# Can I put a dimmer switch on my pool pump?
**URL:** https://boards.straightdope.com/t/can-i-put-a-dimmer-switch-on-my-pool-pump/497684
**Category:** Factual Questions
**Created:** [May 26, 2009, 3:01pm UTC](https://boards.straightdope.com/t/can-i-put-a-dimmer-switch-on-my-pool-pump/497684 "2009-05-26T15:01:50Z")
**Posts on this page:** 20
**Page:** 1
### Author:  [@solkoe](https://boards.straightdope.com/u/solkoe)
#### Post date: [May 26, 2009, 3:01pm UTC](https://boards.straightdope.com/t/can-i-put-a-dimmer-switch-on-my-pool-pump/497684/1 "2009-05-26T15:01:50Z")
I started a thread awhile ago concerning my pool pump and I’m still at it.
The problem is that my 1 HP pool pump is too powerful for the pool. I also have solar heater and the water is pushed through too fast to heat up. I need to slow the flow of water. As far as I see it here are my options:
1. Buy a new pump.
They cost at least $600 and its hard to get the 1/2 HP pump that I need. They simply don’t make them.
2. Change the impeller.
The guy at the pool store said it is possible to put a 1/2 HP impeller in my pump to slow the flow of water. I’m worried that my pump will now run faster and burn the motor out.
3. Put a dimmer switch on my pump.
Is this possible? If I can do it for a lamp, why not a pump?
4. Connect my pump to run at 240V yet plug it in a 120V.
Will my pump run at half speed?
Does anyone have any other suggestions?
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### Author:  [@Really\_Not\_All\_That\_Bright](https://boards.straightdope.com/u/Really_Not_All_That_Bright)
#### Post date: [May 26, 2009, 3:15pm UTC](https://boards.straightdope.com/t/can-i-put-a-dimmer-switch-on-my-pool-pump/497684/2 "2009-05-26T15:15:04Z")
I don’t know why you’d need a switch, since you aren’t going to be adjusting the flow rate.
As for finding pumps, I was in a Lowe’s hardware store yesterday looking for a 3/4 HP sump pump and they hardly had any, but they had tons of 1/2 HP pumps from $100ish.
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### Author:  [@beowulff](https://boards.straightdope.com/u/beowulff)
#### Post date: [May 26, 2009, 3:30pm UTC](https://boards.straightdope.com/t/can-i-put-a-dimmer-switch-on-my-pool-pump/497684/3 "2009-05-26T15:30:22Z")
> [@solkoe](#):
>
> I started a thread awhile ago concerning my pool pump and I’m still at it.
> The problem is that my 1 HP pool pump is too powerful for the pool. I also have solar heater and the water is pushed through too fast to heat up. I need to slow the flow of water. As far as I see it here are my options:
>
> 1. Buy a new pump.
> They cost at least $600 and its hard to get the 1/2 HP pump that I need. They simply don’t make them.
> 2. Change the impeller.
> The guy at the pool store said it is possible to put a 1/2 HP impeller in my pump to slow the flow of water. I’m worried that my pump will now run faster and burn the motor out.
> 3. Put a dimmer switch on my pump.
> Is this possible? If I can do it for a lamp, why not a pump?
> 4. Connect my pump to run at 240V yet plug it in a 120V.
> Will my pump run at half speed?
>
> Does anyone have any other suggestions?
I think you are incorrectly evaluating your problem.
The water isn’t moving “too fast to heat up”. Solar collectors operate most efficiently when they are coolest. In other words, if you slow the water flow down, the water may come out of the collector hotter, but the total amount of heat transferred will be less than a design with more volume. Also, even though the water may be hotter, there is less of it, so the pool will not get as hot as a design that moves more water.
If you want to prove this, just put in a diverter pipe, and re-circulate some of the water away from the collector and back to the pool.
---
### Author:  [@Philster](https://boards.straightdope.com/u/Philster)
#### Post date: [May 26, 2009, 3:56pm UTC](https://boards.straightdope.com/t/can-i-put-a-dimmer-switch-on-my-pool-pump/497684/4 "2009-05-26T15:56:40Z")
I’m not sure if a 1/2 hp pump runs slower than a 1 HP pump anyway. I’m not sure slower is better ( **beowulff** ’s post)
Maybe expectations are too high for the solar heater, and maybe it needs a lot of run time to make a little difference in temps (5-8 degrees). Heck, an all-out strong gas heater will take a long time to make cold water warm. Even gas/electric Pool heaters in pools need many hours to up to the temps, and when it is cold out, the amount of time to move the temps from 60 to 80 could be days.
