Hey guys, I've visiting this site for a while now learning all about ground source heat pumps and would really like to use one on our new home we are building. I've had a manual J and D completed, and now the ground loop design in coming back and I just don't understand how these folks are coming up with their numbers. Here are the details: House size: ~5170 sqft. (I know this really doesn't matter, but thought you would like to know) Location: Mountain area just East of Albuquerque NM (Very similar to Santa Fe, NM) Elevation: 6890' Ground temp: 66-68° (according to all the charts that I have seen) Ground material: Limestone (we had a soils report done) Heat loss: 90,510 Btu/h Loop design will be vertical, we have 2 acres, but there is limestone just under the surface and trenching would be difficult. Some folks (I don't know who came up with these numbers exactly) believe we need the following: Bore holes: 9 Depth: 300' Pipe size: not listed What do you guys think? I know some of you have access to the computer software that can size these loops, would you care to run my numbers? I think that the size should be more like 8 bore holes at 250' each, and that may be over doing it still. Please help, over sizing this loop will put this system out of reach financially!!

That "Heat Loss" seems very high. Who did it and how did they arrive at it? Is there some reason you think you need a ground source heat pump there?

I agree with ICFHybrid. You have:
a) Big house (5170 sqft)
b) Big heat loss (90510 BTU/hr) relative to house size by at least a factor of two vs what is easily feasible.
c) A proposed 8-ton GSHP system with a large borehole array that "will put this system out of reach financially."

You're building a large house, but it's the large GSHP system that will stress your finances?
The design of your exterior shell is so out of touch with current building science that it's design heat loss is yesteryear? You propose a high-tech heating system for a low-tech house. Things are really out of whack here.

If you put some effort into building a better shell, you'll spend relatively littile extra to get that, shrink the cost of the GSHP unit and borehole array needed to support it and which you seem to want to do, you'll get a more comfortable house out of it, and maybe you'll pay less annually in terms of PITE (Principal, Interest, Taxes, Energy).

Gentlemen,

Please ask about the building envelope before assuming.  Here are the assumptions in the heat loss calculation...

1.  Exterior wall (5500 sqft of wall) consists of 2x6 with R-24 high density blown-in fiberglass, and 1.5" of exterior foam (XPS R-7.5)
      a.  So, you can see that the wall construction (in terms of insulation will be fairly good, especially for our region)

2.  There are A LOT of windows, 1060 sqft of windows, with u-value of 0.34 and low-e coating.

3.  Structure totals are 58,095 Btu/h

4.  Ductwork losses and infiltration add an additional 21,554 Btu/h

5.  There is a 0.88 derating due to elevation, bringing the total to 91,510 Btu/h

Why geothermal?

Propane is our only fossil fuel choice and that is around $2.50/gallon here.
Electricity is around $0.11/kWh

I've done the math, and with the tax incentives it does work to our advantage. 

Now, about that ground loop....anyone care to comment on that???

BTW, I prefer this design to an 'igloo Styrofoam cooler'

Giving your location and average limestone, you would need about 1200' of borehole, 3 holes, 400' each, 1.25" pipe is what I would do.

The total on your heat load calc for the building still seems high. If your 91,510 is multiplied by that 0.88 derating factor for elevation and the duct loss/air infiltration loss is then subtracted, that gives your building loss. Without details, it's hard to reconcile your numbers, but based on your living space sqft, wall area, and window area, and making a few assumptions, I can account for only about 2/3 of that 58K BTU/hr building loss. I assumed your living space is on two levels, with a length/width ratio of about two, so that the building might be something like 34x76; that's the only way the living area and wall area make any sense together. Still, that gives a 12 foot ceiling height.

The wall area I took to be gross, including windows. Subtracting that HUGE window area, I have 4440 of net wall area. I used a 70 degree delta T. That R24 insulation gets derated to perhaps R20 to account for framing, then the foam R is added on, so I used 27 for whole wall R. Wall losses come to 11,511 BTU/hr.

The window loss is 1060 times U=0.34 (not even Energy Star) times that assumed 70 delta, giving 25,228, a big part of the total loss.

For attic area (half the floor area of two levels, or 2585 sqft, with an assumed R 49, and the 70 delta gives just 3693 BTU/hr. I assumed an unconditioned attic, with ductwork in it (a terrible idea for new construction), because you included "duct losses," which would not be included if the ducts were in conditioned space rather than in an unconditioned attic.

