Geothermal, Your HERS Index, And Your ENERGY STAR Path: A Builder’s Briefing
I am Dave Hartzell, 47 years in HVAC, 15+ years running my own shop in Kingfisher. This one is contractor to contractor, for builders and raters.
What the HERS Index measures, in RESNET’s own words
- A typical home built to 2006 energy efficiency standards scores 100 on the HERS Index.
- A 1 point change in the HERS Index represents a 1% change in energy use.
- A home with a HERS Index Score of 0 produces as much energy annually as it uses.
- A lower Index Score means a home uses less energy.
It covers heating, cooling, water heating, lighting and some appliances, and RESNET frames it against its Reference Home: 70 is 30% more efficient, 130 is 30% less. A certified rater simulates your plans, then verifies the build at pre-drywall and final.
The point number nobody can source
Trade press and vendor content say geothermal takes a fixed number of points off a HERS score. I am not repeating the figure, because I could not find its source in a single RESNET or EPA document.
It is worse than unsourced, it is the wrong shape. The score is a whole house simulation output measured against a Reference Design Home whose own HVAC baseline changes by climate zone. No fixed point value could be true everywhere.
The reference home is not a strawman
Under EPA’s National ERI and HERS Index Target Procedure, in climate zones 1 through 6 the ENERGY STAR Reference Design Home is configured with an air source heat pump, even when your house has ground source. Only in zones 7 and 8, the far north, is the reference home itself ground source.
So geothermal gets scored against a decent heat pump, not a strip heater, and your envelope, ducts and square footage swing the result as much as the equipment does. Get your climate zone onto the plan set in writing.
How the target itself gets set
The software builds the Reference Design Home per the Expanded Reference Design Definition, calculates its index value, then multiplies by a Size Adjustment Factor. That product is your ceiling. EPA now calls it the ENERGY STAR ERI Target.
The Size Adjustment Factor is a ratio of a size matched benchmark home’s conditioned floor area to that of the home you are building, raised to a fractional exponent and capped at 1.0, so it only ever helps a larger than average house. That exponent has not been identical in every version of the procedure, so do not take it off a web page, mine included. Get it from the current procedure on energystar.gov.
The equipment thresholds are not one number
ENERGY STAR’s Geothermal Heat Pump specification, Version 3.2, sets EER and COP by system type.
| System type | EER | COP |
| Water to air, closed loop | 17.1 | 3.6 |
| Water to air, open loop | 21.1 | 4.1 |
| Water to water, closed loop | 16.1 | 3.1 |
| Water to water, open loop | 20.1 | 3.5 |
| DGX to air | 16.0 | 3.6 |
| DGX to water | 15.0 | 3.1 |
A submittal claiming EER 17.1 without saying water to air tells you nothing about a water to water or DGX machine. The test standard shifts too: ISO 13256-1-1998 for water to air, ISO 13256-2-1998 for water to water, ANSI/AHRI 870-2016 for DGX only. The spec took effect January 1, 2012 and Version 3.2 carries the same date. The version number moved. The EER and COP did not.
Joining, and the two inspections that hit your schedule
To partner you review the Partnership Agreement Terms and Commitments, designate an authorized signatory, identify a Residential Verification Organization, watch the Builder Orientation video, and submit. EPA states there is no cost to partner with ENERGY STAR or to use its promotional materials. Single Family New Homes then runs Join, Design, Build, Certify, Market, with a minimum of two inspections, pre-drywall and final. Pre-drywall is the one that costs money when it is an afterthought: duct and loop penetrations, air sealing and the mechanical rough all have to be right and visible the same day.
Track B and your HVAC contractor’s credential
On Track B, the HVAC Credential path, your HVAC contractor has to hold an EPA recognized credential before he can assess the systems, through the ACCA Quality Assured Contractor Program or the Advanced Energy Contractor Credentialing Program, both delivered by EPA recognized H-QUITOs. Ask every mechanical bidder, me included, which track your rater is on. Finding out after rough in is a schedule problem, not paperwork.
