ᐅ Domestic Hot Water Heating vs. Air-to-Water Heat Pump Efficiency

Hello everyone,

We have been living in our new house for about three months, which is equipped with an air-to-water heat pump that also heats the domestic hot water. There is a buffer tank maintained at 50°C (122°F) and a fresh water station that heats the hot water on demand. The heat pump is currently in summer mode, meaning the circulation pump for the underfloor heating is off, and the heat pump is only heating the water. For the past two months, I have been tracking the electrical energy consumed as well as the heat produced to get a sense of the heat pump’s efficiency, and I am a bit surprised. Typically, in our household of three people, we use about 5 kWh of electricity per day and get around 11 to 12 kWh recorded by the heat meter. That means a ratio of 2.2 to 2.4 times energy output. I expected higher efficiency since the heat pump brochure mentions a COP of up to 5 at 20°C (68°F) outdoor temperature, but that is at a supply temperature below 40°C (104°F).

So, my questions are: what is your energy consumption for domestic hot water generation in a three-person household? Is it also around 5 to 6 kWh? And does anyone have experience and can help me understand where my reasoning about efficiency might be wrong? Could it be that the electric heater is already being used to reach the 50°C (122°F) buffer tank temperature?

Best regards,

Christian

cmsgarb schrieb:
.... That means 2.2 to 2.4 times. I would have expected higher efficiency since a COP of up to 5 was indicated in the heat pump brochure...

Poor system design and sizing!

Best regards

Thank you for your response. May I ask how high your energy demand is for water heating?

Best regards

Christian

I don’t have a heat meter, so I can’t give you my COB. Just this much: why do you heat your hot water tank up to 50°C (122°F)? We have ours set to 41°C (106°F) with an amplitude of 2. This means it heats water again when the temperature drops to 39°C (102°F) and stops once it reaches 43°C (109°F). For our household of four, even the 200L (53 gallons) tank is sufficient.

We need approximately 3,500 kWh per year for domestic hot water production, heating (around 23°C (73°F) target temperature), and controlled residential ventilation including heat recovery. That is about 10 kWh per day.

@Topic: Take a look at your heat pump’s datasheet to see what it specifies for each temperature. It’s certain that with a heat pump, the higher the temperature, the lower the COP. That’s why it only really makes sense when used with underfloor heating or radiant heating systems.

Since May, my electricity consumption has been between 50 and 60 kWh per month. However, my COP is only around 2.7 with a ground source heat pump.