The temperature of our cold water supply is very cold maybe 5C. When this water is mixed into the solar hot water tank it cools the rest of the water down considerably.

We got ripped off for 1200 euros by a company called http://tutiendasolar.com/ (We paid them for a solar hot water system which they were never able to deliver.) So I did not have an appetite to spend any more money.

I realised that if we could increase the temperature of the water supply it would increase the efficiency of the hot water system.

I bought 100 metres of black 32mm PVC agricultural tubing which can withstand 8 atmopheres for 57 euros. The two brass connectors cost 7 euros each so that brings the total to 71 euros.

To calculate the amount of water in the tube you use this formula.

Length of pipe x Pi x Radius of pipe

In our case:

100m x 3.142 x 1.6cm = 502.72 cc

That is 50 litres.

According to my calculations:

If the water in the tube could get to 38C it would be enough for an 8 minute shower at 6 litres per minute.

**Here are some other calculations:**

To compare the cost of heating the water in this tube with an electric water heater we need this calculation:

We are heating 50 litres of water by 35C (from 5C to 40C)

The formula I have is imperial:

weight of water in pounds x temp increase in deg Farenheit /3413 = KWH used by an electric heater.

The metric figures are:

55 kilos = 110 pounds

35C = 95 farenheit

SO

110 x 95/3413 = 3.061822 KWH

The average price of electrity in Spain is 0.19 per KWH

SO

3.061822 x 0.19 = 0.58

Each time the tubes heat enough water to have an 8 minute shower it theoretically saves 58 euro cents.

The supplementary water system will pay for itself after 122 showers.

Let’s assume that there are always 5 people here and they have showers on average 5 times per week. That is 25 showers per week.

This system should pay for itself in about 5 weeks.

According to carbonfund.org the carbon emissions of electricity are :-

0.0005925 metric tons CO_{2} per kWh

If there are 5 people here having 5 showers per week that is 1300 showers per year

If each shower uses 3.061822 KWH that is 3980 KWH saved per year (or 756 euros)

This means that we theoretically save 2.35 metric tons of CO_{2} being emitted per year.

Maybe you can find an error with my figures but it seems that solar water heating makes sense.

See another solar water: How to make a batch water heater