At the labeled 40 percent recovery midpoint, milk cooling returns more heat than the worked example's wash cycle needs. A refrigeration heat recovery unit (RHR) tops out near 140 F, however, below the 160 to 170 F wash-start requirement. For a milking center replacing a propane or electric water heater, storage addresses when that heat is available; a final-rise heater supplies the temperature the refrigeration cycle cannot.
Key Takeaways
A milking center runs two large thermal processes back to back and in opposite directions. Milk leaves the cow near 98 F and is pulled to 38 F for storage; University of Wisconsin-Extension puts that cooling load at 56 Btu per pound of milk. Minutes after the last cow leaves, the parlor fills a wash vat at 160 to 170 F. One process rejects heat outdoors through a condenser. The other buys heat from a propane or electric water heater.
An RHR unit joins the two. The extension bulletins describe the heat recovery mechanism but do not size storage for the whole day, add a heat pump for the final temperature rise, charge it off-peak, or address the mismatch between heat arriving during milking and a short wash fill afterward.
This article works that balance for a 200-cow herd milked twice daily, with every assumption labeled. It does not argue that heat recovery eliminates the final-rise heater, because at 140 F it cannot.
In this article
Milk cooling is the largest electrical load, and most of it leaves as waste heat
The New York State Energy Research and Development Authority audited 32 dairy farms and found milk cooling the largest electrical end use at 25 percent of farm electricity, ahead of lighting at 24 percent and ventilation at 22 percent. NYSERDA benchmarked refrigeration at 0.824 kWh electrical per hundredweight of milk, with a range of 0.40 to 1.8 kWh electrical per cwt. Fossil fuels supplied 76 percent of water heating energy on those farms, and almost two thirds already had some form of refrigeration heat recovery.
The University of Missouri Extension reports that up to 50 percent of water heating energy can be recovered from the heat removed from milk, that a compressor heat recovery unit raises water to as high as 110 F, and that propane water heaters in its retrofit program saved an average of 530 gallons per year. UW-Extension gives the wider published range: an RHR unit captures 20 to 60 percent of the Btu removed from the milk, and its maximum water temperature is about 140 F.
The cooling requirement behind that load is regulatory. The Grade A Pasteurized Milk Ordinance, administered through the FDA and the National Conference on Interstate Milk Shipments, requires raw milk for pasteurization to be cooled to 45 F or less within two hours after milking, with a blend temperature not exceeding 50 F. These limits appear in Missouri's 2 CSR 80-2.070. The FDA lists the 2025 revision as current as of July 2026. Heat recovery hardware sits on the compressor discharge and changes nothing about that requirement.
USDA's National Agricultural Statistics Service reports 2025 production at 24,390 pounds per cow per year, the basis for the 67 pounds per cow per day used below.
How a refrigeration heat recovery unit gives heat back, and where it stops
The compressor on the bulk tank discharges refrigerant gas above 200 F. An RHR unit is a refrigerant-to-water heat exchanger placed between the compressor and the condenser; the gas leaves it 75 to 85 F cooler and the water is warmed. The condenser rejects whatever remains. UW-Extension lists jacketed RHR tanks in 50-, 80-, and 120-gallon sizes, each holding roughly one milking's wash water.
The recoverable fraction is bounded because only part of the condenser rejection can be transferred without raising head pressure. The published range is 20 to 60 percent. The temperature is bounded by the refrigerant's condensing band, so the unit tops out near 140 F. Both limits are properties of the refrigeration cycle, not of any tank behind it. The general case is covered in Refrigeration Heat Reclaim for Commercial Hot Water.
The wash side sets the target. University of California Cooperative Extension specifies a wash solution of 160 to 170 F at the start of the cycle, not below 120 F leaving the system, for at least ten minutes, with pre-rinse, acid rinse, and sanitize at 100 to 110 F. University of Florida IFAS puts the wash sink at a minimum of 160 F at the start of wash-up. An RHR unit delivering 140 F leaves a 20 to 30 F gap on the hottest fill. A final-rise heater remains in the system to close that gap.
