Max temperature of polyethylene water tank?

[Vannautical engineer]

Hello. I'm planning a water heating system in my van that will recirculate the water out of the main storage tank, through a heat exchanger, and back into the tank. (Small van, absolutely no room for a conventional water heating system with a dedicated tank, or even a tankless heater really.)

I'm using a polyethylene RV tank for water storage. My question is, what is the maximum temperature of water that a [size=small]polyethylene tank can safely hold?

From what I see here, it seems to be about 140deg F? Is that a reasonable limit? (I would probably keep the water a little below that just to be on the safe side.)[/size]

[size=small]https://www.usplastic.com/knowledgebase/article.aspx?contentkey=798

Also note that the tank will be vented. So while I am planning on carrying hot water in the tank, it won't be under pressure. I realize that RV holding tanks aren't built to withstand any pressure.[/size]

 

[BelgianPup]

140 degrees Fahrenheit.

But why would you need it that hot? For bathing purposes, 108 to 110 is the norm. Six seconds at 140F will produce scalding burns.

 

[Vannautical engineer]

Ok, great. I had actually not even considered what "normal" residential hot water temperature is. But if the limits of a polyethylene tank are a good bit higher than that, I guess I'm in the clear. However that's good to know, as I might aim for the hot water recirculating back into the tank to be close to 140deg, since recirculating water that's close to 140 back into the tank will heat the entire tank up to around 110 faster than just recirculating water that's just around 110 back into the tank.

 

[bullfrog]

Just a thought why not use a fan and a heater core/radiator that is thermostatically controlled to heat the rear area of the van as well as keep the water at a temperature you want?

 

[Vannautical engineer]

Well, that would be a lot more plumbing and a lot more space taken up by a radiator/fan setup, plus that would remove heat from the water that I'm trying to heat up. The goal of this isn't to heat the inside of the van, I have a dedicated cabin heater for that. It's to get hot water. I just wanted to make sure that a polyethylene water tank can withstand having hot water in it.

It's an interesting thought, I just don't think it's necessary in this case.

 

[bullfrog]

Something to consider is how the weight of the tank is supported. Most likely the plastic tank will be deformed over time especially if you heat the water going down the road. Not only will the temperature make a difference but the weight of the water as it vibrates. I once froze a black tank and was amazed to find how quickly it melted with a heat lamp in single digit temperatures. I have repaired several kayaks and other plastic items using heat. I usually put repair patches in the oven on a piece of aluminum foil and bake them till they become playable. At 180 degrees it is like taffy. I would suggest you try a test piece, if nothing else a piece of plastic pipe that is about the thickness of the tank. Be sure to wear some leather gloves. My kayaks seem to do okay holding shape up to about 120 degrees but that is air temperature and no weight on them, hot water is a whole different deal although plastic hot water lines seem to carry 104 degree water just fine.

 

[MrAlvinDude]

When you plan to heat up water (for hot water storage), you may want to be aware of some pitfalls, about storing warm-only water.

Some pathogens love warm-water, but will be killed off, if you (frequently - or for at least 2-5 minutes before use) can raise the temperature of the water to (or above) 140*F

You might want to look at this wikipedia page https://en.wikipedia.org/wiki/Legionella#Legionella_control
to get an idea of some temperature ranges to be aware of.


Where I live, it has become illegal, to lower the temperaturs of hot-water tank systems below 140*F.

Of cause not all the water in a hot-water-tank will have the same temperature - especially not the fresh (cold) water entering when hot water is being used.
But the majority of the tank (and especially whenever the water leaves the tank) it MUST be at (or preferably) above 140*F.


Anyway, I just thought that I would mention this issue, so you can be aware of it both during planning, testing and use of your hot-water-system.

If you can for instance, make sure that the water, at the heat exchange will get above 160*F, then you may have eliminated all pathogens in the water, and can then be able to store the water at any temperature.

