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Hot Water-energy free

Posted by LazyPup ( on
Tue, Nov 8, 05 at 21:07

I just completed a consulting job to design the Plumbing system for a friend who is building a new home in Arizona.

He is building a 4br 3-1/2 bath with walkin shower and soaker tub in the master bath.

The targeted design parameters were:
1. Instant on hot water at all locations.
2. Maximum water conservation
3. Maximum fuel economy for heating the water.

The end result,,100gal of on demand hot water available at all times, recirculation loop for instant on hot water, (which will eliminate the cold water normally wasted as we wait for the hot to arrive at the fixture) and all of the hot water being heated by heat recovery from his AC system while reducing the operating cost of the AC at the same time.

My first prototype of this system was installed in a 3br in Pensacola, Fla 5 years ago. We initially installed that system in May and the following November when the weather cooled off the homeowner called me to say his water heater was not working. Upon examination we discovered that we had never attached a gas line to the water heater, thus the heat recovery system had met 100% of his demand for the entire AC season.

Follow-Up Postings:

RE: Hot Water-energy free


"all of the hot water being heated by heat recovery from his AC system while reducing the operating cost of the AC at the same time."

How does the heat recovery from the AC work? Can you provide a a more detailed description, or a link describing the equipment required?

Thanks -- Gary

RE: Hot Water-energy free


Heat recovery from A/C . . I'm guessing essentially a liquid cooled condensor . . all that heat for the hot water supply instead of tossing it away into the air. I'm a bit surprised that it would have reached sufficient temp for domestic hot water; unless the user is accustomed to fairly low temps to begin with.

Recovering this heat should help the A/C do it's job better; though I'm betting that in doing so has upset the normal "operating" point of the A/C. I'm no refrigeration guy other than understanding the basic principles of it; but seems to me that temps at various parts of the system are designed in . . and that using water cooled condensor vs. air cooled as the system was originally designed for; is likely upsetting the design point of the whole system. May work very well . . may or may not damage stuff . . but overall sounds like a good way to at least pick up some ( otherwise thrown out ) energy.


RE: Hot Water-energy free

While it has not gained a lot of public attention, AC heat recovery systems have been commercially available to the residential market since the late 70's.

Basically is it a small heat exhanger that has one coil inside of another. The high pressure refrigerant gas line from the compressor to the condenser coil is cut and rerouted through the heat exchanger and back to the condenser.

A line is then connected from the bottom of the water heater (Tee'd off the drain port) to the heat recovery unit where it passes through a coil and is then run back to the water heater and tee's in at that hot water output port. The recovery unit has a small 1/24HP circulation pump built in to circulate the water.

It requires one BTU to raise one pound of water one degree of fahrenheit.
Water weighs approximately 8lbs/gal.

The cold water supply is typically 55degF and the demand temp is 125degF for a differential of 70degF.

Thus in order to heat the water from 55degF to 125degF we must add 70BTU/lb x 8lbs/gal = 560btu/gal.

One ton of AC equals 12,000BTU/hr

The recovery units are typically about 25% efficient, which means they can recover 12,000btu/.25 = 3,000BTU/hr.(There are revoery units available that will approach 75% efficiency).

3000btu/hr divided by 560BTU/gal = 5gal/hr

(Most heat recovery units are rated at 5 to 7gal/hr per ton of AC.) I.E. a typical 4ton AC will consistantly produce 20 to 28gal/hr.

The total operating cost of the recovery unit is a 1/24hp pump motor (typically about 45watts of electrical energy)

Not only does it generate the hot water with wasted energy that is normally blown off into the atmosphere, it reduces the compressor head pressure, thus reducing the energy requirement of the AC compressor. (When the high efficiency recovery units are installed we install a pressure switch on the condenser fan, and the fan does not need to run when the recovery unit is running.)

The initial equipment cost of a recovery unit is typically about $300 plus installation costs.

I have been installing these units since the mid 80's and I personally would not recommend installing a central air without one.

While most HVAC techs dont want to be bothered with it, in truth they are available at any HVAC supply house and are highly recommended by most AC manufacturers.

Run a search on Google or yahoo search for "AC Heat Recovery" and you will see dozens of websites for them.

RE: Hot Water-energy free

I had a HRU installed about 1980 along with a new central system in a home I bought with a big reverse cycle wall shaker chilling and heating the two car garage.
Power Company had a preferred HRU brand and it supplied 100% of hot water for two with a new kid ... during the summer a/c season.
If I recall it supplied some hot water during the winter too with the heat pump, just not 100%. Time Clock juiced the electric hot water heater an hour each, morning and evening during winter months.

Only problem I had was the HRU would occasionally loose its pump output and I'd have to bleed it a tad to kill off that bubble that would cavitate the impeller i guess and stop the flow.

Small little thing... not much bigger than an a/c disconnect. maybe 10 x 18" , 5 or 6 inches deep.

RE: Hot Water-energy free

Thanks, guys.

One can sure learn a lot of good stuff, here.

ole joyful

RE: Hot Water-energy free

Technically interesting, but I have real problems with calling it "energy free", since you're running an A/C unit that, with proper house design & good insulation, should be mostly unnecessary. I'd think it'd be a lot cheaper just to put in a solar hot water heater with (if you're into that sort of waste) a solar panel to provide the power for circulation.