In a Thermal Tank installation, the RHR loop and the wash water occupy separate coils. The heat exchanger is a dual-coil loop of 32 mm DN corrugated stainless steel tubing, 40 to 80 metres, with all connections through the top head plate. The RHR loop charges the stored volume through one coil; potable wash water passes through the other on its way to the final-rise heater.
The stored volume is a closed hydronic charge that never contacts refrigerant, milk-side equipment, or potable wash water. An immersion electrode element rated at 10 kW per tank provides backup heat, and a Modbus register interface takes a time-of-use schedule from a farm controller.
The heat balance per milking: more Btu available than the wash needs
The following balance uses a 200-cow herd milked twice daily. Milk yield is the NASS national average. The hot water volume follows the UW-Extension example: 75 gallons per milking-system wash, consisting of 50 gallons plus margin, and 69 gallons for a bulk tank wash every other day, allocated here as half a tank wash per milking.
UF IFAS reports total equipment wash water near 3 gallons per cow per day on a 300-cow, three-times-daily system. The 110 gallons used here is an example heated share, not a farm measurement. Substitute the parlor's own vat volume and fill count.
At the labeled 40 percent midpoint, the refrigeration system returns 150,080 Btu per milking against a wash water requirement of 100,914 Btu. Even at the published 20 percent low end, 75,040 Btu nearly covers the 77,979 Btu preheat to 140 F. The midpoint is not a measured recovery value.
Per hundredweight, the milk gives back about 2,240 Btu at 40 percent recovery against a wash need of about 1,500 Btu per cwt at this vat volume. Quantity is not the constraint in this example. The 140 F ceiling and the timing are.
Storage closes the gap between when heat arrives and when the wash runs
The RHR unit produces heat while the compressor runs: through milking and for up to two hours afterward while the bulk tank finishes pulldown under the PMO limit. The milking-system wash runs in a ten-minute fill at the end of milking. The bulk tank wash runs after pickup, every other day on the example farm, at a time set by the hauler.
A jacketed 120-gallon tank sized for one milking carries one fill forward and rejects the rest to the condenser. Storage must be sized for the recovered heat to be held and the usable temperature difference. Storage shifts the timing of heat; it does not create heat.
Storage volume to hold one milking's recovered heat: V = Q ÷ (8.34 lb/gal × 1 Btu/lb-F × ΔT_usable). V = 150,080 Btu ÷ (8.34 lb/gal × 1 Btu/lb-F × 85 F) = 212 gal, at an 85 F usable delta from 55 F to 140 F.
Storage volume to hold the day's recovered heat, two milkings: V = Q_day ÷ (8.34 lb/gal × 1 Btu/lb-F × ΔT_usable). V = 300,160 Btu ÷ (8.34 lb/gal × 1 Btu/lb-F × 85 F) = 423 gal, at an 85 F usable delta.
TEHQ publishes module capacities at a 63 F (35 C) delta: 54.6 kWh thermal for the 350-gallon module and 108 kWh thermal for the 700-gallon module. One milking's recovered heat at the 40 percent midpoint is 44.0 kWh thermal, so a 350-gallon module holds a milking with margin at its 63 F delta rating. Two milkings total 88.0 kWh thermal, inside the 700-gallon module's 108 kWh thermal rating at the same 63 F delta.
Sizing for the day lets the bulk tank wash draw on heat recovered hours earlier. Standing loss on the 700-gallon module is approximately 2.4 F per 24 hours. The Thermal Storage Tank Sizing Calculator converts a farm's own vat volume, fill count, and inlet temperature into a module count.
Modules interconnect, ship in panels, and are assembled through a standard doorway. A filled 700-gallon module weighs 6,046 lb on a 60-inch footprint, or 308 lb per square foot. The farm's engineer confirms the floor.
