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,
http://www.engadget.com/2006/12/10/alterna-fuel-rv-takes-family-cross-country-on-47-of-diesel/. 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.
Steve-
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: steve@rvotes.com. 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. (
http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItemVersion&item=261243597059&view=all&tid=1243548330016)
• Bimetal Temp. Switch. 2Pc. $3.29. Only need to use one of them. (
http://www.ebay.com/itm/300744163912?ssPageName=STRK:MEWNX:IT&_trksid=p3984.m1439.l2649) 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! (
http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&item=111128541554).
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".
Bruce.