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Är expansionskärlet fyllt så pass mycket att det bara är en liten luftkudde i det kan det nog bli så. Det är flera hundra liter köldbärare i slingan. Temperaturändringar gör att köldbärarens volym ändras och därmed trycket i expansionskärlet.
Sorry for the English...The fluctuation of the pressure with the temperature of the brine circuit is completely normal. This is due to natural expansion and contraction.The only way to not have this pressure yo-yo is to fit a real fairly large expansion tank with a membrane, but there is not much point, because it works also fine without.However, the sounds in the system are not normal, and this is because you have underpressure in the system.The installation manual for your pump (and also for Bosch Compress 6000 series) states, that you must have 2.5-3 bar pressure in the system.When the brine is cold and pump is working, it is good if you have at least 2 bar. But you have only 0.4 bar, this is why you can have sounds, also the efficiency of the brine pump can be lower at such low pressure.Here is the cutout from the manual for your heat pump:You need a simple filling tool with hose and hand pump, they can be rented, and you need to add some coolant (make sure you know which coolant is used, e.g. 30% ethanol or glycol or some other), or call the technician to do it. Probably not the best idea to put 2.5 bar after the brine has cooled, because in the summer you can have then over 3 bar pressure, but 0.4 is too low.
I simply show you what the manual says for your heat pump.The manufacturer says that it is optimal to have 2.5-3 bar in the circuit. It does not depend on location of tank.If you want to think about the details, then it depends a little about VP placement in relationship to the brine pipes, etc.But I like to think that the manufacturer has a good reason why they recommend this amount of pressure in the KB circuit.Will it work with 0.4 bar? Probably. But you have very little playing room there, maybe towards end of winter it can fall even lower.Since you say it has been working for a year, then it is safe to assume all the air is out of the system, and it is not going to fall anymore at same temperature.I just like to do how manufacturer is recommending, they are usually much smarter than the end user or the installer.I have between 2.0 and 3.0 bar in mine, and everything is good. I have a Bosch Compress heatpump, this is ~same as IVT.