When you're designing your solar thermal installation, you'll need to choose an appropriate pipe size – one which fits the needs of the system and the available installation space and complexity.

Pipe size directly affects the volume of fluid that will be in the system and is itself affected by the required flow. If you choose pipes that are too thin, you'll get far too much friction which will lead to slower flow and a bigger pump to push the fluid around; choose too big and you'll have far too much solar fluid running around the system and higher installation costs.

As a rule of thumb, a solar thermal installation with up to four collectors needs ¾-inch pipes (1.905 centimeters); up to eight collectors will need 1-inch pipes (2.54 centimeters); anything bigger doesn't have a rule of thumb! In any case, you can use the table on this page to calculate the best pipe size for a given flow rate (in gallons per minute).

If you're thinking of using smaller pipes to save money, don't: you'll have to operate a bigger pump to push the fluid through the restricted space, which will cost you more in the long run. It's better to have pipes that are a little too big than too small. Bigger pipes also allow system expansion at a later date.

Fluid Volume Calculations

Once you know what size pipes you'll be using, you can calculate how much fluid there will be in each part of the system at any given time. Depending on whether you choose a drainback or a closed-loop pressurized installation, you'll need to work out how much liquid you need. In either case, you will need the attached table which gives good estimations of the volume of fluid contained by a foot of various types and sizes of pipe.

Drainback systems need a volume of fluid equal to the size of the drainback tank and all the pipes below it (vertically):

Drainback tank volume + (Length of pipes below tank in feet x Volume contained per foot)

Pressurized systems are more complex because the solar loop must always be full. You'll need to start with the collectors, checking how much liquid they hold (the manufacturer or the SRCC will know this). Some collectors can hold as many as 2 gallons each, so don't ignore them!

Next, you'll need to measure all the piping and work out the fluid volume it will contain – for the whole solar loop, not just above or below a given point. Finally, you'll need to check each individual component in the system. Most of them won't hold much at all and can safely be ignored, but some – notably wraparound jacket-style heat exchangers – can contain several gallons, so don't assume anything!

(Length of solar loop pipes in feet x Volume contained per foot) + (Volume contained by solar collector model x # of collectors) + (Volume contained by components)

Expansion Tanks

There's one more important system component to consider here: the expansion tank. All pressurized systems need one of these