Recently, I was working from the basement and I noticed tonnes of condensate water going down the drain, and thought I might re-use it to improve the efficiency of my home AC system.

So I put a condensate pump (tank&float combo unit) in the drain path, and on the condensate pump's output I added a pump capable of high pressure (upwards of 90-100 psi)

After the high pressure pump, I ran hose outside and around the outdoor coils with a high pressure misting aperture on each side. I aligned them so that the mist sprays out and gets 'sucked in' when the fan is running.

I put the whole thing on a relay that only energizes when the compressor is running also.

Winterization aside, I know if I was paying for tap water for this, I'd never save any money. But should this come out ahead since the water is "free" as condensate?

I know some window AC units use this strategy, but I've never seen it in central air.

How can I reliably measure its affect if the outdoor temperature and dew points and even the indoor set-points are always changing?

I do have CTs on all the equipment. The pumps are fairly low current and run for only a few seconds (4-6 seconds) every minute the AC is on. The misting nozels spray about 2/3 of every minute, as the hosing is long and stores quite a bit of pressure.

I've considered adding a pressure tank, and regulator to try to get a consistent pressure with intermittent pumping, but I don't know if that would actually help much. It'd also give me more confidence that the hosing won't burst if the hosing isn't flexed so much, though it's been reliable for a season and a half now.

It's also occurred to me that it might be more efficient to do away with the sump pump's pump, and just use the single high-pressure pump. I only kept the condensate pump's pump in play because it can push air out of the line, and the high pressure pump can't suck air. But I suppose I could move it to the bottom of the condensate tank, and disregard the condensate pump's pump.

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    I’ve wondered about this myself. Nice write up of a clever idea. I hope someone can comment on the expected efficiency.
    – jbbenni
    Jun 7, 2021 at 23:49
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    I'm assuming your central air is already in the shade? If not, give it a roof and it'll save you a ton of money. A lot of installers don't give a crap about your unit sitting in the sun and getting baked when you need it the most.
    – Nelson
    Jun 8, 2021 at 2:07
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    The cost of water is so tiny that tap water should be almost as free as the condensate. However, tap water is mineralized which means by midsummer your A/C coils will be covered with calcite deposits or whatever's in the water. Really you should be using distilled. Jun 8, 2021 at 4:10
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    Condensate = distilled from the air in the home = no minerals...
    – Ecnerwal
    Jun 8, 2021 at 11:01
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    It seems incredibly unlikely that the efficiency gains could even be measured since the amount of condensate is so small. I had an AC whose drain pan leaked so I put a 5 gallon bucket underneath. It collected about 5 gallons/day (give or take). A swamp cooler is going to use something like 3 to 15 gallons per hour so you're at least an order of magnitude away from doing anything effective. That being said I think it's definitely a neat idea to use the condensate water instead of fresh water for this idea. Jun 8, 2021 at 14:05

9 Answers 9


Clamp Power Meter

Buy or borrow a clamp power-meter with a "kWh" feature, that is, the ability to perform an energy measurement.

Not all clamp meters have this, e.g. a "True RMS" meter performs an accurate power measurement for non-sinusoidal signals but still not an energy measurement.

Open the AC unit or the electrical panel (careful, you need to know what you are getting into), and clamp the meter's probe over the live (black) feed of your AC. (If it's a 240V feed there will be black and red, and either is fine, whichever has best access).

enter image description here

Have the meter record the total power over a 24 hour period (more accurately: the total energy), and perform a split A/B test. Test A with sprinkler, and B without and repeat several days. Record the temperature and humidity, and then plot your data, splitting by A/B and sorting it by outside air temperature (e.g. low to high).

Best to perform alternating tests, e.g. in the A/B/A/B order because weather usually changes gradually. Keep the consistent-weather pairs, toss out the outliers like thunderstorm days, cold front days...etc.

I think after 2 days of about the same weather you'll know if there is a difference, and after 10 days you'll be able to somewhat quantify it in terms of money.

I don't know how science or math savvy you are, but if you provide the measurement data, we can help drawing a conclusion from it.

Audio Recording

Alternatively, and this is less reliable, record the sound of your AC for a period of 24 hours. Then in an audio-editing tool estimate how long it is on and off, determining the duty cycle of the fans based on the noise level or recording amplitude (high/low). Place the recording device's microphone near the AC to isolate the fan noise from other noise.

enter image description here

Again perform an A/B split and plot by outside air temperature. If 24hrs is not practical you can try 1 hour around 3pm and 1 hour around 3am and draw a qualitative conclusion, and possibly come up with a quantitative cost impact as well.