A solar heater might be best over a very, very long period of time, with water that isn’t frigid to start with. Maybe it can kick 70 degree water up to 78 working tirelessly over a week – I don’t know, but knowing how gas heaters work, I can get a sense that solar heaters tweak the temp up and help maintain it rather than give it a swift kick in the ass.
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### Author:  [@Bill\_Door](https://boards.straightdope.com/u/Bill_Door)
#### Post date: [May 26, 2009, 3:58pm UTC](https://boards.straightdope.com/t/can-i-put-a-dimmer-switch-on-my-pool-pump/497684/5 "2009-05-26T15:58:03Z")
No, you can’t put a dimmer switch on the pump. Electric motors do not like running at other than their rated voltage, and will not do it, or at least not for long. Switching the impeller is your best bet. The pump speed is set by the motor speed, a smaller impeller will not affect the RPM.
In your case the amount of water pumped is set by the total pressure drop through the system. The pump develops a certain amount of head, and delivers to a set pressure, probably atmospheric plus a small static head, assuming the outlet is submerged.
When you turn on the pump, the flow increases until the frictional losses equal the total delivered head of the pump, minus the pressure at the discharge side. You can roughly figure a square law for friction, where doubling the flow quadruples the friction. A smaller impeller will develop less head, and the flow will decrease.
For small variations in impeller diameter the formula is:
D1/D2 = Q1/Q2 = H1^(1/2)/H2^(1/2)
Where D is the impeller diameter, Q is the flow, and H is the developed head.
That being said, there should be no reason why increased velocity through a solar heater should impact heat uptake other than favorably. The outlet water temperature will be lower at a higher flow, but that should be compensated for by the larger volume. If you make 10 gpm with a temperature rise of 1.0 deg F, you could slow the flow and make 5 gpm with a delta T of 2.0 deg F. At a higher delta T more heat would be lost to the environment, and it would be marginally less efficient, but not so you would notice.
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### Author:  [@solkoe](https://boards.straightdope.com/u/solkoe)
#### Post date: [May 26, 2009, 7:00pm UTC](https://boards.straightdope.com/t/can-i-put-a-dimmer-switch-on-my-pool-pump/497684/6 "2009-05-26T19:00:07Z")
> [@Really\_Not\_All\_That\_Bright](#):
>
> I don’t know why you’d need a switch, since you aren’t going to be adjusting the flow rate.
>
> As for finding pumps, I was in a Lowe’s hardware store yesterday looking for a 3/4 HP sump pump and they hardly had any, but they had tons of 1/2 HP pumps from $100ish.
So I could use a sump pump as a pool pump? I guess a pump is a pump.
---
### Author:  [@solkoe](https://boards.straightdope.com/u/solkoe)
#### Post date: [May 26, 2009, 7:05pm UTC](https://boards.straightdope.com/t/can-i-put-a-dimmer-switch-on-my-pool-pump/497684/7 "2009-05-26T19:05:38Z")
> [@beowulff](#):
>
> I think you are incorrectly evaluating your problem.
>
> The water isn’t moving “too fast to heat up”. Solar collectors operate most efficiently when they are coolest. In other words, if you slow the water flow down, the water may come out of the collector hotter, but the total amount of heat transferred will be less than a design with more volume. Also, even though the water may be hotter, there is less of it, so the pool will not get as hot as a design that moves more water.
> If you want to prove this, just put in a diverter pipe, and re-circulate some of the water away from the collector and back to the pool.
I understand your logic but I know from experience. I had an inefficient 3/4 HP pump running pool water through 1 solar heater. The temperature was 10C higher at the output than the input. I was so happy I decided to buy another solar heater. I connected it in parallel to the first. At the same time, I bought an efficient 1HP pump. The water speed is much greater. The difference in temperature between input and output is much less. Go figure.
My pump is too big anyway. Aside from the heating problem, I want to slow down the flow.
---
### Author:  [@Tim\_T-Bonham.net](https://boards.straightdope.com/u/Tim_T-Bonham.net)
#### Post date: [May 26, 2009, 7:08pm UTC](https://boards.straightdope.com/t/can-i-put-a-dimmer-switch-on-my-pool-pump/497684/8 "2009-05-26T19:08:49Z")
You can’t put a dimmer switch on the pump, and you can’t run a 240V pump on 120V. (Well, you could, but not without damaging the pump.) Pump motors are designed to run at specific voltages.
