Figuring Head

Rich Davis_2

Trying to design a hydronic heating system for my house.
Been reading Siggy's "Modern Hydronic Heating" and the IBR Guide, I'm having a difficult time figuring head on the system. Then I saw the video from Taco about variable speed dif.T pumps and he said just measure the longest loop times 1.5 and multiply by .04. Is that it? Or is there something more? Sure seems easy that way. Maybe it's just me, much earier to understand if shown, than reading.

Brad White_191

Variable Speed

That old chestnut about taking your longest loop x 1.5 and multiplying it by 0.04 is based on a tried practice as follows:

Take your total linear feet of piping, add 50% for fittings, which will give you a total developed length or equivalent feet of piping. Multiplying that number by 0.04 is the same thing as assuming that your "head loss per 100 feet of pipe" is at a maximum 4.0 feet per 100 feet.

There is a rationale to it, you see, but is that your reality?

Personally, I size my piping for not over 2.3 feet per 100 feet and count my fittings by type and quantity. If your installation is "fitting intensive" and less on straight pipe, the figuring is hardly applicable to you.

Now, in the practical sense, I could use that '1.5 x 0.04" function as a ballpark estimate for preliminary design and it probably would cover me just fine. But it may be too much pump. I owe my clients the lowest practical energy consumption which will get the job done.

With variable speed pumping, at least your pump, properly controlled to delta-T or delta-P, will "Be the Pump You Need". If going in your pump is too large, then your range of turn-down will be limited.

Might it work for you? Just might. But unless you take off the total pipe and fittings, you are guessing. Closely, but guessing just the same.

My $0.02

Brad

John Barba_6

Calculating head

Brad is right -- the 1.5 x .04 is an estimate -- as stated in the webcast. It will estimate head based on maximum flow and velocity per pipe size. If your actual flow and velocity is less than the maximum, then the actual head will be lower than what you calculate....perhaps by up to 50%, depending. A Delta T circulator, as long as it's big enough to begin with (i.e. don't expect a 008-VDT to do more flow and head than it can at max speed) will self-adjust to not only the maximum flow required based on supply/return Delta T, but will also self adjust to provide the correct flow with any combination of zones calling.

Feel free to give me a call at Taco (401-942-8000) or e-mail me if I can help further...

John Barba

scrook_2

Brad is right -- the 1.5 x .04 is an estimate...

For a given size of tube or pipe at a given flowrate: one with a loss of 4 ft head/100 ft of tube at a particular flowrate.

D107

and then different kinds of emitters have different

pressure drops, depending on your pipe sizes, yes? a 3/4" line coming into a big cast iron rad might might not gain any head for the length of that rad or might even 'lose' head, yes?

And how do you count near boiler piping and the boiler itself? Or runs where the pipe size changes?

In general how do the actual pipe sizes enter into that formula? I've seen people on the wall quoting different pressure drops per linear ft of 1", 3/4" pipe etc.

That TACO variable speed video is great by the way, really clarifies the subject and seems like a major advance in hydronics.

Thanks,

David

bob_46

Yo Brad

It sounds like Dave is refering to "static regain" in a hydronic system. Have you ever heard of anyone taking regain into consideration in head calculations?...bob

Brad White_191

Hi Bob

No, never thought of considering it. Unlike more compressible fluids such as air, I wonder if water would behave that way. Certainly I can see passing very slowly through a large volume CI radiator rather than passing through 3/4" piping with embedded fins, would conserve some energy, sure.

Interesting thought, but I think the net difference would be the PD of the tubing it otherwise would be.

Brad White_191

PD, Pipe Size, Flow

Hi David

A given pipe size will have a certain pressure drop at a certain flow rate but also at a certain temperature, oh sure, throw viscosity into it...

For example, one inch type L copper tubing carrying 5.0 GPM will have a "pressure drop per 100 LF" of 2.00 feet.

At 6.0 GPM this rises to 2.75

At 7.0 GPM this rises to 3.60

AT 8.0 GPM this rises to 4.46


You can see the exponential increase/decrease in PD versus flow, so any "full flow" circulator will see far less resistance with a nominal decrease in flow rate.

When figuring total pressure drop of the longest run, one would trace it segment by segment, (from tee to tee essentially, those points where flow rate changes).

Starting with the boiler for example (in larger piping but which contains more flow), go on to the secondary distribution, the emitter, back to the return and back to the boiler... Find that molecule of water and ask yourself, which is the longest journey it can take? What valves, fittings and of what type would it encounter?

The fittings have specific pressure drops, expressed either as "equivalent feet of pipe", or at a stated pressure drop at flow rate. Valves are often expressed using the Cv factor, (flow rate in GPM at 1.0 PSI.)

As you note, variable speed pumping is a wonderful thing. I still would prefer to calculate my head loss (it has to be technically defensible you see), but VS pumping is a mitzvah.