Glycol protects an exposed loop against freezing but lowers stored energy per gallon, increases required flow and pump head, and adds an annual chemistry regime. The stored-energy penalty applies only to the volume that holds glycol. A coil-type tank keeps the large stored volume as water and confines glycol to the outdoor loop, trading the storage derate for a heat exchanger approach-temperature penalty.
An outdoor air-to-water heat pump, a rooftop PVT array, and a snowmelt slab share one hydraulic fact: part of the loop sits outside the heated envelope and will see sub-freezing temperatures while the circulator is off. DOE's guidance on heat-transfer fluids for solar water heating states the trade plainly: water has high specific heat and very low viscosity but no protection against freezing. Propylene glycol mixtures protect only if the concentration is maintained and the fluid is changed every 3 to 5 years.
Key Takeaways
DOE's fluid guidance does not cover the storage side. A thermal storage tank is sized in kilowatt-hours thermal, and kilowatt-hours per gallon is the product of specific heat, density, and delta-T. Glycol lowers the first term more than it raises the second. Every gallon of glycol-filled storage therefore holds less heat than a gallon of water at the same delta, and the fluid needs more gallons per minute and more pump head to move the same Btu per hour.
The design decision is where the glycol sits and whether the exposed circuit can use an alternative such as drainback. This article quantifies the stored-energy derate, flow correction, pump penalty, and maintenance regime. It does not size a specific project.
In this article
Glycol guidance exists, but not for the storage designer
The published material comes from three directions, and none addresses stored energy. Dow's engineering guide for inhibited propylene glycol recommends a minimum of 25 to 30 percent by volume for corrosion protection. Solutions below about 25 percent may be at risk of bacterial contamination. Concentrations above 50 to 55 percent are rarely necessary because the less glycol used, the higher the relative heat transfer efficiency.
Boiler manufacturers publish the flow correction. One manufacturer's glycol technical note divides the water flow rate by the solution's specific heat at the mix percentage and working temperature. Its example takes a plant needing 133 gpm with water to about 146 gpm with a 30 percent mix at 180 F and adds capacity if the corrected flow exceeds the appliance maximum. That note is cited for the method; the calculations below use ASHRAE specific heat values.
Trade authors publish strategy. John Siegenthaler and Harold Simmons, writing in ACHR News in March 2017, put the annual yield penalty of the heat exchanger an antifreeze solar loop requires at 3 to 5 percent. They also note that glycol must be protected from breakdown during collector stagnation and describe drainback as the alternative.
Institutional standards show how owners resolve the choice. The University of Michigan's hydronic design guideline discourages glycol on environmental grounds, calls 30 percent ethylene glycol typically acceptable for pumped systems in full winter conditions, and prohibits automatic cold-water makeup in favor of a packaged premix fill tank with a two-stage low-level alarm.
The Michigan State University snow-melting equipment standard specifies a 40 percent solution of inhibited, dyed propylene glycol. It requires piping to be labeled with fluid manufacturer, type, and total system volume within six feet of the fill point.
The stored-energy question is left to the designer. Heat pump buffer tank sizing for commercial systems covers the water-only volume arithmetic this article adjusts.
Where the glycol sits decides what it costs
In a coil-type tank, the heat source never enters the stored volume. The Thermal Tank uses a dual-coil loop of 32 mm DN corrugated stainless steel tubing, 40 to 80 metres long, with every connection through the top head plate. The heat pump, PVT array, or waste heat source circulates through one coil. The stored water, a closed hydronic charge, surrounds it. Potable water passes through the second coil and never contacts the stored charge.
The penalties follow the fluid. Glycol in the stored volume reduces stored energy; glycol confined to the coil loop leaves the stored water at its water rating but still requires corrected loop flow, pump head, and maintenance.
The coil placement is not free. A coil heat exchanger needs a temperature difference to move heat, so the glycol loop runs a few degrees warmer than the stored water at the same heat pump output. That approach temperature penalizes heat pump COP and usable storage delta. The coil architecture trades a storage penalty for a heat exchanger penalty; it does not eliminate penalties. On a solar loop, Siegenthaler and Simmons put the heat exchanger penalty at 3 to 5 percent of annual yield.
