We are new to the geothermal world and trying to decide whether to have it installed in our new construction or not.  Seeking advice from experts in this area.


House in mountains of Colorado (cold winters) at 7500 feet altitude.
Manual J load is 56,000 btu
Electric cost here is 11 cents/kwh (plus 15% tax)
All electric
Radiant heat (hydronic)
House will be well insulated but it is not complete so we don't know how well it will perform (AAC block walls are 12 inches, no insulation - hoping for thermal mass benefit and passive solar)

We actually have a dry well 900 feet deep near the house.

Considerations -
- upfront cost
- how will the system really perform?  what will savings really be?
- Will the cost of pump usage be high (we've gotten mixed feedback about this - what size will we really need and what will it cost to run it?)
-we've heard that if the ground (granite, mountain) is very dry (we had two dry wells so did our neighbors) geo won't work very well. 
- we've heard it might be better get solar thermal (but we already have a big dry well, so...)
- we don't want to find that we have super high electric bills and retroactively put in geo after spending a lot on boiler, etc.  Unsure how to know how the house will truly perform when there is little to compare (no aac houses in area)
- costs to install have varied from $20k to $40k   Not sure why such a wide range
- one co rec only 1.8 amp pump but that seems too small?

Get references from local installers. Closed loop circulating pumps are fairly low power.

If the radiant heat is designed for fairly low circulating water temps and outdoor reset control is used, water to water geo can be very efficient. Solar thermal and geo can coexist.

I can't speak to how your dry well figures in to this; again, seek local references.

Beware oversizing - some might tell you that a 56k btuh load needs a 5 ton (or larger unit), but meeting 100% of the design heating load with geo alone is rarely the most economic choice.

Appears to be plenty of bore.
What temperature is your floor designed for? That's the difference here. Will you be installing cooling as well?
What would we use for auxiliary heat?
joe

OK here goes.
 Lots of unknowns here, like how effective the passive Solar will be and Heating degree days in your region. When I look at Colorado, I see values from 6000-11000 HDD depending on location. It would be nice if the heating contractor also calculated the btu’s per year the house will consume to heat. That would make things easier.

Here is what we know: You are only going to use electricity so your choices are resistance heat or geothermal, you are using hydronic heating. Walls are 12” AACC so figure 12X1.2 r/inch Plus drywall etc will be ~ R 15.
 You are at 7500 ft so likely will not be using gas filled windows. In other words the house is modestly insulated and has a fairly large thermal mass. This means that you can probably get away with a smaller geo system because the house has lots of thermal mass.
The fraction of heat derived from passive solar may or may not be a factor. We don’t know enough to say.
 Manual J calcs yield 56,000 btu/hour at design temp.
Electricity costs 11 *1.15 or 12.65Cents/kwh 1 kwh = 3,412 btus so 1 million btu/s costs 37$  At full load a 56,000 btu boiler will use 16.4 kwh and cost 2.07$ per hour to run. A geothermal system is likely to cost around 0.70$ per hour to run.
You already have a 900 ft hole that was supposed to be your well but has no water (presumably) that you can use for the bore hole.

My thoughts are the following. 1. The house will likely require quite a bit of btu’s to heat. 2. You have the borehole already. 3. Your only possible fuel source is expensive (electricity). Given the very limited info we have, you seem like the perfect candidate for a geothermal system.

I would want to know how many btu’s per year my house was likely to consume, which the hvac contractor can tell you because he has the manual J calcs, and knows the HDD and the design temp. This is enough for a reasonable estimate of btu’s per year needed.   If you know the btu’s, you know the cost per year with resistance heat as well as with geothermal and can decide if it makes economic sense to you

Good Luck

Eric

Eric OP said the J load was 56000. I hope to find out floor design temp and then can compare op cost for them.
j

Floors will prob be at 70
No a/c needed - we're at 7500 feet high in mountains of colorado
Back up electric boiler, if electric out, we're looking at getting back up gas generator on site.

A few notes -

What is complicated is that when we talk to others who live in AAC houses, they say they are suprised how warm it stays/how little they need to heat them.  The AAC walls are R-15 but might perform better than stick with R-19 because of thermal efficiency.  (By the way - no drywall on AAC walls - just plaster, as that is how they are designed, to breathe, etc).

We'll be doing R60 insulation in ceiling, nothing in walls, and have (well, had) really good fiberglass heat mirror windows (but all the krypton already leaked out - very sad waste).  Plus, not a great contractor so there are 7/8 gaps around some of the windows, 1/2 inch around all - and not sure how just using lots of spray foam will do re window/door air loss.

It's surprising the varied info and responses we get from local companies giving bids.  Our plumber definitely wanted to oversize as stated above.  Another guy suggested 1.8 amp pump and 3.3 ton system and not using all 900 ft of hole (which our plumber wanted to use, but other guy said that's a bad idea for numerous reasons). 

If we listen to HVAC guy, our electric bills could be $600-$700/month in winter (winter is long in mountains).  But if we listen to the few people in AAC houses in CO, we'll be surprised.  But if we aren't happily surprised, it's an expensive mistake to make (getting geo after the fact would be much more costly/big loss of equip we'd buy now for electric boiler, etc).

