We have a wood fuelled furnace which heats water in an accumulator tank in the garage. When I turn on the water it sends hot water up to heat the hot water tank in the airing cupboard (which is fed by the cold water tank in the loft)
The main bathroom and our en suite both have shower pumps coming off that hot water tank (they are both situated next to the airing cupboard). But DS1's bathroom is down the corridor, nowhere near the airing cupboard and has an electric shower over a bath.
The hot water for the bath comes out of the hot water tank and so in theory I am guessing that we could have a mixer shower with a pump in there rather than an electric shower. Is there any reason why I couldn't, such as a limit on the number of pumps coming off the tank?
I suspect that within a few years he'll be having a lot of showers and I'd rather him use the hot water from the furnace than increase the electricity bill!
I think the main issue would be the capacity of the cold water tank - this has to supply the cold water to the showers as well as the hot water cylinder. You probably need around 360 litres (80 gallons) to be reasonably confident that the cold water won't run out. If your current tank is smaller, you could connect another to it, providing this is done so that water can't stagnate in one of them. You'll also need to check that this weight of water is supported safely. All of the above is subject to your actual patterns of shower use - if you expect that all three showers will never be used simultaneously then you could probably get away with a smaller cold water tank. You could also consider using a digital shower (e.g. from Aqualisa) as these can operate in a water saving mode.