sizing a recirculating pump

How do I size a recirculating pump? I noticed Grundfos? makes many sizes. I'm using two 1" - 3/4" trunk lines that go to each end of the house and two 1/2" return loops that T into a 3/4" line in the middle and goes back to the storage tank type heater. Each run is about 30-40' to the farthest outlet. How do I size it? GPM's, size of pipe, friction loss? etc. Thanks everyone. Tad

Edit Post

Try the UP1518SF or UP1518SU (F is for flanged, U is for union). Make sure you isolate the pump with shutoff valves and put a check valve and hose bib in the return line. An aquastat and timer isn't a bad idea either.

pump

You only want enough flow to keep the lines hot, so the smaller the pump the better, usually. With a larger pump you will want to reduce the flow to just maintain the temperature, but with two lines you have a problem because they have to be throttled to balance them, but you do not want to reduce the total flow to the pump. To control the velocity, you need a valve between the pump and the water heater. If you control the velocity with the balancing valves you will ruin the pump.