Glycol in the loop does nothing for a water-charged tank sited in an unheated space. A 700-gallon module loses roughly 2.4 F per 24 hours standing, and an 80-gallon module loses 7 to 8 F per 24 hours. Insulation buys time, not immunity. Heat tracing is an option on the module, and siting inside the envelope is the default. Siting and heat tracing, not loop glycol, protect the stored water. The specification page for the thermal tank lists module sizes and options.
The stored-energy derate is specific heat times density
ASHRAE Fundamentals, 2025 Chapter 31 Table 6, as corrected in the Society's published errata, gives the specific heat of inhibited propylene glycol solutions at 100 F as 0.919 Btu/lb-F at 30 percent by weight, 0.878 Btu/lb-F at 40 percent, and 0.831 Btu/lb-F at 50 percent, against 1.00 Btu/lb-F for water.
Dow's density table gives 63.73 lb/ft3 at 30 percent by volume and 100 F, 64.16 lb/ft3 at 40 percent, and 64.53 lb/ft3 at 50 percent, against 62.00 lb/ft3 for water. Taken against water at 68 F, 62.32 lb/ft3, as Dow instructs, the specific gravities are 1.023, 1.030, and 1.035.
The ASHRAE columns are by weight and the Dow columns by volume. ASHRAE lists 30 percent by weight as 28.2 percent by volume and 40 percent by weight as 38.1 percent by volume. Pairing the nominal concentrations therefore overstates density by about 2 percentage points of concentration and is slightly favorable to glycol. The error is smaller than the variation between inhibitor packages.
The published rating of the 700-gallon module is 108 kWh thermal at a 35 C (63 F) delta-T with a water charge. The calculations retain water specific gravity at 1.00 on the published 8.34 lb/gal basis, while glycol specific gravity is taken at 100 F against 68 F water per Dow. Using 0.995 for water would lower the water baseline by 0.5 percent and narrow each derate by about half a percentage point; the published baseline is retained here.
Q = V × 8.34 lb/gal × SG × Cp × ΔT
Q = 700 gal × 8.34 lb/gal × 1.00 × 1.00 Btu/lb-F × 63 F
Q = 367,800 Btu = 107.8 kWh thermal, published as 108 kWh thermal, at a 63 F delta-T.
Q = V × 8.34 lb/gal × SG × Cp × ΔT
Q = 700 gal × 8.34 lb/gal × 1.023 × 0.919 Btu/lb-F × 63 F
Q = 345,800 Btu = 101.3 kWh thermal at a 63 F delta-T.
Q = V × 8.34 lb/gal × SG × Cp × ΔT
Q = 700 gal × 8.34 lb/gal × 1.030 × 0.878 Btu/lb-F × 63 F
Q = 332,600 Btu = 97.5 kWh thermal at a 63 F delta-T.
Q = V × 8.34 lb/gal × SG × Cp × ΔT
Q = 700 gal × 8.34 lb/gal × 1.035 × 0.831 Btu/lb-F × 63 F
Q = 316,300 Btu = 92.7 kWh thermal at a 63 F delta-T.
The derate is 6.0 percent at 30 percent concentration, 9.6 percent at 40 percent, and 14.0 percent at 50 percent. A project that needs 108 kWh thermal of usable storage at a 63 F delta-T with a 40 percent glycol charge needs about 775 gallons of vessel, 10.6 percent more, before any other margin.
These are thermal kilowatt-hours, not electrical kilowatt-hours. Dividing stored thermal kWh by heat pump COP at the design lift gives electrical kWh shifted. That COP falls with the coil approach temperature.
In the coil placement, the same 700 gallons stay water and hold 108 kWh thermal at a 63 F delta-T. The glycol inventory is the coil and outdoor piping: a 40-metre run of 32 mm tube holds roughly 8.5 gallons, and an 80-metre run roughly 17 gallons. Those coil volumes are estimates computed from nominal bore, not published internal-volume ratings. The derate on that inventory is irrelevant to storage because the coil is a heat exchanger, not a store.
Flow rises by the reciprocal of Cp times SG
To carry the same Btu per hour at the same delta-T, a glycol solution must flow faster by the inverse of its volumetric heat capacity relative to water. The boiler manufacturer's note divides by specific heat alone. Including specific gravity is slightly more accurate and slightly less punitive.
