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Improving hot water heating efficiency ... with cold water


April 4, 2013

A prototype water heater system that uses cold water to make hot water pictured with Slater's assistant, David Dawson

A prototype water heater system that uses cold water to make hot water pictured with Slater's assistant, David Dawson

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Apart from heating and cooling the house, water heating is one of the biggest energy drains in the average home. But what if you could literally use cold water to create hot water? That’s just what San Diego inventor Hal Slater claims to have done with the creation of a water heater system that promises to improve water heating efficiency by as much as 50 to 100 percent.

The system works on the basis that cold water supplied to households in temperate climates averages around 70° F (21° C), which the researchers say is 15° to 20° F (8° to 11° C) warmer than it needs to be. By using a small water-to-water heat pump, the system extracts this excess heat from water in a 20-gallon (76 liter) cold water tank and delivers it to a typical 50-gallon (189-liter) water heater.

With funding from a grant from the California Energy Commission, Slater teamed up with a research team from the University of California, San Diego (UCSD), led by Dr. Jan Kleissl of the Mechanical and Aerospace Engineering Department, to test the system. To test real-world performance and determine the effects of different incoming cold water temperatures, they installed three prototype systems in homes in coastal, mountain and desert climates. They also monitored each system for a year to compare performance over different seasons.

Slater told us that all the systems performed better during the summer months when the cold water comes in at over 80° F (26° C), providing more heat that can be extracted. In the winter months in the mountain test home, the incoming water temperature dropped to 55° F (13° C), which is the temperature the cold water tanks are set to. The system basically reverts to an electric water heater at those times.

According to Slater, test results showed an improvement in water heating efficiency by as much as 50 percent over current efficiency leading air-source heat pump water heaters, such as the GE Geospring. However, he believes that with additional refinements the efficiency can be increased to 100 percent, putting it on a par with the best solar water heaters, but without the need for a rooftop solar panel.

Slater has patented the system and is now seeking funds and a manufacturer to commercialize it for single- and multi-family applications.

Source: Hal Slater

About the Author
Darren Quick Darren's love of technology started in primary school with a Nintendo Game & Watch Donkey Kong (still functioning) and a Commodore VIC 20 computer (not still functioning). In high school he upgraded to a 286 PC, and he's been following Moore's law ever since. This love of technology continued through a number of university courses and crappy jobs until 2008, when his interests found a home at Gizmag. All articles by Darren Quick

I suspect that you come out ahead overall even though you are making your hot water "work" harder to bring your now colder water up to temp when you desire warm water for a shower etc, because so much water use is cold water: flushing toilets, watering lawns, drinking water, filling the coffee maker, etc.


Interesting, but i dont see how the water is coming in so warm, if its getting pumped in through pipes that are in the ground, wouldnt they be alot cooler even in temperate regions. I mean that would be horrible to have such hot/warm water to shower with or drink when your already trying to escape the heat.

I feel like im missing something, for one thing why would you even need to heat the water if its coming in so warm. It almost seems like this is a method for creating cold water rather then warm water(well it makes both), I dont know i honestly dont understand this, i know i must missing something.


Interesting idea, I do not doubt there can be an efficiency improvement for certain cases... but I think that overall energy consumption tests should be done before claiming a percentage of improvement. This is why: The cold water in the system will be colder. So when I have a shower and I mix hot and cold water, I will have to use a higher proportion of hot water for the same result, and this would require more energy than without the invention.

Anyway, thumbs up to Hal Slater. Salut!

Mrk White

Mrk White I agree, but a way to circumvent that problem would be to take the "cold" water from the mains system to your shower mixer tap, so you would be using the "really cold" water in the tank for drinking or other cold uses but "street cold" water for showering and maybe the washing machine or dishwasher etc.

Samuel Holden Bramah

i'm jus' sayin' but this guy dint invent this, this kind of thing has been known about for years, of course it works its the laws of thermodynamics. you'd be better fitting a ground source heat pump in your garden, and then linking it with an air source and this water based system in order to produce maximum heat output. jus' sayin'

Graham Aikman

He should have a model that integrates hot water recirculation. also how much cold water has to be brought into the house and used to keep up with usage of hot water i wonder

Ian Madden

This is not a new idea at all. Geothermal system work like this. But water is pumped into pipes running underground a couple of meters where the ambient temperature is constant and warmish through out the year. It then gets pumped through a heat pump etc etc.


How does the efficiency compare to a passive solar water heater?

If for what ever reason I wasn't using solar heat, I would heat the water with the waste heat from the refrigerator, air conditioning, or generator.


