Free Hot Water from A/C

garymunson_2009April 30, 2009

Since summer is about here, I thought I'd repost my support for heat recovery units on central A/C units when you are in a hot, humid climate where the A/C is used a good part of the year. It make no sense to 'waste' all the heat the condenser unit throws off when it can be used to heat your domestic hot water. In Florida, in particular, heat recovery is far more cost effective than any solar hot water can possibly be. In addition to being much simpler and cheaper, a HRU provides hot water much more consistantly and at the time of day the hot water is needed. Most hot water use is in the mornings or evenings when a solar system won't deliver hot make-up water, triggering it's back-up heat source. My opinion is that in Florida, Texas, or other places where cooling rather than heat is a concern, the obvious choice is a HRU on your A/C unit.

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I bought one with a retrofit new central heat pump complete ducting and all install about 1980... was a really cheap option except for its power hookup which they wanted too much for. Utility approved HRU and had the utility on demand load shedding radio/relays on everything for monthly power bill credits too.

Skip a day time clock on the water heater - would skip every day for summer full time ac months. Totally free hot water then.

Occasional air bubble in HRU motor need bleeding. Pretty simple stuff.

Keep your AC filters clean!!

    Bookmark   May 1, 2009 at 10:24PM
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OK--I live in DFW area where the A/C goes pretty much non-stop from end of may through sept--
so how much would it cost to do something like this and do I call a plumber or an HVAC guy or both
does it matter that my outside HVAC units are across my house (basically) from my NG water heaters in my attic space?
just two people right now--me and husband--but we do have visits from daughter and son in law periodically...

    Bookmark   May 23, 2009 at 6:17PM
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The units cost about $400. They make one with dual coils for twin condensors...don't know the price on them.
Shop around for price. The pumps are usually rated for 60' runs. Unit needs to be by the condensor. If you're handy, you can do all the mounting and hooking up to the water heater. Just have an AC guy cut it in to the AC. Mine charged me $250 to supply the small amount of tubing and fittings necessary and the labor to do it. Took him about 1/2 day.Is your water heater a tank type or tankless? You need storage capacity for an HRU to work. The more the better. My old house had a 40 gal WH and was OK, we installed an 80 gal in our new home... much better. A smaller tank can be problematic if you keep the water heater completely turned off during the cooling season like we do.

    Bookmark   May 24, 2009 at 9:52AM
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I'm a very happy brand new user / poster on GWeb. My
background is as a scientist degreed in electronics with lots of
experience in analytical chemistry materials labs. All over the
world I've seen how big and small companies make things. I'm
57 next year and retired at 46 so I could 'learn how to' do it.

Your idea is right on target. The amount of waste .. let me put it
.. there's a great resource of free energy available. But we lack
the vision of what our "historical" grand and great grandfathers
did when they put meat out hung high in freezing weather (to keep the dogs from it) and in summer they put cloth over it and
hung it to dry it.

Cellers , that our 1915 era homes in California are called
basements, were used to store foods that kept well at 65 to 70F.
Onions , tomatoes, more than I know and canned foods were
always available. Poor or rich everyone had food to eat.

My 1915 house is my Vanilla Box which I hope in the next few
years starting soon will be an example and testbed, and data
collection laboratory to show old homes are potentially more
useful when one considers how close the attic is to the solar
panels on the roof and the cooling basement 7 feet under the
earth just 8 steps down from the livingroom level.

Yet no homes are made here with basements where the Earth is
easy to dig. A travesty? Ignorance? Arrogance?

Consider this to answer that... every diamond has a side to which
each observer will say looks the same as the other sides. So
each opinion is needed to make the whole idea hold together.

Let us pray.

And I welcome you who can create automated systems where
working explorers like you, all, can contribute and then those
who helped us with software will tie it to hardware .. like
3D printers(look it up) and we soon can make actual pieces
and parts over the internet for each other's ideas.

And before that put together groups. Try to begin with people's
children and guide them to learn how to teach what they
learn. Can we do it without the public schools? Yes.