The following illustration uses a 16 kW-thermal heat pump and a 10 F loop delta-T. The heat pump output matches the unit paired with a 700-gallon module in the published Puerto Rico laundromat case study. It makes the arithmetic concrete; it is not a design rule. Substitute the design heat pump output.
gpm_water = Q / (500 × ΔT)
gpm_water = 54,600 Btu/h / (500 × 10 F)
gpm_water = 10.9 gpm.
gpm_glycol = gpm_water / (Cp × SG)
30 percent: 10.9 gpm / (0.919 × 1.023) = 10.9 gpm / 0.940 = 11.6 gpm (+6 percent).
40 percent: 10.9 gpm / (0.878 × 1.030) = 10.9 gpm / 0.904 = 12.1 gpm (+11 percent).
50 percent: 10.9 gpm / (0.831 × 1.035) = 10.9 gpm / 0.860 = 12.7 gpm (+16 percent).
Pump head rises for two reasons. In turbulent flow, head scales roughly with the square of flow. The 11 percent flow increase at 40 percent concentration alone adds about 22 percent head in an unchanged circuit.
Viscosity adds to that penalty. Dow's table gives a 40 percent solution at 9.60 centipoise at 40 F and 26.99 centipoise at 10 F, against roughly 1.5 centipoise for water at 40 F. The guide states that glycol solutions typically have lower film coefficients than water under equivalent flow. Viscosity above about 1,000 centipoise below 0 F can make a fluid impractical without larger pumps.
A heat pump defrost cycle or snowmelt startup at 10 F is where the circulator meets the viscous fluid. Select the circulator at corrected flow and corrected head at the coldest operating fluid temperature. Expect the heat exchanger to need more surface or a wider approach to deliver rated capacity on glycol.
The flow and head corrections raise circulator electrical kWh for every hour the loop runs, throughout the system's life. This is an operating cost in the electrical column of the pro forma, separate from stored thermal kWh.
Concentration follows the design low temperature, not habit
Dow's engineering guide recommends a freeze point at least 5 F below the lowest expected ambient temperature. It states that a 35 percent by volume solution is usually adequate for burst protection where the fluid may slush but must not split pipe. The ASHRAE errata table gives freeze points on a volume basis that agree with Dow within about one degree.
A heat pump loop pumped whenever it is cold is a freeze-protection case: the fluid must stay liquid at the design low. So is a snowmelt slab or PVT array that may sit idle through the coldest night. Interior piping that only needs to survive a power loss without bursting can use burst protection.
Choosing 50 percent where 40 percent meets the design low costs another 4.4 percent of stored energy and another 5 percent of flow for no protective gain. The design low should come from the project's ASHRAE design conditions, not a default. A coastal California heat pump loop and a Michigan snowmelt circuit do not want the same fluid.
The following values are for inhibited propylene glycol by volume from Dow engineering guide Tables 3 and 4. Freeze-point entries and protection concentrations are separate values; the latter specify protection to the stated temperature.
Propylene, not ethylene, wherever potable water is one wall away
The chemistry choice is settled by toxicity, not performance. Ethylene glycol has slightly better thermal properties at a given freeze point, and the University of Michigan guideline accepts 30 percent ethylene glycol on closed campus loops. It is excluded from any loop that shares a heat exchanger wall with potable water and from solar water heating entirely in DOE's guidance, which states that ethylene glycol must not be used because of toxicity.
The CDC's Morbidity and Mortality Weekly Report for September 18, 1987 describes two ethylene glycol poisoning events from building systems. A hospital air-conditioning system was flushed with an antifreeze solution that flowed back into potable water used for dialysis, with one death. A firehall heating system was cross-connected to the potable supply; 29 of 354 interviewed picnic attendees met the case definition.
The ATSDR ToxFAQs sheet for ethylene glycol states that ingestion of larger amounts can cause serious illness or death, with breakdown products that form crystals in the kidneys.
Propylene glycol is listed in 21 CFR 184.1666, within the FDA's catalogue of direct food substances affirmed as generally recognized as safe. Dow's guide states that its propylene glycol fluids are low in acute oral toxicity and used where contact with drinking water or food is possible.
A coil-type tank whose second coil carries potable water must never see ethylene glycol in the stored volume or the source coil. Whether a single-wall coil is acceptable between a propylene glycol charge and potable water is a question for the authority having jurisdiction, not for a vendor.