Current Transformer

If you are already employing CTs (Current Transformers) to detect the on/off operation of the AC, you could hook it up to a probed energy meter (so no clamp needed) and use it estimate the duty cycle in a split test. It might take effort to translate CT measurements to absolute energy use (kWh by the AC), but if difficult, the split test results could still be useful as a quantified relative comparison between the cases.

Image: https://9to5mac.com/2019/09/15/remove-silence-in-logic-noise/ (but use any tool that plots the amplitude over a wide time span to quickly determine the duty cycle)

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    @manassehkatz-Moving2Codidact Yes good point, edited.
    – P2000
    Jun 8, 2021 at 1:21
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    OP states (perhaps cryptically) that they already have CTs (Current Transformers) on all the equipment.
    – Ecnerwal
    Jun 8, 2021 at 1:45
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    I don't think the clamp meter will help: The same amount of power is used, with or without, a spray. The AC should run less frequently but it depends, of course, on the ambient temperature. And the ambient temperature is always varying. Jun 8, 2021 at 4:47
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    @PeterCordes yes, correct, I edited it to be more clear about the meter, it's the kWh feature we need. Thanks.
    – P2000
    Jun 8, 2021 at 14:21
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    @ChrisH yes A/B/A/B because weather usually changes gradually. Keep the consistent-weather pairs, toss out the thunderstorm days...etc. Good point, I edited.
    – P2000
    Jun 8, 2021 at 14:31

See other answer for how to test for actual efficiency gain, but you're basically cooling your output coil with a swamp cooler. In commercial settings it's called a free cooler. Water is either misted or dripped over a porous substance that the cooling air passes through. It cools the air as it evaporates, but greatly increases humidity. The concern I would have with your setup is that your outside evaporator/radiator is typically designed to keep moisture away from the internals, other than ambient humidity. There may be concerns of corrosion with large amounts of water, and even if not, it's not clean water, and radiators are often not particularly easy to clean so you could have a problem with condensate buildup or other crud like dust buildup caused by the adhesion of the surface water.

I would suggest that if you want to make use of your "free" condensate, you may want to look into intentionally building a free cooler to run in series or parallel so that you can intentionally mitigate the concerns of misusing the existing unit. It may also be more effective to cool the working fluid directly than to cool the intake air. The free cooling unit on the building I used to work on was about the size of one of the four chilling units and the chilling units only kicked in when the free cooling was inadequate. Cost effect of free cooling was much higher despite the system using paid-for water.

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    In commercial settings, it's called a closed-circuit cooling tower or evaporative fluid cooler. The existing system would be a dry cooling tower. A free cooler is something completely different, that's if he bypassed his AC's compressor.
    – user71659
    Jun 8, 2021 at 8:28
  • Would mold and/or mildew have a tendency to build up on the coils as well?
    – JS.
    Jun 8, 2021 at 23:49
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    "It's not clean water". I'm not sure I agree. It's condensate which is, essentially, distilled water.
    – SiHa
    Jun 9, 2021 at 6:23
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    @SiHa: Have you ever looked in the condensate collection and drain area of an AC? :-P The water is essentially clean when it condenses, but then nasty stuff starts growing in and around it. Jun 10, 2021 at 1:56
  • @R..GitHubSTOPHELPINGICE, Yes, but I read it as the condensate was being piped direct to a tank, and pumped from there. Maybe I misunderstood.
    – SiHa
    Jun 10, 2021 at 6:29

Disclaimer: This question is about Air Conditioners. The "You" discussed below is an air conditioner owner. This answer is not about heat pumps, which are a different class of equipment (operating on the same scientific principle, but there the similarity ends).

Well, an aircon owner would likely ruin their condenser coils* by letting water evaporate on them. But other than that, Mrs. Lincoln, you're onto a very sound idea.

However, my proposal is that you should use equipment purpose-built for that, rather than use equipment contrary to factory advice and contrary to its UL certification. While you may theorize that there should not be a problem, if there is a problem, you will have no one to blame but yourself for a potentially 4-digit repair. (and a reminder: someone working on the freon loop needs licensure to do so).

Really, you should do exactly what many large commercial installations do: replace the condenser with a freon-water heat exchanger - that kit is readily available in the marine world, because boat A/C doesn't dump heat into outside air, but rather, seawater!

Since you don't have seawater, you should have a service water loop (and condensate is a great source for makeup service water, since it's almost distilled.) Once the A/C system has made your service water hot, you cool it down at a cooling tower. There are several styles.

