AC Heat recovery for Hot Water?

haus_proudOctober 9, 2008

We replaced our central AC about a year ago. I think it's a 2.5 or 3 ton unit (for a 1550 square foot one story home in North Carolina). I anticipate that the gas water heat will have to be replaced soon -- it's about 10 years old. When I replace it, is there some way that I can recover the heat thrown off by my AC to contribute to my hot water system? I know the technology is available. What I want to know is how do I go about figuring out if it makes sense financially to do it -- how long the pay-back, etc.?

Thanks for your guidance.

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Here's some data and links on HRU's. Keep in mind you also need a regular water heater as a storage tank. I have an 80 gallon electric tank unit in my house that stays shut off all summer (we are in cent Fl.). I've also included my thoughts on tankless systems.

In hot climates a proven strategy that works well is a heat recovery unit attached to your AC unit. It provides free hot water and also increases the efficiency of the AC a couple of SEER points by reducing head pressure (and electricity draw) on the compressor. You're paying for your A/C to move a great deal of heat out of your house, you might as well use that heat to provide you with hot water. Adding one of these to an older AC unit that still runs well is a good way to cut $30-$50 off your monthly power bill during AC season (which here in cent Fl can be about 9 months). The neatest thing about this solution is that it's cost-negative. In addition to being free hot water as opposed to other 'high efficiency' heaters that still consume energy, it actually lowers your cost of air conditioning. Here is a manufacturer's site for two of these and a power company document about it as well. The second mfg site has a neat calculator for your savings...but my experience shows it to be somewhat under-optimistic. I don't think it's taking into account how a hru will improve your AC's efficiency. There are claims that they don't work with higher efficiency unit, but I have two HRU connected to 13 SEER heat pumps that provide plenty of hot water.

Tankless water heating is also a problem area for accurate claims of savings. A gas tankless heater can be much more efficient since it eliminates the flue that passes through the center of a conventional tank heater and becomes a chimney to carry away your water's heat once the flame shuts off. Electrics are another story... Big savings are claimed that are not supported by data. No studies are shown that give specifics telling you what make/model/year tank unit is being compared with the advertised tankless. Maybe they use an ancient cast iron tank heater insulated with sawdust? Verifiable details please. Somehow electric tankless heaters dodge the requirement for the yellow government 'EnerGuide' label that every other appliance must have. If they had to carry it, their claims would show to be false. Claims of better durability because of 'no tank' are a red herring also...the 'tank' still exists, only it's now smaller. Buildup from hard water deposits may be more of an issue with tankless, possible surcharges from the power company later due to high instantaneous current draw, heavy wiring needed, new construction with planned tankless can make retrofit of tank type later very difficult due to lack of dedicated hot water piping, incompatibility with future solar or other alternative power sources due to aforementioned high current draw. These are all serious disadvantages of the tankless electric. All electric hot water heaters are 100% efficient at heating water. That's a function of an electric element immersed in water. The only added efficiency of a tankless is the reduction of standby losses which are very minimal in new technology insulated tank heaters. Put your hand on one and you'll see they pretty much run at room temperature...very little standby loss.

Another myth is that shading a A/C condenser saves '10-20% or more. The reality is not much. Similar situation to a garden hose left out in the turn on the water and what comes out is scalding hot....but in a few moments the water is cool...sun impinging on something will make it very hot but that is a function of 'accumulated heat'. What heat is striking something and being carried away by a fan is very small. See the following link about this subject...

A free, online calculator that can help you plan upgrades is here:

    Bookmark   October 10, 2008 at 5:03AM
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I see you are in North Carolina. You may not be in a hot enough area to benefit from an HRU. Your better option may be a tankless gas unit. I'm sure more will chime in on this.....

    Bookmark   October 10, 2008 at 5:06AM
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I'm in North Carolina. I have a geothermal system and we pass the hot water thru a hot water storage tank before it passes back to the ground. The hot water heated from cooling our house actually heats the water in the tank to 110 degrees before passing back to the ground outside to be cooled back down.

