Calculating Dynamic Head Pressure

corgi11

I see that the easy calculation comes out to about 6ft head per 100 ft pipe run.

Doesn't the pipe diameter make a difference in the calculation. I would think that for the same distance a pipe of a smaller diameter would have greater resistance?

Jamie Hall

What easy calculation?

Head loss per unit length is a function of pipe internal diameter, pipe internal roughness, and velocity of flow.

Flow quantity is a function, in turn, of pipe internal diameter and velocity of flow.

Look up Hazen-Williams or Darcy-Weisbach for more information.

corgi11

Easy calculation that I have seen is length of run, then add 50%, then multiply by .04. Comes out to 6 ft head per 100 ft of pipe run.

Thanks for all your help Jamie.

hot_rod

head is the mechanical energy contained in a fluid. A circulator adds head, components in the loop remove it.

Blame, or credit Dennis Bernoulli with the origin of the term in his 1738 book “Hydrodynamica” and what is still accepted today as Bernoulli’s Equation.

Pressure is a measurement of force. So, actually there in no such thing as head pressure 😏

This issue if Idronics give you more info on understanding and calculating head energy

https://idronics.caleffi.com/magazine/16-circulation-hydronic-systems

Jamie Hall

https://forum.heatinghelp.com/discussion/comment/1832185#Comment_1832185

But you have completely left out the flow rate and size of pipe. Or the reference you are using has. That just won't do.

LRCCBJ

Just provide us with the total length of pipe…………out………….and back. Provide the diameter of the pipe and confirm you're using copper or iron pipes.

Then we need a flow rate. This is determined by what circulator you choose and its capability which depends on the resistance in the piping. There are charts that show what each circulator can do at various resistance to flow. You do need to have some idea of how much you need in GPM or you cannot get to a conclusion. The required GPM depends on the radiation. Also, this value may change if you are using a zoned system. You need more flow if six zones call versus if one zone calls. This now suggests the use of a circulator that changes speed depending on the load.

jesmed1

https://forum.heatinghelp.com/discussion/comment/1832185#Comment_1832185

That's the "rule of thumb" formula for sizing a circulator. It works because most older hydronic systems (before PEX, etc) had average pipe sizes and flow rates that meant that, if you sized a circulator for the head loss using that rule of thumb, your circulator would end up be sized about right.

Dan Holohan talks about using that rule of thumb on pg. 97 of Classic Hydronics. He says he used that formula sizing circulators for 19 years and never got a complaint.

The "rule of thumb" only works for sizing because it sizes the circulator for the WORST CASE head loss in the system. And as long as a circulator sized for that worst case is installed, it will also provide adequate flow to all the other lower-loss pipe runs.

However, Jamie is correct that if you wanted an exact, ACCURATE number for the head loss in any given pipe run, you would need to know the specific parameters he mentioned.

corgi11

Thank you all for your responses.