Girl-Friendly Explanation of the Aqua-Hot Heating System on your RV Bus

Editor’s Note: I truly don’t want to offend any wives by my title above. It’s meant in gentle humor. There are flocks of tech-savvy females on the RV circuit, but I believe the majority are like me. When I started out in 2008, I was mechanical systems clueless. I didn’t even know what a generator was. I was scared of my thermostat. How-to articles started out so far over my head, they were like foreign languages. So for all of us non-techie babes, I have written this article. 

Well girls, I have to confess I totally didn’t understand the Aqua-Hot heating system on my RV bus. I just flipped it on and wallowed in the nice hot water. But if you really understand how it works, you will be in awe of its beauty and complexity. This system really is a miracle. And it’s not over our heads at all, if you can just get a girl-friendly explanation of it.

All RV’s need heat when it gets cold.

Most RV’s have propane heaters (one in front and one in back.)

This RV has an Aqua-Hot system.

The propane heater is identical to the one in your stick house. It uses propane to keep a tank full of hot water ready at all times. This is a very energy-needy system. In cold weather, your propane tanks can run out of fuel every seven days. It’s a huge hassle to keep refilling these tanks. On a small RV you have to take the tanks out and drive to the store. On a big bus you have to pull up stakes and drive your bus to the store. If you decide to stay in one place all winter, you would definitely want to rent a larger tank and set it up.

The Aqua-Hot system, on the other hand, does not keep a tank of hot water ready. It works on demand, heating water just ahead of you as you are using it. What a great idea!

The Aqua-Hot has three different ways to heat itself up.

It can use electricity from shore power, or diesel fuel from your main engine’s gas tank, or heat coming off the engine itself while you are running down the road. Or a combination of these!

Last winter we were forced to park in the back yard of our old house and the long driveway is lined with vicious tree branches and tight curves, so we didn’t want to go in and out of there any more than necessary. With Aqua-Hot system of heat, we did not have to pull of stakes for the entire winter of below freezing weather. We used about 70 gallons of diesel from our 150 gallon tank.

Not only does “Mr.” Aqua-Hot have three different ways of heating coming IN to the system, he has two different competing demands on him going OUT of the system.

He has to give you both heat AND hot water for your showers. When you combine all of these you get a duke’s mixture of possibilities, depending on how cold it is outside, how many people need a shower, whether you like your room 65 or 70 degrees. Or maybe it’s August and you’ve got the air conditioners running full blast.

Don’t be intimidated. Just approach the thermometer switch on the wall and set it to 70 degrees.  Now walk over to the panel above the driver seat and turn on the Aqua-Hot system with the switch that says “Electric.” Let’s assume for this day you are parked and plugged into 50 watt shore power.

The system has a little tiny plastic sensor in the living room that measures the temperature. Its 62 degrees. He sends a signal to the brain, “Hey, we need more heat out here.” The Aqua-Hot then turns on its electric heating element and begins cooking a tank full of antifreeze (50% water, 50% poison stuff) to 170 degrees. Then it pumps the hot antifreeze throughout the bus to small heat vents/outlets/registers. Just inside these vents are cute little bitty radiators with lots of surfaces. A fan comes on and blows air across these very hot surfaces, then out the vent hole, heating your room.

Now all is well, and you are at a comfy 70 degrees. But wait! There’s more! I said this system was complex and amazing.

This electric heating element continues to work as long and hard and as he can, no matter how cold it gets outside, but there is a limit to his ability. Because, did somebody say “Your turn, honey.”  Now you need shower, and where is all that hot water going to come from?!

Back at antifreeze central, just picture a long snaky pipe filled with clean water. It winds its way through the antifreeze tank, so it also stays heated to 170 degrees, same as the antifreeze. Turn on the hot water faucet on a warm summer day, and you will easily  have enough hot for two showers without any extra effort. But on a cold winter day, and he is working hard to keep the antifreeze at 170 and keep the rooms heated up. If you drain off all the hot water for a shower, you pull a bunch of ice cold water back into the pipes around the heater core, and you are giving that electric element the shivers.

