Glycol vs Antifreeze Valves: How to Protect a Monobloc Heat Pump from Freezing
*Conceptual comparison of a glycol-water circuit and a treated-water circuit with freeze protection valves.*
A monobloc air-to-water heat pump puts part of the heating water circuit outdoors. That is what makes monoblocs simple to install, since there's no refrigerant pipework to run into the house. It also means that, in freezing weather, water sits in a heat exchanger and pipework exposed to sub-zero air.
Built-in frost protection can reduce risk while the unit has power, the controls are enabled and the required water flow is available. Power loss or interrupted circulation can leave the outdoor circuit exposed during a cold spell.
There are two common answers: fill the system with **glycol**, or fit mechanical **antifreeze valves**. Installers, manufacturers and homeowners still disagree about which is better, and the honest answer is that it depends on the project. This guide explains how each method works, what each costs you in performance and maintenance, and which factors usually decide the choice.
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Why a Monobloc Heat Pump Needs Freeze Protection
In a monobloc, the water-side heat exchanger (where refrigerant heat passes into the heating water) sits inside the outdoor unit. Flow and return pipes then carry that water through an outside wall into the building.
If water stops moving in sub-zero conditions, it can freeze in:
- the unit's plate heat exchanger
- outdoor flow and return pipework
- any external valves, strainers or fittings
Water expands as it freezes, which may crack a heat exchanger or split pipework. A failed plate heat exchanger is often one of the more expensive repairs on an air-to-water unit.
The built-in frost protection, and its limit
Many air-to-water heat pumps include **automatic frost protection**, such as starting the circulation pump at low temperatures. Sensor thresholds, standby behaviour and any use of the compressor or backup heater depend on the model.
This works well, but it depends on two things:
1. **Electrical power** to the unit and its controls
2. **Water circulation** through the outdoor heat exchanger
So the scenarios that freeze protection really has to cover are fairly specific:
- a **power cut** during a cold night
- a **failed circulation pump** or controller fault
- the system being **switched off or isolated**, for example in an unoccupied holiday home
- **blocked flow**, for example a closed valve or a clogged strainer
Glycol and antifreeze valves are two different ways of covering these failure cases. They are a safety net behind the unit's own protection, not a replacement for it.
*The highlighted zone shows typical outdoor exposure. Include any unheated indoor sections in the site assessment.*
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Option 1: Glycol (Antifreeze Fluid)
How it works
Glycol protection lowers the freezing point of the system water itself. A heating-grade glycol, usually **inhibited propylene glycol**, is mixed with water at a concentration chosen for the site's minimum expected temperature. The selected concentration provides its specified freeze or burst protection even when circulation stops, provided the fluid remains within its rated conditions.
Propylene glycol is generally preferred for heating systems over ethylene glycol, which is the type commonly used in vehicle antifreeze and is much more toxic. Products made for heat pump and heating systems also contain corrosion inhibitors. Automotive antifreeze should not be used.
Advantages
- **Protection that doesn't need power.** The fluid stays protected through a power cut, pump failure or shutdown.
- **No water loss.** Nothing drains away, so after a freeze event the system can usually restart without being refilled.
- **Well understood in very cold climates.** Glycol is a common choice where long sub-zero periods are expected and the cost of a frozen heat exchanger is high, for example in many commercial projects.
Trade-offs
- **Lower heat transfer.** Glycol mixtures generally have a lower specific heat capacity than plain water, so for the same heat output a higher flow rate may be needed.
- **Higher viscosity and pumping energy.** The mixture is thicker, especially when cold. Pressure losses in pipework and heat exchangers rise, which reduces the available external head from the heat pump's pump and may increase pump energy. *(See our guide on whether a heat pump's built-in circulation pump is enough → [internal link])*
- **Possible efficiency penalty.** Higher pressure drop and lower heat transfer can reduce system efficiency slightly. How much depends on concentration, temperatures and hydraulic design.
- **Cost.** Large systems such as underfloor heating, buffer tanks or long pipe runs may need a significant volume of glycol.
- **Ongoing maintenance.** Glycol can degrade over time, and inhibitors are consumed. Concentration and condition (for example pH) are typically checked periodically, and the fluid topped up or replaced as needed.
- **Handling and disposal.** Draining a glycol system means collecting and disposing of the fluid properly, not pouring it down a drain.
Freeze protection vs burst protection
Fluid suppliers distinguish the onset of ice formation from the conditions associated with pipe rupture.
- **Freeze protection** refers to keeping the fluid liquid and pumpable down to a given temperature. Confirm pumpability, minimum equipment flow and restart conditions separately, since liquid glycol can still be highly viscous.
- **Burst protection** refers to preventing pipes from rupturing. Below its initial crystallisation point, a glycol mixture may turn into an ice-crystal slush. A burst rating applies under specified conditions. Ice slush can restrict circulation, and the equipment may need to warm up before restart.
