System Pump sizing (Primary loop)

BMod

I have a Lochinvar WHN399 piped as a secondary loop (close T's) using the supplied pump as a boiler pump. I then have a primary loop, 3" black pipe with a 20 gallon buffer tank in line. The pump is capable of around 120 GPM, down to about 40 GPM by use of a VFD. Loads are piped secondary also (close T's) circ pumps on every zone. Loads are water to air heat exchangers, building infloor heat, and outside snowmelt. My question is what GPM should I run the primary loop pump at (system pump)? I am thinking 40 GPM. I know the faster I run the pump the less of a temp drop there is between loads. Any help is appreciated. Thanks.
Also the reason I went with 3" primary pipe size, is because in the future there may be another WHN399 boiler added. So for now the piping is way over sized.

Paul S_3

Total btuh of system and design delta t of system....im thinking 40 gpm for 400k btus....at 20f delta t

GPM=BTUH/ DELTA T X 500

SWEI

What altitude? What design temp? These will help you determine a reasonable net output.

I'd run it at 30-35°F ΔT if conditions permit (including proper VFD tuning.)

BMod

1200 feet, no exact design temp as infloor and water to air exchangers are existing, no details about length of loops, ect. Old boiler was about 125 to 130 degrees so will start with that and fine tune.

SWEI

399k input, no altitude derating, assume 95% = 21.7 GPM @35°F ΔT or 25.3 GPM at 30°F ΔT.

For a short primary loop on a 399 firetube, a Taco 0010 or a Series 100 works great. The WH can control the pump speed, so I'd suggest either a 0010-VVF3 or a 0010-F3 with a Nimbus.