Now he’s crying in pain, and reaches the limit of his electric capacity. Mr. Aqua-Hot  must choose whether to give you heat or hot water. The guys who built this system already told him, “If you get overloaded, forget the heat and just make hot water only. We don’t want the lady of the house hitting ice water halfway through her shower.”

But this is where the genius comes in! When you get in this situation, you just walk back to the panel over the driver seat and you flip the other Aqua-Hot switch, the one that says “Diesel”, and you will hear, from the bay underneath the bus, the sound of a diesel flame heater kicking in. You will come to cherish this quiet, gentle whoosh as the sound of luxury. This is “on demand” on steroids. Within two minutes, you have enough steaming hot water to shower your grandson’s entire little league team. You can now take a long hot shower on a cold winter day, but as you know, we RV’ers are way too sensible for that. And you will probably continue to take your little Navy shower just as always. And by the way, when you take a shower in the wintertime, I advise you to always switch the diesel switch on BEFORE you get into the shower. It’s no fun streaking through the bus soaking wet with no regard for your windows and neighbors.

To keep from scaring you early on, I failed to mention that your bus also has the words ”heat pump,” on the thermometer, which is a whole different system from your Hydro-Hot. We actually never use this heat pump. It is part of the air conditioning system. It uses Freon to heat instead of antifreeze. The only advantage we can see to the heat pump is that it won’t lower the temperature of your antifreeze and your hot water core. On the other hand, the heat pump only works well when the outside temperature is 40 degrees or above. So we’ve always been in the habit of using our furnace only. But would love to see a comparison of electricity usage on heat pump versus furnace.

When we were stuck in Oklahoma for the winter (I’ll explain why below) it drops below freezing at night and sometimes dips below zero. We found out that the electric heater kept us cozy in the daytime, but needed help after sunset. At bedtime we kept the electric heater on, but had to switch on the diesel boost and let that diesel burner run all night long to keep our bus at 65 degrees. Steve estimates we used 70 gallons of diesel over that bitter winter. With our 150 gallon tank, we could easily have lasted through a Canadian winter. It was a great blessing to us not to have to drive to the gas station once all winter. Compare that to propane tanks that must be filled once a week.

We also had to wrap heat tape around our water hose, and leave the faucets dripping all night. Our water hose still froze up on bitter cold nights but the next morning the sun would shine and by early afternoon the hose would thaw. Many times the hose was buried in a blanket of snow, and this actually insulated it from cold, as non-intuitive as that sounds. You can ask the bunnies and polar bears about that.

A couple times our water froze right inside the bay and we found out that ice cold air was sneaking into the bay through the big opening of the sewer hose. It was just like leaving a door open! On extra cold nights we had to go out and unhook the sewer hose and cap off the black tank. And that ended that.

Even if our thermostat on the furnace is set to the “off” position, Mr. Hydro-Hot knows he has to keep making hot water for us. Whenever the temperature of the antifreeze drops below 140 degrees, the electric burner kicks on and reheats it, even on a hot summer day. When we draw hot water, this pulls fresh cold water into the pipe, cools down the antifreeze and signals the burner to kick in. The element heats to a maximum of 170 degrees and then floats, similar to a battery floating to maintain full charge.

Now let’s say we pull our electric plug outside and park our bus on BLM land atQuartzsite,Arizona. We are dry camping for two weeks. What fun for us, and for our flexible, ingenious heating system.  We have no electricity, so we might as well turn our thermostat to the OFF position. We would never attempt to do “dry camping” in a cold climate. But here in Quartzsite, let’s say it’s a balmy 70 degrees outside so we don’t need heat anyway. We can even open our windows. But we would like a little hot water for showers. And how do we turn the lights on?  Hmmm?!

The answer is – generator – that thing your husband rushes to Lowe’s to buy when bad weather hits and power goes out all over town. Your generator runs on diesel so you don’t want to squander it, and it makes noise, so you don’t want to run it at night when other people nearby are trying to sleep. So start it up in the morning and power will flow from generator to inverter, converting that power to electricity. It flows through all the outlets marked “inverter.” Now you can run your lights, coffee pot., toaster oven, radio, computer and big screen TV all at once. The generator will also recharge your house batteries. Once you turn it off, you will still have six fully charged batteries so you can run your TV, computers and lights quietly for the rest of the evening. .Run your generator a couple hours in the morning when you get up, and a couple hours in the early evening, and you are sitting pretty until your diesel runs out, or your dirty tanks get full.

