Domestic Hot Water (DHW) Tank Calculation

Hello everyone and welcome to the new episode of the HVAC Education Hub Channel.

In this episode, we'll cover how to calculate domestic hot water tanks in your household when you have a heat pump or some other low-temperature source. The first thing to remember is that to have quality domestic hot water tank heating with heat pumps, you need to have a big coil surface of the tank.

The coil surface must be 0.2 m2 / 1 kW or for small models, a minimum of 1.5 m2. If you compare this with your gas boiler or some other traditional fossil fuel source, this coil surface could be also 1 m2, but for the heat pump, it must be much bigger.

Also, approximately 40 to 50 L per person in your household is roughly calculation to calculate how big your domestic hot water tank must be. So in our example, we'll have four persons and 200 L hot water tank. This is the mass of our tank. We have a specific heat capacity of water (SHC), which is 4186 J/kgK. Also, we want a target temperature in a tank of 50 C. And our main water supply temperature is 10 C. So, we have all the data to calculate how much energy we need.
If we put this into calculation, we will have around 33 million joules of energy:

E = m x SHC x (Tout-Tin)
= 200 x 4186 x (50-10)
= 33 488 000 J

This is not capacity or power. This is energy. If we want power, we need to see how much energy per second is this – J/s = W.

If we want to know what is the time we need to divide energy with power. Let's see an example with a 7 kW (7000 W) unit. When we divide our energy with power, we have 4784 seconds (or 79.7 min). So we need approximately 80 minutes to heat, 200 liters tank from 10 to 50 C.

If we use a bigger heat pump capacity, for example, 12 kW (12000 W). And if we divide our energy with this, we have 2790 seconds (or 46. 5 min). So, less than 1 hour, around 45 minutes, to heat 10 to 50 C. This is important only for the first filling (when we fill the water tank, when we start up the unit).

After some time, we will usually re heat the tank, so we will never go below some point. For example, if we put re heat temperature of -8 C and our set point is 50 C – when the water drop below 42 C (50 – 8 = 42), our heat pump will start re heating process again.

Thank you for watching. Subscribe to my channel and follow me on LinkedIn. Feel free to send me an email or message with feedback or questions so we can continue learning about heating systems together.

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