Need Help with Calculating Total Head Loss for Pump Sizing and Recommendations

[NEPoolaholic]

Newbie to forum. Great place! Have enjoyed the reading and all of your expertise!

After reading the primers on this forum and at other sites, I find I am at a bit of a loss in determining the total head loss of my system and then determining the right size pump for my pool system.

I need to replace a 20+ year old 3/4 HP Hayward super-pump predecessor. Much of the pump is rotting out, and it cannot run long in hot weather without shutting down until it cools off.

While mathematically capable, I've seen many confusing sites and calculators. Considering my system components, I've gotten absurd numbers (of over 200 feet of head). So, clearly something in my understanding and the calculations is very, very wrong! Which is why I joined this forum seeking some answers.

Here is my system and a few questions at the end. Please ask any follow up questions! Thanks in advance!

System:

Pool: 20' x 40' inground, vinyl liner, 8' max depth. Water volume approximately 29,000 gallons.

Pad location: the pad with pump, filter and heater is approximately 6 feet BELOW the pool deck. I'm guessing that the suction and return ports are about 3.5' above the pump, while the skimmer is more like 5'+ above the pump.

Lines: 1.5" black poly (with some damage that has been repaired with couplings). 2 suction (side wall in deep end and skimmer at mid point). 1 return (to eyeball in shallow end). No deep end center drain.

Pipe length: Deep end suction 65 ' to pump. Skimmer suction 50' to pump. Return line 35' from heater. Approximately 26' of piping at the pump/filter/heater pad and accounting for the vertical drop to the pad in all 3 lines. Total length of 1.5" pipe estimated at 176'.

Items impeding flow: seven 90deg elbows; one 45deg bend, one tee, at least seven couplings along the length of the 1.5" black poly. 2 two-way valves. 1 sand filter (Hayward 62 gpm rated) and multi-port valve. 1 heat pump. 1 skimmer. 1 return eyeball. All piping is 1.5" PVC or 1.5" black poly.

Current pump: 3/4 HP old Hayward (like the super-pump) with A.O. Smith motor.

Goal:

I'd like to get a more energy efficient pump. A two-speed pump seems like the right choice, but the Hayward MaxFlo VS SP2302VSP seems like a heck of a lot better in total lifetime cost (even over just a few years). Based on pad wiring, I'd have to stick with 115V pumps, so that leaves out some reasonably priced models (Pentair 340042).

With the current old 3/4 HP pump, I get a decent flow around the pool and the skimmer works fairly well. (I worry about going to a very slow flow that the skimmer will no longer work.

Questions:

• What is the head loss of this system (in several gpm flow rates)? How would I calcultate that?
• What size pump should I be looking for? 3/4 HP? 1 HP?
• At low flow rates, seems like my head loss (although I am calculating it wrong, I think) is way above any pump's curves - even at low gpm's. How can a VS pump handle my system? Or, what am I doing wrong to calculate total head loss?

Thanks again!

 

[jblizzle]

Welcome to TFP!!1

Generally there is no need to calculate head loss and it is not trivial to do so.
A 1HP 2-speed would be fine like the Superflo or Super Pump. 3/4 HP in a 2-speed version are a little hard to find. Although you might be able to find a 2-speed motor to put on your existing pump ... that would be the cheapest route.

Head loss is not constant, it varies with RPM and flow rate ... like I said not trivial to calculate.

 

[JasonLion]

At typical MA electric rates a smaller two speed pump will have the lowest lifetime costs. A two speed costs almost as little to operate as a variable speed, but costs way less up front, which more than makes up for the slightly higher hourly cost.

 

[NEPoolaholic]

Thanks guys! Looking at my system further, I only have 230/240v run for the heater. Adding in electrician cost, breakers, etc. to enable a 230v VS pump makes that avenue totally cost prohibitive. Looks like a 2-speed is the best choice (i.e., Pentair Whisperflo 1HP 2-speed running on 115v).

One question on pump ratings: there are two Pentair Whisperflo models that have the same motor -3/4 HP with 1.67 Service Factor and a 1 HP with 1.25 service factor. As I've read here and elsewhere, there is no difference between the full-rated and the up-rated when the brake HP is the same. Yet still, Pentair makes and sells both motors and the prices are different (WFDS-3 vs WFDS-24). Is this all marketing, since some people will want a "bigger" pump? Or is there another difference?


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[jblizzle]

It is totally marketing. The pumps are identical.

 

[JasonLion]

This is a historical thing. In some parts of the country pump ratings have historically been done one way, while in other parts of the country they have been done another way. Pentair maintains two lines of pumps (full rated and up rated) that correspond to two different approaches to pump labeling in different historical markets. The corresponding pumps in the two lines are identical except for the name plate (as can be seen if you look at the replacement parts list and note that they both take exactly the same replacement parts).