[Richard]

The title alone will get some attention. To make the story as painless as possible, I will lead with the answers.

We were down to about 4 minute showers when on electric. The real culprit and solution was replacing the Grohe shower cartridge. https://www.amazon.com/gp/product/B002ZL...UTF8&psc=1

On this journey, I also replaced the internals of the tempering valve because that seemed to belikely culprit for cold water. https://www.amazon.com/gp/product/B07XNP...UTF8&psc=1

And while I was at it, installed a double wall flat plate heat exchanger. In hindsight this was overkill but it does provide 20+ minute hot showers on electric, and truly endless hot showers on diesel.

   

In this graph you can see that with the old cartridge the shower water temp would rapidly plummet. The blue line is the old cartridge, the green line is the new cartridge, and the black line is 100 F representing cold showers. It wasn't quite as severe on diesel, but honestly not a comfortable shower. Now that I have played with it, the signature of a faulty cartridge is a high starting temp, higher than the setpoint, and a rapid decline in output temperature. 

I first replaced the Watts tempering valve, and collected data in the shower. In hindsight, the faulty cartridge in the shower obscured whatever improvment the new tempering valve innards made. After thinking about it, the better way to test the tempering valve is at the outdoor faucet, hot water only. 

After the Watts repair, and still no joy, I decided to implement the ultimate Aquahot upgrade which the installation of the double wall flat plate exchanger. Here is a poor picture.

   

If you decide to go down this path, here is how I did it but first let me explain what it does and how it does it. The install will make more sense if you understand the background. In it's stock setup the AH heats potable hot water via a long copper coil wrapped around the outside of the AH tank. The purpose of outside is to provide a double barrier from contaminating potable water with ethylene glycol. Works great for safety, but it is not the best heat exchanger technology. The second thing to understand is that when the AH is running on diesel, the B zone circulation pump runs circulating ethylene glycol in the tank and through the bathroom heating loop. It doesn't get hot in the bathroom because the fans do not come on in the inside radiator. The circulation of the EG in the tank greatly improves the heat transfer to the potable water coil wrapped around the outside of the tank. Easy to see if you imagine hot but stagnant liquid in the tank. As you draw heat away the liquid next to the outer edge of the tank starts to cool. By stirring it you keep hotter liquid in contact with the outer skin of the tank which results in more heat transferred to the hot water loop. That's just dandy if you are running on diesel, but most of us like to use the pedestal power to heat our water, after all that's the purpose of the 220 V megawatt heater, right?

The issue is the EG (ethylene glycol) in the tank does not circulate when using electric, which results in shorter shower times. 

A way around the heat transfer inefficiency is to take the circulating EG and pass it through a second add on heat exchanger while passing potable water through the other side of the exchanger. Flat plate exchangers are compact and very efficient, and double wall models provide the potable water protection required. They have been used in Europe to heat potable water for a long time. 

This is how I did it. I will break it into three parts: connection and routing of potable water, connection and routing of hot EG, and how to activate the EG circulation when using hot water. Part one, if you look at the incoming cold water pex line to the AH and follow it into the big stainless cover, you see a 6 inch long run of straight copper. Removing the burner helps a lot in this step. I cut a two inch section of that copper line out, and sweated two elbows in. I ran copper from the water in up to the level of the stainless cabinet and terminated with a 3/4 NPT fitting. I used a braided hot water heater hose to connect the incoming(cold) water to the water inlet on the FPHE (flat plate heat exchanger). I used the second elbow and ran copper the same way to the top of the stainless enclosure and also terminated with 3/4 NPT. I connected this line to the water out of the FPHE. This way cold water comes from the incoming, passes through the FPHE, and enters the AH as prewarmed water. 

Part two. Clamp the 3/4 inch heater hose north of the middle check valve. Remove the hose from it's barb. Install a short section of new hose with a barb to 3/4NPT fitting on the end. Connect this using a hot water heater hose to the incoming EG side of the FPHE. Connect the outgoing EG to the heater hose you removed with a hot water heater hose and a barb. The water side and the glycol side are now plumbed. 

Install a flow valve like this one, https://www.amazon.com/gp/product/B07QNN...UTF8&psc=1 , into the incoming cold water pex line. Provide 12v positive to one wire and run the other wire to the B zone pump relay. It will be easy to spot were to attach since the red (diode) wire from the burner is siamesed here also. This will turn on the circulation pump anytime hot water is used. 

Here are the measured results of shower run times take after the modification. The blue line is showering on diesel power, the green line is on electric power, and the yellow line is on electric but without the circulation pump running.  It took me about four days, but 3.5 days of that was false starts and faulty approaches. What I posted can be done in half a day. 

   

Many thanks to Bill Johnson for the graphs, data collection, and moral support.

[Richard]

That was as clear as mud.

A few pics will help. (Thanks Bill)

This is the tap into the Cold Water loop. 
   

