Firstly throw all your thoughts of domestic plumbing out of the window. In a B&B there is a high chance of multiple outlets in use at once. You need to design the system so that there is sufficient storage capacity and flow to service 6 showers at once plus run the baths. It only takes one complaint from a punter of poor flow and the owner will be on your back.
You're looking at needing 36 to 45 litres of stored hot water per person so assuming 2 people per room plus 2 owners that a possible 20 occupants, more if any of the rooms are family rooms. This means that you are going to need 720 to 900 litres of stored hot water.
You'd be well advised to use flow restrictors on all outlets to lessen the demand and the cost of the restictors will be greater than the savings made by using smaller pipe sizes. If you restrict the shower and bath outlets to 10 litres per minute and all others to 8 l/min you will still get very good flow rates. You can assume that only one outlet will be in use per bathroom so you're looking at a peak usage of 80 l/min by the guests plus you need to add on some for the owners and kitchen so you are probably looking at 100 l/min.
If you run everything off one cylinder you are looking at needing 35mm pipework at the start of the runs reducing as you go. With two cylinders splitting the load you will be able to drop down to 28mm. You also need to look at pumped secondary circulation to ensure that hot water reaches all outlets within 60 seconds of opening a tap to prevent legionalla.
Now you've got you hot water capacity you've got to look at how the water gets there. If you use unvented you're going to need 200 l/min coming into the building. If you havent got that you're looking at storage, booster sets or accumalators. For storage you're looking at 135l per bed space or a possible 2700 litres of water. If you add this to the hot water requirement of a possible 900 litres you are looking at 3.6 tonnes of water in the roof space. I'd suggest that a structural engineer is needed to asses any strengthening works that are required before any work goes ahead.
This is the scenario that you need to put to the owners. If they arent willing to go to these lengths then you need it in writing that any defencies are not your responsibilities.
When it comes to pricing give yourself a lot of leeway. There's loads of scope for things to go wrong in these sorts of jobs.
Mike
