Now to the question: "Is that economical?"

As said in the beginning, this question does not come, when you mention that you are going to buy a new car, in which case the answer has to be "No!". 

But nevertheless here you can say "Yes!"

"here" means the complete project:

• the combination of the replacement of the old gas heating and 
• the installation of a thermal collector system for heating support and warm water preparation  and
• it includes the behaviours of the here living people regarding the usage of warm water (time and frequency of taking showers etc.). Normally we don't take showers in the morning. Therefore it was possible to stop the gas burner for a whole week in March though it was cold at night, because the storage was filled up by the collector (and sun) during the day and the underfloor heating was served by the storage and collector during day and night and/or hours without sunshine. Still in the morning there was enough warm water for washing and heating though there was not enough warm water that all family members could have had taken a shower or bath if they would have liked to do so. The storage was filled up with plenty of warm water about 11 to 12 a.m..

In principle you should be an idealist, if you are going to place a small to medium sized thermal collector for heating support on your roof. In January you should be pleased about 20 kWh of harvested energy. This is the equivalent of about 2 m³ of saved gas; nevertheless 1 Euro (Great! But however you saved 2.5 kg CO₂!). Because in Germany the sun is rare in January the portion of heating support is less than 5%. Here you need an efficient additional heating system.

In principle: If you plan to save money, there are easier ways to do so. Which?

That's easy: Thanks to low gas prices we had to invest 4.10 Euro each day in 2006 (before we started with this project) to have a warm house and warm water for showers. A smoker, who uses to smoke a package of cigarettes each day could save more money by giving up his unhealthy hobby though he has not to invest a lot of money first to do so, unless he needs support by books and courses (like "How to Stop Smoking in 10 Days"). But for a non-smoker it can be a piece of cake and a coke as well which he uses to have as a second breakfast at work in the morning or...or...or...

And we try to reduce the daily costs from 4.10 Euro to 2.05 Euro with an investment sum of 16,000 Euro. If you calculate fast, you will find out, that it would take 21 years to catch it up. Now again: "How can this be economical?"

Here are two remarks to this calculation:

• My first forecasts tend to 55% of saved energy instead of 50%. Then it would take 18 years only.
• Furtheron there is a decreasing supply of oil and gas and a increasing demand for energy of China, India, the USA,... and therefore you have to think of increasing prices for gas and oil. Since 2000 my supplier increased the prices for gas by 9% on average. If you consider this within the calculation then it would take 12 years only. With the help of the following table you can make your own forecast of gas price development:

The calculation does not consider these 80 Watts for the devices (pumps, controller,...) and no maintenance costs. You would have both for a conventional heating as well.

Normally you will finance such a system by a credit, because you don't have the money in your cupboard or if you have, this money could bring interest. If you consider both within the calculation the amortisation period will be extended by two to three years. In our case we had a loan from KfW of 3.2% interest for 10 years and 2 years of grace.

And now: "Is this economical?"

Maybe. Be patient. And our environment will be the winner!

And I think in 15 years we will be happy, if we see 1500 Euro on our gas invoice instead of 3300 Euro.