It's happening here. Right now.

Please contact me at your leisure .. I don't know yet if PM's
are set for me. But I'll check this link again and see what
followups there are. And in time I hope GWeb will be a place
which leads those of us who want to move into the future.

PS. I would like to see a website similar to MAKE and
Instructables which is geared towards teaching .. where we
can download entire outlines of research we've done individually.

If you want help learning to do what I mentioned. About creating
a learning path. Contact me and I'll share an example.

When I get going here I'll post a file showing how to learn, to learn. Another saying is, To do research means to describe each
facet of the diamond's example above.

One Q. Are photos easy to place here on Gweb? I see a place for
a link but I'm currently not using an easy to use photo service.
I would rather use a website and throw together quick easy to
read html pages. I'd like your advice on how to use a photo for
explaining ideas like the OP posted.

Thanks so much for your time - I promise to be more brief.

    Bookmark   May 27, 2009 at 3:41PM
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HRU - Not what I consider the best value especially for a homeowner.

I'm a 57 year old retired , at 46, scientist who worked for many years in the field consulting in
analytical materials labs. I worked with lots of instrumentation and it all had various systems to
move heat or do various things with materials and electricity. I also was exposed thoroughly
to developing methods to test how plants who make things run their production lines.

Anyway I say this kind of product doesn't look like a good value to me for a couple of simple reasons.

1. The initial cost . If it's $100 or less well I can't complain. It's only a box with a tube basically. So what you pay
is money that's out of service for you and not working elsewhere.

2. The freon has to be plumbed into the unit. This is more cost and introduces something I would never do
in a laboratory unless it had more gains than this ... A leak would ruin the A/C unit and cause downtime plus
the cost of the A/C unit. This isn't a good plan. If the unit came with the A/C mfg design I would consider it
a part of the method the plant uses to build the entire unit. But aftermarket just does not ever make much
sense to me unless .. there is no other way. I think there's another way.

3. The efficiency ratings I read are 10%. That is exactly the gain I got when I water cooled my coils on a
home refrigerator unit. I got a similar reading when I cooled a regular home A/C unit with water. The amount
of water to cool my parent's 4 or 5 ton , 1935 (it's still running) A/C unit is about 6 gallons / hour. The amount
of water usage is very low.

So I say this: Use water to cool your coils. You'll lose the heat energy to the water if the water's thrown away
but assuming it's at home the water can be used so it's not wasted. And if you want some heat that's in
the water then plumb a regular $50 heater core or small radiator on the cooling coils of your A/C unit. If
the size doesn't fit .. yes you'll have to reconsider the design. Maybe you can only run 1/4" copper tubing firmly
attached to the coils ..

But you'll be running water through the secondary coolling system .. not freon. And a failure of that system won't
affect the safety or even the operation of the A/C .. as long as you haven't blocked the air flow.

If not for the probable high initial cost . I'm guessing $400 and the plumbing into the freon line I would be positive
towards a unit like this. Which I am via the design ideas above.

4. Another method to cool coils without putting water directly on them is to use a 'swamp cooler' design around
the coils where air comes in to pass over. I realize this won't work as well in Houston or Florida. However air
that is 90F and wet will still carrry away more heat than air that is dry and at 90F. Any gain of humidity with
pads and a bit of water over them will give you some gain of efficiency. And there is the fact that your compressor
will run longer.. ours has been running in 100F Summers since 1935. And I personally know it's been running since
1975 without any maintenance or downtime and the costs of running it are low. This will occur with both plans of
my idea or the HRU.

One things I don't quite get in the marketing is the absolute heat temp of the output of the HRU. If the compressor is
to run cool it 's going to need cool water. The outflow from the unit we have , from 1935, is at about 15F above
the incoming temp. While this would help to preheat water I would not expect hot water nor would I want hot water.
I 'd want my compressor to run as cool as possible.