So it's basically a swamp cooler for the service water. When humidity is very high that will not work, and you're effectively interchanging with ambient air, same as the A/C does not.

Anyway, having an "efficiency" conversation about an air conditioner is almost pointless pointless if you're not talking about heat pumps. The real efficiency win is flipping that changeover valve and collecting heat at 250-600% efficiency during cool weather.

* What's wrong with distilled water on coils? Several. First, surprisingly to novices but well-understood to the nuclear industry, demineralized water is caustic: water wants to be mineralized, or to be more precise, minerals seek demin water more aggressively than they do common tap water, which is already mineralized and thus closer to mineral equilibrium. Normally aluminum coils are "self-weathering" i.e. they develop an oxide layer which arrests further corrosion. But the demin water will attack that oxide, stripping it off, causing progressive corrosion. You can solve this by using nuclear reactor grade materials, which is how it's done with heat pumps that are covered by condensate all the time in heat mode.

Aluminum and brass certainly can and do corrode - anyone who works around these coils for awhile recognizes the white fluff of aluminum turning back into bauxite, and the lime green "schmutz" coming off a copper or brass fitting suffering corrosion.

But even further - if you take a close look at window air conditioners that have been in service for some years, you see a lot of dirt on the condensers. Sometimes you even see enough dirt that plants are starting to grow in it. This got there by accumulated dust. Dust blows through condensers all the time, but when it's wet due to rain, the dust collects and sticks. This dirt tends to hold water, and so this, combined with oxygen + atmospheric impurities + whatever was in the dust, creates a cocktail that accelerates corrosion on the condenser. This is the demise of many window A/C units.

But don't take my word on it, ask an A/C repairman.

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    Of course it is a heatpump system. Why would it not be? Could you elaborate (substantiate) "ruin your condenser coils by letting (0-mineral-content condensate) water evaporate on them"? BTW, heatpump water heaters are even more awesome, in the summer anyway. But not quite in my budget yet, though I do have the floorspace planned out for one. Jun 8, 2021 at 5:07
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    The air in my home, from which the condensate is generated contains no brake dust, nor car exhaust, or any combustion products for that matter. Jun 8, 2021 at 5:13
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    Are you suggesting that the evaporation of water is some sort of an inherently corosive, energetic, or violent chemical reaction? Jun 8, 2021 at 6:01
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    @BillyC.: I don't know what problem Harper is getting at here (and the answer would be improved significantly by clarifying), but I'd guess that having something constantly wet outdoors will make dirt/dust stick to it and accumulate, and eventually collect enough spores for mold or mildew or other biology to start growing. Jun 8, 2021 at 14:31
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    @PeterCordes Added as a footnote. Huge footnote. Jun 8, 2021 at 18:24

If you gather and archive a local source of weather data, and operate the mist system on alternate days for a while, you should be able to gather a pretty good sense of the effect on power use, despite variations in internal and external temperatures & dewpoints. Over time, your data set will have "directly comparable days" as well as "general trends" that you can infer results from without having a fully instrumented test chamber and a few kilobucks to run it.


should this come out ahead since the water is "free" as condensate?

Probably. Your logic is sound.. As others have mentioned, heat pumps are already quite efficient, and the most you can gain from this system is going to be dependent on how much condensate the evaporator produces as well as the outdoor humidity. Unfortunately both of these are somewhat competing factors, but you could still very possibly come out ahead. As it gets more humid, more moisture will make it's way into the home, and you'll produce more condensate at the evaporator. On the other hand the more humid it is, the less readily the mist will evaporate and contribute to cooling the condenser coil. Still... this could very much outweigh the energy consumption of the pump. What I'm less sure about is if it will make a meaningful enough impact on your energy bill to justify the time/money/effort. If you're like me, maybe that doesn't even matter because it's a little fun to engineer little systems like that ¯\(ツ)

Many others have mentioned reliability and clogging up / corroding your condenser, but I don't think this is realistically a big problem provided you clean the coil once in a while (which you should do periodically anyway, and is the first thing a technician will check if you call up for a/c not working). They're designed to live outside after all. The notion that the condensate water is pure and distilled is only partially true.. It may be distilled, but it's mixed with all the junk in the air passing through the evaporator coil. Dirt, pollen, dust, etc will be captured by the drops of water, and flow down into the catch basin. A good hvac filter will mitigate this, and the severity of the problem in the first place will depend on your situation. If you live near the ocean this may be particularly bad as there will be some amount of salt in the air, and you will effectively be putting your condenser into a salt-spray test (obviously not as rigorous a real salt-spray test)