Knowing very little about the system and how our house will perform makes this a hard decision as there doesn't seem to be real numbers, in our case, to go on.   Will ask HVAC for annual Btu.  We assumed his number for peak hours, and halfed that for other time.  Looks from above that you used peak heating load full time which would mean a whopping $48/day for electric  - that seems almost impossible (I hope). 

Where did your $.70/hour for geo come from?

Thanks for this help!

I think you misunderstand...we'd all like our floors to be around 70*F...the question we have is what is the design temperature for the water being circulated in the radiant tubes under the floor. That design temperature will go a long way toward figuring out if geo will work for you and if so at what efficiency.

Got it. Will post that info asap.

Good points, maybe I was not being clear, I just took the design load and calculated what the costs per hour would be at that temp, I was not implying that would be your average hourly cost. That is why I asked for the other info. Basically if you know the design temp for the house and the heat loss from manual J you can estimate how many btu’s per hour per degree the house takes to heat. This is just
(Manual J heat loss)/(70°- design temp) This is crude and usually an overestimation.
Now if you know Btu’s per hour per degree and also the number of heating degree days, you can estimate the maximum heating requirements. Btu/(deg*hour)* HDD*24= total btu’s per year needed. Divide btu’s per year by 3412 = kwh per year for heating. This does not include solar contributions, nor does it account for internal heat gains by people, pets, or appliances which can add up.

Electric resistance heat can be considered 100% efficient so each kwh of heat requires 1 kwh of electricity to generate. A geothermal system has a total efficiency of around 300 % (or better in some cases) so for every 3 kwh needed in heat, you use 1 kwh of electricity.
These are all fairly crude approximations but they can ballpark what costs will be.
The only AACC construction I have seen was in Germany and they use ~ 15” thick block with 3” eps foam on the outside and 3 more inches on the inside.

The one thing the thermal mass will do for you is to allow a slightly smaller heating system because the system allows you to coast on the heat stored in the thermal mass for 4-5 hours of peak demand

The mass effect works in cooling as well. It also provides an additional benefit of carrying sensible load into the night when outdoor air often only has a latent load in the humid southeast.

Good way to ball park Eric, though it may pay folks to know that radiant floor design will affect required water temperature from geo system which will affect COP.
joe

Here are the numbers from HVAC co - The design temperature specified is 115 degrees. The wall R-value is R-20, a 4 ton unit would be capable of the heating load with a little electric heat backup during peak demand.

He also said - The hole drilled is capable of providing 5tons, I would recommend using the holes full potential and hooking up a desuperheater which will preheat your water going to your domestic hot water system. It would further reduce your heating bills in the winter. The difference in cost between a 5 ton unit and a 4ton unit is minimal so reducing the equipment size has little effect on the system price. Estimated annualn load for the house is 101million BTU's.

However, another contractor said we'd blow the hole trying to use all 900 feet so I think the above is wrong. Other contractor wrote this:
-you cannot install geo pipe to 900 feet as the pressure would be 400PSI and the pipe is only rate to 160 PSI. It would blow out the bottom U-bend from the head pressure.
- the contractor may have used a rule of thumb of 200 feet per ton and extrapolated a 900 feet hole to be 4.5 that. In fact, once water reaches ground temp in a well a deeper well does not provide more capacity. While you can get 5 tons out of 5 x 200 feet deep wells, you only get 2 tons out of a 100 feet deep well.
- the intention is to reduce first costs and provide a balance between first costs, operating costs, energy savings. The Peak Demand is 54,000 btu/h, but this is only at design temperature – basically the coldest days of the year. 48,000 btu/hr is your peak load for over 90% of the time. A smaller unit uses less energy when at partial loads than a larger unit (1000 watts

Finally - two more points that might help with advice - we do not need cooling at all (mountains of CO), and another couple with an AAC house in similar situation (size/radiant/altitude/temp) said they only pay $120/month in their propane bills. In CO, propane is about 37% less than electric. Right now.

Is that a true R20, or is that some loaded "R-equivalent" value based on the thermal mass?

The latter will vary wildly with actual vs. assumed/modeled diurnal temperature swings. I would NOT use anything but the steady-state value for sizing a heating system's output, even if most of the time the daily peak-loads experienced by the heating system is lower than it would be in a low-mass structure of equivalent steady-state R.

That said AAC (like ICFs) perform well in high-altitude places where the nighttime radiant cooling makes for GIaNormous temperature swings, and on-average an R15 (ASTM C 518) may well perform like R20 (average), or even R30(peak) depending on which parameters you're trying to measure.

Even though going with a 4 ton rather than 5 may add slightly to aux-power costs, it'll probably perform better from an overall operating cost point of view, with longer cycles and less maintenance. The thermal mass may provide sufficient buffering from the true peaks that you can keep the auxilliary heating elements off and still never be TRULY cold, despite modest undersizing. If the 99% peak is a true 54-56K, a ~10% undersizing won't hurt. The 99% peaks aren't necessariliy even on the days with the coldest AVERAGE temps (although often they are), but in dry high altitude CO the daily peak loads will be way over the daily average on design day, and playing catchup with the undersized system wouldn't take more than a couple hours after sunrise, most of the time.

The R20 is not a number we could verify - it was from the HVAC co, but the published R value for 12 inch AAC block is R15.

Re the comment that radiant heating will affect COP - does that mean the pump costs are higher than with no radiant?

Eric if you want to run the numbers with R15 and design temp 115, that would be great to know.