One of the most famous features of Japan is the availability of hot spring all around the country. If this is an advantage or disadvantage of my new living place in Germany remains yet to be seen, because the hot water close to the surface normally comes along with other earth activity including volcanos and earthquakes… Anyway, I have no source of hot water here other than by gas heating, so at least I wanted to have an outdoor bath resembling Japanese style.
The pot is handmade from high alumina clay, fired at about 1350°C, and shipped from famous Jingdezhen, Jiangxi China, within about 8 weeks. It is a heavy pot, about 350 kg. It resembles the Japanese made bath pots 1:1 but the price is much more competitive and a certain Ms. Wei of the pot company knows how to deal with foreign customers and can manage export of such items as a routine business… Transportation fees in Germany and customs duties, taxes turned out to be more costly than the pot itself.
The essential elements are, (1) the pot, (2) the piping system all made from DN40 glue-fitting PVC-U pipe, (3) a circulation pump (180 Watts, Wiltec 51554, including a filter/strainer), (4) a gas heater (fired by Propan), (5) hot water supply pipe – this is constructed such that it can be drained easily in winter after each use, so it is possible to have a bath also in freezing conditions, made from 18 mm copper pipe and fed by the main hot water system of the house (using natural gas), (6) a regulator system to keep stable temperature.
The water heater has a safety system to switch off the heater after 20 minutes of use, and it has a built-in electric ignition system powered by 2 batteries, 1.5 Volts each. In order to achieve temperature regulation without interfering with the internal circuit of the gas heater, I just switch on and off the 3 V power to the heater.
Normally the heater is only used to keep the water at constant temperature of about 42°C (Japanese baths are really hot…), which takes less than 20 min on-time, anyway, I decided to add a timer circuit that interrupts the heater ever 15 minutes for 30 seconds. So essentially, it can continuously heat the bath even when filling in cold water. Normally the bath is started with reasonably hot water from the house main supply.
Unfortunately, the ignition system by high voltage causes the regulator to hang up. The high voltage sparks change the ground potential it seems, and even some attempts with protection and clean-up circuits (low pass filters) had no permanent effect, when powered from a single supply that is split to 12 V the regulator/timer, and 3 V for the heater. So I now powder the unit from two completely separate powder adapters, and two completely separated circuits, isolated by mechanical relay.
The voltage regulator for 3 V is a simple LM317 circuit, with some more capacitors and features to protect it from any surge voltages.
The temperature regulator and timer, sure you could build it yourself with some microcontroller, display, etc, but no need as the complete module is available for less than 3 EUR mail-order.
The water circuit has the circulation pump. Note that not all the water is passing through the heat, only a certain portion, and pressure is generated by an orifice in parallel with the heater. Temperature sensing is done in the circulation loop. In winter the circulation system can be completely drained and switched-off by appropriate ball valves. So you can still have a bath, but you just can’t heat it by the circulation system (rather need to add more hot water, or just limit the bath to 1 hour or so).
The heat loss seems to be about 1 kW, so we need to run the heater that is using about 1.2-1.4 kg of propane per hour of operation only for some minutes at a time (1 kg propane has about 14 kWh of energy, and we may assume an efficiency of 0.7). Experience shows that is this correct, the heater may switch on every 15-20 minutes of so for about 3 minutes.
Now the system is completely automatic. In normal usage there is no need to adjust any regulators. Just fill in water and switch the bath main switch to “ON”.
Lastly, the water is not just plain tap water but sure enough I am adding bath salts composed such that they resemble my favorite Yamaguchi prefecture hot spring pretty well!