Sizing filter to pump

Terry

Well-known member
Jul 8, 2008
128
Dallas, Tx
I have a 2 hp pump that, after reading here for some time, I speculate is probably oversized for my filter. I have read the info on hydraulics, pipe size, head loss, etc and it’s just over my head, I’ve tried several times, but I’m not getting it. I have to replace some parts on my 16 y/o sand filter and am contemplating just replacing the whole filter.

I know most people would size their pump to the filter, but what if I were to go the other direction and size the filter to the pump- which would probably result in oversizing the filter, but I understand that is a good thing.

My specs: 19,070 gallons total. 2hp AO Smith motor with a sf of 1.3 (model B748). 1.5” plumbing. Main drain, 1 skimmer, 4 return jets, and separate spa that shares equipment via Jandy valves. Have no idea how much pipe is under the ground. Equipment sits about 12’ from the edge of the pool, but the skimmer is on the opposite side of the pool from the equipment.

If I replace the sand filter, what size would be a good match?
 

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teapot

In The Industry
Jul 25, 2009
574
London and France
Your pump sounds about right for that size of pool bearing in mind turnover rates, possible your filter is undersized, Its not possible to over size its just more convenient to not have too bigger filter, space requirements and money etc.

If your current filter is ok I would add a second of the same size.
 

X-PertPool

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In The Industry
Jun 12, 2009
1,385
Exeter, PA
Hayward s244t 300lb sand filter. You have 1.5" underground plumbing so even if your pump has 2" fittings it will eventually be reduced to 1.5" so getting a filter with 2" fittings wouldn't help. I've used the S244t for pools up to 20x50 with no problems. If the pool is smaller than 14x28 then I would probably use a smaller filter
 

Terry

Well-known member
Jul 8, 2008
128
Dallas, Tx
The Hayward s244t is what I have, so I'll see about the replacement parts.

I am a bit confused though, I see signature lines all the time with 24" filters and 3/4-1 hp pumps. I also read about oversized pumps pushing the water through the sand filter too quick. I *think* I understand that due to my spa I will need a larger pump for the jets, and I get that, but I thought maybe that meant I should have a larger filter (even though the pool size doesn't dictate a larger filter).

EDIT:
Kevin, I see you are one with a 1 hp pump on this filter :mrgreen: What is glasspak media?
 

teapot

In The Industry
Jul 25, 2009
574
London and France
Terry said:
The Hayward s244t is what I have, so I'll see about the replacement parts.

(even though the pool size doesn't dictate a larger filter). If I had built your pool you would have had a bigger filter, it's about turnover rates as you said.

EDIT:
Kevin, I see you are one with a 1 hp pump on this filter :mrgreen: What is glasspak media?
Its a copy of Dryden Aqua's AFM which I am using, is AFM any better? it should be but who knows.

http://www.afm.eu/
 

X-PertPool

TFP Expert
In The Industry
Jun 12, 2009
1,385
Exeter, PA
I got a free sample from the maker (enough for a 300lb filter woo!). It comes with pea gravel that you put in first then the glass goes on top. Very thin pieces of glass about the size of sand. I put it in my filter to get rid of it. It works, but haven't noticed a filtration difference. Although I can't really see a difference in the water clarity even when comparing sand to de. The only thing I didn't like was actually handling the glass it looked sharp and was afraid I'd get some in my eyes
 

duraleigh

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Apr 1, 2007
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Mas985 is the very best.....he may be along soon to help.

My guess is your pump is probably producing something around 90 gallons/minute. I that turns out to be accurate, then a flow rate on your filter of about 110 would be just about perfect.

In other words, if your pump produces X gpm, then X*1.2 would be about right for your filter.
 

mas985

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May 3, 2007
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Pleasanton, CA
I doubt you are exceeding the maximum flow rate of the filter but what make/model is the pump? If it is as Northstar full rated 2 HP, my guess would be something around 80 GPM.

Your biggest issue is probably the power bill. Given the spa, you may want to swap the motor for a two speed to save money.
 

Terry

Well-known member
Jul 8, 2008
128
Dallas, Tx
First, thanks for the help. Second, I'm going down fast here, you're going to have to take it down a notch for me to keep up. :mrgreen:

Ok, I thought I learned that in my situation my 1.5" pipes would be my limiting flow rate factor- from memory that they could only carry something like 50 gmp. Specs on my current filter say 62 gpm. My pump, I don't know, I think I gave up before I got to it, but I posted a picture of the label in the first post.

