Shallow Well Power Consumption - GWHP

[Sixlashes]

Submersible Pump Power Consumption - GWHP

I have gone through numerous previous posts and cannot find the answer to this. Please forgive me if I overlooked it.

QUESTION: How much power will a 1/3 hp or a 1/2 hp submerged well pump running at 70% capacity use? I cannot apply the figures I get from Grundfos/Franklin Motors/F&W for my situation. I am confident from reading previous posts that there are folks in this forum that are well versed in this.

BACKGROUND: I am building my home and plan to install a Climatemaster Model 049 - 4 ton geothermal heat pump. I am trying to determine which pump (open or closed loop) will be more economical for the heat sink since the water temperature and power consumption of the chosen pump clearly impacts the overall system EER/COP.

I have an abundance of good water at a depth of 50-55 feet here in NW Florida at year-round water temp of 68-69 degrees. If I utilize a horizontal loop 6' deep, my water temp will vary from the well water temp by approx 10 degrees. This does not take into account ground heat saturation during the long summer cooling load here in Florida which will further lower the efficiency of the closed loop. Will the increased EER/COP values at the more attractive well water temps be worth the added power consumption of the well?

Since I am already installing an irrigation well, the cost of installing an additional well for the water source is more competitive than installing a ground loop. I plan to use the irrigation well as the injection well to minimize the possibility of fouling the screens with the small amount of iron present (assuming air gets into system). I have been told by drillers in the area that pulling large amounts of water at a high rate of flow back out of the injection well should eliminate\reduce clogging of the injection well screens.

I am leaning toward the open system, but do not want the cost of running a well pump to make it more expensive. I am convinced we are on the precipice of vastly increasing utility costs and plan to live in this house for the rest of my life. What sayest thou?

NOTE: This is also posted in HVAC

 

[Sixlashes]

Correction: 4" Submersible Pump- NOT Shallow Well

I used the wrong pump description in my previous title. Bummer.

 

[Sponsor]

 

[Bob NH]

I would go with the well because it will save power even if pumping from 55 ft. Rationale and analysis follow.

If you are using two wells then they are presumably in the same aquifer with water at the same depth.

If you discharge the water down into the well, instead of just dumping it into the top of the well, you should be able to regain at least 20 ft of the pumping head. Therefore, the total "lift" required is about 35 ft. That does not include the head loss in the heat exchanger of the heat pump.

If you have a heat exchanger (tubes in the ground) that heat exchanger is likely to have significant head loss; maybe even 35 ft; which would make it an even proposition for pumping power.

Now consider the benefit of the 10 degree difference in temperature of the water, which will affect performance in summer and maybe in the winter. Compressor power is roughly proportional to temperature difference so a difference of 10 degrees could be 15 percent of compressor power, which is much more than the pumping power.

Now in the A/C mode I estimate that you will be dissipating 48,000 x 1.25 = 60,000 BTU per hour. If the water temperature rise is 10 F you will pump 100 pounds per minute (12 GPM). If you have 10 psi pressure loss in the condenser (A/C mode) the pumping power will be 100*(35 ft + 23 ft) = 5600 ft-# per minute = 0.176 HP. With a wire-to-water efficiency of 50% you would use (0.176 HP * 0.746 kW/ HP)/0.5 efficiency = .263 kW

You will have to make a number based on your pump, but 1/3 HP should do it, especially since they usually have a significant service factor.

The calculations above are an example. Your supplier should be able to give you calculations based on the equipment that he is delivering.

It will be an unsuual well pump that you want. You want something that will be at the maximum efficiency point when delivering about 60 ft of head. They don't make water-supply pumps in that range. If you get a water-supply pump you will be wasting a lot of power.

Also, you don't want a control valve to throttle the flow. The pump should be matched to the system so there are no control valves to waste energy. This might be a good application for a 1750 RPM submersible pump motor if you could find such a thing.

 

[Sixlashes]

Bob,

I was hoping you would be one to post a reply. I have read your other posts with great interest. I just retired from the Air Force and after building our house (goal #1), I am planning to attend University of Southern Alabama in Mobile to earn a civil engineering degree (goal #2). A little late in life, but I want to keep the brain challenged and I have the GI bill to make it a reality. I am looking forward to getting back into the math and will enjoy the practical instruction and projects. The guy who sealed my plans for wind load (required here in Fla) accomplished the same goal. He's an old fart like me (46).

