Hot water options in an electric van

We don’t have a propane tank in our van, so we heat our water using the electrical system. It’s a choice that works for us because we have a large battery and solar system. Here’s a description of our system and the other options you might consider.

We plumbed our van with a cold and hot water tank. The hot water tank is an Isotemp slim square unit. It has a tempering thermostat so the water in the tank can be heated much hotter than would be comfortable to use, then it’s mixed with cold water as it comes out so that it is at the right temperature. That way, the 4 gallon tank can provide many more gallons of hot water.

Isotemp Slim Square water heater (picture courtesy Indel Webasto)

The water stays hot in the tank for a long time – we’ve used it two days later and been surprised at how warm it was.

The tank has a 750W 120V immersion heater element inside it. It also has a separate heat exchanger “coil” that allows hot water from another source like your engine coolant to run through the tank and heat the water in the tank. More on that later.

The 4 gallon tank takes about an hour to heat up to the point where its thermostat turns off. Our unit actually draws 860W from the 12 Volt system (there are inefficiencies with the inverter converting to 120V), so it’s consuming around 71 Amp-hours from our battery each time it heats up from cold.

That’s about 1/10 of our battery capacity, so we’re OK heating a tank of water every day if we want to. Our solar panels are capable of replenishing that energy most of the time. Obviously, we also try and plan ahead and heat up the tank while we’re plugged in to shore power at home before we leave on a trip.

Isotemp do make other tank sizes, and their cylindrical tanks are cheaper. We just didn’t have the space to put anything other than the rectangular one. It fits next to the 3-person bench seat.

If we don’t want to heat a whole tank, we have two other options. We have a travel kettle. Boiling water in that takes 860W for 5 minutes. That’s obviously much less water, but it’s enough to wash dishes or clean up. We also sometimes heat a little water in the pan we’ve used for cooking. Again, that’s really handy for doing the dishes.

We also have a portable solar shower. Actually, it’s just a big black bag that hangs up in the sun. We use the 10 liter MSR Dromedary bag with a shower attachment. It’s sturdier than most of the plastic solar showers you can buy. In the summer this heats water up enough to feel good when you’re cleaning off after a bike ride.

One thing we will do when we have the time is figure out how to use the hot water tank as a diversion load for our solar panels. When the battery is fully charged, the solar controller switches off. All of that sunlight is “going to waste” at that point. Using our Victron equipment, it’s possible to set up a system that switches on the water heater when the battery reaches a certain state of charge. That would continue to use all the available solar energy and heat our water “for free”!

Other options we considered

On-demand hot water

If you have a smaller battery, it might not be able to sustain a 70 Amp draw for an hour. That would limit an electric hot water tank to only being used on shore power. You might be able to get away with using an on-demand electric hot water heater instead.


We chose to go all-electric rather than installing a propane tank. Obviously, vans with propane systems installed can make use of propane boilers either to heat a water tank or for on-demand water. There are a lot of advantages to this approach. The only real downside (other than having to install and plumb a propane tank) is that most permanently installed propane heaters require a large-ish hole in the side of the van to vent.

If you just want hot water for a shower, take a look at the Zodi system. It uses the 1lb green propane tanks to heat water that is pumped out of the storage case or a bucket.
We don’t have one, but we’ve used one. It really helps to dump the shower head into the supply bucket while you are soaping up. That heats the intake water up and saves water that otherwise would go to waste.

Engine coolant system heating

We mentioned that our hot water tank can be plumbed to run a hose from the engine coolant system through a coil inside the heater. That’s a great way to get hot water if you’re driving every day. If you are camping in the same spot for several days though, you won’t have that source of heat to warm your water. It’s a good secondary heat source, but it’s not the right primary heat source for many people.

Diesel hydronic heater

We have a diesel-fired hot air heater (the Espar Airtronic D2) to warm the van up. You can also buy similar heaters that heat water rather than air. This water can then be stored in a tank or can be circulated through radiators in the van to warm the interior.

We chose not to use this system because it takes longer to get the van interior up to temperature than the D2 does, but it is still a viable option. The hydronic diesel heater can be used in combination with the engine coolant system too, so that either source heats the vehicle interior and the hot water.

