If you’ve read the marketing for your heat pump, you might have noticed claims about it having a “quiet” or “silent” mode. Don’t use it.
Your heat pump has two key components that generate noise: the compressor, which is essentially a large motor, and the fan, which you can see spinning. Modern heat pumps are already designed to be extremely quiet compared to older models. In the past, they were much noisier.
Years ago, someone came up with the idea of reducing the speed of both the fan and the compressor to make the unit even quieter, especially at night. While this might sound like a brilliant solution, in reality, it’s anything but.
Let’s break it down.
Suppose your home requires 8kW of heat on a particularly cold day to stay warm. If your heat pump has been sized correctly, it will deliver that 8kW when it’s running at full capacity. On the coldest nights, the system will be working hard to keep your home toasty, and with everyone’s windows shut, there’s no need to worry about outside noise.
However, if you’re concerned about bothering the neighbours with the sound of your heat pump, you might be tempted to switch on the “quiet” mode. But here’s the problem: by slowing the fan and compressor, you’ll be reducing the unit’s output to 4kW. While it may indeed become quieter, your home will also start to get colder – and your running costs will go up. Why?
When the fan speed is reduced, less air is circulated through the unit. The heat pump extracts heat from this air to warm your home, so with less air, efficiency decreases. In cold weather (and really at all times), you want as much air flowing through the system as possible. The more air the unit processes, the more heat it can generate. By running the fans at a slower speed, the coil temperature drops, efficiency plummets, and the unit may freeze more quickly, requiring more defrost cycles. As a result, your heating bills rise.
What do you end up with? A quiet heat pump, an uncomfortably cold house, and increased running costs. It’s not exactly a winning combination.
So why does this mode even exist? It’s simple: it was originally designed for cooling mode.
In cooling mode, the demand for cool air decreases at night when the outside temperature drops and the sun isn’t beating down on the building. The peak cooling requirement occurs during hot, sunny days. At night, limiting the output of the unit to make it whisper-quiet is a smart move, ensuring your neighbours aren’t disturbed while they sleep with their windows open. The unit won’t need to run at full capacity at night anyway, so reducing the fan and compressor speed makes sense for cooling.
But when it comes to heating, quiet mode is a bad idea. Unless you plan to use your heat pump for cooling, you should never activate quiet mode in heating. If you’re that concerned about noise, you’re better off turning the unit off altogether. At least that way, your home will be cold, your neighbours will be happy and you’ll avoid the high running costs.
The real solution? Heat pump software should prevent quiet mode from being enabled during heating, while still allowing it for cooling.
As the post indicates it partly depends whether the heat pump has been sized correctly and also on weather conditions.
Some users have noted savings using Silent Mode where their unit is seriously oversized – especially outside of coldest periods.
I would say that sometimes it is worth having the Quiet Mode on for an Mitsubishi Ecodan if it is oversized as it causes it to run a bit more low and slow.