AC vs DC Pumps. Which Run Hotter?

[Reef Frog]

Let's say we're comparing a standard AC return pump like a Sicce, Eheim or Tunze with a new generation controllable DC pump (Tunze, Vectra). Both are being used as a submersible.

Lets say the the AC Pump moves 600 GPH and the DC pump moves the same quantity (600 GPH). Do the DC pumps put less heat in the water even though they are moving the exact same amount of water? Or is heat output about the same for all? Is DC inherently more efficient somehow?

Reading posts on the topic, many comments seem to indicate that the DC pumps run cooler by nature, while doing the same amount of work. Is this the case? If so, why? What's the science or physics behind this? And is the difference appreciable or so small that it shouldn't be a practical consideration?

 

[D.moller]

I too am curious about this I run a mag 9 in my 65 display and my temps are just a few degrees warmer than I'd like. I suspect the pump is the heat culprit but I'd like more info before I bite the bullet and upgrade to dc.


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[sirreal63]

DC watts are exactly the same as AC watts, 50 watts of DC is the same as 50 watts of AC, they are both 50 watts of heat energy.

 

[slief]

DC watts are exactly the same as AC watts, 50 watts of DC is the same as 50 watts of AC, they are both 50 watts of heat energy.

True but not all DC pumps are created equal in that sense. Some bleed off much of the heat in the power source & or controller. Also, I think AC power generated much more heat watt for watt than a pump that runs off DC voltage. My RD3 230's for example run cold to the touch where as the Super Dart Golds that I replaced were pretty warm to the touch. The Darts pulled 178 watts per my Kill A Watt meter & even with the RD3's running at full power (230 watts), they are still cool to the touch.

 

[sirreal63]

The difference is in cooling, but watt for watt heat is the same. DC pumps do not send extra heat to another dimension, they follow the same laws of physics as AC pumps. A watt is a standard of measure, it does not vary because the sources are different. The RD3 has the motor block cooled by water, the Dart does not. The heat generated is the same if the wattage is the same. The RD3 is cooler to the touch because it is being actively cooled, this keeps the motor cooler, but the heat is still generated.

 

[madweazl]

What does it matter really? If your pump is putting you over the edge for an acceptable temperature level, you got other issues.

 

[D.moller]

So what are the other issues if the pump is the major heat contributor. If the dc pumps expel a majority of their heat at the controller and or power source rather than into the water isn't that a benefit given to running dc.


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[sleepydoc]

In general, the newer DC pumps are more efficient than many of the AC pumps, but it ultimately depends on the motor block and how efficient it is. There are many newer efficient AC pumps on the market.

As was mentioned above, a watt is a watt. Thermodynamics applies. Always. If a pump is consuming 50 watts of power, it's generating 50 watts of heat. A couple caveats, though:

1. DC pumps generally have a controller and some of their power consumption is in the controller, not the motor block. Heat generated by the controller will heat the ambient air, but not the water (at least not directly,) so if you have a AC pump and a DC pump moving the same amount of water and consuming the same amount of power, the DC pump will heat the water less because part of the power is being dissipated by the controller.

2. External pumps - these will heat the water less because they dissipate heat into the ambient air. How much the heat the water depends on the design of the pump and how efficiently it gets cooled by the surrounding air vs transmitting the heat to the water.

What does it matter really? If your pump is putting you over the edge for an acceptable temperature level, you got other issues.

There are many people for whom heat management is an issue. In most cases, the return pump is the major controllable source of heat in the tank since turning lights off isn't an issue. If you are right on the edge, a more efficient pump can definitely make the difference between needing a chiller or not. The difference between a pump that uses 90 watts of power and one that only uses 40 watts is like having a 50 watt heater in your tank.

Switching to a more efficient pump or switching from an internal to an external pump is far cheaper more than one way than buying a chiller.

 

[1jwampler]

My understanding of the question is same GPH (600gph) AC or DC for efficiency. From what I have seen AC pumps draw about the same from the wall to deliver the same GPH. But... The difference is the AC pump (assuming both submerged) delivers 100% of the heat load to the water, where the DC pumps typically dissipate about 1/2 of their heat to the air via external power supplies and controller. As others have said watts are watts, they have to dissipate their heat somewhere, but submerged A/C pumps are self contained and 100% in the water.


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[twon8]

I too am curious about this I run a mag 9 in my 65 display and my temps are just a few degrees warmer than I'd like. I suspect the pump is the heat culprit but I'd like more info before I bite the bullet and upgrade to dc.


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Mag drove pumps run hot. A mag 9 is putting 93 watts of heat into your tank. You don't have to go DC, just getting a smaller and more efficient AC pump would help. I would go with an Eheim 1250 , which uses 28 watts vs the 93 the magrive 9.5 uses. It'll be less flow but still more than sufficient. You could use an 1260 but it's overkill IMO.

 

[Skim]

You may find this informative.

https://youtu.be/R4QztZC5xrQ

 

[Ron Reefman]

I'm no electrical engineer, but aren't some of those watts going into making the motor spin rather than making heat?