As far as hot water, there is no electric element heating except while the generator is actually running. The burner has been trained to shut off whenever the generator shuts off. Otherwise it would drain your house batteries in a hurry. Even if you have the Hydro-Hot electric switch in the ON position while dry camping, the system will not run off your batteries.

So if you want to take a shower, you can do it while the generator is running. Or if you turn the generator off, the water will probably stay hot for 45 minutes or so. But remember your magic switch – the Hydro-Hot diesel, which you haven’t even touched yet. Just flip it on and the magic diesel burner comes on, heating water with diesel fuel from the motor. It doesn’t even need the generator to do its job! Within two minutes you have enough hot water for that ball team. Seriously, it is heating the water on-demand, just ahead of you. So no matter how long you leave the hot water faucet running. . .I wouldn’t, though, because God only gave you a 65 gallon storage tank for your used (“gray”) water. And once that’s full, you’re up a different kind of creek. Also, you are carrying a tank of 90 gallons of fresh water. Be thrifty with your water usage and you will be able to have fun out in the desert for seven whole days before you have to drive into town to refill your fresh water and empty your used water.

Well class, I think that’s enough for one day. You have to agree the Hydro-Hot system is amazing. It does take maintenance and after several years the heating element, pumps or fans may certainly need to be replaced. You can save a lot of money if you work on it yourself, but some systems are easier to get to than others. In the case of our 2005 Tiffen Allegro Bus, Stephen spent a week with bloody arms and hands and he looked like a sausage with his 6’3″

 250-pound frame stuffed into a tiny opening. Here are a few photos of Steve repairing his system.

Best of luck, guys. We couldn’t live a day without our Aqua-Hot!

 *Why we spent the winter in Oklahoma:

We had a photography business in our home for 22 years. In order to sell it in 2008, we had to hold the note for the buyer. He was newly self-employed, out-of-state, and in a bad economy. We only did it because his parents co-signed and had good financials. The buyer defaulted after one year! We offered to settle for $94 when he owed $5600, He chose instead to declare bankruptcy, taking his parents with him.  What a tragedy. We lost our shirts financially, got the house back and parked our RV in the back yard to babysit the house while looking for a new buyer – one with a bank loan.


– Posted by Carol Ann Dwyer

15 thoughts on “Girl-Friendly Explanation of the Aqua-Hot Heating System on your RV Bus

  1. This all sounds great until you have to work on the system. My electric heat stopped working and there is a relay switch attached to a circuit coard that stopped working. The entire circuit board had to be replaced at a cost of $1500. Now the electric element has stopped working again. The design of the casing on the aquahot allows no access to the element. There is a 3″ space that you have to look through with a flashlight and mirror just to hit the reset button. It will drive you crazy just trying to hit this button and chances are the element will be burned out and you will have to replace that. Bottom line is the system works fine until it doesn’t and then it is almost impossible to fix. You won’t want to fix it yourself so you hire an aquahot expert who will charge you $1000 to $1500 just to do basic maintenance or a simple repair because they have design the aquahot to be almost impossible for the owner to work on themselves. Aquahot could have designed the box to allow access openings where you need them, but they are looking out for their repair folks who make a lot of money on this totally absurd design. Aquahot is the most maddening design that I have ever seen. It cost a fortune to fix when it could have all been designed to allow it to be worked on easily.

    • There are a few Aqua-hot tech’s around that will work on the system without breaking the bank. I used John Carrillo who used to work for Aqua-hot and now is out on his own. His website is, or phone is 970-518-3085. I have done some work on the system myself, and it isn’t too difficult aside from the access to it. In our tiffin, the unit is in the middle of the basement and accessed through one of the smaller basement doors. As my wife said, I lost a little blood on that deal because there are a few sharp places in the tunnel. Our system has worked well except for having to replace an electric heating element, a drain tube, a float valve, a combustion chamber and a relay. I do the service each year, (8) years now, which means changing the burner nozzle and fuel filter. I sure would like to have easier access unit, but I sure wouldn’t trade the unit for any propane furnaces. I agree that the parts to repair it can be expensive, but there is a users group out there that has resources that can help trouble shoot and provide sources for less expensive repairs. The address is, and is a great source. Roger Berke is also a certified Aqua-hot tech and very helpful to anyone with “the system”. These guys tend to frequent area’s where there are large groups of Rv’s in the southwest and at other big rv events. We were just a Quartzsite for the big rally and John had a booth there. Good luck with your system. Steve