Assuming what you want is to reduce the volume of water pumped, you could change the impeller size, as people have suggested, or divert some of the water elsewhere: install a Y-fixture, with one side going to the pool, and the other side elsewhere. With an appropriate valve, you can control how much water goes to each side. But you will have to find somewhere else to send the rest of the water. Possibly sending it to a sprinkler system or something, or even resending it through a 1-way valve back into the pump input line, maybe?
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### Author:  [@lazybratsche](https://boards.straightdope.com/u/lazybratsche)
#### Post date: [May 26, 2009, 7:12pm UTC](https://boards.straightdope.com/t/can-i-put-a-dimmer-switch-on-my-pool-pump/497684/9 "2009-05-26T19:12:43Z")
> [@solkoe](#):
>
> I understand your logic but I know from experience. I had an inefficient 3/4 HP pump running pool water through 1 solar heater. The temperature was 10C higher at the output than the input. I was so happy I decided to buy another solar heater. I connected it in parallel to the first. At the same time, I bought an efficient 1HP pump. The water speed is much greater. The difference in temperature between input and output is much less. Go figure.
Right, but with the lower flow of hot water, you’re not adding more heat than with a higher flow of warm water. It won’t feel as warm coming out of the pump, but it’s heating the whole pool just as much (if not slightly more).
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### Author:  [@Squink](https://boards.straightdope.com/u/Squink)
#### Post date: [May 26, 2009, 7:37pm UTC](https://boards.straightdope.com/t/can-i-put-a-dimmer-switch-on-my-pool-pump/497684/10 "2009-05-26T19:37:15Z")
> [@solkoe](#):
>
> The difference in temperature between input and output is much less.
You have to multiply the temperature difference by flow rate, gallons per minute or whatever, to get a measure proportional to the total amount of heat being added per minute.
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### Author:  [@Really\_Not\_All\_That\_Bright](https://boards.straightdope.com/u/Really_Not_All_That_Bright)
#### Post date: [May 26, 2009, 8:14pm UTC](https://boards.straightdope.com/t/can-i-put-a-dimmer-switch-on-my-pool-pump/497684/11 "2009-05-26T20:14:05Z")
> [@solkoe](#):
>
> So I could use a sump pump as a pool pump? I guess a pump is a pump.
I’ve seen several commercial pools that have sump pumps.
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### Author:  [@YamatoTwinkie](https://boards.straightdope.com/u/YamatoTwinkie)
#### Post date: [May 26, 2009, 10:38pm UTC](https://boards.straightdope.com/t/can-i-put-a-dimmer-switch-on-my-pool-pump/497684/12 "2009-05-26T22:38:09Z")
> [@solkoe](#):
>
> I understand your logic but I know from experience. I had an inefficient 3/4 HP pump running pool water through 1 solar heater. The temperature was 10C higher at the output than the input. I was so happy I decided to buy another solar heater. I connected it in parallel to the first. At the same time, I bought an efficient 1HP pump. The water speed is much greater. The difference in temperature between input and output is much less. Go figure.
> My pump is too big anyway. Aside from the heating problem, I want to slow down the flow.
I’m not quite sure you’re grasping this. Let me ask you, what’s going to heat your pool faster (assuming you can fill,heat,&pour one container every minute or so), a thimble of _boiling_ hot water or a large bucket full of merely warm-hot water?
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### Author:  [@solkoe](https://boards.straightdope.com/u/solkoe)
#### Post date: [May 26, 2009, 10:59pm UTC](https://boards.straightdope.com/t/can-i-put-a-dimmer-switch-on-my-pool-pump/497684/13 "2009-05-26T22:59:51Z")
> [@YamatoTwinkie](#):
>
> I’m not quite sure you’re grasping this. Let me ask you, what’s going to heat your pool faster (assuming you can fill,heat,&pour one container every minute or so), a thimble of _boiling_ hot water or a large bucket full of merely warm-hot water?
Right, I understand.
The fact is my pool was heated with one solar heater and a smaller pump. Now my solar heaters (plural) are not working. They do not heat the pool. Regardless, I need a smaller pump. The problem with the heater must lie elsewhere.