I love folks suggesting geothermal as an equal or better alternative. Got a spare $20-30,000?

Ed Campbell

Has anyone heard of thermodynamic panels? It is claimed that they can provide water up to 50°C from one panel about the size of a solar panel. They are basically a refrigerator in reverse.

David Clarke

How is this better than an air to water heatpump? In order to extract a reasonnable amount of heat you would have to use lots of water. Not exactly eco-friendly.


Mrk White's comment is dead on. The amount of energy needed to create the hot water will be less, but the amount of hot water you will end up using will be greater. All that energy you pulled out of the cold water to make the hot water will not be a 100% efficient processes by the laws of thermodynamics. So you'll end up actually using more energy in practice.

Samuel Sebastian's workaround does not seem at all practical. How would you even implement that? A completely separate water line for "semi-cold" and one for "cold-cold"? That's ridiculous and likely very expensive to retrofit all that plumbing. If you're willing to spend that much money, just get solar panels or go geothermal.


Our water comes straight from a well at about 54 deg. Won't work for us. I guess California has warmer earth than we do in the Midwest.


Another workaround would be to simply waste the super-cold water and only use the incoming-temperature water for household use and tempering the hot. While it seems like a horrible solution, it energy costs are much more than water costs, then it might make sense.

Otherwise, there is little value here. For every degree of heat put into a gallon of hot water, a gallon of cold water must be chilled the same amount (assuming 100% efficiency of the system). In the winter, the 55˚ water can only be chilled 23˚ before it would start to freeze. So you'd end up with 78˚ "hot" water unless the cold flow to waste was much, much greater.


sebastion's work around would not be expensive at all, especially when you compare it to the cost of other energy savers, like solar panels and heat pumps...


The mixing hot and cold would eliminate the benefits in that instance (unless, as mentioned above, a separate tank for regular cool tap water was included for mixing) but faucets for drinking water could be extra cold which would benefit ice-makers. Any application where you start off with all hot--such as filling a pot for boiling water would reap the full benefit.

I don't know how much net benefit if any there would be and how it would compare with the cost but it sounds worth investigating at the least.

Sometimes an "invention" is not new (though sometimes what the person thinks is a prior example isn't exactly the same thing) but often these "not new" developments include some improvement to cost or function or safety which makes it feasible or competitive now when it was not before. There are lots of benefits to things that are "not new".

Snake Oil Baron

I'm in SoCal (Riverside) and our water is nowhere near 70deg. . . it's considerably colder than that. . .



Water seldom will enter a home at 70 degrees. Water pipes are buried to a depth of 4 ft. Underground temperatures at 4 ft average about 52 degrees, (unless it is Antarctica or frozen tundra). So how does it get heated to 70 degrees in order for a heat pump to extract the extra heat? In the hottest of weather here in Kentucky, my water enters the house at about 52 degrees.

Don Yarber

Actually the idea is not new, in the 1980’s Dennis Lee’s company was creating super-efficient heat pumps that could pump as much as 12 times more heat energy than electrical energy used to run the compressors ( COP =12). When his corporation moved to Ventura California, His heat pumps were used to heat the Ventura Junior College swimming pool. He later gave a demonstration at an Oxnard auditorium that he could pump the heat energy out of the air to run a Fischer cycle heat engine and generate over 10 KW of electricity in a closed cycle, self-powered unit. Apparently, the Utilities Company didn’t appreciate Dennis Lee’s efforts. They used their clout with the local officials to have Dennis Lee arrested and all his company equipment, research papers and business contracts confiscated. He was held for a year with any trial and then released after his business was ruined.

Herbert Dorsey

Ahah! The Royal Shakespear Company developed a heating system for their Theatre at Stratford upon Avon back in the 1950's, with a measured peak efficiency of 1450% C.O.P.. Those old reverse Carnot and Rankine equations!

Coefficient of Performance, with large heat pumps, can be spectacular. And, having the River Avon at the theatre's back door was a slight advantage! also the efficiency dropped dramatically when heat was most needed, as it is in winter!

Alastair Carnegie

Don't most uses of hot water involve mixing it with cold water to get the desired water temperature? If so, won't making your cold water colder by pumping heat out of it mean that you'll need to use more hot water in the mix? Wouldn't that negate all or most of the savings?

David Schwartz

I have my old primary school essay book from 38 years ago which basically describes this system in terms of of a desert-living creature. That said, I think there is significant potential for enrergy saving in tropical and sub-tropical regions that is not being explored at present. Good luck! SAA


If your washing machine rinses using cold water and the heat exchanger cools it to 13 degrees from 20+ then you have heated some water. Every time you use the cold water, you heat some water in the hot water tank.

