Heat pump hot water tank cooling down quickly
Using Home Assistant and ESPAltherma I have live data of our hot water tanks temperature. My wife noticed that it was cooler than she would have expected. At first we put it down to it being very cold weather and the colder water was cooling it down quicker. Saying, that we weren’t convinced. This is a new tank that was installed along side the new heat pump, so should be well insulated.
I went into the loft where our buffer tank is installed and I could here running water. Not a leak, but not what I expected. My Mum has the same setup and I check with her. Hers was silent.
Buffer Tank
Graph showing tank temperature loss
As you can see above, the tank lost 6c in around 4 hours. The system is meant to reheat when the tank gets too cool, but this only seemed to kick in after a bath was taken. After some investigation I found this setting is called Hysteresis and is set to 8c below the reheat value. At the time, ours was 48c when the tank was heater overnight and 45c for reheat. You have to be in installer mode to change this setting.
I then remembered that the install team had said that from time to time I should check the system pressure. When I checked, it was zero. This can be seen below via the graph on the right.
Tank temperature for the last 4 and 24 hours on the 23rd December 2024. Also, the water pressure.
It is very easy to top up the pressure. It should be between 1 and 2 bar, ideally around 1.5-1.6 bar. I topped it up and the buffer tank went silent. Ever since the hot water tank has been back to normal. I have written a Home Assistant automation to warn me if the pressure goes below 1 bar.
It also led me to write a fun automation for a lamp you can see from the bathroom. It sets the colour to red when the tank is very hot. Green when it is normal and blue when it is cool.
Our hot water setting is now 45c when heating and 42c for reheat.
alias: Set Lamp Colour Based on Temperature v2025.1
description: Change light color based on hot water tank temperature or turn it off.
triggers:
- entity_id: sensor.espaltherma_hot_water_tank_temperature
trigger: state
conditions:
- condition: time
after: "07:00:00"
before: "22:30:00"
actions:
- choose:
- conditions:
- condition: numeric_state
entity_id: sensor.espaltherma_hot_water_tank_temperature
above: 45
sequence:
- data:
entity_id: light.office_lamp_light
color_name: red
brightness_pct: 75
action: light.turn_on
- conditions:
- condition: numeric_state
entity_id: sensor.espaltherma_hot_water_tank_temperature
above: 38.5
below: 44.7
sequence:
- data:
entity_id: light.office_lamp_light
color_name: green
brightness_pct: 75
action: light.turn_on
- conditions:
- condition: numeric_state
entity_id: sensor.espaltherma_hot_water_tank_temperature
below: 38
sequence:
- data:
entity_id: light.office_lamp_light
color_name: blue
brightness_pct: 75
action: light.turn_on
mode: single
