 levels with basement.

Any thoughts would be appreciated.

thx Jim

I would be interested in J- AMI 's response here
My gut says in your climate ( close to balance heat and cool load ) There wouldnt be a problem sizing the system for full load and using the aux . for those days below design temps .
For the most part the reason people design some aux. heat into a geo system is the cost to do other wise . In many cases you can drop a ton of system / and feild size add some aux . save enough money on the install that the increased cost of operation in winter ( net around $50-100 after you figure the system will cost less to run the rest of the year ) that the up front savings are well worth it . Some custumers may have the priortiy of lowering there energy usage ie. carbon foot print , and may want to spend the additional money for the last ton or so , but most want the best ROI and using some aux. is the answer there

Good Luck,
Joe

Joe, I guess you meant $10/sqft for a high end system?

I am building about 5400 sq ft of conditioned space.

I could browse the info I have from one contractor if you need it, but is it right?

The house is a 3-level, daylight basement, main, and loft, with bonus area over the garage.

They seem to think a 5 ton system is needed.

R-38 in the ceiling, and I believe R24 in the walls.  6 inch walls, stick built, and concrete block basement, and insulation under the floor.

JIm

Your success with this will be near totally dependent on the skill and integrity of the installer

yep, <$10, my bad. Any thoughts on the rest of my questions?
j

I can guess at some numbers and tell you that geothermal should save around $800/year in electric costs. This number increases by maybe $100/year if you make the system 7 ton such that the aux heaters never come on. The WaterFurnace quote supplier can easily print out a report for the entire comparison (geothermal, geothermal no aux, ASHP) (plus lots of other data).

If conventional ASHP is more than about $36K, I'd get geothermal.

Never a fan of trying to size for 100% of the load. The extra duct work and loops as well as 2 extra tons of compressor will make operating cost similar but installation cost much higher.
My experience with ASHP's is that usually your payback is about the same time wise, but much, much better over the long haul with geo.
You haven't mentioned how much your ASHP quote was or even fossil furnace. Much of the price is duct work and such that all systems will have to have (so that shouldn't be part of the payback calculation). ASHP's and furnaces also have a cap on the rebate that GSHP's do not. There shouldn't be that much difference in the prices after credits.
j

Good point - make sure that all of the install costs you are using are after tax credits.

Thanks, yes the tax credits are what really makes it a no-brainer. Roughly, I think the Geo will cost about what a full conventional heat pump would have cost. My quotes are a mixed bag. The geo may be even a few K more after credits, but the payoff should not be many years after that, and I can do some radiant floor.

What concerns me is the idea that the well verticle loop will saturate with high load. That suggests to me that I am loop-limited, and that as I get closer to the limit, the water re-entering the heat pump (in heating mode) will be colder, and therfore less efficient. I suspect that a conventional heat pump is pump limited in capacity, and my Geo installation may be loop limited, and that may hint at an efficiency loss vs. load. The air does not load down, but the water loop seems to.

I am an electrical engineer, not a thermal engineer. I am just surmising all this from common sense, in subject matter I am not used to dealing with. Does this all make any sense?

thx Jim

Did the installer mention antifreeze with the three wells?   What would it cost to add one more well?  Could the tonnage and everything else be kept the same when another well is added?  I am curious. 

One supplier is proposing 4 200ft wells with double casing, which he says supports about 6 1/2 to 7 tons. The other proposes 3 330 ft wells with single casing. The first is 45K+, the 2nd is 46K+. #1 is Waterfurnace synergy 3d; #2 is FHP Aquarius, I believe, it is there high-end unit 2 stage.

I planned to ask #1 if he could use dual casing, as that sounds like the cheapest way to add capacity to the loop. #2 already has more capacity, as he sized his system for a separate 5 ton air handler, and a 3 ton raniant floor water to water unit. I suspect #2 will not hit Aux heat as readily as #1, and I have questions out to both at this point.

#1 contractor told me when he has hit aux on previous jobs, it is because the water loop saturates (my words, not his). That makes me think the installation is imballanced. And I think this may be a characteristic on a Geo system, because the Air, in a converntional system, does not saturate. The air is the air, and it is substantially unlimited. I have no doubt that Geo is more efficient. This is just a consideration, and I dont know how to quantify it.

Thats all I know at this point.

Jim

I dont know if antifreeze was mantioned. What does that do, beyond the obvious. I probably thought it was to prevent damage.

I wonder if this means that some installations are undersized or ineffective ground loops, like some horizontal loops. Mine will be verticle loops in mostly rock. I think that is a good thing!

yeah.....no antifreeeze in Alabama might be okay, but don't try it in a heating dominated climate. Heat pumps are designed to operate with water below 20*, that's why the antifreeze.
The loop questions are interesting to me only because of the volume. We've fielded a lot lately.
I've responded repeatedly that you are more likely to have trouble from undersized duct work.
Most of the "new guys" will likely hire some one with experience to put their loops in, but they may not grasp the extra duct requirements.
There are tried and true industry standards for loop sizing and no newbie will do their own, but here is your safegaurd;
Check the International ground Source Heat Pump Association for your installer's (or his sub-contractor's) name.
Also check the references, if your contractor's prior customers are happy you will be too.
.....and will everyone please stop second guessing professional loop installers designs.....please?
Ask about their duct work design.
Joe

You are correct about ASHPs having an unlimited supply of heat to draw from and closed loop systems do have limits. But geothermal still wins overall.

Your loop length sounds right for rock.

Heat pumps operating at 20 degrees? At what efficency? I thought the whole point of Geothermal over Air Source was that the EST (Entering Source Temp) was well above freezing. What makes Geothermal better then Air Source assuming the EST is 20 degrees for both of them?
   

Why would you expect to have brine always above freezing? You can certainly loop the h@)) out of it to keep EWTs that high, but you'll pay much more than you save. On top of that while we design our loops to be in the neighbor hood of 34* at balance point, LWT is still below freezing temp of water.
The efficiencies they threw around when they sold you your heat pump have little in common with the average operating COP which is likely under 3.
J

From what I can tell, when the soil around a loop freezes, thermal conductivity suddenly drops way down. So best to not go much below freezing for the loop temp.

Best not to get too hot either - the soil will dry out (lower thermal conductivity) and shrink away from the tube at some point.