You can also buy a laser infrared thermometer for about 50 euros and when it's below freezing outside, walk around the house with it to check critical points (window and door connections, seals, patio or platform connections to the house, etc.). No matter how inaccurate such a device is: it will show you tendencies in any case. Is it noticeably warmer somewhere than on the exterior wall? The same applies to the attic, if available. Is heat "creeping" out of the house anywhere? But as mentioned before: hot water demand for 6 people should not be underestimated either.