I also found this calculator on AO Smith's website, which, at my (current) electricity rates doesn't seem to make the 2 speed pump that attractive of an investment. It looks like the 2 speed would save me about 3.4 kwh/ day or roughly 34 cents. Good chance I've got something wrong here so feel free to set me straight.

http://www.aosmithmotors.com/calculator ... aspx?id=57

Back to duraleigh's recommendation of 110 gpm filter, it looks like Hayward's 30" 500 lb filter has a 98 gpm flow rate. What would this get me? Cleaner water?
 

mas985

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May 3, 2007
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Pleasanton, CA
AO Smith is the motor manufacture not the pump manufacture. Since you have other Hayward equipment it may be a Hayward pump. But you might be able identify the pump from pictures here. Or take a picture of the wet end and post for identification.

Also, much like filters, pipe flow rate recommendations are just that recommendations and not absolute limits. So flow rates can and do exceed recommended limits. If your filter limit is only 62 GPM, then your pump is likely exceeding that by quite a bit. Like I said before, probably close to 80 GPM. I wouldn't worry too much about the plumbing since those recommendations are somewhat restrictive and are not all the critical except for efficiency. But the higher flow rates can reduce the filtering capability of a sand filter so you might be better off with a larger filter. Also with a large filter, you won't need to backwash as often which is a plus.

A two speed pump will generally use about 1/4 the power/energy on low speed but requires twice as much runtime so overall savings are about 50%. So you have to decide if it is worth it for you.
 

Terry

Well-known member
Jul 8, 2008
128
Dallas, Tx
Ah, ok well, it looks like we are approaching the depths of my idiocy on this subject. :mrgreen: So when people talk about having a 1hp pump they aren't talking about the motor? I've just typed and erased about 15 questions here. I'm so lost I don't even know what to ask. :wink:

Here's my pump, good chance it is original to 1986. Pretty sure it's prior to '94 anyway.
 

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mas985

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May 3, 2007
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The actual performance of a pump head curve is due to the wet end. A motor is simply sized to the wet end load. The problem with using just the SFHP rating of the motor is that pumps can have much different head curves but use the same size motor so it isn't always a good indicator.

It looks to be a Dura-glas or Max-E-Glas pump so flow rates likely to be 70-80 GPM. BTW, what is your current filter pressure?
 

Terry

Well-known member
Jul 8, 2008
128
Dallas, Tx
I was just looking at the Dura-glas and thinking that looked like it, feeling a little better about myself now. :) How can I tell which rated pump I have (3/4, 1, 1.5), or does that matter?

Pressure on skimmer only has been sitting at about 17 psi. I've had zeosand in it and it almost never rises. Pressure goes up when adding main drain and the spa, usually around 22 give or take.

EDIT:
It is the DuraGlas, I was able to make out a couple numbers on the virtually non-existent label: P2R. Doesn't help me with which DuraGlas, though.
 

mas985

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May 3, 2007
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Pleasanton, CA
You probably have a P2RA6G-183L 2 HP Dura-Glas pump. With the filter pressure @ 22 PSI, you should be getting close to 73 GPM. When you shut off the other suction lines, the pressure drops because the flow drops due to higher suction head loss. In that case, you may be getting closer to 67 GPM.

Most filters work better when flow rates are 75% or less of the maximum flow rate so I would size the filter for at least 90 GPM.
 

Terry

Well-known member
Jul 8, 2008
128
Dallas, Tx
I think I'm starting to get it, sort of :)

mas985 said:
The actual performance of a pump head curve is due to the wet end. A motor is simply sized to the wet end load. The problem with using just the SFHP rating of the motor is that pumps can have much different head curves but use the same size motor so it isn't always a good indicator.
I interpret this to mean that the pump, not the motor, determines the gpm flow rate. This flow rate is important to know because it is what we use to determine appropriate filter size. Do I understand this correctly?

Further, pumps are designed to hold a specific hp motor, this is where the “1 hp pump” comes from. I now have a 2 hp motor on my pump- this may or may not be what it was designed for. Thus, I may or may not have a 2hp pump (and its associated flow rate).

Now, the energy usage comes from the motor, yes? If I use the AO Smith calculator for my motor is that an accurate representation of my usage- or does the pump affect the amount of kwh it pulls somehow? Understanding this helps to know what type of affect, energy wise, a new pump would have.

I spoke to Sta-Rite this morning trying to determine which model I have, with no luck. Is there a way you are aware of that I could determine which model (and therefore hp) I have? Maybe something different between the different models that would help narrow it down?

They did mention that air intrusion (which I have) is a sign of an oversized motor. I realize it could be due to several other causes, but with the motor having been replaced I suspect I ended up with an oversized motor.

