Author Topic: Aqua Hot Hack. - lengthy  (Read 8320 times)

Bruce Sieloff

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Aqua Hot Hack. - lengthy
« on: September 09, 2013, 02:27:42 PM »
In light of some recent discussions, I thought I would share this.

I have been having a running discussion with a gentlemen named Steve Alder, who OK’ed his name and email usage, about a fix that one of our members recently effected to his Aqua Hot:

Doug Allman - “it froze last winter and burst the copper lines around the heat tank. We replaced the lines ourselves and the heat transfer compound and now have extremely hot water.”.

I too was discussing the same sort of repair, appalled at the replacement cost, however justified, of an AquaHot unit because of a failed heat exchanger. Heat exchange is a fairly simple process and $7-8K for a replacement unit is really enough to create a search for an alternative solution. Apparently you can fix the original system, per Mr. Allman, or you can try another approach.

Steve had a good deal of experience with Flat Plate Heat Exchangers (FPHE) as a result of a project to run his coach on veggie oil, He mentioned he would be interested in discussing the fix with anyone interested. The following is an edited email exchange, offered as an option…YMMV.

Yes, I have fixed my issue. I did it for about $260. I opted not to do it "right", which would have been a project so huge, I would have never gone anywhere this season. Instead, I simply cut in to my domestic hot water loop, bypassing the AquaHot completely. I purchased a small, double walled, flat plate heat exchanger, and a $22 12V circulation pump, a $3 theroswitch, a $40 mixing tempering valve and went to town. I went to Home Depot and got  SharkBite adapters, a 10' length of PEX. Here's how it works: Using the SharkBite T adapters, I tapped into one of my heat zone's hot coolant send and return. I hooked up the domestic water and the hot coolant to the FPHE. I made sure to incorporate the tempering valve, otherwise I would risk getting 180 degree hot water at the sink. I put the thermoswitch on the Domestic hot water out line and used that switch to control the circulation pump. Essentially, the circulation pump will turn on when it senses the hot water coming out at ~140 and turns off at ~160. This is of course, still tempered by the valve. I went on a 15 day trip with my family of five last month, and the hot water worked FLAWLESSLY! Those FPHE are so small and so efficient, I'm always amazed.

The circulation pump uses only 5 watts and only runs on demand. It is totally silent. There were a lot of details I did when doing this, so if you want to reach me directly, I'd be glad to provide the parts and more details. While this solution is still technically a hack, it some ways, its an improvement because that FPHE will suck out every BTU of the coolant before it would ever deliver cold water at the faucet. My email is: Best, Steve Adler

As far as the fix, it really was relatively very easy.  It was probably no less than 1/20th as difficult as it would have been to try to remove the Aquahot and replace the tube, cement it all back, and reinstall the unit.  Total time spent was probably a few hours - and much of that was figuring out stuff along the way.  

The plan was to abandon the copper tube, Sawzall the CPVC domestic water send and return from the heater and plug it into a FPHE sitting next to the AH.  The only icky part was when I cut into the PEX line for one of my heating zones, I miscalculated the gravity pressure of the coolant - my rag popped out and I got bathed in a couple gallons of coolant.

Parts were:
• FPHE.  I got a Double Walled, Stainless steel unit because it would have potable water next to ethylene glycol.  How embarrassing would it be if I poisoned my family?  While double walled units are hard to find and three times the cost of regular ones, I paid  $155 for mine on eBay (#221267585579)

• Circulation Pump: To use one like the AH uses, I've seen these little pumps as much $900!!!  I did some estimating of heat transfer and figured the flow rate I would need.  A little research and I found a hot water rated, super low power one for $22.   (

• Bimetal Temp. Switch.  2Pc. $3.29.  Only need to use one of them.  (  I added this because I didn’t like the idea of circulating coolant to the FPHE 24/7.  This is an RV, and I boondock a ton, so the thought of wasting even 5 watts didn't sit right with me.  Plus I was concerned that the output domestic hot water temperature would be way too high.  The switch was mounted on the domestic hot water out port of the FPHE.  It turns off the pump when it feels the water coming out is 60 C and turns back on at 40 C.  

• Tempering Valve:  $42.50 - these valves are usually at least twice this expensive!   (
You have one of these already on your AH, but you need another one for this project.  Without this, I would make soup from my children.  This unit was installed at the FPHE and mixes cold water with the hot output to deliver a hot water that doesn't maim.

• SharkBite Ts, assorted fittings, a length of PEX, and some other crap from Home Depot.