Camp stove

Saving the best ’till last (?!), there’s no reason why you shouldn’t just heat water on a small camp stove to use for all your washing needs. In combination with a cheap 12 volt shower head, you can even use this water for showers.

24 Replies to “Hot water options in an electric van”

  1. We bought a BOSS shower system from Mr. Heater. $200-something through Amazon Prime. It has its quirks and isn’t perfect, but well designed, and recharges on our 12V outlet near the rear doors.

    It heats water to about a 40 degree differential, so,we usually just set it up and recirculate it for a few minutes to raise the intake water. The pump drops into one of those blue reliance containers, and we set up a shower curtain that attaches to the back doors when open. Great to take a scalding 110F shower at night in cold temps.

    Used it for 3.5 weeks on a road trip with no issues.

  2. Oh, any plans to hook up the engine heater recirc lines to your IsoTemp? I was looking the same slim one, but still haven’t visualized the details of running the lines back to near the rear wheel well. Seems like a long run on a 170WB, Someone probably has an elegant solution to run them back inside a protective housing (another heater hose likely).

    Would be amazing to use that engine heat and have more or less on demand hot water any time the engine runs for a bit.

    1. So far, no plans to hook up the coolant lines. It would be lovely to have the free heat, but it’s real hassle. Hein on the Sprinter forum put his tank in the engine compartment and that makes a lot more sense if you’re using the coolant lines.

      There’s a Mercedes option for heater pre-fit that just gives you the hoses, stubbed out to behind the driver’s seat. No dealer could get me that option when I ordered the van.

  3. Does your shower get used a lot? We are currently debating whether to have hot water/shower in our 170 4×4 build. The tradeoff is mostly an issue of space and how often we will use it.

    We are evaluating the Webasto Dual Top or Espar Hydronic systems. Alternatively, we would just have hot air but not hot water. The Isotemp in your build sounds great but we only have a 440 ah battery bank, 100w solar, and 6 people in our family, so I don’t think that is an option for us.

    Any insights from your experiences would be really helpful. Thanks!

    1. We use hot water through the shower and/or the sink quite a lot. We do a lot of biking, so it’s good to be able to wash the dirt off after a ride. We also race cyclocross in the winter, and the hot water is really, really nice after a cold muddy race.

      The Isotemp holds its temperature for quite a while because it’s well insulated. If you were mainly doing shorter trips you might be able to just use shore power to heat it up before you left. That’s what we do if we’re going to a cyclocross race from home. However the tank is not large enough for 6 people to shower.

      The thing it can be used for (although we didn’t) is to run an engine coolant line through the Isotemp. That heats the water “free of charge” as you drive along. Hein on the Sprinter forum put his Isotemp under the hood of his van and plumbed it into the engine coolant. That gives him pretty much all the hot water he can use! Again though, with 6 people you’d have to stagger who has showers when.

      The Webasto Dual Top and Espar Hydronic are both well regarded systems. They are more demand-driven than the Isotemp. Rixens is another system to think about. It incorporates the diesel water heater and an engine coolant line.

      Hope that helps.

    1. Tim, we take “navy” showers (turn off the water while soaping up) so it’s hard to say how long it would last on full throttle. We really have never tried that experiment. I will do that when we have some time to play with the system.

      We originally bought the 6 gallon tank. We haven’t needed more water than we use, but the 6 gallon might be nice to have. It’s a trade-off though because the larger tank takes longer to heat up. If you’re after fast hot water, the smaller tank is a better bet.

      1. Thanks for the reply – yes I’m debating with myself what size tank to get and how the system could work. I’m planning to shower every day 🙂 plus washing up. I’m considering various ideas:

        – Hooking up to the vehicle coolant for the heat transfer. Hopefully Transit vans aren’t too tricky for this. Worst case you can turn on the van to idle for 15-20 mins to heat up the tank. Harder if it’s a bigger tank. The beauty of this, with a 2 hour drive you could heat up a bigger tank.
        – Using the ‘generator control’ feature on the Victron Color Control GX to turn on the water heater once the batteries are full. It’s discussed here So as long as my panels are big enough, and the sun is out, we should get some free heating this way. I’m assuming it would need to switch on the inverter and the heater. (I’ll get an electrician friend to help with this)
        – Using a bigger tank, so more energy is stored. It sounds like they stay hot for 24 hours. By having a bigger tank, one good day of heating should provide enough hot water for a couple of days. And assuming short showers, you wouldn’t be heating the whole tank from cold each day.
        – And one crazy idea: imagine using coils of black pipe out in the sun, and a small pump, instead of the vehicle coolant pipes. Not very practical to carry around though…

        1. Tim, hooking into the vehicle coolant system will provide the most “free” heat, especially if you’re using a gas engine which heats up faster than diesel. Just be aware that you might provide too much heat. I remember Hein on the forum had issues with his tank thermostat tripping (and needing a reset each time) because his coolant got the internal temperature too hot.

          We plan on using the tank as a energy sink. Either the Generator Control feature of the CCGX or the relay on the BMV 700 monitor could be used to do this. The inverter would already be on. The CCGX just has to control a 120v relay for the heater.

          The water does stay hot for a long time in the tank. Even if you had used half of it, the rest would stay pretty warm and may only need a slight top-up.

          The coils of black pipe idea is interesting, but if you’re going to use solar heating why not just do it via the photovoltaic panels on the roof? Those work even in freezing weather!

  4. Another idea in a similar vein to the black pipe: use flexible hose with a piece of copper or wetback pipe in the middle, to lay across an outdoor fire, or even a gas flame. This wouldn’t need a huge coil of pipe, and provides an opportunity to heat the water off grid, without sun, and without wasting diesel.

    I’m really trying to solve the problem of sitting in rainy conditions for days and not having hot water. I suspect I’d just drive to a campground and plug into power if it got bad. Thanks for the info, and thanks for an awesome website. It’s very helpful.

  5. Your isotemp tanks sounds possibly like it could fit my needs. I am trying to set my water system up to be freeze durable say 20d F for 48 hrs as well As extended periods at just below freezing in my driveway. (Ski trips and Vancouver island coastal winters). I don’t want to run the diesel heater or run shore power to keep things warm. I also don’t want a fiddly system that makes draining it a pain. Do you think that rules out the isotemp tank option? I fall into the weekend warrior category So quick departures and arrivals are important. I may evolve later into longer to three week trips where more set up time is OK. I would really appreciate hearing your thoughts on whether the isotemp might be a good system for me. Have discarded the boss 2000 Mr. heater option as to fiddly. Currently thinking solar bag showers in summer and sponge baths in winter most likely.

    1. You might want to consider a Wabasto Dual top Evo, as it has a built in tank that automatically drains if it gets near freezing. And pretty quick and getting the water heated up again in the small built in tank.

      The problem with an isotherm tank is even if you insulated it in the caravan, eventually it will freeze, if you’re in really cold conditions for an extended period. However driving would warm it up, and the heat exchange system with the engine would be anti-freeze, so it might be fine.

      Note I’ve never used these, I’ve just been researching them myself.

    2. Tim, if you knew you could keep the Isotemp powered while you were away, and always plugged in when you were in your driveway, then it could work out. BUT there will be the one time where you forget to plug it back in when you get home. You say you don’t want to use shore power, but that’s by far the easiest way to keep the tank frost free at home (heat the tank, not the van).

      Draining is the only other option with that type of tank, and it is fiddly because the air valve is hard to get to and is basically a Philips head screw you have to turn to open it.

      An on-demand heater might work better for you, but you will still have the issue of the water that supplies the tank freezing up. I considered some crazy system to heat the water, then drip feed it back into the water supply tank to keep that from freezing, but it seemed like too much hassle.

      In the end, if you want water in the van you’re going to have to keep the tank(s) at least above freezing point. Your sponge bath won’t be much fun with a frozen sponge!

  6. Thanks both of you above.

    Talking leaving the van for 48 hours at 15 degrees Fahrenheit. I have 448 amp hour AGM batteries and planning on 20 gal cold water tank. With the “cheap digital thermometer” and 12 v hot pad system I should be able to keep my tanks warm for a couple of days if I am parked at a ski resort and staying at a friends place. Especially if I enclose them in a reflextic insulated plywood box. Or am i wrong?