This leaves the water in the tank to be heated up another 20 degrees by our on demand renai heater when someone turns the hot water on upstairs.

I'm not sure what a regular heat pump would do in this climate. I also don't have any winter data as this is a new build/new install.

    Bookmark   October 12, 2008 at 12:52AM
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we are in DFW TX area and use A/C a lot--from reading these posts and couple of links--does this heat exchanger only work with electric hot water heaters?
we have gas ones...

    Bookmark   October 18, 2008 at 12:22AM
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Your quickest payback would be a heat pump water heater. They are not as popular in the states as other countries yet but more and more are starting to build them.

    Bookmark   October 18, 2008 at 10:55AM
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The HRU's will work with any hot water heater with a tank. The old heater then becomes the storage device for the hot water and a back-up source of heat when the A/C-heat pump isn't being used. Mo likes the heat pump water heaters but I feel in a hot climate, the HRU is preferable. You are paying to move the heat anyway with your A/C, might as well not 'waste' that heat.... In effect, the HRU-A/C combo is a 'heat pump water heater'. The HRU also makes your A/C cost less to run... If you already have a gas water heater, the cost difference between using gas and using a heat pump water heater will not be that much, especially if your gas unit is fairly new.

    Bookmark   October 19, 2008 at 4:15AM
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I agree Gary that down south Texas Florida Arizona where you have many cooling months would have a pretty awesome pay off for waste heat recovery and increase seer rating. For the areas that have a pretty good split heating/cooling I would go with the heat pump water heater.

    Bookmark   October 19, 2008 at 12:12PM
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Thank you all for your helpful comments. I have archived this thread and will use it when the time comes for me to replace our water heater. Whether or not we do the HRU will depend on the technical feasibility -- how close to the AC unit does the water heater have to be, what kinds of connections are needed between the two systems, and of course the competence of the installers and the cost.

The added SEER for the AC was a factor I had not considered, and is clearly an important added advantage.

    Bookmark   October 20, 2008 at 10:41AM
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With the advent of PEX plumbing lines the distance isn't much of an issue. The only limit would be the rating of the circulator pump. My unit's pump literature recommends 150' max. Line loss is not an issue since the HRU produces much more hot water than you can use and the heat loss only helps the A/C efficiency.

    Bookmark   October 21, 2008 at 6:25AM
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I just stumbled back over this post and was thinking about the recovery units operation in theory. One thing I hadn't considered is if the unit is pulling heat out of the system that is not going outside it should not only increase the seer rating but also cause a drop in operating amperage with a drop in operating pressures.

I am curious as to what difference in amps it would make with the a/c on with a cool water tank and recovery pump on compared to having the circulation pump off or a hot tank of water with a tong meter or similar on a hot day.

It would be interesting. Its known fact they will heat water but the amperage impact is not.

Anyone think it would make very much of a difference?

    Bookmark   December 17, 2008 at 7:38PM
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Yes it will

Higher SEERS are obtained by bigger coils (mostly) so, draw heat off the hot gas through the exchanger and then send it through the condenser coil; you should by theory see an increase in efficiency, especially on hot days.

(keeping it simple in thoughts)

    Bookmark   December 18, 2008 at 6:39PM
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My 3.5 ton 13 SEER heat pump will drop from 14 amp to 11 amp on a 90 degree day when I switch on the HRU circulator pump (I added a manual switch to the pump circuit to be able to gauge savings). The hotter the tank, the less amp drop and when the thermostat temp is reached, the circulator pump switches off. I incorporated a circulating loop in my domestic hot water system to afford instant hot water and shed further heat, increasing the energy savings. Keep in mind that as opposed to solar, a HRU will provide hot water in the evening and morning when most people shower and do laundry. A solar system won't provide hot water at those times and the back-up elements will switch on to reheat the tank. Even with a large storage tank, remember the cold make-up water will trigger a re-heat thru the electric elements, a HRU will provide heat thru the night assuring a full tank of hot water in the AM at the lowest cost.

    Bookmark   January 13, 2009 at 6:21AM
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