 Essentially I wired the pump in series with the switch, and took the + lead from an electrical terminal that I tested was 'hot' when my master AH switch was on.  This way the circulation pump only can run if I have the heater on in the first place.
 I tapped into one of the heating zone's PEX send and return using the SharkBite T adapters.  These things are a work of engineering art - they are AMAZING and so easy!!

For a buck, I bought a SharkBite removal tool.  This allows me to easily remove the domestic hot water connection at the FPHE to drain the contest (very little) of the FPHE and tempering valve for the winter.  It gets cold here in RI!

I did not touch the electric heater in the unit at all.  In fact, I really didn't touch the AH to do any of this.  I simply cut the CPVC water lines for the Domestic water, and used T's on one of the heat zone's send and return lines.  Your assessment is correct; I don't care where or how hot coolant (sounds like an oxymoron) circulates into the FPHE, as long as it does, I can run fresh water past it and let hot water out.

 I can assure you that the FPHE I used is over kill, as is the circulation pump.  I figured a ten plate would be more than enough, the twenty something plate I got was the smallest double walled I could find.  

The circulation pump also far exceeds the output of the domestic water pump.  I tested the install running 100% hot water out of all of my faucets at the same time yet the circulation pump thermo switch still cycled off the pump for periods of time, indicating that even at full flow rate, the hot side wins, so to speak.  In other words, the flow of cool water from the main RV pump can't supply enough cold water fast enough to keep the FPHE cold.  During the winter, it will come closer, but still - not even close.  As I said, these FPHE never cease to amaze me.

Yes, this is a hack fix.  Of course, I'm the guy who cut into a perfectly good Cummins M11 engine with a bolt cutter and stuck a veggie system in between ;)    

The two main problems I have with my hack solution are:
 1) It requires an active circulation pump for the hot side, where the original copper tube didn't. Not a big deal considering the silent, low wattage of the pump and the Thermoswitch.
 2) I essentially abandoned the original copper tube inside the unit, where it died, and is now left to rot.  Now am now running some PEX out of what used to be a self contained, attractive, stainless steel obelisk ,  It isn't particularly pretty anymore.  At the end of the day, however, the smile on my face proves that the benefits outweigh the losses.  My wife is still bragging to everyone about how "smaaaht" her husband is and how the whole family enjoyed 15 days of piping hot water - and with the $8,000 saved, I can take her on a cruise! ....

I found Steves' approach innovative, indicative of a deep understanding of how heat exchange works, and while his fix is not elegant, it could be. Undeniably, it worked for him, and he did it for $260 in parts. I'm considering squirreling away a FPHE "just in case".
2003 45' Beaver Marquis, 505 CAT and 1 Ridgeback riding shotgun. 8)

Gerald Farris

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Re: Aqua Hot Hack. - lengthy
« Reply #1 on: September 09, 2013, 03:20:23 PM »
Beaver Coach Sales is performing the same retrofit repairs on freeze damaged Aqua Hot systems. However their pricing is higher because they are not using EBay parts and they charge for labor, but it is still a lot cheaper than a new Aqua Hot system.

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Bruce Sieloff

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Re: Aqua Hot Hack. - lengthy
« Reply #2 on: September 09, 2013, 04:53:03 PM »
True, Gerald, and good to hear.

I would probably be trying to fix the internal heat exchanger myself using an appropriate high temp conductive expoy, per Mr. Allman.

But Bend is 400 miles from my house, and I am in Seattle, and that is about $200 in diesel one way. This guy lived in Rhode Island and was stranded the day before his vacation started.

I found his solution very innovative: hack the coolant line to put the FPHE in line so you had hot coolant and hack the not-so-hot water line to pass through the FPHE to gain heat on the way out. Toss in a tempering valve and thermostat so you could adjust the temp.

This is the kind of Yankee ingenuity that needs to be in the back of all coach owners minds, even if they can't tell a disconnect from a disease, so they can explain a proposed solution to the shop trying to repair it, or at least understand have some information in self defense as an alternative solution to replacing the Aqua Hot; which can be offered by a profit seeking shop, or rightfully fretful owner,  as the only reasonable or safe solution. Hence the reason for the post.

Kind of like adding an inline surge protector before the inverter instead of trusting the built in protector in the inverter to do the job.

The more we understand how our systems are designed and really work, the more readily we can survive a disruption in service and get on with a fix.