Looking at my system, I have different components replaced at different times by different companies. I think the pump is original, the filter in ’94 (I think it was originally cartridge), and the pump motor in ’02-03. Good chance none of it is all rated to work together, I’m starting to suspect I may need to just start over.

Another thought. The flow rates you posted in your last post are based on the pump being a 2hp pump, yes? So, if my pump is really a 1hp pump (for example) then the flow rates would be less, right?
 

mas985

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May 3, 2007
12,420
Pleasanton, CA
Terry said:
I think I'm starting to get it, sort of :)

mas985 said:
The actual performance of a pump head curve is due to the wet end. A motor is simply sized to the wet end load. The problem with using just the SFHP rating of the motor is that pumps can have much different head curves but use the same size motor so it isn't always a good indicator.
I interpret this to mean that the pump, not the motor, determines the gpm flow rate. This flow rate is important to know because it is what we use to determine appropriate filter size. Do I understand this correctly?

Yes

Further, pumps are designed to hold a specific hp motor, this is where the “1 hp pump” comes from. I now have a 2 hp motor on my pump- this may or may not be what it was designed for. Thus, I may or may not have a 2hp pump (and its associated flow rate).

More or less. Pumps are designed around available motor sizes. Normally, you will see motors sold in 1/2 HP increments with certain service factors. So a pump designer will make sure that the design comes close but below one of those increments since it would be very costly to then design a new motor for just a single pump design.

Now, the energy usage comes from the motor, yes? If I use the AO Smith calculator for my motor is that an accurate representation of my usage- or does the pump affect the amount of kwh it pulls somehow? Understanding this helps to know what type of affect, energy wise, a new pump would have.

Yes and no. The impeller creates a load (energy required) which is transfered to the motor. The motor then draws enough current to move that load at near 3450 RPM. So really, the impeller and motor efficiency determines the amount of current draw and thus power. This is why you can change out just the impeller to a smaller size which will reduce current draw and power consumption of the existing motor. This is a very cost effective way to downsize a pump. Impellers are usually less than $50 although on some pumps you also will need to change the diffusor so it can be a bit more.

However, motor efficiency comes into play as well. Higher HP motors have higher efficiency because the winding gauge is thicker to handle the higher current so lower I2R losses. But if you underdrive a motor, efficiency drops some because of the inductive fields but it started higher to begin with so net net, the power draw for two different HP motors is about the same for the same impeller at least within a few percent.

Also, since the impeller load changes depending on the operating point of the head curve, the current draw on the motor will also change with the operating point. Plumbing with higher head loss has lower current draw because flow rates are lower. Low head loss plumbing draws more current but the gallons/watt-hr are higher for low head loss plumbing which is why that is desireable.


I spoke to Sta-Rite this morning trying to determine which model I have, with no luck. Is there a way you are aware of that I could determine which model (and therefore hp) I have? Maybe something different between the different models that would help narrow it down?

As I posted earlier, I believe you have a P2RA6G-183L 2 HP Dura-Glas pump. The filter pressure confirms that. My point earlier is that not all 2 HP pumps have the same head curves so they will perform different and consume different amounts of energy although they will be close. In order to predict the performance of a pump on a plumbing system you have to know what the head curve looks like so knowing just the motor size is not enough you need the model # of the pump. Normally, someone will not put a higher HP motor on a pump than needed. However, they might reduce the size of the impeller. But again, the filter pressure is consistent with a 2 HP pump.

They did mention that air intrusion (which I have) is a sign of an oversized motor. I realize it could be due to several other causes, but with the motor having been replaced I suspect I ended up with an oversized motor.

I think they must have meant an oversized pump. A lot of people use pump and motor interchangeably but they are really different. Oversizing the motor has absolutely no effect on the wet end. However, an oversized pump will have flow rates and head loss which may be too high for the plumbing and will create high suction and draw in air.

Looking at my system, I have different components replaced at different times by different companies. I think the pump is original, the filter in ’94 (I think it was originally cartridge), and the pump motor in ’02-03. Good chance none of it is all rated to work together, I’m starting to suspect I may need to just start over.

Another thought. The flow rates you posted in your last post are based on the pump being a 2hp pump, yes? So, if my pump is really a 1hp pump (for example) then the flow rates would be less, right?
Unless someone replaced the motor with a larger one or reduced the impeller size, then I suspect that it is indeed a 2 HP pump and the filter pressure seems to confirms that. It would be much lower if the pump was smaller.
 

Terry

Well-known member
Jul 8, 2008
128
Dallas, Tx
Thank you so much for taking the time to help me with all this! I am on my way out at the moment, but will be spending some time this evening digesting everything.
 
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