Burst protection usually needs a lower concentration than freeze protection at the same temperature. That makes it tempting, but it isn't always enough for a heat pump. Slush in the outdoor plate heat exchanger may still be a concern, and some manufacturers specify a freeze-protection level rather than burst-protection. So the concentration shouldn't be chosen from the lowest air temperature alone. It also depends on whether the system must keep circulating at that temperature and what the heat pump manufacturer requires for the heat exchanger.
*A burst-protection rating addresses rupture risk under the stated conditions. Confirm low-temperature pumpability separately.*
A simple illustration: how much does the flow rate change?
*This worked example uses assumed fluid properties to illustrate the calculation. Use the selected fluid supplier's density and specific heat at the same reference temperature and specified concentration for design.*
Assume a **10 kW** heat pump designed for a **5 K** temperature difference between flow and return. The required volume flow rate follows from:
> Flow rate = Heat output ÷ (Density × Specific heat × ΔT)
| | Plain water | Illustrative glycol-water mixture |
|---|---|---|
| Density | ~1.00 kg/L | ~1.03 kg/L |
| Specific heat | ~4.19 kJ/kg·K | ~3.9 kJ/kg·K |
| Required flow at 10 kW, 5 K | ~0.48 L/s (~1.72 m³/h) | ~0.50 L/s (~1.79 m³/h) |
In this example, the required flow rate rises only by about **4.3%**. The bigger effect is usually **viscosity**. A glycol mixture is noticeably thicker than water, and the difference grows as the fluid gets colder. Pressure drop through pipework and the plate heat exchanger therefore rises by more than the flow-rate change alone would suggest.
We deliberately don't give a single "glycol adds X% pressure drop" figure. The real increase depends on fluid temperature, concentration, pipe sizes and whether the flow is laminar or turbulent. It needs to be calculated for the specific system, and the result checked against the heat pump pump's available external head.
Full-system glycol vs a separate glycol loop
In some designs, glycol is kept only in the outdoor circuit, separated from the indoor heating system by a **plate heat exchanger**. This limits how much glycol is needed and keeps the indoor system on plain water. However, the extra heat exchanger adds a temperature difference between the circuits. The heat pump may then need to run at a slightly higher flow temperature, which can reduce efficiency. Size the exchanger for the required duty and approach temperature. Verify the outdoor loop's minimum flow and water-equivalent thermal volume for defrost, and account for pumping, expansion and safety provisions on both circuits.
*Conceptual layouts. On the separated system, the plate heat exchanger transfers heat between two independently circulated fluids.*
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Option 2: Antifreeze Valves (Freeze Protection Valves)
How it works
Antifreeze valves are mechanical, temperature-sensitive valves, usually fitted on the flow and return pipes close to the outdoor unit. When the water temperature at the valve falls to its opening threshold, the valve **discharges water**, and this is intended to keep water from freezing in the exposed section.
How well this protects the outdoor heat exchanger and pipework depends on correct valve placement, pipe layout, drainage and air-entry paths, and the manufacturer's installation requirements. A poorly placed valve may leave water trapped in sections it cannot drain.
The valve itself operates without electrical power. For example, the [Caleffi 108 series technical brochure](https://www.caleffi.com/sites/default/files/media/file/01376_en.pdf) specifies water as the medium, with nominal opening at 3 °C and closing at 4 °C. Check the exact variant, including any ambient sensor for cooling operation. Follow the specified orientation, discharge clearance and air-entry requirements, and check for trapped water around high points, low points and isolation valves.
Advantages
- **The system can run on plain water.** No viscosity or heat-transfer penalty, which supports system efficiency and pump performance.
- **Lower fluid cost and simpler water treatment.** Standard heating water treatment applies, rather than glycol management.
- **No power required.** The valves work during a power cut.
- **Accepted by many manufacturers.** Several heat pump manufacturers list antifreeze valves as an acceptable alternative to glycol in their installation instructions. Requirements vary, so the manufacturer's guidance should always be checked.
Trade-offs
- **They work by losing water.** Once a valve opens, water is discharged. Afterwards, the system usually needs to be refilled, repressurised and bled of air before it can run normally. The exact recovery steps depend on the system design, so the heat pump and valve manufacturers' procedures should be followed. If nobody is on site, for example in a holiday home, the system may stay out of service until someone attends.
- **Installation details matter.** Valves need to be positioned and oriented according to the valve and heat pump manufacturers' instructions, typically close to the unit and with a clear, freeze-safe discharge path. Poor placement may cause them to open when not needed, or not protect the parts that matter.
- **Unintended opening.** Valves may open during cold weather even when the heat pump is healthy, for example if low-temperature water sits in the pipework at low flow. Each nuisance discharge means water loss and possibly a call-out.
- **Other freeze-prone sections may still be at risk.** The valves protect the section they drain. Other exposed components, such as external pipe runs further from the unit, may need separate consideration.
- **Periodic checking.** Like any mechanical component, they benefit from occasional inspection, and some manufacturers may recommend replacement intervals.
