WDVS, blown-in insulation, copper water pipes, transition from oil/pellets

Hello everyone!

Last week we viewed a house that was offered to us for purchase. The purchase price is more than interesting for this property, especially since the location is absolutely perfect.
Now there are a few small questions, to which I would first like to provide some information. We viewed the house for about 2 hours to get an initial impression. Here is the info:

250 m² living space,
partial basement,
approx. 13 m eaves side, 11 m gable width,
year of construction (approx.) 1970,
36 cm masonry (estimated),
2-layer masonry (partly plastered) --> probably only an air gap
Heating (oil) from Buderus (boiler built in 1987, burner approx. 10 - 15 years old, according to the chimney sweep a thermal efficiency or whatever it is called of 6)
oil tank 16000 (!) liters
windows approx. 10 years old (K-value 1.1)
facade and roof in good / very good condition
previous owner always had everything done by professional companies (known to me and really conscientious companies), zero own labor
we have friends in the neighborhood of the house who have confirmed this to us
the house is located in West Lower Saxony (if that is important)

Now to my questions:

Was it already customary in the 70s or could it be that the water pipes were already made of copper? Or did this only come later? Copper is more durable... or am I mistaken?

This value of 6 mentioned by the chimney sweep... can anyone make sense of it? What does this value indicate? The chimney sweep approved the heating system a few weeks ago without any problems.

I assume an air gap of 7 cm in the 2-layer masonry. Is it worth insulating this air gap with blown-in insulation? Or do I need a wider air gap to have an insulation effect that justifies the costs? Is there a m² price (just roughly) for this blown-in insulation (standard), so you can get a rough idea? (I would like to preserve the facade... therefore I am reluctant to opt for external thermal insulation composite system)

Is it generally worth switching from oil to alternatives (pellets)? Storage space would be available. On many websites I find no concrete numbers when comparing costs. Is heating cheaper with oil or pellets (based on consumption)? Could the boiler (if still suitable) be converted into a combustion chamber for pellet heating or are the combustion chambers completely different? Roughly what costs should be expected for pellet heating (living space 250 m², unfortunately I cannot provide m³ details yet)?

The masonry chimney has 2 flues. Unfortunately during the viewing I did not pay attention to which flue the heating is connected. Is it allowed to connect a stove / fireplace on the heating flue on the ground floor (heating power 7 kW with high chimney connection)? Or is this generally prohibited?

Questions and more questions... I was able to answer most of them myself but here I reach my limits! I would be very grateful for any factual answer.

Kind regards,

Neubau2013

Hello everyone!

Last week we visited a house that was offered to us for purchase. The purchase price is more than interesting for this property, especially since the location is absolutely perfect.
Now there are some small questions, to which I would first like to provide some information. We viewed the house for about 2 hours to get a first impression.

[...]

Questions upon questions... I was able to answer most myself, but here I reach my limits! I would be very grateful for any factual answer.

You can ask all these questions to the expert, who should of course and definitely (!) be present at the second viewing appointment. They can answer them and also know where to look in order to provide answers.
Such experts can be found online, partially specialized in such assessments. A bit cheaper is looking in the yellow pages and hiring an expert nearby. Usually, you will quickly find one at civil engineering offices, since these civil engineers are often also building surveyors (and energy consultants) at the same time. They are not expensive either; for me it was 2 hours (hourly rate just under 100 euros). Absolute peanuts compared to the purchase price and potential pitfalls!