Back to the issue; a Grundfos 10S03-6 is a 1/3 hp pump with a listed 13 gpm at 60 feet of head. However; they list it's power consumption as 7.2 full load amps @ 66% efficiency. Would this be an adequate fit? Would you power consumption calculations apply to this pump? I am starting to see through the haze and would say yes.

Its a little far over on the curve, but available for purchase. Would a cycle stop valve be helpful to regulate the pump's flow to what the coil requires, 11-12 gpm? It seems guys in this forum highly recommend it. Would it keep the pump from overheating? Running at this head would put it 2-3 gpm from its max output.

The pressure drop of the coil is rated at 3.8 psi at 70 degrees. I will need no more than 130 feet of 1" to 1-1/4" pipe and a half dozen 90's to get it done.

I am indeed returning the water to an irrigation well of the same depth about 70 feet away. That should ensure no heat contamination of the first well. I am planning to pipe the injection output to below the draw down level in the second well to reduce oxygen as much as possible to reduce fouling.

I appreciate the help.

 

[Bob NH]

13 GPM at 60 ft of head = 13 GPM x 8.34 #/gal x 60 ft = 6505 ft-# per minute = 0.197 HP = 147 Watts water power.

At a wire-to-water efficiency of 50%, that is 294 Watts electric.

Volts x Amps x Power factor = Watts. Say Power factor = 0.8.

Amps = 294/(230 Volts x 0.8) = 1.6 Amps

If it's a 115 Volt motor that is 3.2 Amps.

It looks like a reasonable pump but I would ask the seller of the Grundfos pump to guarantee the Watts for the pump.

I would not use a CSV. Any kind of control valve dissipates energy and in order to have a control range you must oversize the pump. You are better off to let any excess flow/pressure drop be dissipated in the heat exchanger (evaporator in A/C mode) because higher flow reduces the temperature difference and thereby reduces compressor power. You just need to be sure that the velocity isn't so high that you get erosion in the heat exchanger, which shouldn't occur with a well-designed system.

A little more pumping power will usually be returned in lower compressor power unless the system has been perfectly optimized.

I have not had occasion to try one yet but if you need to control flow I would consider using one of the single-phase to 3-phase converters that cost about $200, and give you capability to adjust the speed. You can set it at a fixed frequency other than 60 Hz and then let the pump run without any control valves. Grundfos probably sells a variable speed drive for about $1000 which is the $200 unit in a box with some controls and a Grundfos label.

PS: Where did you retire from in the AF and what did you do? I spent my career developing weapons for the Air Force. Minuteman and MX/Peacekeeper re-entry systems, and Sensor Fuzed Weapon (CBU-97 and CBU-105). The strategic missiles (Minuteman and MX were from Norton AFB, and Sensor Fuzed Weapon was run out of Eglin AFB.

 

[Sixlashes]

Looking around, either the aforementioned Grundfos or a similarly rated Gould pump are my only easy options. I agree to let the additional flow run through the coil. In the big picture, it is minimal. My only concern is running the pump at its low head/high (flow) end and overheating it. I assume (spelling?) the power consumption would need to be tweaked a little to account for the extra gpm. You have taught me how to do that.

The heat pump manufacturer recommends a flow controller. I don’t really know if that is to reduce flow from a unified household/heat pump water system or what. If an additional 10-15% of flow erodes their coil, something is wrong. The higher flow should increase the head and choke off the higher flow rate anyway. Right?

My years in the AF were all spent working communication/guidance/radar systems on heavy aircraft. McChord AFB in WA, RAF Mildenhall in the UK and last but not least (actually, yes the least) Grand Forks AFB in ND. I had to retire to get out of there. The wife had enough. Sorry – no offense intended to you northern tier folks. I ended up a maintenance superintendent consumed with logistical and personnel issues. As you can imagine, I was spending quite a few nights’ lodging in tents in exotic far away lands. That’s when the wife got tired of 30 below and going solo with all of the fun challenges that presents.

Also - would you please look at my other post in the HVAC section - "Clothes Dryer Makeup Air - A Better Way?" Tell me if I need the nice men with the straight jacket...