So when you do want to use hot water it doesn't need to be heated so much. Plus your cold water is actually cold.

In places like Crete, Thailand Austrailia this makes sense.

Taking heat from water in pipes is a lot easier than having a huge Air Heat pump.

Karsten Evans

In the first world most people have a good heat pump in their kitchen. The refrigerator manufacturers should put a hot water tank on their product.


In the midwest our water is almost never that warm, with an exception perhaps at the peak of summer. The big question is what do you do with the cold water? Most uses mix the cold water with warm water to get to the desired temperature, and at that point the efficiency of this device goes negative.


Depending on how hot you like your shower and how adjustable the thermostat is NO MIXING.


it is broadly use in large hospitals--- remove heat from the water used for air conditioning and with heat removed warm up the water for showers -old system my friends -

Peter Perletti

Great idea, ground source heat pump without having to bury a load of pipes!


What a coincidence! The kid and I spent all day yesterday analyzing water heater efficiency and redesigning it. Sure, this "heat pump" might cut the energy cost of heating 50%, but the actual savings after cost of equipment would be zero. Additionally, you mix cold and hot water to get warm bath/shower water. if you chill the cold water, then you need to increase the hot water supplied to get the warm water. Here is what my 15 yr old son came up with. 1) put pvc check valves on the input and output of the water heater. This slashed radiant loss through those two pipes by 90%. 2) put a rubbler vent flap on the chimney flue. This stopped cold air from falling down the chimney into the tank between runs. These didnt decrease the cost of heating...they decreased the cost of maintaining a tank of hot water by $60 a year at a one-time material cost of $15. 60 X 20 years = $1,200 saved per heater. $1,200 X 100,000,000 U.S. residential heaters equals $120 billion saved...WHERE'S HIS X-PRIZE!?


I work for a midwestern state weatherization program funded by the DOE and we can only approve replacement items that pay for themselves in energy savings. For several years water heaters have been notoriously lacking in progress comparable to (for example) 95+ direct-vent furnaces and condensing boilers. So with this history, there's already a strike against water heaters. Trying to find a better method by using unrealistic input temperatures doesn't help. Along with others here, I'd have to agree that this sounds like it's being tested in controlled and optimistic conditions, and is not representative of what many of us actually experience.


Provided the heat-exchange fouling problem could be resolved satisfactorily, in most climates the systems would work better for most or all of the year if the heat was recoverd from the waste (gray) water rather than from the incoming water. Provided the water is discharged above 0°C that should be OK. (It is much easier to extract heat from water with a low delta T than it is from air - or dry ground.) Lindsey Roke

Lindsey Roke

A criminal amount of water may be wasted if you needed a large amount of warm water. If it's balanced by your cold water usage it'd be fantastic :) I imagine if you're having a shower and mixing hot/cold water you'd run a divert straight from the mains instead of your cooled water tank. Leave the cold water for loo flushing etc. You could always regain energy from ambient energy, if you really wanted. That's the way it got warm in the first place. I'm sure he's not the only one using mains water as a convenient source of heat ;) But the first to patent it is the one that counts, lol.

Craig Jennings

As an add-on to a ground source heat pump or air source heat pump, it may well supplement a heating system. I would think that the cold water would need to be circulated a bit, adding to your water use costs

Ulaanxuu Bold

Heat does not travel through the ground very fast so if you run warm water through the pipe the ground around the pipe warms but if in winter you run ice water through the pipe the ground around the pipe cools.

re; Craig Jennings

This does not run the cold water down the drain after extracting some heat from it.


@Slowburn I agree with you. Why do refrigerator manufaturers put LCD screens, and other nonsence in their designs, but then just waste all that heat that is generated. They should not only have a hot water tank, they should also have a compartment to keep your food warm.

Paul Anthony

Taking about making used of the room of improvement they had and put it into a good use like improving their methods! Usually the idea to a water heater's design for splitting cold, inbound water from hot, confident water is that it depends on the most crucial that warm increases to do the hard part. The position of the heat-out tube at the top of the tank does the rest. I believe they crack something out of it that makes a new statement of a water heater.

Edea Krammer

The presentation of the device is weak, as I can’t think of ANY place that has 70F intake water.

But... used to recycle heat from showers, washing machines, and dishwashers...

It’s a WINNER !

Dump all the “gray” water into an insulated tank, suck the heat out of it and preheat the potable water before use.

Then recycle the “gray” water to water lawns, gardens, etc.

California may have to mandate systems like this just to survive.

William Carr
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