My home is to be included as one of the featured residences in this coming Fall's Dallas / Ft. Worth Renewable Energy Tour.

For each home, a summary writeup is required for each RE technology.  For me that means I have to create a summary for geothermal and solar PV.

Below is the draft I've written for geothermal.  I thought I'd 'dry run' it here first.  Keep in mind it's written for a wide audience, ranging from those who know little about geothermal ("I don't have a natural gas connection at my house - can I still install geothermal?") to expert HVAC installers ("What kind of borehole field heating are you experiencing from year to year?")

I live in a very hot but only mildly humid climate, so the emphasis is deliberately heavier on cooling than it is on heating.  For my location, good loop design focuses on keeping the water below certain temperatures, and doesn't have to focus on how cold the water gets in the Winter.

Please feel free to make any helpful comments that come to your mind.  And, you're welcome to use this for any purpose you desire - no credit required or asked for.

Best regards,

Bill

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Geothermal Heating & Cooling System Summary

Residential Structure

Plano (Dallas), Texas  75074

 

 

Overview

 

Seasons: Year-round heating & cooling

 

Energy Type Required: Electricity (10.1 ¢/kWh) - no natural gas required

 

Size of Residence: 3400 square feet, single story

 

Average Monthly Heating/Cooling Cost: $60 (596 kWh)

 

Thermostats: WaterFurnace Programmable (daytime: 77° living / 81° bedrooms; night time: 81° living / 74–76° bedrooms)

 

Energy Monitoring: Web Energy Logger (WEL) – http://www.welserver.com/WEL0043/

 

 

System Summary

 

Installed: July, 2007

 

Technology: Geothermal-source heat pump

 

Number of Heat Pumps: 2 WaterFurnace Envision – 3 ton and 5 ton, both with dual-speed compressors and variable speed fan blowers

 

Water to Refrigerant Loop (Outside): Single loop, vertical wells, no antifreeze

 

Heat Pump Packaging: Self-contained (blower, compressor, heat exchanger, coil in single package – equipment is indoors; no refrigerant lines outside of package)

 

Location of Heat Pump Unit(s) and Air Ducts: Inside attic

 

Refrigerant Gas: R410A

 

DeSuperHeater: Available but not connected

 

Aux Heating Capability: Electric heat strips not installed

 

Number of Independent Air Distribution Zones: 4

 

 

Technical Details

 

(all numbers 'as built' or actuals, unless otherwise noted)

Design Temperatures: +1° F outside record low and +112° record high for Plano, TX

 

Cooling Capacity: 8 tons maximum; typically runs 28 - 79 KBTU/hr (2.3 – 6.6 tons); 650 KBTU/day heat rejection needed on 100°+ days

 

Structure Performance: 516 sf per ton of cooling capacity.

 

Heat Pump Performance (including blower and pumps): 21 EER (5 ton unit); 5.4 COP (3 ton), 4.8 COP (5 ton)

 

Inside Humidity Control Performance: 40 – 50% RH during cooling season

 

Lot Size: One-half acre

 

Water Loop: 8 bore holes (wells), each 300 ft, spaced at least 20 ft apart

 

Water Pipe: 1” high-density polyethylene (HDPE), approximately 1 mile in length; 60 psi static pressure

 

Loop Water Temperature Extremes: 61° (Winter) – 83° F (Summer)

 

Earth Heat Absorption/Supply: 80,000 KBTU annual heat absorption (cooling), 18 KBTU annual heat supply (heating)

 

Pumps: Up to 3 Grundfos UP26-116 (1/6 hp, 385 Watts each - spec)

Water Flow: 14 - 30 gpm

 
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With the knowledge that you have now, would you still use 8 tons for a 3400 sqft house in Dallas?

Looks good Bill.

Just a few thoughts:
Aux Heating Capability: Electric heat strips available, but not installed.
Heat Pump Performance - you may want to mention if figures are actual or specification.
Was your system design temperatures based on the record high and low for your area? It appears that they were.

Posted By jonr on 07 Jul 2011 06:10 PM
With the knowledge that you have now, would you still use 8 tons for a 3400 sq ft house in Dallas?