These three totals (11,511 + 25,228 +3693 give only 40,432 BTU/hr. While my assumptions are of course just that, I'm having a hard time trying to see what else might be in there to increase the total by almost half. Something still doesn't seem right. Also, that 21K for duct losses and air infiltration says that either the ducts won't be sealed carefully (shoddy installation?) or the building won't be especially tight, or a bit of both. Perhaps you can fill in the details.

[Edit: I just looked at your attachment; it does seem to be sort of two living levels.]

Suggestions would be to reduce window area to something more reasonable and perhaps install better windows, with lower U, make sure all ductwork is within conditioned space, and do a good job of making the house very tight against air infiltration. Having a large house doesn't have to mean it will necessarily have a huge heat loss.

For reference, my house in central NH (CZ 6) has close to 4000 sqft of living space on two levels (gross; about 39x56 outside dimensions), facing a lake and mountain range for view, and is heated/cooled by a two-ton GSHP. The heat loss model shows about 22K BTU/HR, but it seems to run lower than that, as it hasn't yet had to go to second stage on the heat pump to keep the house at temp.

Electricity is around $0.11/kWh

Hmmm. I'm building 6900 square feet of residence with 1000 sf of window and an additional 400 sf of glazing in a sunroom. HDD is about 5700. Construction is quite similar to yours, but looks like yours might even top me. Interior volume is about 78,000 cubic feet. Heat loss is somewhere between 32K BTU and 40K BTU, probably on the lower end based on preliminary results we are seeing. My electricity is also $0.11/kWh and geo would be insanity here.

Even if you are in Taos, a temp delta of 70 seems a bit excessive.

All more reason to feel comfortable about 1200' borehole instead of 2700'. PM me and I can run the exact numbers for you. what is your zip code? how many people in the house? Forced air (manual D)?

BTW, I prefer this design to an 'igloo Styrofoam cooler'

The "REAR" of that design better be facing South....

Hey guys, thanks for your replies.

Here are a few more details...

Window u-value I cited is a worst case average of all the windows for the project, as you know each window has a different u-value (size, and style differences), ours range from 0.32 to 0.34 depending mostly on size and grille pattern.  In addition, we are at high elevation and we cannot use an argon fill between panes, we will have to use air, which lowers the insulating value (sucks, I know, but those are the breaks living at high elevation).  I have 3 quotes for triple pane windows and they are all minimum of 2x higher than my double pane windows I'm quoting here.  The decrease in heat loss would not allow a large enough decrease in the HVAC equipment size for a good trade.  In other words, triple pane window + smaller HVAC would cost more than double pane windows + larger HVAC.  The heat loss reduction would only be about 12,000 btu/h. 

All the ducts will run between basement and first floor.  Some (~2/5) will run in the crawl space under the first floor, but that should be conditioned space (that is what the architect says and what I told the heat load engineer).  Maybe I should double check on that loss.

Wall area was total wall - windows and doors.  So, in the calculation example above that would need to be upped.

The 'rear' is facing east so that the large pitched roof is facing south for a solar PV array. 

@ICFHybrid, if electricity is $0.11/kWh, for our heat loss, and 100% conversion efficiency I would be paying ~$6613/year in heating costs, for ground-source heat pump I would be paying ~$1538/year.  Electricity is the most costly energy source/MBtu, granted the capital outlay is far cheaper, but with the tax incentives and install costs I believe I still come out ahead.  Now, if we had access to natural gas that is by far the cheapest whole solution, but still more expensive in annual heating cost than ground-source.  Of course this all depends on prices in your area. 

More details...
The deltaT in our calculation was 62°.  (Outdoor temp of 10° indoor of 72°). 
Heating degree days in our area is 5605 averaged over 3 years.
We should modify the ground temperature to approximate the average yearly air temperature = ~54°.

Anyone know if we should be using a derating factor for high-elevation with a forced-air ground-source heat pump?

Thanks.

Just remember whoever designs this is going to be responsible for performance. As Doc pointed out before, with his zip code you might have ambient temps vary by 40F within 60 miles. Local guys know how to design for local issues, well meaning bloggers may be ignorant to.
I am not assuming anything, but I know many people tell me the heating system is out of reach yet they still have a budget for 5 figures of granite on 6 figures worth of cabinets.
Comfort is often the place where folks decide to cut the budget.
The real question is if the load is that high, doesn't that put propane out of reach?
Good luck.

