There are two advantages of a gas absorption heat pump over an electric heat pump when temperatures drop below zero: it has a burner, like a boiler, and it uses an ammonia-based solution, which evaporates even at extremely low temperatures (-33°C).
How low can the outdoor temperature be for a heat pump to operate?
For those of us who are already familiar with heat pumps, it is clear that below a certain outdoor temperature their operation becomes not only inefficient from an energy perspective, but in many cases actually impossible for a very large proportion of the models available on the market.
This is because traditional refrigerants used in electric heat pumps typically have operating cycles limited to around –10°C, with several models already stopping at –6°C. These are not polar temperatures, but conditions that can easily occur on any colder-than-average winter day in the Po Valley. After all, the design temperature for Brescia is –7°C — and we are certainly not talking about the Alps.
When I say that these models stop, I do not mean that their performance merely declines; that already happens once temperatures drop below 0°C.
I am referring to a complete shutdown of the unit, meaning a total interruption of the heating service.
In other words, if we expect temperatures to drop below these thresholds (even for just one day), we run the risk of having no heating precisely on that particularly cold day, because the unit is operating outside its allowable conditions.
Heat pumps in winter – are they worth it?
The answer is a resounding yes, when we are talking about gas heat pumps. These solutions ensure high efficiency even when temperatures fall below zero, without compromising indoor comfort.
Unlike electric heat pumps, which may suffer a drop in performance or even shut down in extreme conditions, gas heat pumps maintain operation thanks to an integrated burner.
This means that even when the mercury drops to truly low levels, a gas heat pump continues to operate effectively, delivering reliable, consistent heat.
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The issues with electric heat pumps
Manufacturers of electric heat pumps have, understandably, sought and continue to seek to extend the operating range of their units as much as possible. However, there are physical limits linked to the characteristics of the refrigerant that simply cannot be overcome. This is why electric heat pumps are often equipped with backup electric heaters (i.e. emergency or safety devices) to ensure service continuity in particularly harsh conditions. Alternatively, electric heat pumps are installed alongside traditional backup boilers, significantly increasing the initial investment cost and still leaving room for potential issues (a boiler that only operates occasionally may be more prone to problems due to long periods of inactivity).
In addition to the risk of shutdown, it should be borne in mind that the closer the system operates to its limits, the more the heat pump’s performance deteriorates, to the point of falling well below that of a traditional boiler (not even a condensing one). This is because the refrigeration cycle can no longer extract sufficient heat from the environment, and the heating service is supported (where provided) by an electric heater that warms the system water.
It is easy to imagine what this means in terms of electricity consumption and power demand.
As if this were not enough, at temperatures close to or below freezing, heat pumps also face the issue of coil icing, because the refrigerant inside is at a lower temperature than the outdoor air. As a result, condensation forming on the coil quickly turns into a layer of ice that prevents effective heat exchange.
This issue affects all air-source heat pumps. To resolve it, electric models usually reverse the operating cycle, effectively running the heat pump like a refrigerator: heat is removed from the system (which should in fact be heated) and transferred to the coil in order to melt the ice.
To speed up the process and avoid supplying cold water to the end users, some electric models also rely, once again, on an electric heater to warm the water (which the unit cools throughout the defrosting phase), with the obvious consequences in terms of energy consumption and electrical load.
Cycle reversal inevitably involves relatively long recovery times (tens of minutes) before heating operation can be fully restored once the coil has been defrosted. These delays can translate into potential service disruptions for the connected users.
Gas heat pumps in “sub-zero” conditions – the advantages
And what about gas heat pumps? Are they any different, or do they also “start shivering” below zero?
To answer this, let us look at the fundamental differences between electric and gas technology: gas heat pumps always include a burner, exactly like a boiler, and secondly the refrigerant used in gas heat pumps is a water–ammonia solution.
So what does this mean in practice?
Have we ever seen a boiler refuse to start at –20°C simply because it is cold? Unless the water in the pipes or some other component is frozen, the answer is no—and a gas heat pump behaves in exactly the same way.
Even under the harshest conditions (and we will shortly see just how harsh), even if it is unable to recover heat from the outdoor air, a gas heat pump can still rely on its burner to transfer heat, via the refrigerant, to the system water.
So, in the worst-case scenario, a gas heat pump behaves exactly like a boiler, continuing to deliver heating under all conditions, with a minimum efficiency that remains around 100% (and therefore higher than that of a condensing boiler operating under the same conditions).
And what benefit does the water–ammonia solution bring compared to the refrigerants used in electric units?
It is worth noting that ammonia evaporates at –33°C. This means that this value represents the minimum temperature at which the refrigerant can no longer recover heat from the environment, and therefore only close to this temperature do the unit’s performance levels drop to the 100% efficiency mentioned above.
This seems a remarkably low limit, so much so that we have gas heat pumps installed in Val di Fassa, where design temperatures range between –12°C and –15°C. It may not be Sweden, but it is still very cold!
As also confirmed by the data tables in the design manuals, the efficiency of a gas heat pump—unlike that of electric models—varies only slightly with outdoor temperature, even when it drops well below zero. In any case, even under the most critical conditions, when any electric heat pump would have shut down long ago, an efficiency that is still higher than that of even the best boiler operating under the same conditions is guaranteed.
And what about defrosting (coil icing)?
Thanks to its refrigeration circuit, which is completely different from that of electric units, a gas heat pump can divert part of the hot refrigerant to the coil while still maintaining 50% of its heating capacity, without reversing the operating cycle.
This ensures that the heating service can be maintained at a temperature compatible with user comfort during the short time required for defrosting (around 180 seconds). After that, the unit returns very quickly to its normal operating conditions, ensuring a rapid recovery of its usual excellent efficiency levels.
In short, thanks to gas heat pumps, we do not run the risk of being left in the cold precisely when outdoor temperatures are at their harshest—without the need to add additional equipment or drastically reduce efficiency. On the contrary, we continue to maintain performance levels that are sufficiently high to ensure real savings compared with other technologies, under any conditions.
With gas heat pumps, warmth is guaranteed in every condition.
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