The Tank › Air Generate heat exchanger experiences
- February 23, 2011 at 1:06 pm #15324
I have used the Air Generate Air Tap heat pump mounted on top of an electric water heater. The unit failed after a few weeks use and a new on was shipped to me. I have not installed it yet and here is why.
This heat pump is akin to a window mount AC unit and with its condenser coils mounted inside the water tank. The condenser is a long soft copper tube loop that you feed into the tank via the cold water inlet port. This is all easy enough to do as the copper loop just bends around to fit the curves of the tank.
The problem occurred when i extracted the copper heat exchanger loop. I use well water but the amount of build up of slime and whatever on the copper tube when I pulled it out after less than a month was amazing. I was able to wipe it off easily with a rag and the copper itself was not badly corroded. The anode rod is new. It seems to me that the heat conductivity of this copper loop will be drastically effected by what ever this build up is and it seems likely to get worse as the deposits increase.
My question is: Won’t this pretty much undermine the utility of this add on heat pump negating much of the added efficiency?
Before I install this new heat pump I am considering building a separate preheat tank for the the heat pump to mount on and building in an access port so the copper coil/heat exchanger can be cleaned off.
Am I nuts? Has anyone else seen this?
ScottFebruary 23, 2011 at 5:16 pm #15326
Scott, I think you have valid concerns.
My dad had a 250 gallon ambient pressure tank installed in Dec 1983. He heats the water with 5 solar panels. The tank water does not come into contact with domestic water. The advantages I see: When introducing “new” water to heat you bring in more minerals and if your supply is other than a deep well you also bring in organics. By heating the separate tank water you eliminate these two future problems.
Dad’s tank water is also treated with a “safe” algaecide. Since the tank is at no pressure any leak would be into the tank anyway and not into the plumbing.
The WH preheat coil is just below the water line horizontal in the tank. It may be a finned tube. You would place your heating coils in the bottom of the tank. Or use a finned tube from the heat pump in the bottom instead of the factory coils. With the heat source in the bottom you would heat the whole tank. The WH preheat coil in the top would try and cool the whole tank.
This will be a good project. You might consider having a sight glass external to the tank, an inverted “U tube” vent at the top, appropriate fill and drain valves. I would protect this investment with one of Randy’s powered anode rods (order from this site).
You might find a solar company already having a tank to meet your needs.
DavidFebruary 23, 2011 at 5:26 pm #15327
I have some concerns about having anything made of copper inside a glass-lined steel water heater. I’m thinking it’s going to eat up a sacrificial anode a lot faster than normally. The powered anode might throw enough current to protect exposed steel or not. Anytime you start using a powered anode in a different way from originally intended, you’re in virgin territory.
Randy SchuylerFebruary 23, 2011 at 6:02 pm #15329
Scott may research and might find a powered anode better suited to this project than the powered anode you sell. But yours might be just right. I know that underground petroleum tanks installed for the last 10+ years at gas stations are protected with powered anodes. The principle is the same: force enough current at the proper voltage to stop nature in her tracks.
DavidFebruary 23, 2011 at 6:51 pm #15330
You’re quite right, David, and there is a good chance that a water heater will last forever with an impressed-current anode in a normal situation.
It’s the copper coil that’s worrying me. If you put copper and steel together in water, the steel gets destroyed in short order. If there is nothing but a bit of exposed steel and a powered anode, no problem. But what about a whopping big copper coil?
A powered anode produces a lot more driving current than a sacrificial anode and is self-adjusting. But like I say, nobody knows what will happen with this combination because the makers never envisioned such an application.
Randy SchuylerFebruary 23, 2011 at 7:19 pm #15332
I’m really curious now. I’ll find out from my dad who made his tank, if they are still in business, and the tank and internals specifications. He has had it since 1983 with zero tank problems.
DavidFebruary 24, 2011 at 4:00 am #15334
Wow! Thanks so much for the thoughtful and insightful replies. I am a mechanical engineer but must confess to ignorance of much of what Randy and David are speaking about above. I have not spent a lot of time thinking about the chemical/ionic aspects of the Cu-Fe issue.
