Heating installation company - Achieved more than agreed

I think that "threatening with a lawyer" was not what was meant here, but rather a consultation with the same.

For this, it would probably be advisable to be able to assess the legal situation well. Especially when (unfortunately) no agreement is reached. ;) Only then do the disputes start and all statements and every piece of correspondence come to the table. In unclear situations, it will then have to be interpreted what the parties presumably intended to agree on and what now applies.

Have fun with hobby lawyering. ;)

I generally agree with you – but in this letter I described all points factually. And this cost me nothing except a little brainpower!

This is about an amount of around €2000. If I consult a specialist lawyer and he reviews the documents and gives me a recommendation, I’m already out nearly €1000. If then through legal advice it turns out that I have poor chances of success, I still have to bear these costs.
How do I know this so precisely? I had such a case two years ago – €1000 for legal advice is nothing there.

If the company does not agree with my claims, then you simply negotiate anew and maybe agree on a mutual compromise in the middle of the amount. Only if this attempt fails would I reconsider legal advice or bite the bullet and pay the amount.

After all, I currently have enough to do with the collection already ;-)

If I visit a specialist lawyer and he reviews the documents and gives me a recommendation, I am already almost 1000 € down.

No idea which lawyer you have, but I would give him a piece of my mind. ;)

I'm pulling your ears!

If it stays as it is now, I see the risk that the KfW calculation will become invalid.. It should definitely be recalculated, otherwise you end up committing subsidy fraud or something like that..

but only if the change was also known to the TE (and this could be proven) - ignorance (especially since it was carried out differently than commissioned) offers some protection from punishment, right? Ok, the letter to the heating company would then probably be more of an incriminating than an exculpatory indication...

He IS aware of the danger, he wrote it here. Whether that is ultimately provable is another matter, but one should exercise a healthy dose of caution.