What glycol costs and what it saves
Storage capacity below is expressed in thermal kWh at the stated delta-T. Divide by heat pump COP at the design lift to determine electrical kWh shifted. Circulator consumption is a separate electrical cost.
Regulatory and market timing, as of the publish date
As of October 2026, the ASHRAE Handbook errata covering the 2022 to 2025 I-P volumes corrects the propylene glycol specific heat table in the 2025 Fundamentals Chapter 31, noting that the SI table had been printed in the I-P chapter. Designers working from a printed 2025 I-P Fundamentals should pull the corrected table.
As of October 2026, DOE's heat-transfer fluid guidance directs readers to the local authority having jurisdiction for fluid requirements. Code treatment of single-wall versus double-wall heat exchangers between a transfer fluid and potable water varies by adopted code and amendment. Confirm with the AHJ before specifying the coil arrangement. No statement in this article is a code determination.
California commercial projects carry seismic anchorage and HCAI considerations on the vessel that are independent of the fluid. Buffer tank sizing for California commercial heat pump systems covers them.
Nine checks before you specify the fluid
Frequently Asked Questions
Does glycol reduce the capacity of a heat pump buffer tank?
Only if the glycol is in the tank. Stored energy is mass times specific heat times delta-T. A 40 percent propylene glycol charge at about 0.878 Btu/lb-F and 1.030 specific gravity near 100 F holds about 9.6 percent less heat per gallon than water. A coil-type tank with a water charge and glycol confined to the source coil keeps its full water rating, while retaining the coil's approach-temperature penalty and the glycol loop's flow, pump, and maintenance penalties.
How much more flow does a glycol loop need?
Divide the water flow by specific heat times specific gravity. At 40 percent propylene glycol near 100 F, that factor is about 0.904, so a 10.9 gpm water loop becomes about 12.1 gpm. Head rises roughly with the square of flow, about 22 percent in this example, before the viscosity increase is added.
What concentration of propylene glycol does an outdoor heat pump loop need?
Enough to hold a freeze point at least 5 F below the design low temperature. Dow's table puts nominal 30 percent by volume at about 9 F, 40 percent at about -6 F, and 50 percent at about -28 F. Dow recommends a 25 to 30 percent minimum for inhibitor performance and calls concentrations above 50 to 55 percent rarely necessary.
Can ethylene glycol be used in a buffer tank that also heats domestic hot water?
No. DOE's solar water heating guidance states that ethylene glycol must not be used because of toxicity, and the CDC has documented poisonings from building heating and cooling systems cross-connected to potable water. Use inhibited propylene glycol, which is listed among FDA's substances affirmed as generally recognized as safe, and confirm the heat exchanger wall requirement with the AHJ.
How often does hydronic glycol need to be tested or replaced?
Dow recommends an annual sample for systems in operation, holding pH between 8.0 and 10.0 and replacing fluid that falls below pH 7.0. DOE states that antifreeze fluids degrade and should normally be changed every 3 to 5 years. Dark color, an oily layer, a burnt odor, or sludge indicate that the fluid has broken down.
Is drainback better than glycol for a PVT or solar thermal loop?
Where the array and piping can be pitched continuously back to an indoor reservoir, drainback removes the glycol fluid, the heat exchanger and its 3 to 5 percent annual yield penalty, and the glycol maintenance regime. Where continuous pitch is impossible, or the loop must run while below freezing as a heat pump loop does, glycol with a coil-type tank is the workable answer.
Put the glycol where the freeze risk is and nowhere else
Glycol is a freeze-protection fluid and should be confined to the piping that can freeze. Every gallon of storage that holds it gives up 6 to 14 percent of its capacity at 30 to 50 percent concentration. Every circulator that moves it works harder, and every system that contains it carries an annual chemistry schedule.
A coil-type tank with a water charge puts the glycol in the coil and outdoor run, measured in tens of gallons, and leaves the 700-gallon store at its full 108 kWh thermal at a 63 F delta-T, accepting a coil approach temperature as the price. That is thermal storage capacity; divide by heat pump COP at the design lift to determine electrical kWh shifted.
Size the store from Cp times SG wherever glycol is in the stored volume. Correct loop flow and head, choose concentration from the design low rather than habit, specify propylene glycol wherever potable water is one wall away, and write the testing schedule into the O&M manual. The client should use this information to verify with their engineering services that the proposed solution will satisfy the project's goals.
