Silent mode, NO! not on heat pumps.

If you read the marketing for your heat pump it will tell you it has a quiet or a silent mode. DONT use it.

Your heat pump has 2 bits inside that make a noise, the compressor, basically a big motor, which nowadays lives in a sound insulated box and the fan which you can see whizzing around. But potentially you can hear the wind and a motor. Nowadays both of these are super quiet but back in the day they were much noisier.

Years ago an idiot came up with the idea that you could ask the unit to slow down the fan and slow down the compressor to make the unit quieter at nigh time. It sounds like a brilliant idea if you have no idea what you are doing.

Lets explore:

Assume you have a house which needs 8kW of heat on a cold day to keep it nice and warm. Your installer has sold you a correctly sized unit which will give you the 8kWs you need. So on the coldest night of the year your unit is going to be flat out keeping you nice and toasty. Its highly likely that you and the neighbours will have all the windows shut, remember its cold out there.

If you are worried that the neighbours might be upset about the noise the heat pump is making you could hit the quiet button. All this does is slows the fans and the compressor reducing the output of the unit to 4kWs, its lovely and quiet and everything is good, except the house will get cold and the run cost will rise. Why?

If you limit the fan speed the amount of air going through the unit reduces, we suck the heat out of this air to heat the house, so less air means we cool it more and the efficiency falls. In cold weather (well actually all the time) you want as much air going through the unit as possible. More air means more heat for us to catch. If you run the fans slowly the coil temperature falls and with it the efficiency, in cold weather It also freezes up quicker because the coils are colder so you do more defrosts. Hence up goes the run cost. What a perfect combination, happy neighbours, a cold house and a high run cost. Pure genius.

So why would anyone put this mode on the unit? Its simple. It was designed for cooling mode. Its a hang over from when the units were air conditioners, lets explore.

In cooling mode the demand for coldness falls at night, there is no sun beating on the building and the temperature outside is lower. So you don't need the full capacity of the unit at night. Peak cooling occurs when its hot and sunny.

In cooling Its a good idea to limit the output of the unit at night to make it whisper quiet, it keeps the neighbours happy, they cant hear your units running while they try to sleep in their stifling bedroom with the windows open. The unit was never going to run flat out at night anyhow, but just in case it decided to, you can limit the fan and the compressor so it stays silent.

So unless you plan to cool your house with the heat pump you should NEVER run it in quiet mode. If you are really that worried about the noise, switch the unit off at least then you only get the cold house and the happy neighbours but not the high run cost.

The clever thing to do would be for the software to not allow quiet mode in heating but to allow it in cooling.