The principle of heat recovery is very straightforward – passing hot combustion gases over a heat exchanger of some kind to extract as much heat as possible before venting them to the atmosphere. In practical terms, at its simplest, this can involve little more than routing the flue through other rooms in the house and allowing some of the heat to be transferred and warm up the air – albeit slightly. However, once you choose to go further than this low tech approach, things can soon become a good deal more complicated.
Cool the flue too much, for instance, and it may not draw properly and the efficiency of combustion will suffer – and depending on the type of your fuel you’re using, tars, acids and even water may begin to condense in the flue and cause you problems. Designing a system to suit really is a job for the professionals.
There are a few proprietary systems available which are intended to help energy savers reclaim their otherwise “lost” heat, but from memory, I think most of them are designed around enclosed wood-burning stoves rather than open fires as such. Of course, the same general mechanism should work in both cases, but actually fitting the necessary heat exchanger might be more problematic – and it’s inevitably something you’d need to discuss with the manufacturer to be absolutely sure.
I suspect that a lot is going to depend on the particular circumstances and construction of your pub, so I think you’re going to have to get someone to have a first-hand look before they can give you any really useful advice.
Another area to be clear on at the outset is how any work of this kind will place you in terms of legal requirements. I’m sure you’re much more up on this sort of thing than I am, but it occurs to me that being a licensed premises and – since you mention kitchens – a food retailing establishment too, there are likely to be a few extra hoops for you to jump through, over and above any of the usual concerns of planning and building regulations. It’s always a good idea to check things out with your local council before you get too far down the line – and they can often be a useful source of advice too.
The bottom line is that when it comes to heat recovery, there isn’t really a “one size fits all” solution since everything pretty much hinges on the individual case and, as the saying goes, the Devil is in the detail.
That said, it’s a great idea and would make a tremendous addition to the green credentials of your pub, so it’s certainly something worth exploring further – and the best of luck for the project!