Jonr, what would your conclusion be?  Here's some numbers for you for today:

Today is a day out near the fringe of a design temperature day - it reached a high of 107°.  By midnight my GSHP system will have probably transfered about 745 KBTU from my structure to my front yard for the 24 hour day.  All the while doing this on first stage cooling only.

Best regards,

Bill

Posted By geome on 07 Jul 2011 06:56 PM
...Just a few thoughts:
Aux Heating Capability: Electric heat strips available, but not installed.
Heat Pump Performance - you may want to mention if figures are actual or specification.
Was your system design temperatures based on the record high and low for your area? It appears that they were.

Geome, got 'em, including editing what I wrote here yesterday to start off this thread.

Many thanks!

Best regards,

Bill

Posted By a0128958 on 08 Jul 2011 09:48 PM

Posted By jonr on 07 Jul 2011 06:10 PM
With the knowledge that you have now, would you still use 8 tons for a 3400 sq ft house in Dallas?

Jonr, what would your conclusion be?  Here's some numbers for you for today:

Today is a day out near the fringe of a design temperature day - it reached a high of 107°.  By midnight my GSHP system will have probably transfered about 745 KBTU from my structure to my front yard for the 24 hour day.  All the while doing this on first stage cooling only.

Best regards,

Bill

Bill are you suggesting that never leaving first stage in extreme conditions is good design? Careful we don't invent the single stage heat pump.
j

Posted By joe.ami on 09 Jul 2011 06:27 PM

Bill are you suggesting that never leaving first stage in extreme conditions is good design? Careful we don't invent the single stage heat pump.
j

No.  I believe I paid a lot of money for capacity I don't need.  I don't understand why I couldn't have had a design that uses fewer (vertical) holes, less pipe, fewer / less powerful pumps, and smaller heat pump units.

Instead, what I have is a system that never gets out of 1st stage, even on a 106° day where 750 KBTU is rejected to the earth, that doesn't have any problem maintaining 40 - 45% inside RH, and that has loop water that never gets hotter than 84° on a 24 hour rolling avg basis.

(Keep in mind that I'm in Dallas so everything is 'flipped' from your challenges up North.  Heating in the wintertime is an after thought for my location.  The critical loop temperature is at the high end, not the low end where I never see loop water colder than 62°.)

I think what it really amounts to is I don't know what the cost trade off was between the up front capital cost I paid for the excess capacity I (think I) have, versus the utility bill I now have for heating and cooling ($60/month on average).  I'm assuming my avg monthly heating/cooling cost is lower because I think I have excess capacity.  But I don't know what the break even point is for the excess capacity cost versus the lowered utility bills.

Best regards,

Bill

I'm assuming my avg monthly heating/cooling cost is lower because I think I have excess capacity.  .

Best regards,

Bill

Not really, your bigger loop might give you extra savings (small ones...) but your bigger equipment will run less efficient overall. Bigger pumps, less comfort, especially in A/C. You might be thrilled by what you have, but you don't know how much better it could be....

IMO, there is a blend of correct statements in the foregoing few posts

1) Bill's system are oversized, to a point where they are effectively single stage systems

2) Two stage systems run VERY efficiently in low stage

3) Bill is experiencing both very low utility bills and very good dehumidification owing to high efficiency and long runtimes of units operating in low stage

4) The extra expense incurred by oversizing may take decades to pay off (if ever) via lowered operating costs

Posted By engineer on 11 Jul 2011 10:50 AM
IMO, there is a blend of correct statements in the foregoing few posts

1) Bill's system are oversized, to a point where they are effectively single stage systems

2) Two stage systems run VERY efficiently in low stage

3) Bill is experiencing both very low utility bills and very good dehumidification owing to high efficiency and long run times of units operating in low stage

4) The extra expense incurred by oversizing may take decades to pay off (if ever) via lowered operating costs

Agreed exactly.

The two pieces of information I don't know is:
 
1) what was the incremental capital cost for additional holes, additional pipes, and additional tonnage capacity associated with any oversize capacity.

2) what is the incremental lower operating cost I have today as a result of having oversize capacity.

Since I don't know this information, all I can do is assume I paid for more than I needed, but, I don't have the facts to support the statement.

Best regards,

Bill

1) You probably don't want to know...

2) <10% (pure conjectrure on my part)