Hot Tub Solid Fuel Water Heather

My daughter woke up one morning and said "we should make something to heat the hot tub by burning wood" challenge accepted!

First we made the first prototype out of some old reclaimed 15mm copper pipe with solder fittings.

We sized the heat exchanger (or copper pipe) to fit out BBQ tray. We cut and soldered the piping to give a good surface area for the flames to warm the water.

We connected the heat exchanger to some hosepipe and a submersible water pump (that we already had for emptying paddling pools) with a temperature rating of 40deg. The water pump is on the colder side of the system, i.e. pressurising.

The system worked well for about 2 hours and then the solder failed

We upgraded the design to utilise compression joints, we also shaped the heat exchanger to sit over the fire rather than below, this made it much more efficient and we've now used it several times.

Obviously having an open fire at ground level is dangerous, so we put the tray in our BBQ and put is near a wall so it is protected. Also - it is important that the system is manned at all times, should the pump stop for any reason or the hose have a blockage or kink the system will overheat.