Anyway, I have a Jebao DC12000 pump that has been running as an external pump on my CL system for 3 years now. It consumes 85 watts and runs at 12000 liters/hour (3200 gallon/hr) 24/7/365 and has no venting or cooling fins. The temp inside my stand is 81F and the case of the Jebao pump is 88F.

So where are those 85 watts of heat?

 

[sleepydoc]

Watts are a measure of energy (actually energy per unit time.) Part of that energy is heat and part of it is water movement. If you had a 100% efficient pump, all of it would be water movement, but those don't exist. Regardless, if you have two pumps moving the same amount of water, that component of the total energy expenditure is the same and you can ignore it for the purpose of comparison.

With your Jebao pump, part of the 85 watts are being consumed (and dissipated) by the power supply - if you put your hand on it I bet it's warm to the touch. The rest are being used by the pump itself. Of the power being used by the pump itself part of it is heating the water running through the pump, and part of it is heating the air around the pump (assuming that the ambient temp is less than 88º F) As I said above, the relative proportions of these routes of dissipation depend on the design of the pump. Cooling fins allow for more efficient transfer of heat to the surrounding air and reduce the amount of heat added to the water.

So short answer: part of the 85 Watts is in the tank and part is in the air around the tank.

 

[jayball]

I'm no electrical engineer, but aren't some of those watts going into making the motor spin rather than making heat?

Anyway, I have a Jebao DC12000 pump that has been running as an external pump on my CL system for 3 years now. It consumes 85 watts and runs at 12000 liters/hour (3200 gallon/hr) 24/7/365 and has no venting or cooling fins. The temp inside my stand is 81F and the case of the Jebao pump is 88F.

So where are those 85 watts of heat?

On a submerged pump all electrical watts eventually turn into heat in the form of friction losses aside from the actual heat produced by the pump.

On an external some of the heat is dispersed into the air.

 

[gcarroll]

My understanding of the question is same GPH (600gph) AC or DC for efficiency. From what I have seen AC pumps draw about the same from the wall to deliver the same GPH. But... The difference is the AC pump (assuming both submerged) delivers 100% of the heat load to the water, where the DC pumps typically dissipate about 1/2 of their heat to the air via external power supplies and controller. As others have said watts are watts, they have to dissipate their heat somewhere, but submerged A/C pumps are self contained and 100% in the water.


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I'm curious how you came to the conclusion that "DC pumps typically dissipate about 1/2 of their heat to the air via external power supplies and controller"? How did you measure this?

 

[gcarroll]

Great info sirreal63, sleepydoc, and jayball. Way to separate fact from fiction!

 

[theatrus]

I'm curious how you came to the conclusion that "DC pumps typically dissipate about 1/2 of their heat to the air via external power supplies and controller"? How did you measure this?

Even the Jebao pumps don't have nearly that inefficient of a controller. I'd expect about 10% of the energy is lost by the power brick and the controller (for a 50W pump this is still 5W, and those components unless actively cooled would be quite warm).

There are a few other differences with AC vs DC pumps - a lot of air cooled AC pumps (PanWorld, etc) have a two-stage drive coupling system - the main AC motor spins a cylindrical permanent magnet which then is magnetically coupled to the impeller. Rotating the magnet mass in air in addition to the impeller will add some load to the system. Of course, a DC pump has both a rectifier+SMPS and an inverter to contend with (but efficiency of these components is always improving).

If you are using any ball or gate valves on your AC pump system to control flow, a DC pump has a higher chance of being more efficient as their key advantage is speed control. The DC pump can be throttled down in speed with a directly proportional energy consumption, which you can't do in an AC pump (they are always synchronous with the AC frequency). The AC pump would be working against higher head pressure in a reduced flow situation.

 

[gcarroll]

Even the Jebao pumps don't have nearly that inefficient of a controller. I'd expect about 10% of the energy is lost by the power brick and the controller (for a 50W pump this is still 5W, and those components unless actively cooled would be quite warm).

That is more along the lines of what I would expect and that is a long ways from 50%.

 

[Ron Reefman]

Regardless, if you have two pumps moving the same amount of water, that component of the total energy expenditure is the same and you can ignore it for the purpose of comparison.

With your Jebao pump, part of the 85 watts are being consumed (and dissipated) by the power supply - if you put your hand on it I bet it's warm to the touch. The rest are being used by the pump itself. Of the power being used by the pump itself part of it is heating the water running through the pump, and part of it is heating the air around the pump (assuming that the ambient temp is less than 88º F) As I said above, the relative proportions of these routes of dissipation depend on the design of the pump. Cooling fins allow for more efficient transfer of heat to the surrounding air and reduce the amount of heat added to the water.

So short answer: part of the 85 Watts is in the tank and part is in the air around the tank.

The power supply is considerably warmer than the pump itself, and the controller is basically room temperature. So that part makes sense. Less heat at the pump which is what most of us are looking for in the first place.

 

[gcarroll]

AC vs DC Pumps. Which Run Hotter?

Don't forget that by design, the pump is still cooled by the water flowing through the pump. Just because the outside is cool, doesn't mean the pump is not heating up the water. An example of this is the RD3 with its anti like loop that more efficiently cools the pump. This could also answer why the pump is cooler on the outside.