I need a new pump motor for my 28k gallon pool. I replumbed the piping in the pool house last year with a lot more valves, bends and couplings than it had before. Using an online blog at INYO pools that walks you through the resistance from various components I have come up with a 115' feet of head ( there are a lot of elbows, 45s couplings, etc.) I use the generic values for things like skimmers, main drains, sand filter and heater. They might vary a bit for my exact units but I doubt it will be significant. Anyway, 115' is off the charts at the 60gpm rate the filter is rated for at 1.5 inch pipe, which is what the pool has. Currently the pump has a 1.5 HP motor, but there is a 2.5 hp motor available at a good price and it seems like my not very efficient pipe layout and the estimated lengths of runs from the pool house to the pool is making a big motor necessary. So my question before I buy is- Does 115' feet of head sound plausible or is it so out of whack that I've made a major error in calculations? The pool is 16 by 32 plus a 12'diameter cove so my calculation of all the runs came to about 250'. That plus all the equipment and fittings adds up fast. Any useful advice is gratefully received.
Calculating Feet of Head
- Thread starter Ehanson
- Start date
It doesn't work like that. Feet of head is not based on the length of the plumbing (at least not directly).
What is the filter pressure?
If anything, you should downsize the pump.
What is the filter pressure?
If anything, you should downsize the pump.
Pressure is reading about 12 PSI. But I was wrong about the current pump motor. I swapped in the pump from the spa jets when the 1.5 hp motor went down. This one turns out to be 3 hp! is it common to use a very high horse power for spa jets? So when you say it doesn't work like that, what part isn't correct?
It is not trivial trying calculate the head loss and really does not serve any purpose in trying to do so. Flow rates generally should not matter.
What exactly are you trying to do?
What exactly are you trying to do?
I'm trying to determine the size of motor I need for my pool pump. The motor I was using has burned out and needs replacing. I understood from reading this article: How Do I Calculate Feet of Head for My Pool? - INYOPools.com, that a number of factors go in to making sure that you have the correct motor for the work it has to do based a number of variables that create friction and impede flow.
What is the motor to be used for?
Are you replacing only the motor or the motor and pump?
Circulation and filtering, a small 1HP (2-speed), would be plenty.
If also for spa, then you need to go bigger.
Note if just replacing the motor, it has to match (or be bigger) than your existing impeller. Or you have to also get a smaller impeller.
Are you replacing only the motor or the motor and pump?
Circulation and filtering, a small 1HP (2-speed), would be plenty.
If also for spa, then you need to go bigger.
Note if just replacing the motor, it has to match (or be bigger) than your existing impeller. Or you have to also get a smaller impeller.
A pool doesn't have a single number for feet of head. The feet of head depends on the flow rate.
If you graph the feet of head vs flow rate on a graph, you get a curve called a system curve.
A system curve is an exponential curve that starts at 0,0 and goes up exponentially by a square factor.
If you double the flow rate, the feet of head goes up by 4 (2^2).
The system curve can be placed on the same graph as various pump curves (available at manufacturer's website). The operating point for any pump is where the pump curve intersects with the system curve.
Your pressure can be converted into feet of head by multiplying by 2.31.
12 x 2.31 = 27.7 feet of head for the pressure side.
3 hp would create more than 12 psi, so the pump might be on low speed of two speeds or have a smaller impeller.
There is also a suction side head that can be measured by a vacuum gauge.
If we estimate the suction side head to be 14 feet, the total head is about 41.7 feet. But that's only at the current flow rate. Increasing or decreasing the flow rate will change the head by a square factor.
So, all that being said, what do you need the pump to do?
For example, you need to circulate the water but do you need any special flow for spa jets, solar etc?
If you just want circulation, then a smaller pump would be plenty. You can do a two speed pump or variable speed.
If you graph the feet of head vs flow rate on a graph, you get a curve called a system curve.
A system curve is an exponential curve that starts at 0,0 and goes up exponentially by a square factor.
If you double the flow rate, the feet of head goes up by 4 (2^2).
The system curve can be placed on the same graph as various pump curves (available at manufacturer's website). The operating point for any pump is where the pump curve intersects with the system curve.
Your pressure can be converted into feet of head by multiplying by 2.31.
12 x 2.31 = 27.7 feet of head for the pressure side.
3 hp would create more than 12 psi, so the pump might be on low speed of two speeds or have a smaller impeller.
There is also a suction side head that can be measured by a vacuum gauge.
If we estimate the suction side head to be 14 feet, the total head is about 41.7 feet. But that's only at the current flow rate. Increasing or decreasing the flow rate will change the head by a square factor.
So, all that being said, what do you need the pump to do?
For example, you need to circulate the water but do you need any special flow for spa jets, solar etc?
If you just want circulation, then a smaller pump would be plenty. You can do a two speed pump or variable speed.
To answer both of your replies Jason and James, I'm planning on just replacing the motor and it will be used only for circulation and filtering. I based the calculations on a flow rate of 60 gpm which would filter the water in 8 hours. James, your explanation helps a lot. I'm thinking that the big 3 hp motor may have been an earlier fix by someone before my time (we bought the house 5 years ago but the pool is nearly 50) and I wonder if they didn't size the impeller for the high horsepower motor. As I said, the pump I'm using is the one that was for the water jets in the spa. Would a high horsepower motor with a small impeller be the setup for that use? Either way, I'd like to use the smallest motor I can to circulate and filter for money saving reasons.
Turnovers do not matter. Could take 2 hours or 12 hours. For chemical distribution, only need to run for 2-3 hours.
To just replace the motor you have to match the original. If you do not have the original, you have to match the impeller (hopefully you can find a part number).
You can put a large HP motor on a small impeller, but not the other way around. So, you have to make sure the impeller is sized for a smaller motor if that is the path you want to go.
You could also take the opportunity to install a 2-speed motor and run on low most of the time for more savings.
To just replace the motor you have to match the original. If you do not have the original, you have to match the impeller (hopefully you can find a part number).
You can put a large HP motor on a small impeller, but not the other way around. So, you have to make sure the impeller is sized for a smaller motor if that is the path you want to go.
You could also take the opportunity to install a 2-speed motor and run on low most of the time for more savings.