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    If located by the ocean, your condenser coils are already directly exposed to actual salt spray. The number of responses here that appear to believe an outside condenser is "magically dry" until you spray water near the fan intake is kinda mind-blowing. They literally sit out in the weather. They suck in mist, fog, spray, rain, pretty much anything the world has to throw at them.
    – Ecnerwal
    Jun 8, 2021 at 15:24
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    @Ecnerwal " They're designed to live outside after all. "
    – Aaron
    Jun 8, 2021 at 18:05
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    @Ecnerwal, the argument I've heard is that its fine for the AC to get rained on once a week, but it wasn't designed to get rained on every single time it runs for 6-9 months of the year. Now is that true? I'm not sure, but that's the argument.
    – JPhi1618
    Jun 8, 2021 at 20:06
  • Ours suck in grass too! All of my neighbors have sprinkler systems that water their lawns with potable water. Most of our houses use the same hvac equipment. (All built by the same builder sequentially). About a third of the houses sprinklers spray the houses and ac units every day, and yet in the 8 years I've lived here I've yet to see mineral buildup or corrosion that i could see from the street. Perhaps the natural rain washes off mineral deposits from their sprinklers. I'm concerned about the dryer vent being close to outdoor air conditioner. 3 ft. I make sure AC is off when cleaning Jun 9, 2021 at 14:40
  • @BillyC. Well, you wouldn't see mineral buildup from the street. Mineralization is an issue because you're using evaporative cooling. When water evaporates, the minerals MUST be left behind. Ask anyone with a hard-water well system what that does to your faucets, tub, toilet etc. -- the minerals are maddeningly difficult to remove, and that's just casual evaporation, not process evaporation. Jun 9, 2021 at 17:31

Safer than going to the distribution box is to use a true RMS AC current clamp meter such as this: https://www.amazon.com/KAIWEETS-Multimeter-Auto-ranging-Temperature-Capacitance/dp/B07Z398YWF/ together with a current splitter, such as this: https://www.amazon.com/Amprobe-ELS2A-AC-Line-Splitter/dp/B001DPR0FE/

Unfortunately, clamp meters (especially cheap ones as above) tend to be not accurate and may give an unstable reading. The unit above does not have an interface for data collection, either - you should be better off averaging the readings over time.

There are devices that allow data logging, such as WattsUp .net (discontinued but available on Ebay) - it can measure current up to 15A on a 120 V single-phase line and send data every X seconds via USB or Ethernet.

If the current draw from the AC motor does not change within the clamp meter error, recording the audio as suggested above may be indeed a good option. Each day the weather will be different so to know whether there is an effect from water misting, you would want to run two identical A/C units - one misted and one not.


As another poster mentioned, the only way to know if there's any appreciable gain in efficiency is to measure it. The physics says the system should be more efficient since you're lowering the temperature of the condenser. How much, and if this is measurable at all are hard to determine by theory alone.

Rather than setting up complicated tests over multiple days, hoping you've replicated the weather, and then crunching a lot of data and doing serious statistical analysis, I'd suggest using a more direct approach.

Get out a set of AC Gauges and measure the pressure on the high side with your cooling setup, and without your cooling setup. With the cooling setup you should experience a lower pressure than with it. Lower pressure should mean lower temperature, and greater efficiency. With these pressure/temperature differences in hand, I'd make a bet that you could translate this into an effiency gain. Of course, this only applies to at the day you tested it. Efficiency gain is of course going to depend on how much water you're cooling with, ambient temperature, and humidity levels.

Of course, all the normal safety rules apply here. If you don't know anything about HVAC, you could potentially burn your skin with refrigerant when connecting/disconnecting the AC Gauges, or electrocute yourself if you touch a bare wire. Adding water to the mix likely increases these risks. As AVE from The Youtube might say... Not To Be Operated By Fuckwits.

  • The problem with your advice is #1, opening up the freon loop requires licensure these days (and the equipment, and the supplier relationships to do coolant recovery/recycling/destruction, no more discharge into the atmosphere)... and #2, it's an inefficient and indirect measurement technique when OP already has a direct technique installed (the CTs). Jun 9, 2021 at 18:29
  • Well, that would explain why we disagree a lot on this site lol. We seem to have very different opinions about the mechanisms by which laws get made. Jun 9, 2021 at 21:43
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    By the way, 40 CFR 82.150 et.seq. And that's all Montreal Protocol stuff, so any signatory will have similar regs. Which is everybody. Jun 10, 2021 at 4:36
  • Sorry, I thought I had called out 40 CFR 82.154 specifically, which gets right to brass tacks. Jun 11, 2021 at 17:51

I can answer from experience rather than comparison or speculation because I have done the very thing you describe, starting with a proof of concept using a window unit. This perhaps should be a comment; however, answering is the path to gain reputation to do that so here you go...

