With the growing number of government subsidies for heat pump installation, more and more companies are jumping on the bandwagon and selling you a complete heat pump installation, often air-to-water because it's the easiest to install and the most economical for you.
However, not all of these companies are able to ensure proper dimensioning. Many are capable of sizing by eyeballs, as they say. As long as the installation is on a plain, and in a climate that's not too harsh, it can be done without too much damage. But in many cases, if the power is insufficient or overestimated, your comfort will probably deteriorate, and wear and tear on the machine will accelerate.
In this article, after looking at the role of power in a heat pump installation, we'll look at the causes and consequences of insufficient power, and end with a few solutions to remedy the situation if you find yourself in this situation.
The heat pump and the power it develops
A heat pump is a thermodynamic machine capable of capturing calories from an external medium such as air or water, and releasing them to the water in your heating network or to the air in your room (depending on whether it's an air-to-water or air-to-air heat pump).
Its power is the number of Kilowatts it develops and can deliver to your home to combat heat loss from your envelope AND meet your domestic hot water needs if it also takes care of that.
Before deciding on a heat pump, it's important to clearly calculate the approximate requirement and round it up to 10 or 20%, to be sure of meeting the heating + domestic hot water requirement.
The power of the heat pump is determined by considering the most extreme conditions of the year, so that even in these conditions you can benefit from the advantages of the heat pump to heat you only with it without having to resort to a back-up.
If the heat pump's output is underestimated, during periods of high demand for heating + domestic hot water, even if it's at its maximum capacity, it won't give you satisfaction, and you risk going cold or running out of hot water.
A good heat pump installer will often recommend that you first reduce the heat loss from your home, in order to reduce the power that the heat pump will have to deliver, and thus reduce the cost of your investment.
Power calculations are based on a heat balance sheet, if available (in the case of new buildings). In the case of a renovation, the power calculation is based on the previous consumption of fossil fuels, by looking at how much gas or oil you used to consume, and making the equivalence with Kilowatt Hours. With this amount of KWh required, and considering that a heat pump statistically runs for a certain number of hours depending on its geographical location (plain or mountain), then by dividing it we obtain the power in KW.
Why a heat pump isn't powerful enough
There are several reasons why a heat pump may not be powerful enough:
Installer experience and qualifications
If your installer doesn't have the required labels for heat pump installation, and works with unofficial suppliers (wholesalers or semi-wholesalers), no one on the team may be able to determine the exact power you need to cover your heating and hot water.
The examinations to be passed to obtain the heat pump labels, oblige the companies to be trained on the correct dimensioning.
An inexperienced installer can also make mistakes in the installation, leaving you with an installation with missing parts or essential components fitted the wrong way round.
You can see in this testimony what can happen in this case!
An aging heat pump, with worn or poorly maintained components
If your heat pump is getting on in years, or if you rarely carry out the recommended maintenance (as with a car), then vital components such as the compressor may no longer function as they did in their youth, and the values theoretically achievable by the new machine may no longer be achievable today.
That's why regular maintenance every two years is so important.
Extreme weather conditions
If you live in a place where weather conditions can be really extreme, often in the mountains, then the power may not be sufficient.
If, for example, the temperature is -20°C, most heat pumps will operate in downgraded mode. In other words, they'll keep running, but won't deliver the set temperature required. Instead, they will provide a few degrees below. This is why, in the mountains, it's often a good idea to complement the heat pump with a backup boiler. Most heat pumps also have an electric backup for extreme cases, but this may not be enough if it's really cold.
However, depending on the region, this type of situation can be anticipated and discussed with the owner. You must then accept a downward drift of the ambient temperature if the conditions become extreme, knowing that this is rare and never lasts very long.
The most common cause of underpowered heat pumps is undersizing and poor design.
A breakdown or failure of the heat pump simply
As with any machine, a component may fail. The compressor, for example. You may have a refrigerant leak, or a clogged heat exchanger.
In short, call in your refrigeration installer or repairman to identify the causes of the problem.
Heat pump not powerful enough: consequences
If you find yourself unknowingly with a heat pump that is not powerful enough for your home, the consequences can be as follows:
A lack of comfort
When the temperature drops below 0°C, your heat pump may no longer be sufficient to heat you. You will realize this during your first winter.
If this is your domestic hot water system, it's all the more important to avoid finding yourself in this situation.
Premature wear of components
If the heat pump is undersized, i.e. not powerful enough, it will keep trying to meet the required setpoint temperature, but will not succeed. This means it will run all the time, non-stop.
In a normal situation, with proper sizing, when the set temperature is reached, the heat pump components go into standby mode, saving energy and preserving the machine's lifespan.
If your heat pump isn't powerful enough, it'll be under constant strain, its lifespan will diminish, and your investment will suffer. You'll probably never see the period when your investment is compensated.
An exorbitant bill
If your equipment is equipped with a backup electric heater, it will keep starting up to meet the required setpoint temperatures. So you'll simply be heating electrically. And you'll be amazed at how much higher your bill will be than when you were running on gas, oil or standard electric.
Solutions to avoid an underpowered heat pump or to compensate for it
If you suspect that your heat pump isn't powerful enough, before blaming anyone else, you need to analyze the causes of the problem.
Good, certified installers always oversize machines by 10 to 20%. You need to take a margin of approximation to make sure you're comfortable. Is this the case for you? Check it out, either by comparing the heat pump's consumption in kWh with your previous consumption of fossil fuels or pure electricity, or by checking what the heat balance or other energy certificate at your disposal says. If you didn't do it at the time of sale, do your own investigation.
Always call on an installer who has all the labels, RGE in France for example, GSP in Switzerland, RESCERT in Belgium, etc. This one has gone through a training course that insists on these dimensioning points.
If you're aware that the current period is very cold and that your machine can only maintain the setpoint with the help of an electric resistor, and you're cold, then the resistor may be burnt out, incorrectly wired, or there may have been an oversight in the wiring of this resistor. It may also be that its power is too low, in which case you can add another stage of resistance or install a more powerful one.
It's also possible that your outdoor unit is completely frosted (block of ice on the evaporator), if the defrosting system isn't doing its job properly. In this case, the system is having great difficulty pumping enough heat outside to keep you warm. The defrost cycle must be repaired.
Always perform regular maintenance on the heat pump to prevent malfunction.
About the author
Juliena mechanical engineering graduate and specialist in climate engineering since 2009, has become a writer specializing in renewable energies, with expertise in heat pumps and photovoltaic solar panels for individual housing.