    • Jan, I am A Factory Authorized service tech,
      If you are paying that kind of money for a minor repair or annual service, You really need to find another Service Tech

  2. Great article…Yes when something goes bad with them it can be a pain but when they work, like you explained in this article they are genius. My husband has tried to explain the operation but I think it takes a woman to explain it so I can understand it and now I get it! BTW my husband is an RV guy and hates to see a customer with a problem with their Aquahot but I don’t think there is anything out there to replace this system so it is well worth the repair. Thanks for taking the time to post this! It gives us all a better understanding or a marvelous system

  3. I leave my electric switch on, but I’d doesn’t seem to heat the water. I’m in Texas were it has been in the 80s for about 2 months so I know the water isn’t that cold coming out of the ground. Should the electric element be heating the water? Also, should I always leave it on?

    • Yes it should heat the water. Yes you should always leave it on, unless you are paying for the electricity (If pinching pennies, you COULD turn it off, but then if you want a shower or do dishes, you just flip your aquahot diesel switch on and wait 5 minutes. We normally keep the electric switch on year round. It only works when you are plugged into electric or have generator running. In summer we always have enough hot water. In winter, when we want to take a shower in cold weather, we usually flip on the diesel switch on and wait 5 minutes for the water to heat up. We can get two showers back to back but we don’t tarry. The diesel is especially helpful in winter if the main furnace has just been running – this uses up all the hot water. (On the wall thermostat, your aquahot heat mode says “Gas heat,” whether it’s electric or diesel.) If you can’t take a shower on electric only, then something is wrong with your system. Usually if the electric heating element is bad, the electric system won’t work at all. But it will work fine on diesel. It’s probably one of the 4 stir pumps gone bad. This is too darn complicated for human beings!!! Give us a call! did you get my email?

  4. Carol Ann, excellent post on the Aqua-Hot system, and fun to read too. We totally enjoy Aqua-Hot on our 2002 Monaco Dynasty (38 foot Jack). We get an annual maintenance at a fair price at Premier RV Services in Junction City, Oregon. Last winter we spent time at Canyon Vistas RV Park in Gold Canyon and I met an excellent mobile Aqu-Hot technician, Orman Claxton, who fixed my ‘neighbor’s’ Aqua-Hot system. So good techs can be found, and at a fair price!
    I haven’t seen a post from you in 2016. I hope you are still out on the road enjoying your motorhome lifestyle!

    • Hi John, thanks for passing along the names of two great Aqua-Hot technicians! We will sure use them if in those areas. Sad that i can’t get more writing done. We are in Oklahoma for a few months. Wouldn’t want to miss the tornado season (tongue in cheek.) Take care.

  5. I am needing someone to repair my Hydro hot in my Country Coach. I live near Mineola, Tx. I am permanently parked. Any suggestions?

  6. I am. In need of a repairman for my hydronic heat system in my MH. I am near Mineola, Texas. Does anyone know of someone in this area

  7. Hi folks. I have acquired a used RV with a diesel aqua hot system. I have found it to be a ridiculously complicated system that has an, at best, 10 yr life cycle. The crowning glory is that the injector has to be serviced every 100 hrs, that’s once a week folks and those guys get $130 an hour. It cost me $800 to have a service tech look at it, he didn’t fix it. I’M yanking the piece of junk out and replacing it with something else. I can’t stress strongly enough, do not buy one of these.

  8. Loved your article. You got one MAJOR thing wrong: the name is MRS AQUA HOT. Anything that can multi task so well and work tirelessly without complaining MUST be a MRS! Don’t you agree?

  9. My husband just found your site and it is wonderful. we have been trying to figure out how the aqua hot system works and you did a fantastic job of explaining. Thank you for your web site

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