 

[Valveman]

I agree with Bob's calculations but, I disagree with his practical application. You only need a CSV on a pump if the flow rate is varied. If the pump runs the house, irrigation, and the heat pump, a CSV is a must. If the pump is dedicated specifically for the heat pump, you don't need the CSV. I have a 3 ton heat pump of the same brand, and more flow seems to help. I hope it doesn't erode the coil. If too much flow is a problem, you can valve it manually with a ball valve or flow control solenoid valve.

If needed, valving manually will reduce the energy consumption as much as varying the speed. I would never use a phase converter, or a variable speed pump. You are trying to save energy and money. Varying the speed does not save any energy unless the pressure can be reduced. Your pump will always need to produce 35' of head, so the head or pressure cannot be lowered, which is the only way to make a variable speed pump effective.

Even if a variable speed could save some energy, I hear from many people who have used them, that continued and regular maintenance and replacement of the variable speed pump, is a major expense. They say that is doesn't do any good to save $50 per month on power, if they have to replace a $1,000.00 pump every 18 months or so. So the best way to save money and energy is to use a pump that will last a long time, even if it uses more energy.

The 10S03-6 pump you are looking at is a good pump. I have a 7S03 Grundfos in my heat pump well. Mine actually runs through the heat pump and then feeds a stock tank, so the pump runs 24/7/365. The last one lasted 14 years that way, and I had to put in a new one last year. I think the only reason mine only lasted 14 years, is because it is a weak well and before I put in the heat pump and added a little restriction, it pumped the well all the way down and was actually pumping air as well for all that time. Yet it still lasted 14 years being abused this way. I expect to get a lot more than 14 years from the new one, since I now have it valved back so it can't pump the well dry.

The most important thing about the pump, is that it needs to be dependable and long lasting, so do not use a variable speed pump or phase converter!

 

[Sixlashes]

Valveman,

The manufacturer recommends a simple flow restrictor to throttle the flow down to recommended gpm. Does this save money by reducing the amount pumped, or just put additional load on the pump? As stated before, I am concerned that the pump will be pulling too much power and overheat with the extremely low head it is pushing.

 

[Valveman]

Most submersibles, including the ones you mentioned, are non-overloading. Even if they are in a barrel and pumping wide open flow, they will not overload. And yes, if you use any kind of valve to restrict the flow, the power consumption will decrease. The more you restrict the flow the lower the amp draw but, it will use a little more energy per gallon produced. A pump sees more flow as additional load. More pressure from a valve decreases flow, which reduces the load, and therefore reduces the amp draw.

I would keep the flow as high as possible to make the heat pump more efficient. However, if you can get the same efficiency while using less flow form the pump, the energy used by the pump will be less.

 

[Sixlashes]

Thanks Bob NH and Valveman.

One last piece to the puzzle - Does a 1/2 hp pump use much more power pumping the same head and volume as a 1/3 hp pump?

Thanks.

 

[Bob NH]

Yes, for reasons described below.

If a 1/3 HP pump is adequate to deliver the flow and pressure required for an application, then throttling the 1/2 HP pump to meet the requirements of the system will use more energy than the pump that matches the system requirements, but a little less than the power it would use if not throttled.

The energy lost in a throttling/control valve is wasted.

The energy lost in pressure drop in a heat exchanger, evaporator, or condenser may serve a purpose because it improves the heat transfer rate and thereby reduced temperature differences.

If you vary the speed of the motor you are effectively changing the pump. Characteristics of the pump are related to speed in the following way:

Flow is proportional to speed.
Pressure/head is proportional to the square of the speed.
Power is proportional to the third power of speed.

Therefore, if the pump speed is reduced to 90% of the speed for the published performance, then the new characteristic will be 90% of the flow, 81% of the head/pressure, and 72.9% of the power.

 

[Speedbump]

Before figuring out the difference between a 1/3hp and a 1/2 hp look at the Service Factor on the motor. You may find that the 1/3hp is the same or even greater than the 1/2hp. My 1/2hp shallow well jet pump has a 1.9 Service Factor which makes it actually a .95hp. Bigger than the Big Box 3/4hp pumps.

bob...

 

[Valveman]

I understand why BobNH thinks that way, that is the biggest misconception in the pump industry. However, a pump that must always lift or produce a certain amount of head, cannot be slowed down enough to save any energy. Because you lose head by the square of the speed, and a water well application always requires a certain amount of head, variable speed pumps cannot save any more energy than a CSV.

A valve simply makes a pump work further left on it's curve. Which makes the pump think it is in a deeper well and therefore produce more head and less flow, which makes the pump use less energy.

