Generally speaking the specifications of a circulator pump depend on three things, the pressure loss of the pipe work expressed in metres head, the actual height difference of the system in meters and the flow rate required.

Manufacturers of pumps provide performance graphs, on one axis is the height difference of the system and on the other is the flow rate. There may be two or more lines to look at on such graphs, and they respond to the speed settings on the pump. As a rule you should look to size a pump so that it can meet the specification on its minimum, or mid setting. That way you have spare capacity to cope with changes in the heat transfer fluid and sediment caught in filters etc.

If you've read the pipe sizing blog you will know how to work out the pressure drop figure of your pipe work, and know your likely flow rate too. For our pump sizing exercise we'll use the solar thermal example again:

We need our pump to deliver a flow rate of 4 litres per minute; this is converted to litres per second by dividing by 60.

We also need our pump to overcome a height difference of 2 metres, and if we use 15mm smooth tubing, a pressure drop of (0.021 x 20m) = 0.42m

The figure of 0.021 comes from the manufacturer’s tables, and combining the two results gives 2.42m. The pump selected is described as having a '6m head' so we've got 3.58m of spare capacity.

Easy eh?

Well not quite, although the pump is described as having a 6m head we need to check its performance graph to see if it can achieve 2.42m at a flow rate of 0.066 litres per second ( 4 litres per minute divided by 60). The first problem to overcome is one of units. Manufacturer’s graphs often use 'Q' instead of litres per min. 'Q' is expressed in metres cubed per hour. To arrive at a figure for 'Q' we need to multiply our litres per second figure by 3600 (seconds in an hour) and then divide by 1000 (litres in a meter cubed)

'Q' for our system with a flow rate of 0.066litres per second is therefore 0.2376 cubic meters per hour. 'Q' is normally on the horizontal axis and height in 'm' is on the vertical axis. Checking the graph reveals that our solar rated circulator pump can meet our requirement

When sizing pumps remember that the pressure drop associated with pipe work is much greater for flexible stainless steel pipe!