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### Author:  [@JFLuvly](https://boards.straightdope.com/u/JFLuvly)
#### Post date: [May 26, 2009, 11:07pm UTC](https://boards.straightdope.com/t/can-i-put-a-dimmer-switch-on-my-pool-pump/497684/14 "2009-05-26T23:07:27Z")
What about a VFD (Variable Frequency Drive) You could probably pick one up for about 150 bucks. Also I think the size of the pool will play a role in how much water has to be moved to keep it clean, ie: the pool needs the surface water rotation for the skimmer to operate properly.
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### Author:  [@Bag\_of\_Mostly\_Water](https://boards.straightdope.com/u/Bag_of_Mostly_Water)
#### Post date: [May 27, 2009, 12:02am UTC](https://boards.straightdope.com/t/can-i-put-a-dimmer-switch-on-my-pool-pump/497684/15 "2009-05-27T00:02:55Z")
You can buy 1/2 hp pool pumps. [Here’s a Pentair WhisperFlo](http://www.poolgeek.com/Pentair-WhisperFlo-Pump-P3488.aspx) with 1/2 hp available in standard performance and standard performance full rated.
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### Author:  [@Blake](https://boards.straightdope.com/u/Blake)
#### Post date: [May 27, 2009, 12:17am UTC](https://boards.straightdope.com/t/can-i-put-a-dimmer-switch-on-my-pool-pump/497684/16 "2009-05-27T00:17:57Z")
> [@solkoe](#):
>
> Right, I understand.
> The fact is my pool was heated with one solar heater and a smaller pump. Now my solar heaters (plural) are not working. … They do not heat the pool. The problem with the heater must lie elsewhere.
Exactly. The probelm isn’t with the pump or the flow rate. Something you did changing pumps/installing the new heater has screwed things up.
> [@](#):
>
> Regardless, I need a smaller pump.
Can ask why? hard to suggest alternatives if we don’t know whey you need a smaller pump.
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### Author:  [@solkoe](https://boards.straightdope.com/u/solkoe)
#### Post date: [May 27, 2009, 2:22am UTC](https://boards.straightdope.com/t/can-i-put-a-dimmer-switch-on-my-pool-pump/497684/17 "2009-05-27T02:22:33Z")
> [@Blake](#):
>
> Can ask why? hard to suggest alternatives if we don’t know whey you need a smaller pump.
I need a smaller pump because the flow rate through the pool is too high. Water swirls in the pool.
Debris gets caught in the whirlpool and settles in the middle of the pool. This actually makes the pool difficult to clean. The debris gets distrurbed when I vacuum the center of the pool then settles again in the center about an hour later.
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### Author:  [@Dag\_Otto](https://boards.straightdope.com/u/Dag_Otto)
#### Post date: [May 27, 2009, 3:30pm UTC](https://boards.straightdope.com/t/can-i-put-a-dimmer-switch-on-my-pool-pump/497684/18 "2009-05-27T15:30:42Z")
Easiest thing to do is to install a butterfly or globe valve on the discharge side of the pump and throttle the flow.
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### Author:  [@JFLuvly](https://boards.straightdope.com/u/JFLuvly)
#### Post date: [May 27, 2009, 4:41pm UTC](https://boards.straightdope.com/t/can-i-put-a-dimmer-switch-on-my-pool-pump/497684/19 "2009-05-27T16:41:32Z")
I don’t think installing a valve will work because if you decrease the size of the orifice you will increase the velocity of the water, almost like pinching off a garden hose, it’s been awhile so I am not absolutely sure about that.
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### Author:  [@YamatoTwinkie](https://boards.straightdope.com/u/YamatoTwinkie)
#### Post date: [May 27, 2009, 4:54pm UTC](https://boards.straightdope.com/t/can-i-put-a-dimmer-switch-on-my-pool-pump/497684/20 "2009-05-27T16:54:15Z")
> [@JFLuvly](#):
>
> I don’t think installing a valve will work because if you decrease the size of the orifice you will increase the velocity of the water, almost like pinching off a garden hose, it’s been awhile so I am not absolutely sure about that.
No. Any time you put a disturbance in the flowrate (as in a narrow orifice, or a valve), you introduce a head loss into the system, which reduces the flowrate. The water velocity at the local _point_ of restriction will increase, but then the flow has to “expand” once it passes the restriction and enters into the large diameter piping, which slows it back down. Try pinching a garden hose about 15 feet back from the tip and seeing if the velocity still comes out faster.
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