The final rise: heat pump versus resistance, with the assumptions shown
Potable wash water leaves the storage coil below the stored charge temperature. With the charge at 140 F and a 10 F approach temperature assumed for the coil, the preheated water reaches 130 F. The final-rise heater lifts it to 165 F.
Final-rise energy per milking: Q_rise = V × 8.34 lb/gal × 1 Btu/lb-F × (165 F − 130 F). Q_rise = 110 gal × 8.34 lb/gal × 1 Btu/lb-F × 35 F = 32,109 Btu = 9.41 kWh thermal, for the 35 F rise from 130 F to 165 F.
Resistance-heater electrical input, with efficiency assumed to be 1.0: E_res = Q_rise ÷ efficiency = 9.41 kWh thermal ÷ 1.0 = 9.41 kWh electrical per milking.
Heat-pump electrical input, with COP assumed to be 2.0 at 165 F delivery: E_hp = Q_rise ÷ COP = 9.41 kWh thermal ÷ 2.0 = 4.70 kWh electrical per milking.
The COP of 2.0 at 165 F delivery is a labeled assumption chosen to expose the calculation's structure. Few air-source heat pumps deliver 165 F water, and those that do run at reduced COP at that lift in a cold milkhouse. Replace the assumption with the manufacturer's rating at the design delivery temperature and winter ambient. Delivery temperature versus COP, including hot water storage for CIP, is covered in Industrial Heat Pumps With Thermal Energy Storage.
In the first configuration, a heat pump charges a second storage zone or module above the wash temperature during off-peak hours, and wash water takes its final rise through that coil. This is the method in Load Shifting Hot Water Off-Peak. The all-in-one 700-gallon system packages a tank, heat pump, and controls; its heat pump's suitability for the required delivery temperature must be confirmed rather than assumed.
In the second configuration, the 10 kW immersion element raises the top of the stored charge during milking so the coil exit reaches wash temperature. Element time = Q_rise ÷ element power = 9.41 kWh thermal ÷ 10 kW = under one hour per milking. This assumes the element heats the top zone and the charge is permitted to reach the temperature needed for a 165 F coil exit. The intended operating mode and permitted temperature range require confirmation.
Without heat recovery, a propane water heater supplies the whole 100,914 Btu per milking. The calculation uses an assumed 85 percent heater efficiency from the Purdue audit guide example and 91,452 Btu per gallon of propane from EIA.
Propane consumption without heat recovery: Fuel = Q ÷ (efficiency × 91,452 Btu/gal). Fuel = 100,914 Btu ÷ (0.85 × 91,452 Btu/gal) = 1.30 gal per milking = 2.60 gal per day = 948 gal per year, using 730 milkings per year.
Value of propane displaced by heat recovery plus heat-pump final rise: S_propane = annual propane displaced × propane price = 948 gal × $2.598/gal = $2,463 per year. The price is the EIA residential propane average for the week of October 5, 2026. This configuration uses the labeled heat-pump COP assumption of 2.0 at 165 F delivery.
Electricity added for heat-pump final rise, assuming COP 2.0 at 165 F delivery: E_year = E_hp × annual milkings = 4.70 kWh electrical per milking × 730 milkings = 3,431 kWh electrical per year. Cost = E_year × electricity tariff = 3,431 kWh electrical × $0.1239/kWh electrical = $425 per year. The $0.1239/kWh electrical tariff is an example from the Purdue audit guide, not the farm's measured rate.
At the assumed heat-pump COP of 2.0 at 165 F delivery, the net on these inputs is about $2,040 per year on the water heating line. With a resistance final rise, electricity doubles to 6,862 kWh electrical per year and the net falls to about $1,610 per year. Neither figure includes circulating pump energy, avoided heater standby loss, or any change in refrigeration electricity.
The Missouri program's 530 gallons per farm sits below the 948 gallons here because its farms were smaller and recovered to 110 F rather than 140 F. Replace every input with the farm's own fuel price, vat volume, and inlet temperature before quoting a number.