*Conceptual illustration. Confirm actual valve locations, drainability and restart procedure in the product manuals. Thermostatic valves normally reclose automatically as the sensed water warms.*
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Glycol vs Antifreeze Valves at a Glance
| | Glycol | Antifreeze valves |
|---|---|---|
| **How it protects** | Lowers the freezing point of the system fluid | Discharges water at low temperatures to reduce freezing risk; effectiveness depends on correct installation |
| **Needs power?** | No | No |
| **After a freeze event** | No discharge by design. Confirm fluid condition and circulation before restarting after suspected freezing | System usually needs refilling, repressurising and bleeding |
| **Effect on efficiency** | May reduce heat transfer and increase pumping energy | Plain water, so no fluid-related penalty |
| **Hydraulic design** | Recalculate required flow and pressure drop for the selected fluid | Standard water-based design |
| **Fluid cost** | Can be significant in large systems | Low |
| **Ongoing maintenance** | Periodic concentration and condition checks | Periodic valve inspection |
| **Typical concern** | Degradation, cost, performance penalty | Nuisance opening, water loss, unattended sites |
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Other Freeze Protection Measures
Glycol and antifreeze valves aren't the only tools. Depending on the project, designers may also consider:
- **Backup power** sized for all components required by the model's frost-protection sequence, including any heater or compressor it calls for, and for the planned outage duration
- **Short, well-insulated outdoor pipe runs**, which reduce the exposed volume and heat loss
- **Electric trace heating** on external pipework (note that this also depends on power)
- **Isolation and drain-down procedures** for buildings that will be left empty through winter
These measures often work alongside the main method rather than replacing it.
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How to Decide: The Factors That Usually Matter
There's no universal right answer, which is why the debate continues. The choice usually comes down to a few project-specific factors.
**1. Manufacturer requirements.**
Start here. Some manufacturers require one method, accept both, or advise against one, and warranty terms may depend on it. The installation manual for the specific unit should be the first reference.
**2. Climate and design temperature.**
In mild climates where freezing nights are occasional and short, the risk window is small. Where long periods well below freezing are expected, the consequences of a failure are higher, and glycol is often favoured, particularly on commercial projects.
**3. Power reliability.**
Assess the credible outage duration and how quickly exposed water can cool under site conditions. Include loss of circulation and planned shutdowns in the assessment.
**4. Occupancy and response time.**
An antifreeze valve that opens in an occupied home is an inconvenience. In an unoccupied holiday home or a remote site, it may leave the building without heating until someone can refill the system.
**5. System size and hydraulics.**
Large water volumes make full-system glycol more expensive. Glycol's higher pressure drop also has to fit within the pump's available head, so the hydraulic design must be checked rather than assumed.
**6. Efficiency priorities.**
For well-designed low-temperature systems where efficiency is the main goal, avoiding glycol can help, provided the remaining freeze risk is acceptable and properly managed.
*Check equipment requirements, climate, power reliability, occupancy and hydraulics before selecting a strategy.*
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Frequently Asked Questions
**Does an air source heat pump always need glycol?**
Not always. Many monobloc systems run on plain water with antifreeze valves or other measures, and some installers rely on the unit's built-in frost protection together with reliable power and good design. Whether glycol is needed depends on the manufacturer's requirements, the climate and the risk profile of the site.
**Can I use car antifreeze in a heat pump system?**
No. Automotive antifreeze is typically ethylene glycol based and is not formulated for heating systems. Heat pump systems generally use inhibited propylene glycol designed for heating applications and compatible with the manufacturer's requirements.
**What concentration of glycol is needed?**
It depends on the lowest temperature the system needs to withstand, and on whether it only needs to avoid bursting or must also keep circulating at that temperature. The concentration is usually chosen from the glycol supplier's data and the heat pump manufacturer's guidance, with a safety margin. Stay within the fluid supplier's effective concentration range and the equipment manufacturer's allowable limits. Check cold-start viscosity and minimum flow.
**What happens after an antifreeze valve opens?**
The valve discharges water from the outdoor section. Once temperatures rise, the valve typically closes again, but the lost water usually has to be replaced and the system repressurised and bled of air before normal operation. Recovery steps vary by system, so follow the heat pump and valve manufacturers' procedures.
**Can glycol and antifreeze valves be used together?**
Use the medium specified by the valve manufacturer. A water-only valve can discharge the glycol mixture if fitted to the same circuit. The Daikin manual cited above prohibits combining the two in the same circuit for the models it covers. For a design with separate circuits, check compatibility and protection independently for each circuit.
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Continue Exploring
**🏠 Related Products**
- Nordtherm Air-to-Water Heat Pump Range → https://nordthermglobal.com/products/r290-commercial-air-to-water-heat-pump-50-100kw-high-temp
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Planning a Project in a Cold Climate?
If you're specifying a monobloc heat pump for a site with freezing winters, we can help you review the freeze protection approach alongside flow rates, pump head and system volume. Share your design temperature, approximate system volume and outdoor pipe length, and our engineering team will be happy to talk it through.EMAIL：[email protected]