You decide. I have not run extensive testing. I'm using mostly experience but not any calculations on the HRU except
the specs I've seen.

I have an idea. Seriously. Why not put a 100 gallon tank of water in your car and use your engine to heat it to about 180F
as you drive around? Then when you get home you'll have a day's worth of very very hot water 'for free'. Seriously.

This is the exact design used for carpet cleaning vans which use engine heat to heat the steam either preheating or
fully heating. I don't know how they can get to 212F without the engine overheating but they do , for about $3000, offer
a heater that runs off the engine and heats the clearning water. why not do the same and then 'plug in your water tank'
when you get home for an whole night's worth of hot water ?

There are ideas out there. They're not dumb. Just weigh the cost and risks.


    Bookmark   May 28, 2009 at 11:24AM
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I have nothing to add to this, just that it's an excellent idea!! Also the idea of the car is not stupid, but you have to consider that you have to move the extra weight around too, costing extra fuel.

    Bookmark   May 29, 2009 at 3:10AM
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While I have a limited knowledge about HRUs for A/C, I have been told that the HRU works best on older less efficient (more heat generated) AC units. It truly is a viable technology. Energy efficient power plants capture the exhaust heat generated to make steam for their steam turbines. I had an energy efficient heat pump installed about 4 years ago and was told (mind you told) that it would not be a good candidate for an HRU. I did not research further.

As to solar water tanks, I did have one installed last year. With the tax rebates, state rebates, electric company rebates the cost was cut in half. My electric usage drops about 26% for 9 months of the year. During the winter it is not as efficient. Unless there are multiple heavy cloudy days in a row, I have enough hot water for my use. I shower every morning and there is always hot water available even with a teenage daughter who takes excessively long showers. On a clear day the solar collector will raise the temperature of the 80 gallon tank from the mid 80s at the bottom of the tank to 154 degrees. I stop it there since the water at the top of the tank will be over 170 degrees and the tank is rated for 180. That is a lot of BTUs captured.

    Bookmark   May 29, 2009 at 3:41PM
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To start with, let me state I am not in the HRU business...have no connection to it. But I have used them for decades. The $400 figure pretty much covers the cost of the materials, any labor is extra although most handy people can do all but the tying into the condenser. My experience is a steady $50 reduction in monthly power consumption. In fact, the way I can tell something has failed is my power bill jumps up. The circulator pumps usually poop out every 6-7 years as do the identical pumps on solar collectors. Sometimes they last longer, sometimes not. In any case, no damage is done except for the dead pump. The system does not require circulation of water to prevent damage...once the freon lines are connected, the A/C can be returned to service without connecting the water line as long as the pump is not wired. The construction of these units are the model of simplicity. 2 copper tubes, one for water, one for hot gas, soldered to each other, wrapped in a coil, and embedded in foam insulation. Were either to fail (never have seen that), you would be presented with an obvious leak, nothing more. I find the efficiency increase to the A/C unit to be about 20%. Easy to see by putting an ammeter on the compressor wiring and turning the circulator pump on and off. On my unit, average current draw on a 90 degree day dropped to 9 from 11 when the pump was turned on. About 20%. However.....this doesn't take into consideration that for about 9 months of the year, my water heater is switched off. Another considerable savings. As I understand your post, you are advocating a 'total loss' water cooling scheme. This would be nowhere as efficient as you either have to pay to pump or buy the water used and you are getting no free domestic hot water. Keeping condensor coils wet has been discussed here before with the general consensus that the increased corrosion of tubing and especially fins is detrimental to the system. The water in the car is clever but as Guido said, the extra 1000 lbs or so would seriously affect fuel economy. How about a small water cooled stationary engine driving a generator running a heat pump and using the radiator for even more heat? Might be efficient but I like the HRU for it's simplicity and ability to operate for long periods of time with no interaction required by me.