@joe.ami

That is exactly right. Propane is expensive, I can have the sum of the capital outlay and yearly heating costs be equal between propane and geo and the tax incentives are a bonus that makes it advantageous, besides does anyone see propane getting cheaper in the next 30 years?

Let me assure you that HVAC is on our must have list...we will not be doing the super high end cabinets or even granite counter-tops. I believe we have our priorities straight.

Trouble is we have a small, but growing geo market here in NM and local guys are still figuring things out a bit.

Building a house and putting together a cohesive budget is a tough job, and I have done my research and believe that I have a good feel for size and costs. Now, when one of those items (ground loop) comes back 50%-100% larger and 3x the cost, than you believe it should be, that is a problem.

Trust me, I am not balking at the high-cost of a high-end HVAC system, I am trying to straighten out why the loop field was designed SOOOO large, and thus costs more than planned.

@ICFHybrid, if electricity is $0.11/kWh, for our heat loss, and 100% conversion efficiency

I'm using an air source heat pump. It does run on electricity. However, "conversion efficiencies" should be well in excess of 100%....My electricity pricing is the same as yours.

The 'rear' is facing east so that the large pitched roof is facing south for a solar PV array.

It's a little hard to determine from the perspectives employed, but I will note that PV panels can be mounted darn near anywhere to get Southern exposure. Your design, however, will have markedly different performance depending on orientation, particularly with those XXL windows.

where in NM. I know a talented M.E. in Corunna who has a geo system under his belt and a son in the Biz......joe something 8)

HVAC load calcs can vary so much between different types of structures and locations.I wouldn't be surprised if the numbers are correct but I would certainly get a copy of the calculations and review for errors.Energy costs will only increase with time so building your home to be more efficient is in your best interest. Post the calcs on here so we can see them.

Joe, we are in the Albuquerque area, going to be building just east of Albuquerque in the Mountain area.

I have had three (3) load calculations done, one was way high (118kbtu/h one low 63kbtu/h and one right in the middle 90kbtu/h.  So far, I think the one in the middle is closest to what reality will be, but I should double check on infiltration and duct losses. 

Posted By therik on 27 Feb 2013 01:48 PM
BTW, I prefer this design to an 'igloo Styrofoam cooler'

Not that there is anything wrong with a Styrofoam cooler

I too have an excess of window to wall ratio, but hey!
This is a differennt issue, but in my experience, where we have triple pane windows with low-e, there is not a lot of solar gain through them. There is also a decrease in the qualtiy of light through triples w/ low-e.
And other than support for your ambitious project, I have no input on your loops. All the best!

We wanted GSHP and PV, spent the budget so no PV for us.

If the system needs 9 holes it needs 9 holes (probably doesn't) No PV for you (or less anyway)

ChrisJ

Thanks for your input everyone.

Heard back from my engineer.  The structure was modeled as 'tight' in the manual J program, and the rest of the infiltration is due to mandatory venting that needs to occur at some CFM for a structure of a particular size.  Sure, I could reduce that amount (8644 btu/h) by using HRV's but it wasn't modeled that way, and that is just trading one cost for another....I take that back, I would have lower operating costs. 

The assumptions on the ducts were in conditioned space and some (2/5) in unvented crawl space.  I guess the modeling program has some standard numbers it assumes. 

From what I have had the opportunity to learn from a particular experienced installer is that for the conditions where we will build (56° ground temp, 1.6 Btu/h-ft-°F), and the heat loss (90.5 kBtu/h) we should have something like 1500' total bore depth vs. the 2700' the original bit was targeting. 

So, that equates to bore depth of ~200'/ton.  Seems like that is on par with what others are doing.

Now, the original bid of 9x300' bores would mean that I would need ~358'/ton, seems like a lot for a good conductor like limestone. 

In addition if I did 9x300' bores I would have enough capacity to run a home requiring 162 kBtu/h (13.5 tons)!!

I think these guys giving me this bid simply said, hmmm...I think since the equipment labeled xx60 and xx48 that means that I need 9 tons of capacity, and then said 300'/ton sounds safe, and gave me the bid. 

I really only need to extract what the home will lose at the design deltaT which is 90,510 Btu/h, right??