Another energy efficient pump questions thread...

[SeaCorey]

I need to rebuild or replace this motor, which sounds to me as though its bearings are shot. It's 3-4 years old and I'd like to replace it with a more efficient unit. I've read a bunch of the threads here about efficient pumps and 2-spd vs. variable speed motors and was leaning towards the B2981 b/c I'm wired for 115V.

I'm surprised that the B2981 specs 12.4A at high speed while my single speed pump "only" pulls 9.8A. I assume I'll need the high speed to run the navigator and to at least prime the 1-story rooftop solar heater. The pool is free-range (no screen cage) and I do get a lot of leaves/oak squigglies so I'll likely need the navigator running at least a couple hours a day.

Is the B2981 a good choice for this setup? Is the low speed sufficient for the solar array (once primed on high speed)? Am I going to realize any overall savings with the increase in power consumption at high speed? Or should I just replace the bearings in the pump I currently have?

Thanks TFP!

 

[jblizzle]

Solar heating works best at higher flow rates. So, when I need solar, I run on high speed. It might work on low speed, but it will not heat nearly as well. In the spring and fall, I run on high speed most of the time and enjoy the lower running costs in the winter and summer when I do not need solar running.

If the 2 pumps are the same SFHP (what are the service factor and HP ratings of the 2 pumps?) then they will use about the same amount of power on high speed. Low speed will move half the water for 1/4 the energy cost.

 

[SeaCorey]

Thanks jblizzle, here're the #'s:

3⁄4~.10 3450/1725 rpm 115V 12.4/2.2A 1.67 SF 1.25 THP B2981 new pump
3/4 HP 3450 rpm 115V 9.8A 1.0 SF existing pump

 

[pooldv]

12a at 120v is only @1500 watts which will make the pump less than 1-1/2hp. Doesn't seem like it could pump enough on low to make the solar panels efficient.

There is not much cost difference to run those the old pump vs the new pump on high. Plug in your electric rate and hours per day for actual costs.
New pump on high
1500watts = 1.5kWh x 12c per kWh = $0.18 per hour running cost x8=$1.44 x30 = $43.20/mo
Your old pump
1,176watts = 1.2kWh x 12c per kWh = $0.144 per hour x8= $1.152 ×30 = $34.56/mo

so, if the new pump uses 1/4 the electricity on low it will cost $10.80 per month on low. Decent savings during summer and winter.

 

[JasonLion]

There are a couple of different things going on here that can confuse the issue.

First, there are energy efficient motors and regular motors. The energy efficient motors save about 10% of the energy usage. This is the one you can actually find out, and have it make a difference. Everything else kinds of cancels out in this situation. Replacing just the motor with a larger motor won't change the energy usage noticeably, despite the fact that the larger motor has a higher max current rating.

The second is that a motor is rated based on the entire range of equipment it could be connected to, while a pump knows that the motor is driving an impeller on a pump. The motor may draw more current or less current depending on what it is connected to.

Then there is SF (service factor). The listed HP is kind of a fiction developed for marketing purposes. The actual power available is is SF * HP, or SFHP. SFHP can be directly compared between motors, while listed HP can not be directly compared.

Finally, the actual power draw depends on the impeller. A large motor with a small impeller will only draw a little power. Put on a larger impeller it will draw more power. Aside from the energy efficient or not issue, the impeller determines how much power it will actually draw (given a fixed plumbing setup).

 

[jblizzle]

So from the numbers, the new pump is a bigger pump (0.75*1.67 = 1.25 SFHP) than your old one (0.75*1.0=0.75 SFHP) and thus the reason you are seeing a higher amp rating. You are comparing a 1.25 HP pump to a 0.75 HP pump ... once you remove the number playing games.

One note though is that those Amp ratings are not the true running amps, generally steady state running the power usage is less than that plate rating.

 

[pooldv]

I did not know about the service factor factor.

True that a pump will not draw max amps when operating in steady state, usually about 20% less. An electric motor typically draws about 1000 watts per hp. So, 1250 watts for 1.25hp pump and 750 watts for 3/4hp pump. Minus factors for all variables listed like impeller size, head pressure, back pressure, etc. That is a pretty good estimate if the manufacturers don't publish typical running amp draw.

If the new pump only draws 2.2 amps on low speed that is pretty good. That would drop the operating cost to around $7.60 per mo at 8 hr per day at 12c/kWh.

 

[mas985]

An electric motor typically draws about 1000 watts per hp.

That's only when fully loaded. Remember ratings are at max load. But if you put two different rated motors on the same load, as in this case, the two motors will draw about the same power even though they have different ratings. The power draw of an induction motor is dependent on the driven load and not the rating. So if you put a 5 HP motor on a 1 HP impeller, it will draw about the same power as a 1 HP motor.

 

[pooldv]

Ok, it is finally sinking in! I went back and reread the OP and see that he is replacing the motor only and not the whole pump. Electricity and motors are sneaky! Just when I thought I was getting it figured I get a new curveball! Thanks for the continuing education.

 

[SeaCorey]

Holy moly, nothing worthwhile is simple, is it?
Given all of the above I'm leaning towards the B2981, wiring the 2spd switch myself and actuating it manually when the solar loop and/or navigator are needed, and using the same impeller i currently have. Now to review the instructions how to wire that switch correctly... Long term plan is to upgrade from the simple dial timer to one that will run at high for an hour or so 2ce a day for the navigator.
But first, when I add an automatic controller for the solar loop can it be wired to kick the pump into high when it opens the valve?

 

[JasonLion]

You will need at least a basic automation system to switch a two speed pump to high automatically when solar heat is called for. Single purpose solar controllers sometimes have the ability to turn on a single speed pump, but not the ability to change speeds on a two speed pump.