The comparison below uses the same 200-cow herd milked twice daily. The jacketed RHR arrangement is what most farms already have. Day-sized storage changes the timing; moving the final rise to electricity changes the fuel.
What the economics depend on
Regulatory and market timing, as of the publish date
The FDA's NCIMS model documents page lists the 2025 revision of the Grade A Pasteurized Milk Ordinance, dated June 2026, as current, with the page updated July 13, 2026. The cooling requirement—45 F within two hours and a 50 F blend limit—is carried into state rules; Missouri's 2 CSR 80-2.070 is one example. Heat recovery equipment does not change the refrigeration system's obligation to meet those limits.
USDA Rural Development's Rural Energy for America Program lists energy efficiency improvement grants of $1,500 to $500,000 at up to 50 percent of eligible cost, with refrigeration and HVAC among the named categories and an energy audit required. As of the page's April 9, 2026 update, the grant window was listed as closed while guaranteed loan applications remained open. No eligibility statement is made here; the farm's USDA Rural Development state office and its advisor determine eligibility and timing.
EIA collects its residential propane price only from October through March. A summer analysis carries forward the prior March figure or uses the farm's own invoices.
Eight checks before specifying anything
Frequently Asked Questions
How much heat does milk cooling give back for hot water?
UW-Extension puts the cooling load at 56 Btu per pound of milk cooled from 98 F to 38 F and reports that an RHR unit captures 20 to 60 percent of it. At the labeled 40 percent midpoint, not a measured recovery value, that is about 2,240 Btu per hundredweight, or 0.66 kWh thermal per cwt.
What temperature does a refrigeration heat recovery unit reach?
About 140 F maximum, according to UW-Extension; Missouri Extension cites units reaching as high as 110 F. Wash water must start at 160 to 170 F, so a final-rise heater remains in every configuration.
How big should a dairy hot water storage tank be?
Large enough to hold the recovered heat from the milkings that precede the next wash, at the usable delta the RHR loop delivers. For two milkings at the labeled 40 percent recovery midpoint, the worked example gives about 423 gallons at an 85 F usable delta. A single 700-gallon module holds that recovered heat within its 108 kWh thermal rating at a 63 F delta.
Can a heat pump replace the propane water heater on a dairy?
It can supply the final rise to 165 F if a unit rated for that delivery temperature is selected and its COP at that lift and the winter ambient is confirmed. In the worked example, the final rise is 9.41 kWh thermal per milking, or 4.70 kWh electrical at a labeled COP assumption of 2.0 at 165 F delivery.
Does heat recovery change the milk cooling requirement?
No. The Grade A PMO requires raw milk cooled to 45 F or less within two hours of milking, with a blend temperature not exceeding 50 F. An RHR unit sits on the compressor discharge and does not alter the bulk tank's duty. A unit that raises head pressure can slow pulldown, which is a reason to size the exchanger and water flow correctly.
Size for the day, finish with electricity, and keep the loops apart
The extension arithmetic on milk-cooling heat recovery has not changed in twenty years because the physics has not: 56 Btu per pound removed, a published 20 to 60 percent recoverable range, and a ceiling near 140 F. What changes is what sits behind the RHR unit.
A jacketed tank sized for one milking carries one wash fill forward. A modular tank sized for the day carries the morning's heat to the afternoon bulk tank wash. A suitably rated heat pump or a scheduled immersion element, with its operating mode and permitted temperature confirmed, supplies the final rise the refrigeration cycle cannot. The closed hydronic stored charge remains separate from refrigerant and potable wash water.
On the labeled assumptions above, including the 40 percent recovery midpoint and a heat-pump COP of 2.0 at 165 F delivery, a 200-cow twice-daily herd displaces about 948 gallons of propane per year for roughly 3,400 kWh electrical per year for the final rise. That electricity figure excludes circulating pump energy and any change in refrigeration electricity.
Every input in that result belongs to the farm. The client should verify with its engineering services and dairy inspector that the proposed solution will satisfy the project's goals.
