Solar water heater guide for Kerala homes: ETC versus flat plate, sizing, installation mistakes and cleaning

A solar water heater is the one solar product we describe as invest and forget. Once it is on the roof it needs no electricity, no fuel and, beyond a wash every few years, no attention, and a good one lasts ten to fifteen years. Kerala homes use a lot of hot water: bathing, washing, the kitchen. Heating it with an electric geyser at today's unit rates is the most expensive way to do it. The comparison with electric geysers is in a separate article; this one is about choosing and installing the heater itself, drawn from three of our installation videos.

How it works

A solar water heater absorbs the heat in sunlight, not the radiation that a photovoltaic panel uses, and stores hot water in an insulated tank. The current technology is the evacuated tube collector, ETC. Each glass tube has a three-layer coating of copper, aluminium and silver on an inner tube; sunlight heats it, the water inside the tube warms, rises into the tank, and cooler water sinks back to replace it. The circulation runs on its own all day. The older type, the flat plate collector with a copper absorber, has largely been replaced in Kerala by ETC, which is more efficient for the price.

The tank is a flask: an inner stainless steel vessel, glass wool insulation, an outer shell. A good tank holds water at around 65 degrees and loses only a few degrees over seven or eight hours, so water heated in the afternoon is still hot for a morning bath and hot enough for the kitchen. The inner tank should be 304 grade food-safe stainless steel, and most brands give five years of warranty on it.

Sizing: 100 litres is the smallest for a reason

A 100 litre system suits a family of four for daily bathing and kitchen use. With six members, or parents living with you, go to 150 or 200 litres. There is no smaller size, and the reason is not marketing. The heater sits on the roof and feeds a bathroom two floors down through a three-quarter inch pipe; the first 15 to 20 litres you draw is the cold water standing in that pipe. Take 30 litres for a bath and half of it is pipe water. A 50 litre heater would be mostly pipe.

Which is also a habit: use the hot water in one planned session, the morning routine, rather than a little at breakfast, a little at lunch and a little at night, each time flushing the pipe cold. Homes that use it in bursts through the day rarely feel they get their money's worth. Larger systems, 500 to 2,000 litres, exist for hostels, hospitals and factories and are a different design; for a house, 100 to 200 litres is the range.

The installation rule most complaints trace back to

A domestic solar water heater is gravity fed. Water enters from the overhead tank, and the heater only fills if the overhead tank's bottom is above the heater's top; the pressure that reaches your taps is set by that height difference. Keep at least a foot between the bottom of the overhead tank and the top of the heater. Most of the calls we get about a heater giving no hot water, or a weak trickle, are heaters installed level with or above the tank.

Where the roof cannot give that height there are low-profile bed-type models, which sit lower but also deliver lower pressure downstairs. And there are pressurised systems fed by a pump, which can go anywhere but cost roughly three times as much. Decide whether the house has, or needs, a pressure pump before you choose the model.

Valves, vents and pipe

Three small parts decide whether the heater behaves. First, a non-return valve on the cold inlet, always, so that hot water cannot push back into the cold line. If your bathrooms have mixer taps or diverters, fit a second non-return valve on the hot outlet too; mixers leak across when they close and the hot line ends up cold. With plain separate hot and cold taps, one valve on the inlet is enough.

Second, the air vent. The tank breathes as it heats, and steam and air leave through a vent pipe that must stand at the same height as the tank, straight up. A common mistake is to fit an elbow and run the vent sideways; that traps air, and an airlocked tank can burst. Third, the hot water run should be CPVC, the pipe made for hot water, not ordinary PVC.

First fill: not in the sun

Evacuated tubes are ordinary glass, not toughened, and a dry tube that has been sitting in the sun is hot. Fill cold water into a hot dry tube and it can crack. The first fill after installation, and any refill after the system has been drained, should be done before six in the morning or after six in the evening. Tube breakage is not covered by any manufacturer's warranty; a tube that fuses, which shows as the coating turning milky white and the tube no longer heating, is usually replaced, but a broken one is a purchase. Site the heater where nothing can fall on it.

Water quality and cleaning

Ask the brand what water hardness it accepts; many specify a limit around 600 parts per million. Hard water and, in parts of Alappuzha, water with a heavy mineral content leave a layer at the bottom of the tank that leads to corrosion. Feed the heater the cleanest water you have, especially if the hot water is used in the kitchen. Then, every three or four years, have the tubes pulled out and washed; a solar water heater grows algae rather than collecting dust, because of the water, and it needs a scrubber and soap, not a hose. An uncleaned heater loses efficiency exactly as an uncleaned panel does.

What to look for when buying

Match the materials to your location. Near the coast, insist on stainless steel or aluminium stand parts, and confirm that what is sold as stainless actually is. We fit aluminium stands as standard because the corrosion life is so much better than mild steel. Look at the tank shell, the tube diameter and count, and the tank size, because manufacturers vary all four to compete on price. Above all, buy a brand that services in your district: the heater will outlive many of the companies that sell them, and you will want a tube replaced in year eight. A standard 100 litre system from an established brand was around ₹19,500 when we filmed this in 2024; prices move, so ask for a current quotation, but a much cheaper unlabelled one is rarely the bargain it looks.

The electric backup coil

After three days of continuous rain a solar water heater does not deliver hot water. For most families that is a few mornings a year of the old geyser, or a cold bath. For houses with elderly members who need hot water every day, most heater models have a provision for an electric heating element. It is a plug-and-play coil with its own thermostat, which holds the tank at about 65 degrees, and it fits into a port on the tank in place of a blanking washer. It costs about ₹3,500 to ₹4,000, needs a weatherproof plug point with an earth, and can be switched on only when the monsoon demands it. It is not a must-buy; it is the option that makes the heater dependable for the five or six weeks a year when the sun is not.

If you are building a house, run the hot water plumbing for a solar heater even if you are not buying one yet. The heater can be added later; the pipes cannot be hidden later.