Glycol Vs Frost Valves in Domestic Heating Systems

Two clients asked me recently about installing or topping up the glycol in their heating systems ready for winter in the Alps. Given the implications of this are that the whole heating system (including underfloor pipework and radiators) would then be running on the Glycol/Water mixture I thought I'd put this post together to explain our position on this.

What Problem are we solving?

Frost protection on the hydraulic liaison pipework between inside and outside units on a heat pump is required for one very specific scenario: sub-zero outside temperatures and where the heat pump exterior unit has failed to operate normally or has had a power cut. If the heat pump is operating normally, then the insulated pipework would be carrying warm and hot water around the circuit, so there is no risk of freezing. It's when the heat pump shuts down that the risk scenario occurs. The risk is frozen water in the pipes causing a burst which would be a costly and inconvenient repair.

What are the solutions?

As per the title of this post, we can either fill the system with a Glycol/Water mixture which has a lower freezing temperature than water, or we can use frost valves to automatically drain water from the system if the specific problem scenario occurs. Either will prevent a burst pipe, but which is best?

Glycol vs. Water: A Numerical Comparison for Heat Transfer and Pumping Power (a.k.a Efficiency)

The efficiency of a heating system depends on the heat transfer fluid used. The two most common fluids are water and a glycol-water mixture. Here we will provide a numerical comparison of these two options in terms of their heat transfer efficiency and the pumping power required.

Heat Transfer Efficiency

The heat transfer efficiency of a fluid depends on its specific heat capacity and thermal conductivity.

Water: Water has a high specific heat capacity (around 4.18 J/g°C) and thermal conductivity (0.6 W/m°C), which makes it excellent for heat transfer.

35% Glycol-Water Mixture: A typical 35% propylene glycol mixture has a lower specific heat capacity (around 3.63 J/g°C) and thermal conductivity (0.4 W/m°C).

Given these figures, the heat transfer efficiency of a 35% glycol mixture is approximately 13% lower than that of water, and its thermal conductivity is about 33% lower.

Pumping Power

The pumping power required for a fluid depends on its density and viscosity.

Water: Water has a density of about 1,000 kg/m³ and a viscosity of 1 cP (centipoise) at room temperature.

35% Glycol-Water Mixture: A typical 35% propylene glycol mixture has a higher density (around 1,036 kg/m³) and much higher viscosity (around 3.8 cP at room temperature).

Given these figures, a pump moving a 35% glycol mixture will require approximately 3.6 times more power to overcome the increased viscosity compared to water, assuming all other factors are equal.

Summary (Glycol mix vs Water)

While glycol mixtures provide freeze protection, they also reduce heat transfer efficiency and require more pumping power compared to water. Glycol is toxic and needs careful handling, as well as regular testing to ensure its concentration and effectivness. Therefore, the decision to use a glycol mixture should carefully consider these trade-offs, particularly in systems where energy efficiency and operational costs are significant factors.

So, if Glycol makes heating less efficient, then what about the Frost Valve?

Typical Frost Valve

The water flows side to side in normal operation.

The lower part is the sensor and valve actuator, it lets the water out to prevent a risk of freezing.

The top part is a vacuum breaker to let air in to replace the water during drainage of water.

Pros:

No Glycol needed so heating system can use water.

Simple operation.

Comparable cost to Glycol at initial installation.

Cons:

Whilst some maintenance can be required the interval is typically 5-10 years and it can be undertaken online (replacement of top and bottom sections).

Space required below valves to allow drainage.

Pipe runs must drain naturally when valve is opened (no U-Bends) to allow total protection.

Conclusion

At Alpine Heating we prefer to engineer the problem away completely, and would typically specify a Heat Pump with a refrigerant connection between the inside and outside units. (R-32 Refrigerant has a freezing temperature of -136 Degrees C!)

If this is not possible then we would recommend the installation of frost valves over glycol.

Then