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Aquahot cold water solved
#1

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 and Rhonda Entrekin
99 Newell, 512
Maverick Hybrid Toad
Inverness, FL (when we're home Cool )
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Messages In This Thread
Aquahot cold water solved - by Richard - 01-29-2020, 07:50 PM
RE: Aquahot cold water solved - by Richard - 01-30-2020, 05:22 AM
RE: Aquahot cold water solved - by bikestuff - 01-30-2020, 06:19 AM
RE: Aquahot cold water solved - by Guy - 01-30-2020, 08:02 AM
RE: Aquahot cold water solved - by Tim wade - 06-03-2023, 06:01 PM
RE: Aquahot cold water solved - by Trainer - 06-03-2023, 07:29 PM
RE: Aquahot cold water solved - by Jack Houpe - 06-04-2023, 04:16 AM
RE: Aquahot cold water solved - by Richard - 06-04-2023, 04:29 AM
RE: Aquahot cold water solved - by Tim wade - 06-04-2023, 01:06 PM
RE: Aquahot cold water solved - by Richard - 06-04-2023, 02:15 PM
RE: Aquahot cold water solved - by Trainer - 06-04-2023, 02:59 PM

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