Geothermal sizing from district energy interval data, Stone Point High School

Peak load, annual energy, and ΔT from 26,064 hourly BTU meter readings for 2023–2025, cleaned of meter errors, as the basis for a first ground-source heat pump concept for the high school and activity center. A 140-ton heat pump plant with district energy kept for peaking covers 94% of the 7,424 MMBtu annual heating energy and needs about 123 bores at 500 ft.

Hourly data Jan 1, 2023–Dec 31, 2025Stone Point High School and Activity Center, Stone Point, MinnesotaGross floor area 170,000 SFPrepared Nov 5, 2026

Summary

Peak load

33 of the 50 highest hours are startups after a weekend or break

The highest measured hourly load is 4,194 kBtu/h on Monday, Feb 3, 2025, 6–7 AM, at −17 °F. Without the 3–8 AM startup hours on the first school day after a weekend or break, the peak is 3,272 kBtu/h, 22% lower, at −14 °F in January 2023.

Test an earlier, staged morning warm-up before setting the peaking capacity and the district energy contract demand.

Meter data

The raw maximum, 17,903 kBtu/h, is a meter error

The reading for Jan 21, 2025, 6–7 AM, the day after Martin Luther King Jr. Day, holds the accumulated energy of five missing hours. The file has six such catch-up readings, 240 missing hours, including 215 in a row from Feb 26 to Mar 6, 2024, and a stuck reading for 31 hours in February 2025.

Left uncleaned, the peak would be 4.3 times too high. The energy is spread over the outage hours.

Delta T

ΔT is below 50 °F four to five months a year

From November to March 2025, ΔT is 58–72 °F, but from May to September 2025 it is only 31–49 °F, with return water at about 145 °F in June–August. The service rules require a monthly average ΔT of at least 50 °F and apply a low-ΔT surcharge below that.

Check the domestic hot water recirculation loop and the DHW heat exchanger before setting heat pump supply temperatures.

Load, energy, and ground-source heat pumps

Choose a period and a tab. The load duration curve shows how much of the annual heating energy a heat pump plant of the chosen size covers and what that means in bore length. The other tabs show the peak hours, the meter errors, and the monthly ΔT.

View
–Highest measured hourly load
–Annual heating energy
–ΔT, flow-weighted
–Equivalent full-load hours, energy ÷ peak

Load duration curve

Heat pump District energy for peaking Design peak

Energy fraction vs. capacity fraction

The dot is the selected heat pump plant. Capacity fraction is relative to the selected design peak.

Inputs

Design peak
Net annual heat extraction per foot of active bore. Peak extraction is limited to 35 Btu/h per ft.
Active bore depth

First concept

Energy fraction, heat pumps–
Heat from heat pumps–
Heat pump electricity–
District energy for peaking–
Total bore length–
Number of bores–Borefield sized for heat extraction only
Borefield footprint–

Sources and method

Sources

  • SPDE_BTU_Meter_HW-0417_Hourly_2023-2025.csvStone Point District Energy hot water BTU meter HW-0417: kBtu, gallons, supply and return temperature per hour, Central Standard Time26,064 rows
  • NOAA_LCD_Stone_Point_Municipal_Airport_2023-2025.csvNOAA Local Climatological Data, hourly dry-bulb temperature26,304 rows
  • SPDE_Hot_Water_Service_Rules_and_Rates_2025.pdfMonthly average ΔT of at least 50 °F, low-ΔT surcharge below that; demand charge on contract demand22 pages
  • ENERGY_STAR_Portfolio_Manager_Statement_2025.pdfGross floor area 170,000 SF, heating by district hot water4 pages
  • C-101_Site_Plan_Stone_Point_HS.pdfPractice fields north of the building along MN 396, about 4.0 acres1 sheet

Method

  1. The readings were checked against the expected number of hours, 26,304. Missing hours, catch-up readings after outages, and stuck readings (six or more identical values in a row) were flagged.
  2. An energy signature was fitted for each hour of the day and day type (first school day after a weekend or break, other school day, weekend or break) against the running 24-hour mean outdoor temperature from NOAA. Missing and stuck hours were replaced by the signature. Catch-up readings were spread over the outage hours following the signature profile, so the energy is kept.
  3. Peak load uses measured hours only. Annual energy is the sum of the cleaned data.
  4. ΔT is flow-weighted by month: Btu ÷ (8.33 lb/gal × gallons). Stuck hours are left out.
  5. The heat pump energy fraction comes from the load duration curve. Ground heat extraction is heat delivered × (1 −1/COP). Bore length is the larger of annual extraction ÷ the annual limit per foot and peak extraction ÷ 35 Btu/h per ft.

Assumptions and limitations

  • Annual energy is not weather-normalized with heating degree days. The average year is the mean of 2023–2025.
  • Loads are hourly averages. Instantaneous load can be higher.
  • The borefield estimate is a heating-only screening check with no summer heat rejection, since the interval data has no cooling load. It is not an ASHRAE design-method calculation. Final sizing needs a formation thermal conductivity test and borefield design software. The annual limit per foot and the 35 Btu/h per ft peak limit are screening assumptions for a geothermal designer (licensed PE) to confirm.
  • ΔT is on the district side. Building heating hot water temperatures are not in the data; check terminal units against the lower supply temperature of water-to-water heat pumps.
  • In Minnesota, bored geothermal heat exchangers need a permit from the Minnesota Department of Health and must be installed by a contractor licensed for them, under Minnesota Statutes chapter 103I and Minnesota Rules chapter 4725.