Why do it this way. Because I wanted to retain the tempering valve setup and not put untempered water into the coach. 

Here is a pic of the finished install. Sorry it's already insulated.

   

What I failed to mention in the first post was why go to all this trouble. The AH is rated at 1.5 gpm. The house pump on my coach puts 2.25 gpm through the shower. I wanted my bride to have a nice hot shower without futzing with throttling the water.

Third potential question, what is the purpose of installing the flow sensor and turning on the pump when using hot water. The pump will already turn on if using diesel, but if just using electric to heat the AH the pump does not turn on. The heat exchanger does not work without the circulation pump running the hot EG through one side. So......I wanted my bride to have a nice long shower while plugged into the pedestal without having to futz with water output adjustment or turning a pump circulation switch on. Mission accomplished. Happy wife, happy life. 

If I were doing this again. I would try just installing the flow switch to turn on the circulation pump to see if that provided the results I was looking for. The switch, pex cutters, pex crimpers, and pex rings would be less than a 50 buck investment. I would try this before going the FPHE route. 

And finally, follow your own advice. I say think horses not zebras when you hear hoofbeats. When we started getting cold water in the shower, I convinced myself that the coils inside the AH had come loose from the tank and were not working properly. That thought led me down the FPHE route. In hindsight I should have done more methodical troubleshooting before beginning.

[bikestuff]

Great write up! Going from a 4 minute shower on Diesel to essentially unlimited showers on diesel OR electric is a HYUGE difference! I think that when someone complains about not enough hot water they should read and understand this post.

The upgrade modification using the flow valve seems like a good one. This should help squeeze all the heat in the AH tank into the fresh water where you can use it.

As someone who spent the big bucks to replace my AH tank / windings, I can’t help but think that the double wall flat plate heat exchanger would have solved my problem. It turned out that my tank had leaks near the top of the tank. This would allow coolant to leak out leaving an air pocket in the tank. Since air is a good insulator, my effective heat exchange was greatly diminished. Adding a heat exchanger would have converted more of the heat in the tank into heat in the water. Oh well. I guess I will file this upgrade away in case my tank springs another leak.

On a personal note: I was privileged to witness and work with Richard on this project. Not only did I learn about the AH (IN DETAIL), I also picked up another skill at Richard’s Man School. PIPE SWEATING 101. He also shared with me a trick that allows you to sweat pipes that have water in them. Anyone who has worked in service plumbing knows what I am talking about. And NO it does not involve the yellow MAP gas. You will have to contact Richard and see if he will share that trick with you.

[Guy]

Just guessing that Richard's trick in solder pipes having water in them involves a loaf of bread?

[Tim wade]

Hello Richard, question. I’m trying to tackle this also. Why not just cut the pex and run the cold water into the FPHP? And then cut the pex down low and run the return as close to the copper as possible without cutting into the copper? I guess I’m just trying to not cut the copper. Thanks in advance.

[Trainer]

Bravo Richard, again. As Richard shared, the Aqua Heater is rated at 1.5 gallons per minute. Since my Poortravel has a 1.5 gallon per minute shower head, I have unlimited showers on diesel.

Only Newells have the second 240 volt, 4500 watt electric element which helps a lot in heating the coolant. This element has been exclusive to Newell since the first Aqua Hot was specified by Newell. Since my heater has only the one 120 volt, 1650 watt electric element, I have to be quicker if I want to take a shower on electric only. And this must be in warm weather when no interior heating load is on the electric element also. So, you all be proud of the extras that comes with Newell ownership.

In very recent years, Aqua Hot Heating Systems, has begun to manufacture the AHE 675 heater with the second 240 element. Only a tiny few are sold to a manufacturer other than Newell.

Richard has shown how to enjoy the more luxourios 2.5 gallon per minute shower head. Richard's abillity to upgrade stuff is stellar. Way above my paygrade for sure.

[Jack Houpe]

I was reading this thread from 2020 first of last week and ordered a flat plate heat exchanger like Richard has done, along with the 8k btu diesel water heater I have already installed I hope to create something of a on demand hot water system. The 8k btu water heater goes in parrallel between the engine in and engine out feed to the aqua hot then in series with the flat plate heater exchanger like Richard did, then I will take the output side of the flat plate heat exchanger in series of the hot water manifold.

I hope this picture will help see what I am thinking. I just noticed I have the in and out on the diesel heater backwards flow, I will change the in and out of the little circulation pump.

[Richard]

Tim,

I don’t know why cutting into the Pex would be an issue. I got hung up in my thinking of heating the water after it left the AH, but in hindsight that was an unnecessary constraint.

I will say three years after this mod, that I still get high fives from the bride when she takes a long shower for this modification.

[Tim wade]

Thanks guys,

This is the correct red wire you speak of above right?

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[Richard]

Refresh my corrupted memory chips, what did I say was the purpose of the red wire? If you are speaking of the circulation pump wire, I cannot tell from your picture. Maybe Rudy knows it by heart.