Or if you can purify the water before it goes into you heater system - could be a UV section of pipe. This might also make it possible to store the water at any temperature.

 

[highdesertranger]

I am curious , what is this heat exchanger your talking about? what will provide the heat for it? and lastly why do you need so much hot water at any one time? Highdesertranger

 

[MrNoodly]

[size=small]...while I am planning on carrying hot water in the tank...[/size]

Between uses, water in the tank will return to the ambient temperature unless the tank is highly insulated. Then when you use the system, the water that has been warmed by the heat exchanger and recirculated to the tank will cool to the temperature of the remaining water — until all the water has run through the heat exchanger at least once, probably several times. So to get a little warm water you have to heat all of it. That is, unless you use the water coming straight from the heat exchanger without recirculating it, as with an on-demand water heater. But then that water, once used, would go to a gray water tank rather than back to the clean water tank.

 

[Vannautical engineer]

Thanks everyone, these are all good things to consider.

As far as support for the tank, I have it laying directly on the floor of the van on its largest face, so that's good. It's only about 10" tall when laying in this orientation, so hopefully the sides won't bulge out too much. If they do I will probably just run a ratchet strap around the outside of the tank to support the sides.

That's a very good point about the possibility of pathogens growing in the water.

The heat exchanger is a liquid to liquid plate heat exchanger (stainless and copper only, no lead) that will heat water using the engine coolant. Really the only reason I'm going to the extent of doing this is that I'd like to have some sort of shower setup in the van.

I don't need all this hot water, but I do not want to go to the extent of having a whole separate storage tank just for hot water because it's a very small van (Transit Connect) and space is at a premium. I'm not worried about all the energy it will take to heat the entire tank since it's just waste heat from the engine, and if I heat the water while I'm driving for use when I park, the amount of energy to heat the water is pretty much unlimited.

Based on all these issues, I'm thinking that maybe instead of recirculating the water back into the tank to heat up the entire tank, I might just use the water coming directly out of the heat exchanger. The amount of coolant flow through the heat exchanger will be regulated, and I'm going to have a coolant temperature sensor on the inlet of the coolant side of the exchanger, so I can regulate the temperature that the exchanger runs at. Only downside is that the engine will have to be running or at least the engine will need to be hot any time I use the hot water, but this is probably ok. T[font=Tahoma, Verdana, Arial, sans-serif]here will be an electric coolant pump, so the engine doesn't actually need to be running to circulate coolant through the exchanger, but the coolant would at least need to be hot if I go this route. But again that's probably acceptable for as little as I plan to use the hot water.[/font]

 

[XERTYX]

What about a 2.5 gallon rheem household hot water heater? Its designed to hold hot water, it can be fitted with a 12v heating element if you can mount a thermostat inside of it, or using a "hott rod" type heating element you can use AC like its designed for with an inverter while using far less watts than the standard ~1400 watt heating element it comes with. They're around $189 at home depot.

 

[Vannautical engineer]

I thought about all of these options when I was planning how I was going to get hot water. Initially I thought about a 110V heating element, decided that would be really inefficient since it would go through an inverter, and require a lot of electricity. Then I thought about using a 12V heating element next, but again that would require a lot of electricity.

And at the end of the day all you're doing with any electric heating element is pulling energy from the engine. But the engine is continually putting out plenty of energy in the form of heat all the time. So the most efficient way to get heat from the engine is to pull it directly from the coolant. I've already mounted the heat exchanger and run coolant lines the length of the van to the engine bay, so I'm already pretty committed to this. Just ironing out the details, but I think it has the potential to be a great way to get hot water.

 

[XERTYX]

I have been researching water heating options myself recently. I stumbled across this video. He fits a 12v 125 watt element into a Coleman cooler. I believe he achieved 120°F after 1h 140°F after 2hrs. 



