Air Conditioner heat recover for hot water

RCinFLA

Anybody using central air conditioner energy recovery add-on heat exchanger for hot water?

Interested is what units you have experience with and which ones are good and not so good. The cost of the unit would also be nice. Looks like they run between $400 and $500 for the heat exchanger.

I have been researching the details, doing calculations to weed out exaggerated claims. (dusting off my college thermodynamics book).

My preliminary evaluations shows that here in S. Florida the opportunity to get a sizable amount of heat recovery hot water is quite good. It actually surprised me. My calcuations based on my known air cond run time in summer shows I could heat up to 80 to 100 gallons a day from approximately 75 degs to 100 to 120 degs F. That amounts to $40 to $50 worth of electric equivalent cost per month compared to pure electric heating element. (assuming I can consume that amount of hot water)

Comparing this to a solar hot water heating system it is equilvalent to solar hot water panel area of about 4 sq. meters here in S. Florida. Looking at the cost comparison of initial purchase, it is about equal, maybe a bit less for the energy recovery system.

As with solar panel heating, the best way to yield the most hot water is to feed the heat exchanger into a preheating reserve tank that then feeds the input of a conventional electric hot water heater. From my days of building solar hot water heaters I know of issues injecting preheated water into the cold input port of a conventional hot water heater. It creates a lot of temperature gradient stress on the enamel coating at the bottom of the steel tank, creating stress fractures in enamel that then results in tank rusting out. This is mitigated by modifying the cold input injection piping to port the water off to the side preventing it from being shot directly at the bottom of tank. There is another solution the uses injector port attachment that replaces the normal tank drain spigot.

One balancing part of the equation is when weather is cooler and you don't have so many hours on the A/C you don't make energy recovered hot water but you also don't consume the large amounts of power for A/C so there is plenty of reduced kWH's to offset the cost of direct electric heating water needed.

I would assume that A/C manufacturers frown on this modification (break line between compressor output and condenser coil) and likely would void any remaining warranty (which mine coverage time has expired)

http://www.hotspotenergy.com/residential-heat-recovery-water-heaters/

LucMan

Re: Air Conditioner heat recover for hot water

Not much of a demand here in NY for desuperheaters, not enough A/C run time.
Grocery stores on the other hand have them piped into their refrigeration rack systems to supply most of the the HW needs for the store.
Make sure the HX is cupronickle, copper HX have a short life span in my experience in domestic water use.
Check the condenser on your A/C, several manufacturers use aluminum coils and there is very little copper at the discharge of the compressor that you could cut into to repipe to the desuperheater. You don't want to cut into the aluminum piping it's very difficult to braze or solder the aluminum piping and I don't recommend even trying.

Dave Angelini

Re: Air Conditioner heat recover for hot water

With solar hot water the best way I have found to avoid damaging the tank from really hot water entering the tank is to just shut down the system when the water gets to 140F. You can also trigger a shut down when the collector water gets too hot. Hot water in the colector is avoided with a drainback system as the water does not sit in the collector. Water at 200F or higher stagnet in the collector and a controller signal to pump is what can really hammer any tank over time.

I think you are right that a manufacturer would definately void anything that lenthens the compressor output line.

TheBackRoads

Re: Air Conditioner heat recover for hot water

Sounds like a great add for all you in the heat and sun! Post a review and some pictures if you do decide to install a unit!

tonystewart

Re: Air Conditioner heat recover for hot water

When I upgraded to Trane 16 SEER units the installer explained the new units would not produce enough heat to power my old recovery units, therefore I upgraded to Solar Heating and have been more than pleased, especially in the winter when the old systems didn't make hot water anyway.

Good luck with yours

T in Lake Worth FL

RCinFLA

Re: Air Conditioner heat recover for hot water

tonystewart wrote: »

When I upgraded to Trane 16 SEER units the installer explained the new units would not produce enough heat to power my old recovery units, therefore I upgraded to Solar Heating and have been more than pleased, especially in the winter when the old systems didn't make hot water anyway.

Good luck with yours

T in Lake Worth FL

In Lake Worth FL that is probably not true but it depends on how the A/C is achieving their SEER performance.

In general, R410a (Puron) performs poorer then R22 when outside temps are above 90 to 95 degs F and better below 90 degs F. R410a has higher compressor output pressure and temperature then R22 which would imply better performance for hot water heat recovery. Gas density is about the same, slightly more on R22 but not significant for heat transfer improvement.

If the SEER improve is achieved with better (generally larger) condenser coils then the head pressure will be lowered and associated superheat gas temp will be lower, resulting in less heat transfer in water heater exchanger and more heat dissipated in condenser coils.

BTU's are BTU's so for a given rating A/C the same amount of heat energy is given off from R22 or R401a in the water heater exchanger plus condenser. The difference can only be in the ratio of which of the two gives off most of the heat energy. When the head pressure/temp is lower then more heat will be expelled by condenser.

The best way to get the most heat energy through the water heater exchanger is to use a preheat tank so the water temp through the exchanger is lower. You get more volume of water heated to a lower temp and net more heat energy captured. The electric direct element may have to bring the temp up to 120 degs but overall there will less kWH's used to heat the used volume of water to 120 degs F. There will be a point of diminishing yield if your hot water useage is small.

Some hot water heat exchangers don't turn on the circulating pump unless the gas line is above 140 degs F. They have fixed temp switches.

BURNINGDAYLITE

I have taken a step farther in heat recovery using a HOTSPOT desuperheater: 
I just installed a 15 SEER heatpump using an expansion vale on the indoor unit (evap). I found that in Nebraska I can get good heating BTUs even in lower outdoor temps. The hotspot desuperheater installed in the compressor discharge line works fantastic. I have a 30 gal. holding tank of cold water inlet to the domestic water heater and looped through the desuperheater. I am able to get a temp raise from 55*f to 120*f in about 2 hr. I am going to try some reverse engerineering on the water storage unit. It makes much more hot water than we ever need during the day so I have purchased a slab water coil 14"x20"x3.5". It has the ability to produce 80,000 BTU @ 180*.
I have a small 300 watt pump circulating water with a control valve to adjust for a total delta of 15 - 20* heat rise on air in and air out.
I put a bypass valve in the loop to the coil. to disable the coil when temp. drops below 100* When no heat is in demand the system is in line with the domestic hot water heater. The math is fuzzy but I have increased the efficiency of the system as the ref. to water by a substantial amount as discharge air temp. is 20* hotter while it is in operation. I find that the heat recovery to the holding tank is still very rapid and has not taken away from our desuperheat. In cooling season I make so much hot water I shut off the domestic water heater completely.        

BURNINGDAYLITE

jmoheng@hotmail.com