Anybody who has put the two type systems side by side, as I have, can see that the energy consumption is the same. You simply can't slow the variable speed pump down enough to lower the power consumption below the valve controlled pump, or it is no longer spinning fast enough to get water to the surface of the well.

Then consider that the CSV controlled system will last 20 years, and the variable speed system will need a $1,000.00 or more in repair ever 5 years. The extra 3 or 4 thousand dollars you will spend keeping the variable speed pump running, is due to the energy used to mine, manufacture, transport, install, and recycle all the electronic equipment in a variable speed pump and controller. So even if a variable speed pump could slow down your electric meter a little bit, it is far outweighed by the energy wasted in manufacturing of short lived equipment.

Every dollar you spend on equipment is also paying for energy used to manufacture. So the more you spend on equipment, the more energy you are wasting. You just don't actually see it our your electric meter, because what you pay for the equipment, is for the energy that has already been wasted during manufacturing.

At high flow rates a variable speed pump uses more energy than a standard pump. This is due to the power consumed by the electronic controller itself, and the loss of motor efficiency for running the motor with pulsing DC power, instead of normal sinusoidal AC power. Also a variable speed controller causes harmonics. Which aside from the problems harmonics causes to other electric equipment in the area, power rich with harmonic current cause the motor to use more energy.

See the attached curve that shows a variable speed or VFD, using more energy at high flow rates, that usually outweighs the small amount of energy saved at lower flow rates. What this tells you is that on the average, CSV and VFD use the same amount of energy. It also tells you that if you never use higher flow rates, a smaller pump will save more energy than a VFD.

 

[Bob NH]

I don't agree with the previous post. Let's look at a case that is based on the needs of the system being discussed.

The requirement, within the range stated by sixlashes, is 12 GPM at 60 ft of head. The comparison works for anything in the range.

The candidate pump is the Goulds 10GS05R. The pump curve given at the Goulds site is 15.6, 14, 12.4, 10.4, and 6.5 GPM at 60, 80, 100, 120, and 140 ft of head. http://www.goulds.com/pdf/7310.pdf

If you take the case of 14 GPM at 80 ft of head and scale it to operation of the pump at 51.9 Hz, it delivers 12.11 GPM at 59.86 ft of head. The "power in the water" at 44,253 ft-#/min = 1 kW, is 137 Watts.

Now let's look at the condition if a CSV is used to limit the flow to 12 GPM. If the flow is throttled to 12 GPM the head will be 104 ft and the "power in the water" will be 235 Watts.

If the "wire to water" efficiency is 50% and the converter is 95% efficient then the pump with speed control will use 135/(0.50*0.95)=284 Watts and the pump with CSV will use 470 Watts.

I don't have efficiency data for the pump, and it is usually not published for these small pumps. However, both conditions are near the nominal operating range for the pump and it should be pretty close to the same for both conditions. The pump efficiency difference is not great enough to make up for the difference in power required to pump the water.

The pump works fine at the lower speed, delivers enough head and flow, and uses only 60.5% of the power (including converter losses) required to get 12 GPM with a CSV.

The speed control also offers an opportunity to see if increasing the water flow will reduce the total power consumption of the pump and compressor.

 

[Valveman]

Even if it worked in the real world that way, you are still missing the biggest waste of energy, which is the variable speed pump. Depending on how many hours per day the pump is running, you are only talking about a difference of about 6 bucks a month, or $360 every 5 years. This means the system would have to last more than 15 years without any maintenance, just to pay off the initial $1,000 extra up front cost of the variable speed system. It would still have to last 8 years to pay itself off, when the variable speed systems get cheap enough to only cost $500 extra up front cost. If you have to work on the system every 5 years on average, you are just getting further upside down on any possible savings.

Bobs calculation shows energy savings because he is using a ½ HP pump that is always slowed down to a little more than a 1/3 HP load.

If you were using the pump for the house as well as the heat pump, you would need to be able to vary the flow, be it with a variable speed or a CSV. Then the curve I gave earlier is correct in that a variable speed pump uses more energy than a CSV system when the flow rate is high. This means the CSV would be saving more energy, when everything is using water, and the flow rate is high. It also means the VFD would be saving some energy, when nothing but the heat pump is on, and the flow rate is low. So over the average the power consumption of the CSV verses the VFD system would be equal, and the real savings would be for the system that cost the least and last the longest.