Verifying effectiveness on actual equipment

Other answers have covered instantaneous measurement in detail. Since evaporation happens rather quickly, just turning off the water supply while the compressor is running should be an effective measurement strategy. My observation in the POC was that the compressor current draw was most affected (more below).

For a season-long assessment — There are plenty of home metering systems for around $100 that use current transformers to measure individual circuits, showing consumption online or within an app. For example (not a recommendation)...

Your local historical weather data is readily available from wunderground.com or your own PWS if you have one. Your utility bill may also include "cooling degree days" or an equivalent standardized measurement of temperature over time.

Real-world implementation

I set up a very simple solenoid valve connected to the city water supply on my split system after the proof of concept. (An 18k unit, well out of warranty so I don't care as much if it caused rapid failure.) Recycling condensate posed too many practical challenges in my situation.

The somewhat surprising result — including to a professional HVAC tech who saw it — was that there has been no obviously accelerated deterioration of the condenser coil as a result of the water. It has been in place over more than four years. I clean it once per year without special equipment or chemicals, which is adequate but leaves room for improvement.

With chlorinated city water there was nothing additional needed to keep things from growing any more than they would without it. Commercial systems, I believe, use some kind of biocide. For this sort of setup also consider the common wisdom that sunlight is the best disinfectant (though algae like it).

Validating what (seems to be) a good idea - the POC

In the POC, dripping or spraying water on the condenser resulted in approximately 10% reduction in power draw from the window A/C unit. That was essentially current draw, with slightly improved power factor. I measured using an AC clamp meter, and also with a kill-a-watt. A relatively small amount of water was all that was required, and more did not result in greater gains, even when it was essentially flooded.

The window unit makes a good POC because:

  • It did not incorporate condensate recycling / evaporation
  • It runs flat-out, i.e. 100% power all the time
  • Power requirements at ~500W are easy to measure (kill-a-watt)
  • The size makes experiments easier

Some other notes:

  • I made measurements in a variety of conditions (all within normal operation) and found essentially the same results.
  • Water spray made essentially no change in the evaporator (inside) temperature. Presumably that was because the a thermostat attached to the evaporator coil switched the compressor on or off.
  • The particular unit had a fixed opening (i.e. capillary tube) rather than more sophisticated modulation of refrigerant flow based on pressure or temperature, so there were no effects from that. (The TXV in a conventional split system modulates refrigerant flow based on pressure and temperature.)

There may be some differences for a rotary vane (scroll) compressor vs. the reciprocating compressor in the window unit, but that is beyond my knowledge.

Could this put you ahead by consuming water to reduce electricity?

A relatively small flow of water is required to keep the condenser wet, and it only flows while the unit is running. City water here costs $4.32 per 1,000 gallons. Electricity, as consumed, is about 11 cents/kwh (that means all per-unit charges: generation, transmission, delivery, metering, tax, others like fuel adjustments).

For a very rough estimate, assume:

  • 0.1 gpm flow for the spray
  • 10.4 RLA 240v compressor (actual 18k BTU compressor rating)
  • 25% duty cycle (compressor runs 25% of the time)
  • No compressor modulation (on or off, full RLA)
  • Ignore blower and other consumption
  • VA == watts (not true, but makes for easy math)

In 30 days, there will be 180 hours of compressor runtime, which will consume 411 kWh of electricity and will cost $45. In the same time, 0.1gpm will use 1080 gallons of water that will cost $4.67.

In sum, a more or less even trade of $4.67 worth of water to save $4.50 worth of electricity. I showed all of the math so anyone can repeat them with costs for their location and consumption for their equipment.

That number is useful input to calculations for re-using condensate where pumps consume electricity and chemical treatment is necessary to prevent scaling, corrosion, biocontamination, etc. Hopefully that will inform further measurements of flow rate, effects in other conditions, etc.


I don't know about the electrical cost saving aspect but one thing to consider for sure is that your plan involves spraying dirty water onto your condenser coils.

A condensate reservoir is a breeding ground for all sorts of nasty things. You ever notice how a condensate reservoir and output line always gets discolored?

Now you wish to continuously spray this on your condenser fins?

Even if the water was crystal clear you would be exponentiating the rate at which your fins get clogged with outdoor dust since you'd be providing a surface for dust to more easily latch on.

You might have to clean your condenser 3-5 times per year instead of once every 1-2 years and that will eat into your savings.

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