If you are using the LND implementation, there's an anchor reserve for the channels you have open. Here you can read more about anchor reserve https://lightning.engineering/posts/2021-01-28-lnd-v0.12/ It is like an UTXO with max 100k sats that must be kept. So if you have open channels, that must be.
Is not about Umbrel. Umbrel is just a dumb web UI interface, is not the node itself. Is about the LND code.
So I suggest to always keep only one UTXO with 100k sats in it, as a reserve. Then you will be able to spend a whole UTXO to open channels. I know, is kinda stupid, but until Eltoo will be implemented, we have long walk to go...
Thanks
It didn't say anything about the 100k sats maximum. Is it the same if you have 2 channels or 200 channels?
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Is usually 10k sats/channel, but no more than 100k in total, if you keep it in one UTXO always. Re-organize your UTXOs and always keep one UTXO with 100k. Never use that UTXO for opening channels. And all will be just fine.
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