RE: Hot Water-energy free

One fact to remember, He said the head pressure was reduced on the AC condenser. This is a very important fact when in a hot climate. The life of the equipment is extended, and even more important is the efficiency of the unit increases with the lower head pressure. Based on a loose conversion of SEER, I regularly see efficiencys that would equate to a SEER in the high 20's with a water cooled coil. My equipment is rated at 9.2. When talking about design parameters, the HVAC manufactures design for climates in the low 80's, while hot climates are in the 100's. The difference is night and day.

For many years window units had drain holes for water. Using a coil loop into the bottom of the unit increases SEER, which increases efficiency. The water is evaporated by the hot coil.

When I asked the head of R&D of a major HYVAC manufacturer why water was not part of condenser cooling, he said industry just doesn't do it. His company follows industry. LENNOX.

RE: Hot Water-energy free

"Technically interesting, but I have real problems with calling it "energy free", since you're running an A/C unit that, with proper house design & good insulation, should be mostly unnecessary."

This statement may be correct for people who live in northern climates or in caves, but for those who live in the sun belt and prefer to keep their living quarters above ground AC is not a luxury but rather a way of life.

Rheem manufacturing is one of the leading producers of both AC and water heaters as well as the heat recovery "Desuperheater units". They have been trying since the early 80's to convince people to take advantage of this technology but most techs in the HVAC business just prefer to continue business as usual.

I am both a certified Master Plumber and a Master HVAC tech, and in my opinion one would be a fool to build a home and not include heat recovery in the design, especially when you consider the initial cost of the equipment is under $300 for a system that will reduce the operating cost of the AC unit, extend the overall service life of the AC and provide all your hot water at the same time,,,where is the down side in that?

RE: Hot Water-energy free


Up north here in IL, is heat recovery of an A/C cost-effective? Our HW heat is natural gas, A/C is electric. I don't have the numbers, but, based on your experience, how far north does it make sense to do heat recovery on the A/C?
I want to do solar HW if the State of IL starts offering its 50% rebates again - and if I can find a plan/someone to do it... but, I am also interested in heat recovery.


RE: Hot Water-energy free

In the Southern sunbelt regions for ten or eleven months a year the daytime temps can range well into triple digits by day and often only cooling to the upper 70's or low 80's at night. In those areas it is not unusual for AC to have an average duty cycle of 80% or greater 24/7 therefore heat recovery will work extremely well.

By contrat, in northern regions in mid summer you may occassionally see upper 90's by day but in general the daytime temps range in the 80's and night time temps in the 70's, therefore your AC duty cycle is typically about 30% during the day for a short three or possibly four month cooling season. Given that the concept of heat recovery relies upon the excess heat being moved by the AC, this system can only function when the AC is actually running. With this in mind, Heat recovery in your region would probably not be a good investment.

RE: Hot Water-energy free

Recycling is a broad term, but in many instances brands a working product as a goodie-goodie two shoes concept. When real money is needed in large quantities to see a small savings, it just don't make sense to spend the money. I have been interested in the concept of solar hot water for many years. The problem I saw, was the systems were very expensive, and required upfront cash, that was difficult to justify, when the car needs new tires. I recently observed a used HW system installed on a 4500 sqft house. I keep asking about savings, and can not get a straight answer. For the project to be real, it needs to be cheap.

As to heat recovery coils, they are not very common in Dallas Texas for the last 20 years. I have seen a few junk coils for gas HW tanks, that had $1000+ price tags on them, and they just didn't work all that well. If there are some real numbers that are verifiable, then I would like to see them. I have an invitation to attend a workshop for recycled solar HW, this December, and I am considering attending. To use recovery on a HVAC condensor and save $60 a month of gas sounds to good to be true for a cost of $300. Maybe for $2000. Put up or quit trying to jazz me.

RE: Hot Water-energy free

Bob Brown,

In my previous post I stated the equipment cost was $300.

Actually the current price of the unit from Rheem Manufacturing is $299 and is available through any authorized Rheem dealer.

According to the "Energy Star" website relating to "Desuperheters" they are saying the installed cost is currently ranging $400 to $700.

They also show a table comparing the energy savings by numerous locations around the country.'HVAC desuperheater'

Now if all of this sounds to good to be true, and if you would feel more comfortable paying $2000 let me know. I will be glad to come install one with a high end price tag more to your liking.

RE: Hot Water-energy free

"...a system that will reduce the operating cost of the AC unit, extend the overall service life of the AC and provide all your hot water at the same time,,,where is the down side in that?"

Did I say that? On the contrary, it seems like a good idea for those who live in places where A/C is used frequently. No, my objection was to the use of the word "free", which is so easily misunderstood.

As for the downside, look at the original post, where one of the features of the design is "Instant on hot water at all locations." So putting in something as wasteful as that, then telling the 80% or more of homeowners that don't understand the technology that they're getting free hot water by running their A/C... Just how many do you suppose will open a window or two to kick on the A/C so they can get more "free" hot water?

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