Unvented cylinder in cellar: is it allowed?
I've been getting contradictory info on if it'd be possible to have an unvented cylinder in our cellar.
Currently, we have a gas boiler heating a small-ish hot water cylinder in the airing cupboard (in our 1st floor bathroom). If we upgrade to a bigger cylinder (as we'd need for a heat pump), this'd mean losing the whole cupboard, which isn't ideal. Alternatively, it could go in our attic. However, I'd rather avoid this as it's more of a nuisance for maintenance, would need us to hack open a far bigger attic hatch to get it in, and/or may be too far anyway (we have high ceilings and a 2nd floor above ground too).
So I was really hoping we could put it in our cellar, which is pretty large and would be a great place for it. But we've been told this can't be done, as unvented cylinders need discharge pipes that must go out above ground to drain off; one can't rely on a pump to get the (potentially 100⁰) water out quick enough. However, online I've seen a small number of suggestions saying it's possible.
So, my question is if having a pressurised cylinder in the cellar is allowed and, if so, what special accommodations one needs to make for it. Also, on the discharge pipe, how high up do they exit from the cylinder? Perhaps if we raised the cylinder, it could be high enough to reach the exit pipes? We currently have a sink and washing machines down there, which pump water up and out to this pipe.