It’s fairly simple. Outdoor unit: PUD-SHWM100YAA (modified to PUD-SHWM120YAA) & indoor unit EHST20D-YM9D equipped with FTC6 . The pump is running on Room Temperature using Wireless Controller.
The indoor unit has 2 boosters built in: 3kW & 6kW. Down to -15C the outdoor unit is enough to meet the required flow temperature. Now lets say that outdoor temp goes down to -17C. The flow setpoint is 45C, but being so cold outside the outdoor unit cannot provide more than 40C. That’s when the Heat pump decides to kick in the 3kW booster to help raise the flow temperature. As soon the flow reaches 43C the booster turns off. When the flow temp decreases the booster kicks in again. So it’s on and off based on the flow temp.
Now imagine -20C outside. The outdoor unit cannot provide more than 37C flow, but the heat pump wants 47C. First kicks in the 3kW booster and helps the flow up to 43C. Then 3kW booster turns off and the 6kW turns on to provide additional power for the flow to go up. They are not on all the time, they turn off when the flow setpoint is reached. On and off every few minutes.
But when we get -28C outside both boosters stays on all the time even during the defrost cycle.
For me it is kind of a big deal to know when the boosters are on because I run automations i Home Assistant that starts and stops the electrical car charging based on temperature outside and if the boosters are on or off. I have 3 phase 25A at home but still not enough when god open the freezer.
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