Primary / Secondary pumping size

aaron_4

1) How long should your Primary loop be in your primary / Seconday pumping designe?

2) In John Siegenthaler book Modern Hydronic Heating 2nd Edition, he talks about sizing the Primary circulator. How do you know what the temperature drop is. He uses 15 in the example.

Thanks Aaron

Dan_15

By the time you pipe in all the valves, the circ., expantion tank, zones, low water cutout, unions, etc. it will be long enough, Mike

pitman9_3

Delta T

> John Siegenthaler book Modern Hydronic Heating

> 2nd Edition, he talks about sizing the Primary

> circulator. How do you know what the temperature

> drop is. He uses 15 in the example.


The temp drop is a function of how much heat is extracted from the water and the flow rate. You can determine the heat extraction by examining what's connected to the system.

Next think about what a btu is. The amount of energy required to raise 1 pound of water 1 degree Farenheit. A gallon of water weighs about 8.25 pounds and a flow rate of 1 gpm means 1 gallon a minute will flow through the system. 8.25 times 60 equals 495 pounds of water per hour. So we've moved 495 pounds of water through the system in an hour if the flow rate is 1 gpm. Call it 500 for easy figuring. That's 500btuh if the flow is 1 gpm and the deltaT (dT) is 1 degree Farenheit.

Now you can look back at your system emitters. Let's assume they will extract 100,000btuh from the water at the water temp you're considering and you determine a specific pump will move 10gpm. Remember each gpm is 500 pounds of water so 10gpm equals 5000 pounds of water moved per hour. Divide the heat output of 100,000 by 5000 and you get 20.

So the dT is 20 degrees F if the emitters extract 100,000btuh and the flow rate is 10gpm.

If you want to shoot for a specific dT divide the total heat extracted by the dT and that will give the pounds of water moved per hour. Divide that by 500 to get the gpm.

Sounds more complicated than it really is. Once you work it out a few times it's pretty easy.