That would be the

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world leaders Rinnai Gas system LPG mounted out side,
You would need to run pipes in the wall or through the wall to your shower head, copper or stainless/ or if you can get it right out side your shower head one of those Stainless braides connections.
About B12,000 for the unit plus i would estimate about 3-5000 to fit and supply copper pipes/ stainless, with mixer valvue, plus replaster/ render wall and paint. Budget B15,000, plus buy a LPG tank, you will need a lease a 20Pound cyclinder
Special orders from Global house can deliver order from BKK, was going to get one, but the price, and partly solar system I will use decided not to buy, I happy with 4-5 lt a minute water flow from the electris with the solar boost for a few month/ week a year
http://www.globalrinnai.com/thailand_hp/index.htm