Batch Water Heater – Free Hot Water

Probably the cheapest, simplest way of getting free hot water from the sun is a solar batch heater. All you have to do is use a tank of water placed in the sun which will act as a pre-heater for the water which normally goes into the hot water system.

In our case we used a 150 litre immersion heater tank which had stopped working. I am sure that in many countries there would be recycling centres which would let you have old immersion heaters for free. You can even chain them together in order to make a very large capacity pre-heating system. The only possible drawback could be that if you live in a very cold climate they could freeze up in a prolonged cold spell. In this case it would be a good idea to disconnect them during the coldest months.

To make some sort of calculation about the energy that my batch heater could expect to absorb I asked a question on a forum. The answer is here My tank is about 50cm radius by 1 metres length so it probably has a power intake of around 400 watts.

According to my own energy calculator it would take over 11 hours to get the water to shower temperature.

“It would take 11 hours 27 minutes to heat 150 litres of water from a starting temperature of 15 C to a final temperature of 40 C when 400 watts is applied. The energy consumed is 4.58 kWh. The cost in Spain would be 0.87 euros.”

It is not possible to give exact figures but our current setup consists of a 50 litre black PVC pre-heat tube, the 150 litre batch heater in this post and a 150 litre thermosiphon solar heater.

One one day last week that was enough for 5 powerful hot showers at the end of a full day of sun. After that the water ran cold.

The first thing to do with an immersion heater is to strip off the insulation. I decided to only strip off the insulation which is facing the sun and leave the rest. I cut off the mild steel covering with an angle grinder then hacked away at the foam insulation with chisels. In the photo above Shep is helping Hannah (our very pleasant Canadian volunteer) to strip the foam off.

I put the tank in an old cold frame made out of plywood. I sprayed expanding builders foam behind the tank.

The window pane is from an old house before we had double glazing. It was very simple to connect the water to the tank using flexible plumbing fittings. The water that goes into the tank is pre-heated in a long piece of pvc tubing mentioned in another post

The only cost involved was the flexible plumbing tubes for just a few euros.

I estimate that this could save us over 120 euros per year.