I hope I'm near Bend if my unit goes belly up, if not, at least I am forearmed.
2003 45' Beaver Marquis, 505 CAT and 1 Ridgeback riding shotgun. 8)

Doug Allman

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Re: Aqua Hot Hack. - lengthy
« Reply #3 on: September 10, 2013, 01:37:46 AM »
Our repair cost us more time than it did money, and we still have the aqua hot in its original configuration.  

Copper line 150', 2 90 degree copper sweat elbows, 1 copper sweat coupling (100' copper to the 50' copper tubing) heat transfer mastic and new insulation and the many 1/8" pop rivets to hold cover in place. Total cost $300.00.  

The copper tubing is refrigeration tubing as well as the copper sweat connectors. The heat transfer mastic is the same used by AquaHot when they manufacture the unit. The insulation was a high density fiberglass which we wrapped with a heavy grade trash bag and taped in place - contractor grade at Menards.

AquaHot in Colorado told us we could not fix the unit in particular that we could not get the copper wrapped tight enough. Really, that just got the challenge off to a better start.

We searched the web for the mastic which we really did not know what it was but sort of looked like furnace cement. On second call to a supplier the lady that answered our inquiry about the mastic they sold told me right away when I mentioned AquaHot that they sell the heat transfer mastic to AquaHot.

I ordered 2 gallons but in reality you could do the job with one gallon as we completely encased the copper tubing and had a 1/2 gallon left over. AquaHots original manufacture only beds the tubing on the tank and some squeezes out in between the tubes.

First get unit out of Coach, remove all old copper on tank,, grind off all mastic down to bare metal - just needs to be clean, not shiny.

Then there are three threaded holes in the end plate of the unit tank, in the control area. We fashioned a temporary frame to bolt the unit to and clamped this all to a barrel filled with water. With the clear end of the tank sticking up we inserted the tubing into the control area, spread heat transfer mastic over the first 5 inches of the tank with putty knives and then started the ring around the rosie wrapping the tubing on the tank. Kept spreading the mastic and wrapping and within 20 minutes we had the copper tubing wrapped onto the tank. Stuck the other end into the control area and let the unit set for two days while the mastic hardened. (If we had run hot water thru it it would have sped up the drying process significantly.)

Using a barrel allowed access to wrap the copper tubing while walking around it. Trying to move the cumbersome tank to wrap the tubing is not the way to go - trust me.

There is not a whole lot more to it than having several friends and some suppliers for parts. Yes, removing and reinstalling the unit is a time consuming process, so find a good natured mechanic
that is willing to help. the unit is also akward and heavy but with a floor jack and some strong youngsters it is not a problem.

When you wrap the tubing in the mastic the tubing being perfectly tight is not an issue. The heat transfer mastic is the critical part as it does exactly what it is supposed to do. We have very hot water.

I am going to have to replace or repair the tempering valve so I would suggest that this should be completed during the process as it is easiest to get at when burner is out.

Anyone wanting to call me in this regard is more than welcome.     231-218-1119 cell.   Leave me message and I will return your call as I do not always answer area codes I do not usually receive calls from as someone is always trying to sell me something for my business I do not want.

We had never seen an Aquahot much less knew how it was made to work. We do now.

Bruce Sieloff

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Re: Aqua Hot Hack. - lengthy
« Reply #4 on: September 10, 2013, 04:10:25 AM »
Congratulations on cracking the code!
If you could share the brand of mastic that would help in getting closer to OEM sources.
As Steve put it, "Anything made by the hand of man can be repaired by the same".
Great work by both of you.
Doug you maintained the integrity of the system with your fix, certainly a more involved fix from a fearless labor standpoint than Steves approach, but I'm handing the spot fix award to the FPHE.
Interestingly, the Aqua Hot forum is pretty emphatic about hacks and fixes not working. Perhaps to warn off folks who might tackle more than they should.
I doubt either one of these fixes will cause Aqua Hot many replacement sales, us DIYers would crack the case anyway since there is literally nothing to lose, and those whose bent is to let the experts do it wouldn't be swayed by either solution.
I, on the other hand, am impressed with both repairs.
Now if I could just find someone who can teach me to weld....
2003 45' Beaver Marquis, 505 CAT and 1 Ridgeback riding shotgun. 8)

Doug Allman

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Re: Aqua Hot Hack. - lengthy
« Reply #5 on: September 11, 2013, 11:24:28 PM »
The heat transfer cement or "Thermal conducting cement" is made by CHEMAX CORP, New Castle Deleware.   800-804-4596