I am using the 22mm takeoffs that used to go to the heatsink rad from my Rayburn Range and running them to the Hot Water Cylinder.

Due to various reasons - Core Drill Bit being SDS Max and my SDS Drill being SDS-Plus; not wanting to knock big holes into wall with SDS Chisels and then having to try and patch them up (I am a truely **** plasterer/patcher), etc. I am wondering if there is any absolute reason why I cannot drill a 20mm hole using a Drill Bit I have already and just have a 13" section of 15mm pipe going through the wall and using reducers as need be.
I really do not want to be spending money on more drill bits or core drills as this is likely to be my last bit of plumbing or building work for quite a while and I need to get it finished this week!!

I am sure this is not best practice and not recommended, BUT will it still work?
Rayburn is gravity, not pumped. It is used very little (costs WAYS to much to use!) and I really don't care if reducing to 15mm for a while impacts efficiency - As long as it does not create any kind of hazard, that is all that really matters (if I could run the Rayburn without any connection to a cylinder, I would not bother with any plumbing to it!).

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I went to a bookstore and asked the saleswoman, "Where's the self-help section?"
She said if she told me, it would defeat the purpose.

It's quite simple to make the hole bigger with a 20mm bit.
I wouldn't reduce it, especially on gravity.

You could always get the back boiler taken out and replaced with a fire brick.

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It is some kind of ceramic glass back thing and given the price of rayburns, prefer not to mess with it - looking to sell the house soon and it will be a 'feature' - how well the 'feature' works is another matter!! long as the feature does not blow up!

I could try making hole bigger but it is a right faff due to the length and I am losing interest TBH! looking for easy way out!

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I went to a bookstore and asked the saleswoman, "Where's the self-help section?"
She said if she told me, it would defeat the purpose.

I could put a bigger bit in the car for Saturday night

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Thats no way to talk about your lady friend

It's not for me.

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the 20mm drill needed an SDS Chuck - which I had but has now overheated and fused open so bit won't lock

So took the easy option .... went next door and asked neighbour if he had any big f***-off drill bits - and he had a 30mm SDS Core Drill Bit

Job Done

_________________
I went to a bookstore and asked the saleswoman, "Where's the self-help section?"
She said if she told me, it would defeat the purpose.