However, you will notice that I recommend using a 1/3 HP pump (10S03-6) without a CSV or a VFD. This will save about 20 cents per day over any other way of producing that particular flow rate, and will be the absolute longest lasting possibility. As BobNH has so graciously preached in the past, there is no more efficient way of pumping water than to correctly size the pump to the load to start with.

 

[Bob NH]

I'm a DIYer as well as an engineer. The system that I am talking about is based on a part (the Hitachi X200-004NFU) that costs $147 + shipping. There should also be some saving on the motor.

http://www.hitachi.us/supportingdocs/forbus/inverters/Support/SM-E256P_X200.pdf
http://www.driveswarehouse.com/c-149-drives.aspx?categories_id=21

The payback would be about one year where I live. The rate is $0.15/kWhr and 200 watts x 5000 hrs per year saves $150.

 

[Valveman]

I don't understand Bob. Have you changed your mind about correctly sizing a pump to the load, being the most efficient way to pump water? You have preached this to us countless times about irrigation systems. Why would you not want a heat pump system to be the most efficient it could be, just like an irrigation system? We have already determined that slowing down a ½ HP pump, will draw a little more than a 1/3 HP load. This will use more energy than using a correctly sized 1/3 HP pump. Which if the heat pump efficiency stays the same, can be throttled back to pull a little less than a 1/3 HP load. This will save another $150 a year over using the variable speed pump.

Then you must consider how hard a VFD is on the motor. Harmonics, voltage spikes, resonance frequency vibration, skin effect damage, and many other problems are produced by variable speed drives, which shorten the life of the pump and motor considerably.

Not only that but, the following are some of the precautions stated in the manual of the VFD you linked to. It basically means you have to put the unit in a dust free, air conditioned environment. Even then it says you will have to replace the unit at least every 5 years, and possibly more often.


"Avoid installation in areas of high temperature, excessive humidity, or where moisture can easily collect, as well as areas that are dusty, subject to corrosive gasses, mist of liquid for grinding, or salt. Install the inverter away from direct sunlight in a well-ventilated room that is free of vibration. The inverter can be operated in the ambient temperature range from -10 to 50°C.(Carrier frequency and output current must be reduced in the range of 40 to 50°C.)

Because a DC bus capacitor deteriorates as it undergoes internal chemical reaction, it should normally be replaced every five years. Be aware, however, that its life expectancy is considerably shorter when the inverter is subjected to such adverse factors as high temperatures or heavy loads exceeding the rated current of the inverter. The approximate lifetime of the capacitor is as shown in the figure at the right when it is used 12 hours daily (according to the " Instructions for Periodic Inspection of General-Purpose Inverter
(JEMA).)Also, such moving parts as a cooling fan should be replaced. Maintenance inspection and parts replacement must be performed by only specified trained personnel."

You would have to be a really good DIYer to be able to work on this kind of equipment, and even then the maintenance will eat up any possible savings. Most people can't even finish reading all those technical instructions, much less be able to understand them. Most engineers can't understand this stuff either, or they would realize how much of it is bad news.

Using a normal 1/3 HP pump is the best way to save energy and make this system last. Bob's calculation of 5,000 hours a year, would mean that this VFD system would need to be replaced about every three years according to their instructions. If it is not in a dust free and air conditioned room, it will need to be replaced even more often than that. Premature replacement of short lived equipment is the biggest waste of energy.

 

[Bob NH]

I don't understand Bob. Have you changed your mind about correctly sizing a pump to the load, being the most efficient way to pump water?

You aren't reading the question posed by the original poster. He is apparently not able to find a pump that matches the flow/head requirements of his system.

Engineers find solutions to meet requirements. The most economical solution to serving the GSHP is to use an "adjustable speed" pump system. After all, the usual reason for installing a GSHP is to save energy and resources.

The "adjustable speed" capability of the pump also allows the pump to be adjusted to optimize the system and manage any variations in operating conditions or expected performance.

There is a great benefit to being able to adjust the speed by a few percent either way to take care of the "unknown unknowns" that arise when one is a pioneer in a developing technology.

 

[Valveman]

You aren't reading the question posed by the original poster. He is apparently not able to find a pump that matches the flow/head requirements of his system.

Engineers find solutions to meet requirements. The most economical solution to serving the GSHP is to use an "adjustable speed" pump system. After all, the usual reason for installing a GSHP is to save energy and resources.