    Bookmark   May 29, 2009 at 4:03PM
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As for the high efficiency A/C not benefitting from an HRU. My experience is that this is not true. If you examine high efficiency units, you'll see that the pressure differentials are different since they are now using 411 gas but this doesn't have much bearing on the unit's efficiency. The temperature at the compressor discharge is still as hot as older, lower SEER systems. The efficiency increase comes mainly from the MUCH larger amount of condensor coils the new units have. Since pretty much everyone's using scroll compressors now, that's no longer an issue in the efficiency there. I really think the mfgs have made a effort to discourage HRU installs since it's obviously cheaper and would cut into sales of the high end high efficiency units that can cost 2-3 times as much as a 13 SEER one. Nothing wrong with solar heaters but they generally cost much more and can be unsightly. Long-term reliability can be about the same as long as the collectors are good quality. As I said in my last post, the pumps generally are identical and require occasional replacement. And..they don't increase the A/C's efficiency.

    Bookmark   May 29, 2009 at 4:20PM
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all these comments did was confuse me--
that is the trouble with trying to be more energy efficient--there are so many cross currents of what is/is not truly efficient that for someone without an engineering or practical application background, it is worse than shopping for something like a new fridge...

I have read about cooling your condensor coils to max efficiency/SEER ratings...and that too seems logical

the problem is that when someone like an average homeowner tries to find someone to install any of these options you run into resistance because the contractor often is even more unfamiliar with what you want to achieve...

    Bookmark   May 30, 2009 at 10:38AM
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I was on board til someone mentioned spending $50/mo to heat their water. I'm on propane and only spend less than $15 for two seniors, so I'm thinking the payback just isn't there, but a good idea for some. New construction would probably be the most cost efficient.

    Bookmark   May 31, 2009 at 6:03AM
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Srercrcr: Re-read my post..the savings I'm quoting is with the water heating saving of about $20 a month (electric water heater) and about $30 a month in more efficient A/C operation. $15 a month cost for propane water heating is pretty good..

    Bookmark   June 1, 2009 at 5:05PM
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I appreciate the reply from the original poster.

I have only a moment to reply. So I'll mention what I can.

The water is not wasted. It's run into the garden where we would have to water anyway during the hot summer.

Corrosion is a concern but I truly wonder how true today it is compared with 40 years ago only because the entire coil is all aluminum now with no galvanic (diff. metal) corrosion. Aluminum stops corroding when it gets a layer of aluminum oxide on it. (That's the hard stuff you grind with) . If actual water spray is a worry I've heard of products which use a 'swamp cooler' design using pads to cool air with water. So it's wet air but the water never condenses thus no wet coils. Plus let me reiterate that the design of the A/C unit , for my Panasonic 9000 BTU says, the 'slinger fan' is made to splash water on the coils. In wet climates , Houston / Florida, you get more
water from the air. In dry climates we don't .. so it made
sense to me to add water. Overflow , if any, went to
the garden. Or a holding tank for later use.

One crazy thing I see here is A/C units on roof with coils facing West or east. The sun blazing into the coil at
the end of a 100F day made no sense. How dumb is
And yes. most contractors know only what the big companies sell and make money on. New technologies are not something companies make money on because they would always be changing to upgrade. They settle on a design and stick with it. If you go through the European countries where various laws and development ideas are allowed to vary you see dramatic differences in energy ideas. Where gas has been $4 / Liter for years Europe is deep into saving energy so we almost have to
create mini societies if we want to have the best in savings. Or we have to be educated to do it ourselves.
Sad really sad.

One poster said it's confusing and it is. Typically the best
you can do is think about something and apply common
sense after you understand how it works. Beyond that
the gains, in today's technologies, are minimal.

PS the 10% efficiency gain figure I got for the HRU was from the mfg website. I had no experience of the product except from this thread and what I read there.

If you get 20% great.

One cool thing , off topic a bit, you all might enjoy are
the videos from BBC called Grand Designs where 9 seasons of 12 shows are covered. Many homeowner designs are covered. And their site is at

Here is a link that might be useful: Grand Designs

    Bookmark   June 9, 2009 at 11:24PM
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