How do you calculate head?

[Rangeball]

This is something I need to get a handle on.

I have a 21,200 gallon 16'x30' IG vinyl pool. A skimmer, main drain in the center of the 8' deep end, and one return. Pump is currently an AG Pentair Dynamo 1hp (long story).

The pump sits on the deck about 1' above the water surface and 6' from the pool. I estimate the skimmer piping runs 15' about 1.5' under the water surface, then up 2' and over 2' to the pump. Main drain runs 10' 8' under the water surface, then up 10' then over 2' to the pump. Return runs down 4' then 36' 1' under the water surface to the pool.

Any guesses? Please educate me on how this is calculated. I'm trying to get a precise idea on my GPM flow, but the chart is curved and based on "total head".

Thanks :)

[mshumack]

I printed a couple of pages off the web with very detailed instructions on calaculating head both on a working system (with pressure and vacuum gages) and for new construction (pipe size, lenght of runs, number of fittings, etc.) It was pretty invloved. Unfortunately, now I can't find it. I'll look some more when I get a chance tomorrow. For now, you can start with this page http://www.poolplaza.com/pump-head.shtml
Good luck
Mike

[mas985]

The easiest way is to assume 45-65 feet of head.

The most accurate way is to measured it:

Total head = Return Head + Suction Head

Return Head = 2.31 * Filter PSI
Suction Head = 1.13 * Pump Vacuum (inches mercury)

If you do not want to purchase a vacuum guage then you can assume that suction head is about 40% of the return head (only works for no solar or extra return features) so

Total head ~ 3.2 * Filter PSI

The hardest way and probably least accurate is to use the plumbing configuration. Problem with this is not all PVC pipe is the same friction coeficients and it is difficult to find friction factors for every plumbing element.

[GraceByDesign]

Oh, great. Not only do we need a chem major to understand all this pool stuff, now we need advanced math degrees, too! :rolleyes:

Grace

Avid reader of this forum
but no pool... yet!

[Rangeball]

Oh, great. Not only do we need a chem major to understand all this pool stuff, now we need advanced math degrees, too! :rolleyes:

Grace

Avid reader of this forum
but no pool... yet!

Really.

I'm almost sorry I asked :(

[Rangeball]

Mike, Thanks for the link. I'll give it a look.




If you do not want to purchase a vacuum guage then you can assume that suction head is about 40% of the return head (only works for no solar or extra return features) so

Total head ~ 3.2 * Filter PSI



Thanks for the info.

By filter PSI I assume you mean what my filter gauge reads? Normal operation is 10 psi. So 3.2 x 10 psi = 32 head?

And right now my filter psi is 14 (lot's of cottonwood lately), so head has increased to 44.8?

Is that right?

[mas985]

Without a vacuum measurement, that is about a close as you will get. 32 is pretty low for head. Must be large pipes and a weak pump. Do you have a head curve for that pump? Just curious why the GPM is for that head.

[duraleigh]

Mark, Rangeball,

Yeah, my normal psi is 11....so 35tdh for my pool as well. I have a full-rated 1.5hp starite max-e pro, 2" suction pipe, and 2 - 1.5" return pipe.

I have always assumed mine (tdh) is low because my sand filter is so large....140gpm rating (950 lbs of sand)

That tdh on my pump curve works out to right at 100gpm.

[tenax]

holy crap! that is a big filter you have there...i can't even picture it! great beer keg!;) :eek:

[Rangeball]

Without a vacuum measurement, that is about a close as you will get. 32 is pretty low for head. Must be large pipes and a weak pump. Do you have a head curve for that pump? Just curious why the GPM is for that head.


Here's the data sheet and head curve PDF for my pump-

http://www.pentairpool.com/misc/pdfs/pumps/dynamo_pump.pdf

It's a 1 hp motor. It's also for an AG pool, and I have an IG. This is the pump that came with the pool when we bought the house, and when it burned out 4 years ago this is the same pump the pool store replaced it with (long before I started paying attention and asking questions :( ).

All my piping is 1.5". I'm trying to calculate GPM, which is why I needed to know the head :)

I have been thinking of a 3/4 HP whisperflow, but as long as this one is working, I don't need to spend the money.