The "adjustable speed" capability of the pump also allows the pump to be adjusted to optimize the system and manage any variations in operating conditions or expected performance.

There is a great benefit to being able to adjust the speed by a few percent either way to take care of the "unknown unknowns" that arise when one is a pioneer in a developing technology.

#1
I do understand the question, the 10S03-6 (1/3 HP) does match the requirements, and may even need to be throttled a small amount.

#2
Engineers find the most complicated and elaborate way to make something work. A pump man will find the most dependable, longest lasting, and simplest solution to a problem. Saving energy and resource is the main reason for using a GSHP, and you don't want to spend more money on the resources to make it work, than you could ever save in energy.

#3
"The reduction in flow is no longer proportional to speed; a small turn down in speed greatly reduces flow rate and pump efficiency. A common mistake is to also use the Affinity Laws to calculate energy savings insystems with static head. Although this may be done as an approximation, it can
also lead to major errors." Hydraulic Institute

#4
There is no great benefit to varying the speed a few percent. In fact the side effects of varying the speed can greatly outweigh any possible energy savings. To an engineer reading the sales brochures, VFD sounds like a good solution. To a pump man with decades of experience and thousands of installations to back him up, VFD is more trouble than it is worth, and rarely last long enough to even consider a pay back.

 

[Sixlashes]

You guys are giving me a crash course in fluid dynamics. As I see it, both setups have their advantages & disadvantages. First off, the only purpose of this pump system is to provide water to a 4 ton geothermal heat pump. While energy efficiency is not my only consideration; it is at the top.

Advantages of the VFD with a 3 phase pump:
1. 1/3 hp pump assemblies are no longer available (Goulds, Grundfos, F&W)
2. I need three different pump volumes – the VFD offers 16 logic-selectable speeds (thermostat/dehumidistat controlled through 24 vac switching relays)
3. Soft start
4. Slower speed easier on bearings and pump end
5. Lower power consumption when pump is running at less than rated output
6. Flexibility in changing pump output to unknown variables (i.e. filter assembly if water contains high particulates or change in water depth in well)
7. Precise (minimum) flow control minimizes potential fouling of injection well
8. Precise (minimum) flow control minimizes coil erosion
9. Less water draw down in well due to lower pump volumes

Disadvantages of VFD and 3 phase pump:
1. No manufacturer (Goulds, Grundfos, F&W) offers a 3-phase pump assembly under 1 hp, and then only as a special order
2. VFD cost - $159 (1 hp model required due to increased current requirement of submersible pump motor – per Hitachi)
3. Waveform causes motor vibration and heat produced power loss -- Output side AC reactor recommended to smooth output waveform ($85)
4. Complexity
5. Impact of lightning on electronics - UNKNOWN
6. Recommended maximum length of leads to pump – 10 meters
7. Possibility of interference with other electronics, i.e. home entertainment
8. Mounting location for VFD - heat/humidity controlled environment preferred

As stated in both setups, I cannot purchase the required pump assembly. I need to purchase a motor and pump end separately either way. Either pump assembly option costs roughly the same. Also. the cost of the VFD and reactor would be roughly the same as additional valve(s) and flow controller(s) to mechanically regulate flow.

My required flow rates are 12 gpm in full load, 11 gpm in part load and 1.6 gpm in dehumidification mode (ClimaDry) see http://www.climatemaster.com/pix/res_lit_buttons_r2_c3.jpg. While the difference between full and part load is negligible (still, potentially 500 gallons per day), the difference in running the unit in humidity removal mode (spring & fall) is huge. While the unit adjusts cooling flow to get the internal water temperature up for the air reheat coil, the cooling flow could fall below the pump’s recommended minimum flow of 1.2 gpm for short periods of time. In either setup, I will need a differential pressure bypass valve to ensure minimum flow of 1.2 gpm per motor manufacturer.

The impact of the humidity removal mode did not dawn on me until today. I apologize for previously leaving out that detail.

If my goal is efficiency – the VFD is the answer. If simplicity is the goal, mechanical choking with a constant volume pump setup fits the bill. Is the efficiency of the VFD system worth the added complexity?

I do have the background, courtesy of the US Air Force, to build either system and maintain it (the VFD). On the other hand, I don't want to be forced to constantly tinker with it.

Am I on the right track? Anything to add?