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My AC unit has gone out in the middle of winter, should I fix now or later?

We recently upgraded our home and installed a new boiler and central heating system but we're running into an issue with the boiler losing pressure. Once it's pressurized to about 1.5 bar, it works perfectly fine while it's on. But when the thermostat turns off, the pressure drops again. We've had the boiler inspected and there’s no unwanted water in it and no leaks from the pressure release outlet. This is happening really often and sometimes the boiler shows 0 bar until we repressurize it. I've checked all the radiator valves and visible pipes, and I can't find any obvious leaks. There aren’t any pipes beneath the main floor either and there are no damp spots on the ceilings. Any advice would be greatly appreciated!

We have a forced air system. In the summer the main floor is always a comfortable temperature but the basement is cold.Any thoughts?