No, its not possible to add an indirect system to an unvented one. However, you could add the indirect system as a separate hot water system and feed it heat from the secondary output of the unvented cylinder.
"The customer didn't want to pay for a much bigger cylinder, so I'm going to pair it with an equal sized cylinder."
Logic check! The customer doesn't want to pay for a bigger cylinder and minor plumbing adjustments but he's prepared to pay for second cylinder and rather more extensive plumbing adjustments. Can't compute that frankly.
"It's not a presurized system, it's fully vented with an indirect cylinder.""
Ah - OK in your first post you said it was unvented whih is why O&G responded the way he did I suppose.
BTW you do know this work is notifiable to the LABC don't you?
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