Hot Water Heater Clean Out

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I’m curious about the standard practices for removing sediment from hot water tanks. Is there a recommended schedule for doing this to keep everything in check? I understand that the hardness of the water in the coach can influence this, but I assume it’s a common maintenance task for everyone.
Kevin

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How exactly do you plan to flush that tank, Kevin? I chlorinate the water periodically, circulate it, and then drain everything, but I doubt that really clears out all the sediment. I’ve also thought about changing the anode, but I struggled with the fitting and was hesitant to break anything. When I get back to Florida, I’ll probably be a bit more, let’s say, determined with it, especially since I’ll have better access to parts and a proper workspace.

My water heater is a 20-gallon model that looks just like a standard household water heater, but it’s actually a marine model—a Raritan. I don’t believe it was original to the setup.

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I have a ’99 Marathon with two 10-gallon Atwood heaters. When I reached out to them, they recommended disconnecting from the water supply and the bus, then adding 6 gallons of white vinegar and 4 gallons of water to each heater. I’m supposed to turn on the 110-volt heat and let it sit for 24 hours before draining and flushing. Instead of separating the two heaters, I just disconnected them from the supply and the bus. I made some adapters to hook up a garden hose for flushing in either direction—what a hassle!

Before adding the vinegar, I did a quick flush to test it and was shocked by the amount of white sediment that came out. Today, I’m planning to do the drain and flush and then repeat it once more. I’ll report back on how much sediment I find in this next flush.

I’m trying to improve the heat transfer from the Webasto to the tanks because the hot water from the Webasto isn’t as hot as when I use the 110-volt or while driving. Atwood mentioned that sediment buildup can hinder effective heat transfer.

Additionally, I discovered that Marathon uses a dual heat transfer unit. You can run the coolant through the tube that goes inside the water tank or the one that runs along the outside. Marathon informed me that they always use the outside tube to prevent any coolant mixing with the house water in case of a leak. Unfortunately, mine was plumbed to the inside tube. Thankfully, I didn’t have a leak, but the coolant flow was too slow, causing the Webasto to shut down early due to overheating.

To identify the blockage, I bypassed the water heaters after trying various fixes, including a new recirculation pump. When the water heaters were out of the loop, the Webasto ran perfectly. I then discovered that the Atwood water heaters had a nylon coating on the first two inches of the tubes, which was breaking down and causing restrictions. The tubes on the outside don’t have this nylon coating. Luckily, I moved everything back to the outside tubes, and the Webasto is functioning much better now. However, I’m still not convinced it’s perfect since I’m not getting the water as hot as I think it should be. This could still be a heat transfer issue.

I’m also monitoring how many times the Webasto cycles during the heating period. I need to flush the tanks out first before refiring the Webasto for another test, which will be tomorrow—what a fun project!

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Kevin, aren’t the heat exchangers used by the Webasto the same ones that limit its temperatures to around 160 degrees, while the engine coolant typically runs at 195 degrees or higher?

Additionally, the electric elements are located inside the tank, so there’s no exchange happening except right at the water contact on the element.

It’s also worth noting that we observe water temperature differences because the bus operates at a much higher temperature compared to the Webasto.

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Jon, yes, the heat exchangers are the same, but if there’s a restriction slowing the coolant flow, the Wabasco will trigger an overheat shutdown at around 190 or 195 degrees. The water heater allows for coolant to run through either interior tubes or the exterior ones welded to the tank. The internal option is more efficient for heating the water, but it poses a safety risk if there’s a failure since it could mix with your shower water.

I understand that when you’re driving, the water heats up significantly, but the Wabasco should still provide enough hot water for a shower. I’d expect the water temperature to be around 105 to 115 degrees, but after running the Wabasco for an hour, I measured only 92 degrees, which isn’t sufficient.

I flushed the tanks today and was shocked by the amount of white sediment that came out. I’ve since added 12 gallons of vinegar mixed with water and will flush again tomorrow. After that, I’ll reconnect everything, refill the tanks, and fire up the Wabasco to see if there’s any improvement. It feels like I’m making one step forward and two steps back, but I’m holding out hope for progress—it’s not that complex, or at least it shouldn’t be!
Kevin

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It sounds like you might be dealing with a restriction in the coolant flow around the hot water tank. Some owners have experienced their overpressure pop-off valve opening when the tank reaches engine coolant temperatures, often due to improper pressurization of the bladder in the expansion tank. Without a bladder, there’s no room for the hot water to expand, which can cause hydraulic pressure to trigger the safety valve.

Instead of addressing the root cause, some individuals end up partially closing a valve on the engine coolant line, which significantly restricts the flow.

Cleaning the tanks is definitely a good move, but I recommend checking the pressure in your expansion tank as well. With the water system pressure at zero, you should aim for around 30 PSI in the expansion tank. This helps reduce the frequency of your pump cycles (while making it run longer when it does engage) and ensures there’s enough expansion capacity for your hot water, especially if there are closed valves in the system.

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Jon, I’ve checked all the valves to ensure they’re open. I’ll also take a look at the PSI in the bladder and aim for 30.

Thanks again,
Kevin

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Kevin,

You’ve inspired me to clean out my tanks. Thanks a lot!

Regarding the hot water heaters, can you clarify which ports are for the Webasto—are they on the top or bottom?

Thanks,
Mike

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Which bus Mike?

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Dale,

That was way harsh.

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Mike,

The middle ports connect to the inside of the tanks, while the bottom ports run along the outside. Another interesting tidbit I discovered today: Marathon modifies the wiring for the top water heater so it activates first. It stays on until the water reaches about 140 degrees, then it shuts off and the bottom heater kicks in. The bottom heater operates with the standard Atwood controls, while the top one has a different control setup installed by Marathon.

I’d love to know which ports your coolant runs into from the Wabasco. For all the Marathon owners with 98-00 vintage units, please share your setups.

Thanks a lot for the info—Wabasco-Atwood heater 101!
Kevin

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Kevin, where do you buy the Vinegar by the gallon?

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Smart & Final restaurant supplies.

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Kevin,

I’m having a hard time locating the right size connectors to hook up to the Atwood for flushing the system. Could you share the sizes you used? I’ve checked at Camping World, Home Depot, and a plumbing supply store, but no luck so far.

Thanks!
Mike

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MM & Kevin,

Does Marathon include a water heater as an additional option or as a backup to the Aqua Hot system?

Tuga & Karen Gaidry

2012 Honda Pilot

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