[Rangeball]

Looks like 60 gpm, turnover in a bit under 6 hours. I run my pump 24/7. I'm contemplating cutting that in half, but the pool is spotless, so it may be cheap insurance :)

[mas985]

If you do go with a new pump, it will be probably more powerful than the one you have so a 3/4 HP is probably more than enough for you. Your head will likely increase as well as your flow rate.

[Rangeball]

When I talked to Pentair last year after I discovered I had an AG pump, they suggested a 3/4 hp whisperflow.

From what I've been able to tell, I don't know how much it would save me in electricity. The 1 hp dynamo pump I have is rated at 12 amps. I believe the 3/4 whisperflow is 11 amps. Am I missing something? This is all new to me.

[mas985]

I think both of those are at 115v if I am not mistaken so the power usage will be slightly lower for the new pump as well as your turnover allowing you to run the pump for a shorter period of time saving even more.

Have you considered a two speed pump? The low speed will save you even more. Even with doubling the turnover for low speed, you will still save ~30% in energy costs over the 1 speed pump. The extra $100 for the 2 speed would be paid off in a few months.

[cygnusecks]

Wouldn't it be most accurate to mount a vacuum gauge just before the pump inlet, and a pressure gauge just after the pump outlet? Also, I assume that you want the solar ON (i.e. not bypassed) when taking the readings, as this will slow the pump down the most.

[mas985]

Yes it would be more accurate but not many people have the extra guages at hand nor feel comfortable doing the measurements so I was giving him an approximation from calibrations on my pool (i.e. suction 40% of return).

As for the solar, the head approximation of 3.2 * filter PSI breaks down so a direct measurement would be best. The pressure rises on the retun side but the suction side tends to drop a bit as the flow is reduced. Suction then becomes less of a percentage of the return side. The problem is this varies widely with the type of solar panels. I have seen PSI rises from 3 PSI to 10 PSI depending on the panels. It is very difficult to come up with a rule of thumb with solar. However, given the PSI with and without solar, a better calibration can be done with head equations to give a more reasonable number.

[Rangeball]

I think both of those are at 115v if I am not mistaken so the power usage will be slightly lower for the new pump as well as your turnover allowing you to run the pump for a shorter period of time saving even more.

Have you considered a two speed pump? The low speed will save you even more. Even with doubling the turnover for low speed, you will still save ~30% in energy costs over the 1 speed pump. The extra $100 for the 2 speed would be paid off in a few months.


Thanks.

Yes, both have 115v motors.

I currently run my pump 24/7, so I'm getting 4 turnovers a day. I could cut my use by 50% going to 12 hours :)

I'd really like to try doing so, but I'm having the most trouble free year ever since owning the pool, and am afraid to fix it if it ain't broke.

Even though my pump is AG, I'll use it until it dies, as it's doing a fine job on my pool. Just trying to be prepared for that day, cause, you never know :)

[mas985]

Well, if it helps any, I was running my pump for a 1/2 turnover during the winter and now I am runing it a 1 1/2 turnovers per day and the water is crystal clear. However, my bather load is light.

One thing you might want to try, is to lower the run time gradually. Drop a few hours each day until you hit 12 hours so you can monitor any changes.

[Rangeball]

I may do that. I think there's an intermatic timer at my in-laws that they use for Christmas lights, I'll see if I can borrow it and do some playing around.

We're coming into the hottest weather, so it should be a good test :)

[Rangeball]

By filter PSI I assume you mean what my filter gauge reads? Normal operation is 10 psi. So 3.2 x 10 psi = 32 head?

And right now my filter psi is 14 (lot's of cottonwood lately), so head has increased to 44.8?

Is that right?

Mark, on the bold-

Is that right? And if head has increase, why am I getting less flow at the return?

Thanks :)

[duraleigh]

And if head has increase, why am I getting less flow at the return?

Head is resistance....so, less flow.

[Rangeball]

Duh... Exactly what the flow charts show :(

I had it backwards in my memory. Thanks :)

Interesting.

A 5 psi increase cut my turnover rate in half...