How to Size a Solar Thermal Space Heating System

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A solar thermal space heating installation with storage is one of the most complicated systems to size. There are so many variables: storage size, collector array size, expansion tank size, piping size, heat exchanger size and heat load requirements. There are two ways of working out how big everything needs to be: the complex mathematical way or by rules of thumb.

Mathematical Method

Almost everyone who takes on the daunting challenge of working out their specific needs ends up turning to computer analysis software to help them. Unfortunately, the programs that do the job are very specialized, not widely distributed and, consequently, very expensive.

The good news is that almost every installer has analysis software of some kind, so you can pay a small amount for a survey and get a much more detailed look at your needs. The programs work by asking all the right questions, working out the complex mathematical formulae behind the scenes and popping out a set of results.

Rules of Thumb

Most rules of thumb are applicable to a general set of standards: a cool-to-cold climate (around 8,500 heating degree days per heating season) in an averagely efficient building, properly insulated and with the basic groundwork done to replace windows, close up cracks, plug bad drafts and so on.

  1. Start by working out the biggest possible system for your available space and work down from there.
  2. The collector array ratio is 2 square feet for every 10 square feet of floor space on the main floor of the building. Add 10% to the collector array size for a second storey. This gives a contribution of 40%-50% of the overall yearly heating load.
  3. The heat storage tank (liquid, not sand) ratio is between 1.2 gallons and 2 gallons per square foot of collector array. If your location gets plenty of sunny days in the heating season, aim for a higher ratio.

Remember that you will need to adjust these figures – sometimes by a great deal – if your situation varies from the basic assumptions, whether in climate or building efficiency (better or worse). It is always worth talking to a local, experienced installer as they may have more accurate estimates to work from for your particular region.

Combined Systems

If you're installing a combined hot water and heating system, remember that you can benefit from economies of scale: although the system itself is larger, the heat losses from piping and other components – none of which are major, nor can be stopped – get smaller in relation to overall energy production. This means they also become smaller in relation to the size of your collector array, which in turn means you can install fewer collectors for the same result.

For example, if your calculations tell you that you need 9 collectors for your space heating and 2 more for your hot water supply (a total of 11), you can downsize the hot water part by as much as 25% and go for 10.5 collectors instead. It doesn't sound like much, but every little counts!