The loop is a site decision, not a comfort decision
Loop geometry is nearly invisible to the heat pump. A peer reviewed simulation of vertical versus horizontal slinky ground loops at matched diameter and pitch found under 5% difference in thermal performance. What the machine sees is entering water temperature and flow, not the shape of the trench. ClimateMaster’s own commercial manuals put loop flow between 2.25 and 3.5 gpm per ton, with a 2 gpm per ton minimum once entering water temperature drops below 50F. Do not accept a loop package without the design flow written down.
Budget the loop around $3,000 per ton and figure roughly 200 bore feet per ton. Watch the unit: drillers bill BORE feet, not pipe feet, and a U bend puts two pipes down one hole. I do not own a rig. I bring in a driller I trust, and in Oklahoma he has to hold a commercial driller and plugger license from the Oklahoma Water Resources Board. An abandoned vertical field gets plugged to Clause 10.9 of ANSI/CSA/IGSHPA C448.3, not the generic well spec.
The money, said straight
The federal residential geothermal tax credit, Section 25D, has expired. The IRS says it is not available for property placed in service after December 31, 2025, and paying early does not save it, because the expenditure counts when installation completes. If a buyer’s budget still carries that credit, the budget is wrong. Send them to their own tax professional.
Commercial and multifamily runs under Section 48, not 48E, because a heat pump moves heat and does not generate electricity. One route to the increased rate is a project under a small output threshold, stated in thermal output with no IRS tonnage equivalent published, so your CPA settles it, not me.
Utility money is real and local. Kingfisher, Okeene and Watonga are municipal utilities, and the municipal heat pump program pays up to $800 per cooling ton. Up to is a cap, not a guarantee, and it requires a desuperheater. In Garfield and Major counties, Alfalfa Electric Cooperative pays $1,050 per ton on ground source against $150 per ton on air source. Confirm the payee with the utility first. Any of this can be financed.
What your buyer pays after closing
A residential service call is $210 flat, $99 dispatch plus $111 diagnostic, and the $111 is not credited toward the repair. Take the estimate within 10 days and the $99 comes back. A geothermal diagnostic is $348 flat, weekday or after hours, no credit and no refund. Emergency is $309, and light commercial up to 15 tons is $389 flat, day or night. There is always a dispatch fee and always a diagnostic fee.
Plans to hand a buyer at closing: One Visit Spring or Fall 360, $199, no repair discount. Spring and Fall 360, $360, 10% discount and two air filters installed at the visits. Full Year 360, $499, 10% discount, filters all year, and the emergency trip charge waived twice a year, though the member still pays the $111 diagnostic. Estimates on new systems are free.
Where I work
Geothermal statewide in Oklahoma. Kingfisher, Canadian, Oklahoma, Garfield, Major, Logan and Blaine are my daily service counties, not the same thing as where I will design a loop field. Master HVAC License, IGSHPA accredited. 4.8 stars, 300+ Google reviews. Call 405-375-4822 and bring your plan set.
Questions builders actually ask me
Will geothermal get my house to the ENERGY STAR target by itself?
No, and anybody telling you it will has not run the model. It is a whole house number: envelope, ducts, water heating and house size all move it.
How many HERS points does geothermal take off?
There is no such number. The score is a simulation output against a reference home whose own HVAC baseline changes by climate zone.
Which EER and COP do I have to hit for ENERGY STAR?
Depends on system type. Under Version 3.2, water to air closed loop is EER 17.1 and COP 3.6. The other five are in the table above and are not the same numbers.
Does my HVAC contractor need a credential for me to certify?
On the Track B HVAC Credential path, yes, before he assesses the systems, through the ACCA Quality Assured Contractor Program or the Advanced Energy Contractor Credentialing Program. Ask your rater which track you are on before you award the mechanical.
Is the federal tax credit still there for my buyer?
Not the residential one. Section 25D expired: not available for property placed in service after December 31, 2025, and paying early does not save it. Commercial runs under Section 48, a CPA question.