Randy; You say “the makers never envisioned such an application” I assume you mean the makers of the water heater. I assume Air Generate did think of this but so far I confess to not being very impressed with their tech support staff knowledge level.
I had not considered your idea of a closed preheat system. My plan was to preheat the domestic water in the storage tank. But now I see that your idea makes more sense. The additional heat exchanger for the domestic water preheat is only a small complication. I envisioned using a poly tank meant for agriculture use. Some of these have large access ports on top. Temps would be no more that 100F I imagine and pressure will be atmospheric.
Thanks again guys and keep the ideas coming.
ScottFebruary 24, 2011 at 6:32 am #15335
Your last post:
Temps would be no more that 100F I imagine and pressure will be atmospheric.
I don’t know what Air Generate advertises as to maximum temperature. A day or two away from home and the tank could get quit hot unless you cut the heat pump off at 100F. 100F is not exactly usable hot water but it helps. Since heat pump water heating is so much cheaper than resistance I would want all of it I could get.
Poly softens at high temperature; fiberglass does not. My Marathon is fiberglass. I heat off-peak only and therefore have temperatures set to 150F. Marathons are rated to 185F. And fiberglass eliminates galvanic concerns. Enough from me. Check
Note: Marathon now makes a two-port Marathon Solar Storage Tank.
Marathon may also know where to get heat exchangers which screw into their oversize element ports.
105 gallon is as big as Marathon makes (I have this one). Cheapest place I’ve found to purchase Marathons is
The Marathon for solar applications may be special order so you may be able to order direct.
DavidFebruary 24, 2011 at 9:31 am #15338
What I meant was the makers of the powered anode. It was meant to function in a glass-lined steel water heater in place of a sacrificial anode. But people keep coming up with other uses for the device without considering the implications. I sell them, but I don’t want to sell one and have it backfire on someone.
I hadn’t thought about the Marathon in this regard, but having a plastic tank would solve the copper/steel/anode issue completely.
Randy SchuylerFebruary 25, 2011 at 6:11 am #15347
David and Randy:
Do you think it is likely that the slimy deposits I found on the copper heat exchanger loop were at least partly caused by the Cu-Fe reaction that Randy points out? I would have thought the glass lined tank would have kept the Fe out of the picture especially in a brand new tank. Do you agree that these deposits will impact the heat transfer of the heat exchanger especially as they seem likely to build up?
Would it be your opinion(s) that the use of the Marathon plastic tank would likely eliminate this build up?
You are correct that it would be ideal to extract as much heat (via raising the temp of the H20) as possible from the heat pump. I was looking at compromise that would allow the use of a non ferrous tank and access to the heat exchanger coils for cleaning. The Marathon tank or WH looks promising. Thanks fort the tip.
ScottFebruary 25, 2011 at 8:11 am #15348
I think if Fe were the problem it would not be coming from the new tank. But what would small amounts of Fe in the water do? I worked for 15 years around GE boiling water reactors. The reactor used “pure water”. However, even the smallest impurities concentrate over time when the water is boiled away. So the design included a reactor water cleanup system.
Yes, any fouling of surfaces interferes with heat transfer.
Whatever, the cause of the crud, I think the Marathon with no water exchange (and algaecide) would eliminate it. No Fe source from a tank and no other sources of Fe or other stuff if the Marathon is a holding tank of stagnant water.
You would want to vent the Marathon to atmosphere. So consider the need to possibly add a little water every so often due to minimal evaporation. If you build a loop seal in the vent line, condensation should fill it and stop “boil off”. The final vent could have a cap or pipe plug with only a 1/16 inch hole in it. This would take evaporation down to almost nothing.
If you do this project, post again and let us know the results. The proof of the pudding is always in the eating!
DavidFebruary 25, 2011 at 12:37 pm #15351Larry WeingartenParticipant
Hello: Crud on copper coils might be simply hardness from the water being deposited there. Two mechanisms are at play. One is the anode working overtime to protect the copper and plating out minerals from the water in the process. The other is the temperature difference which, (just like on electric elements) causes water hardness to form on the hotter surface. My two cents 😎
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