If you fit a 12v 20a thermostat/relay onto a rheem tank and if your solar charge controller has a programmable load timer set it to switch on an hour or 2 before you plan to shower every morning and off afterwards (include a toggle switch to override it on cloudy days so you dont drain your battery) 

If you have enough solar it could be sustainable without using any heat exchanger, if you do use a heat exchanger it could also be an option to augment your heating options by capturing and storing some thermal energy from surplus solar.

Do your own due diligence etc and so forth. This is just what I have thought about doing.

 

[Vannautical engineer]

Not saying it's impossible for it to work, but don't underestimate the amount of energy needed to heat water. It's a lot.

Play around with this calculator.

https://bloglocation.com/art/water-heating-calculator-for-time-energy-power

 

[XERTYX]

Oh ok. Well i wish you the best of luck. Just look into the rheem tank even if you only plan to use engine heat. It would HAVE to be more insulated than a fresh water tank. The RV tank has 1 advantage as I see it. Its square and not cylindrical.

 

[Vannautical engineer]

Ok I will. I watched some of that youtube video. With that 12V 12A heating element, it took him two hours to heat only 2.5 gallons of water.

12V at 12A is 144W. To be able to produce 144W of electricity for two hours, you will need a decent amount of solar and very consistent sun for two hours straight. And after all that, you're only getting 2.5 gallons of hot water.  :-/ I dunno, seems pretty iffy to me.

 

[XERTYX]

Yes I know heating water.. or anything with electricity takes a lot of juice. But if you have an ass load of solar you could use surplus. I'm planning on 500-600w on the roof. I'm also planning on using a relay to power it when the batteries are full off when the voltage is less than 12.7.

I'm planning on using a 240v 500 watt element (when using 240v element on 120v supply the output is 25%) a separate small inverter on the load of the charge controller ON at say 8am, OFF at 10am. Then placing a 200 watt thermal heating pad mounted with a thermostat under the insulation ON at over 12.7v OFF at under 12.7v

I got that idea from Everlander on YouTube. He uses arduinos to switch his off at 80% battery voltage. He has over a killawatt of solar on his rig and he was wasting potential energy even during the winter.

 

[Vannautical engineer]

Yeah if you have a surplus of solar energy, then by all means do it. But I will not have enough roof space to have that problem on a Transit Connect. 

Also like others have mentioned, I would be a little concerned about growing bacteria in my water if I was heating it this way. If you do not have enough consistent electricity coming in to heat your water, you may wind up with water that takes a long time to get up to a high enough temperature, or water that just floats around a temperature that's not quite hot enough to kill off bacteria. Something to think about...

 

[bullfrog]

My motor home used a 6 gallon propane water heater that also had a built in engine coolant heat exchanger. It was very well insulated and worked great for taking a hot shower at the end of a days travel saving a lot of propane and only required two coolant lines that ran to the water heater. Heat was directly delivered to the insulated tank and no electric pump was required as well as no additional potable water lines needed to be run. It was expensive and when it came time to replace it I bought a regular hot water heater and used a 12 volt heating element. I would turn it on while traveling and at the end of the day turn it off just before stopping for the night and immediately take a shower. That also worked well. Lots of the Jeep forums address using a heat exchanger for out door showers but most end up with a small on demand propane system. Good luck, let us know how it works out for you! In my opinion a heavy insulated cooler will work best as a water tank as long as you seal and vent it. Another plus is it is easy to clean the interior or replace if necessary. I never use hot water for drinking or cooking.

 

[XERTYX]

Yeah that is true. My idea is still in the infancy of its theoretical stage anyway. Haha. But I have thought of being able to use it once I am able to build a cabin off grid as well.

Using a larger tank and having say 2kw of solar I could easily divert the surplus to the tank while keeping the 125 watt element always on. It wouldnt use 125 watts consistently as the thermostat would kick it off when up to temp.

Then using a relay during surplus hours to power a 200 watt pad usually used to keep lithium batteries warm in the winter a cabin install is totally feasible. A mobile application... maybe not.