Heat Pumps: Power, Noise and Condensation

Friends, I had a visit the other day from a couple who were considering installing a heat pump in their home, but were concerned about the noise.

To get the heat pump to operate, I ran the hot water for 10 minutes and then requested a hot water ‘boost’ using the app on my phone.

We then stood around the heat pump chatting until the visitors started to get cold. The reason? The heat pump had started up and was blowing cold air over their legs. But they had not heard a thing!

I told them to wait – and slowly the heat pump speeded up and became audible. But it was not what I would call ‘noisy’. In the garden, 5 metres away – you would not be aware of it as a separate sound against the (quiet) suburban background.

In fact, the need for heat pumps to be quiet constrains their design significantly and actually determines their physical size! It would be possible to make heat pumps differently – but they would be either noisier or drippier!

Let me explain…

Click for a larger version. How a heat pump works. A fan rotates and blows air out of the heat pump cabinet. This draws in air which flows over a so-called heat exchanger. This consists of many small diameter pipes containing coolant. The coolant absorbs heat from the air which is later delivered to the house.

Thermal power and air volume

When designing a heat pump, the first thing one needs to know is the thermal power the heat pump must deliver: Let’s say its 6 kW.

If it operates with a coefficient of performance (COP) of 3, then 2 kW out of those 6 kW will be from the electrical motor, and 4 kW will be extracted from the air.

Heat pumps obtain this energy by cooling outside air by roughly 3 °C using a so-called heat exchanger. The heat capacity of air is (more or less) fixed, ~ 1 kJ/kg/°C (source) and 1 kg of air occupies a volume about 0.83 cubic metres.

So, if the heat pump extracts heat from 0.83 cubic metres of air per second, cooling it by 3 °C, then it will extract 3 x 1 kJ = 3 kJ of heat per second i.e. 3 kW.

So to achieve its target of extracting 4 kW of heat, it must pass 33% more air over its heat exchanger i.e. about 1.1 cubic metres of air.

Air speed and noise

Heat pump noise arises from air flow over and around surfaces, and the noise increases with the speed of air flow.

A heat pump can draw a given quantity of air over its heat exchanger in (broadly) two ways.

  • By increasing the speed of air flow over a given area of heat exchanger
  • Or by increasing the area of heat exchanger and keeping the air speed low.

In practice, the faster the air flows, the noisier the heat pump becomes.

So when more heating power is required, manufacturers can speed up a fan a little to increase air speed, but  generally they increase the area of the heat exchanger.

Click for a larger version. Heat pumps made by Vaillant. In order to extract more heat while keeping the air speed low, heat pumps need to be physically larger to accommodate larger area heat exchangers.

Heating Power and Condensation

The heating power of a heat pump is linked directly to the volume of air it passes across its heat exchanger, and the amount by which the air is cooled.

So one other option for increasing the heating power extracted from the air while maintaining low air speeds (i.e. low noise) is to cool the air more.

However when air is cooled, then depending on…

  • the air temperature,
  • the initial humidity, and
  • the temperature drop,

…water may or may not condense. The larger the temperature drop, the more likely water is to condense.

Water condensation is not especially harmful, but at low temperatures, condensation can freeze around the heat exchanger and stop the heat exchanger working.

Heat pumps can detect this and intermittently melt any ice on the heat exchanger – but this makes the operation of the heat pump less efficient.

To cope with condensation all heat pumps are equipped with a drain which allows condensed water to simply drip out the bottom of the casing. This is why it is important to mount heat pumps level – so the designed draining port is actually at the lowest point.

But where does the water go after it drains away?

Allowing water to just drip on the ground – and potentially freeze is not a great idea.

Plumbing the drain into an existing drainpipe may seem adequate but it is not. In winter, when the heat pump is operating below zero, this will freeze and may cause icy spillages, and blockages.

So best practice is dig a ‘soak-away’. For my heat pump we used a ground auger to drill a 15 cm diameter hole a full 1 metre deep. We then filled this with small stones.

The drain hose from the heat pump has a 30 cm long internal heater that prevents icing until the condensate is about 15 cm below ground level. Hopefully the temperature there will be above 0 °C!

Click for a larger version. Arrangement for removing condensation from a heat pump. The casing must be level and water is drained away from the lowest point in the cabinet into a soak-away. The drain is heated along its length to prevent it freezing up at low temperatures.

How much condensation is there?

The amount of condensation depends on many factors but because I knew you would ask, I wrote a spreadsheet to calculate it. (Excel .xlsx file: Calculation of Condensate Volume)

A typical output is shown below. The graph shows the number of litres per day of condensation for a heat pump which delivers 6 kW of heating when the external temperature is 0 °C.

This calculation assumes the relative humidity of the air is 85% and that the temperature drop across the heat pump heat exchanger is either 3.5 °C or 7.0 °C – potentially extracting double the heating power.

In this case the larger temperature drop causes a roughly 10-fold increase in the rate of condensation

The reason for the shape of the curves is that:

  • At low external temperatures, the heat pump must run at high power and so extract heat from a larger volume of air.
  • At low external temperatures, the amount of water in the air is much less than at high temperatures.

Together these two factors combine to produce maximum condensation at temperatures between 5 °C and 10 °C.

Click for a larger version. The graph shows the amount of condensation (litres per day) expected when a heat pump is operating at the external temperature shown so as to maintain an internal temperature of 19 °C. The thermal power at 0 °C is 6 kW and heat pump is assumed to cool the air by ΔT = 3.5 °C  or by ΔT = 7.0 °C. The relative humidity of the air is assumed to be 85%. Notice that cooling the air more drastically increases the amount of condensation.

Non-combatants may wish to stop reading here.

But for those interested, I will explain the calculation below.

Click for a larger image. Spreadsheet for calculating the amount of water which condenses from a heat pump. The text below explains each column in the calculation. The actual spreadsheet is downloadable from a link in the text.

The basic inputs are the shown in red text with a yellow background.

  • The desired internal temperature (19 °C)
  • The thermal power required to maintain 19 °C when the external temperature is 0 °C. (6000 W = 6 kW)
  • The Coefficient of performance of the heat pump (3) which is assumed to be constant.
  • The humidity of the air (85%)
  • The amount (ΔT) by which the heat pump cools the air (3.5 °C)

Column 1: shows the external temperature.

Column 2: shows the temperature demand, the difference between the internal and external temperatures

Column 3: shows the thermal power required to heat the dwelling, assuming it is proportional to temperature demand.

Column 4: shows how much thermal power must be extracted from the air based on the COP.

Column 5: shows the volume of air per second that must be cooled by ΔT in order to extract the required heating power. More air flow is required at low temperatures as the heating demand increases

Next we work on the humidity

Column 6: shows the specific humidity of saturated air with the numbers entered from a data table. This expresses the maximum density (in grams per cubic metre) of water that air can hold without condensing.

Column 7: shows the the same quantity as column 6 but derived from a formula designed to closely match the actual data. This allows me to interpolate between the points in the data table.

Column 8: shows the specific humidity of the air under consideration i.e. with relative humidity less than 100%.

Column 9: shows the specific humidity of saturated air which is ΔT colder than the external temperature.

Column 10. If the specific humidity of the actual air (Column 8) exceeds the specific humidity of saturated air at its new lower temperature, then condensation will occur.

Column 11. If condensation occurs, then the excess water (the difference between columns 8 and 9) will become liquid.

Column 12. Expresses the condensation per cubic metre in terms of condensation per second.

Columns 13, 14, 15 and 16. Expresses the condensation rate in terms of litres per second, per minute, per hour and per day respectively.

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