    Say 50 watt 12 volt hotpad runs for half the time or 24 hours – draw is 50W/12V x 24 hr = 100 amp hours. about 50% of my available power. Just got to be confident the hot pad and theromostat are reliable.

    Van is insulated with 1.5 ‘ thinsulate giving a few hours grace. I would also (if I was smart) arrive with the Isotemp tank heated up during the drive thus giving off heat inside the water tanks box.

    At home (-3 F for 5 days would be a cold snap) just plug it in, (not that big a deal) and have shore power (or in future solar panels) keep the battery bank topped up. Am I wrong?

    May as well just plan on installing an Isotemp tank right? … given that the freezing issue can be handled with battery power on board.

    If I decide to stay for four days start the van up, heat up the interior and turn on the hotwater tank and then let it cool down slow. Might be good to have a thermometer inside the tank box to know the temp.

    Love to hear your thoughts…

    1. Tim, starting the van up won’t give you much heat unless you drive around for a while. The diesel engine takes a long time and some actual load to warm up. If you’re running engine coolant to the hot water tank then it should probably just about get the water hot. I think it would be hard on the alternator to heat the tank electrically when the engine is just at idle.

      The cold water tank should stay above freezing for 48 hours if you have the 12v heating pad on it. The isotemp doesn’t give off much heat – it’s really well insulated. Of course, it’s giving off some but not really enough to help warm the cabinet it’s in.

  7. Nice write up! Do I understand that you heat the Isotemp slim square unit with just your battery pack? If so, what size is it? Do you have any other appliances on it?


    1. Ryan, we can either use shore power or the house battery to heat the isotemp unit. We have a 700Ah / 9kW lithium battery running through a 2kVA inverter. We have a microwave and an inversion cook top as well, but we don’t run either of those at the same time as the water heater.

  8. I’m installing the Isotemp Slim Square in my conversion right now. Did you cap or seal your unused engine coolant connectors in any way?

    Thanks for all the great write ups on your site — we’ve referenced a lot of your content in our build.

    1. Chris, I’m glad you’re making use of the write-ups. We did not do anything special with the engine coolant connectors. We just left the plastic covers from the factory in place.

  9. Hey!

    Any update on how long of a hot shower you’re able to get with the 4.5gal Isotemp tank? Considering that or the 6 gal. I assume with the thermostatic mixing pre tank and the ultra high heating temp (160 degrees right?) you can get quite a bit. Any data you might have gathered would be much appreciated!



    1. I bought a 25L Isotemp tank (6.6 gal) and I get 3.5 minutes of full pressure hot water through a standard household showerhead, using a sureflow 11litres/m pump. I’m using a mixer tap, so it’s easy to use half pressure and turn it off/on without loosing the set temp. Well worth using one! In America I usually only see the two tap mixers.

    2. Kipp, we typically get two navy showers with no reduction in water temperature, using a low-flow shower head to conserve water. There’s enough hot water left after that for doing the dishes, etc. We never bothered running tests because it never ended up being an issue.

      If we had the room, we would have gone for the 6 gallon unit but in retrospect it wouldn’t have been necessary for how we use our hot water system.

  10. Just wondering, a few years down the road from setup with the isotemp slim if you still like it and if you ever figured out how to use the isotemp as a diversion load for the solar panels or the alternator when the batteries are full. We are trying to limit our batteries to 400ah lithium and have the isotemp and an induction stove. Assuming we aren’t using both every day it seems do-able, especially if we get the diversion load system working.

    1. The Isotemp Slim has been great. If we had the space/a better design, I’d use a regular cylindrical Isotemp instead, more from a purchase cost perspective than a volume of water perspective.

      We never spent the time to figure out a diversion load, but we feel very confident switching the heater on during the day when it’s sunny out and we have relatively good state of charge. It heats up pretty fast (a couple of hours max) and then just clicks in and out as its thermostat decides it needs a top-up.

      Our battery capacity is overkill. I’m pretty sure that 400Ah lithium would be sufficient. With the diversion load as well, you’d be absolutely fine!

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