Caravan Water Tanks - Full or Empty for Storage?

💦 Caravan Water Tanks FULL or EMPTY when Stored?

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Whether or not you should leave the caravan water tanks full or empty when being stored, is not something you even think about when buying a new van. But, the question will no doubt pop up a little further down the line.

When you are in between trips, it’s important to come up with a water storage system for the caravan. In other words, is it best to keep the caravan water tanks full or empty when stored?

There are two different schools of thought, where some say that leaving water in the caravan tanks may encourage algae growth, while others believe that leaving the caravan water tanks empty is the true cause of mould build-up.

Personally, we always left the tanks full when we were staying somewhere on mains water for an extended period, even though we knew that we wouldn’t be needing the tanks. But, I have questioned whether it’s the right move or not. None of us have ever gotten sick (touch wood), but it’s always good to be armed with the cold hard facts.

In this article, both of the myths are put to the test using scientific reasoning.

Here’s the short answer to the question of whether or not you should leave your caravan water tanks full or empty for storage…

It really doesn’t matter whether you decide to store your caravan water tanks full or empty because bacteria and algae can grow both with and without the presence of sunlight or oxygen.

Even if you fully empty your tanks, it’s not possible to completely empty them of all moisture (unless it air dries over a long period of time). Just like if you leave your water tanks full, they will never be completely full as there will always be an empty gap at the top.

Let’s delve deeper into why leaving the caravan water tanks full or empty doesn’t matter as we thought.

And, if you’re parking up your van for the off-season, check out these 30 Tips for Caravan Storage.

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Should You Leave Water in Caravan Tanks?

The main piece of advice that you’ll hear on the subject of water tanks while storing the caravan is that you should keep them full. The theory being that by eliminating AIR and SUNLIGHT from the tank, “nasties” such as mould, mildew and algae aren’t able to grow.

Let’s look at the science behind the theory…

AIR: Do Bacteria & Fungi Need Oxygen to Grow?

The need for bacteria or fungi to have oxygen for growth is a common misconception. Let’s explore how these guys can still grow in your caravan water tanks even if they’re full.

There are two types of bacteria:

  1. AEROBIC – Bacteria that needs oxygen to grow
  2. ANAEROBIC – Bacteria that can survive without the presence of oxygen

I’m no scientist, so I won’t expand on it here, but if you want to delve deeper, have a look at the physical requirements of bacteria.

The most typical complaint by travellers who get sick from water is caused by the bacterium E. Coli (gastroenteritis). Funnily enough, E. Coli is a facultative anaerobe – meaning that it can grow if oxygen is present, but it also has the ability to switch to fermentation if oxygen is absent.

Okay, so we’re getting a little technical here, which is not my favourite kind of rabbit hole, but it’s easy to see that not all types of bacteria and fungi need oxygen to grow.

❗️ MYTH: Bacteria and fungi need oxygen in order to grow
FALSE: Some bacteria and fungi will grow even without oxygen being present

SUNLIGHT: Will Bacteria & Fungi Grow Without Sunlight?

Sunlight Forest

Each bacterial and fungal species have their own optimum environments for growth. Some like acidic conditions, while others prefer alkaline or neutral conditions. Some get their energy from sunlight, while others don’t need it.

The fact is, many bacterial species will adapt and are flexible enough to grow with or without sunlight.

❗️ MYTH: Bacteria and fungi need sunlight in order to grow
FALSE: Not all bacteria and fungi need sunlight to grow

Storing the Caravan with Full Water Tanks

Inline Water Filter
Filling up the Water Tanks

Even if we go with the theory that says keeping the tanks full reduces the risk of bacterial and fungal growth, there is still a margin of error.

It’s impossible to completely fill up the water tanks. There will always be that bit at the top that doesn’t fill up and there will always be air bubbles in the water.

❗️ MYTH: Keeping the water tanks full reduces bacterial & fungal growth
FALSE: Caravan water tanks are never completely full

Storing the Caravan with Empty Water Tanks

Some people swear by emptying the tanks while their caravan sits in storage between holidays. But it’s really tricky to completely empty them of all moisture.

There will still be a puddle of water stuck in the bottom of each tank, which won’t run out through the tap or the bung. Plus, there will still be droplets of moisture on the roof and walls of the tank, which will take a long time to evaporate unless you live in a very hot, dry climate.

Empty the Caravan Tanks
Emptying the Caravan Tanks

Creepy Crawlies

To dry the tanks out completely you’d need to leave the caps off to allow all moisture to evaporate. But then you risk the chance of creepy crawlies taking up residence in your tanks. You could put a bit of mesh over the opening to reduce which critters can get in there, but it’s not foolproof.

Dry Scale

Other people have reported that the scale on the inside of the tanks dries up and then flakes off. The problem with the ‘flaky residue’ is that it can clog up your fittings and compromise the future cleanliness of your drinking water.


Another thing to consider when looking at if you should you leave water in your caravan tanks, is your local climate. If you experience very cold winters where the pipes may freeze, then emptying your tanks is the best bet. Otherwise you’ll probably find yourself having to replace burst pipes before you can hit the road again. Don’t forget that this includes your Hot Water System.

❗️ MYTH: Keeping the water tanks empty will eliminate bacterial & fungal growth
FALSE: Caravan water tanks are never completely empty unless you live in a very hot, dry climate
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Cleaning Your Tanks

In the four years that we’ve been living in our caravan, we’ve never once cleaned the tanks with any cleaners or solutions.

I’ll admit, before we knew anything about ‘tank health,’ we considered using the Milton Steriliser Tablets designed for baby bottles and the like. In hindsight I’m glad that we never did. Word on the street is that your tank water will taste like antiseptic for yonks afterwards – no thanks mate!

There are other products on the market such as Tank Cleen, but we’ve never tried them so I can’t comment on those.


Household Bleach

Using household bleach (unscented with no additives) is the best and cheapest solution for cleaning caravan water tanks. Bleach is widely used for cleaning water and water storage vessels as it will kill up to 99.9% of the nasties.

Cleaning Caravan Water Tanks with Bleach:

  1. Empty the tanks & water lines
  2. Add bleach to the water tanks
  3. Refill the tanks with water
  4. Turn on the taps to fill all of the water lines
  5. Leave for 24 hours
  6. Empty & flush the tanks
  7. Refill the water tanks for use

For the full guide to cleaning your caravan water tanks, plus correct dilution ratios, check out the article below.

Cleaning RV Water Tanks →
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Will the Water Stay Fresh in the Tanks?

I guess when you look at how many people around the world drink from rainwater tanks every day, it makes you realise that leaving water in a tank is not that big of a deal.

I mean, do they fret about the quality of their water every time they go away for holidays or if the tank has been sitting half empty as they await the next downpour to fill it back up again? I think not.

Outdoor Tap

If you really want to know about the exact condition of the water in your tank you’d have to consider the following points:

  • What temperature has the water been sitting at?
  • Were there pre-existing germs in the tank?
  • What was the quality of the water source when you filled it up?

Chances are you’re not going to be able to answer those questions.

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Tips For Keeping Your Drinking Water Great

Drinking Water

The most important take away point is that the quality of water going into your tanks is the key to good tank health.

  • Always fill up your tanks from good quality water sources
  • Filter your water as it goes into the tanks with an inline filter (eBay)
  • Filter your drinking water at your sink with an under-bench filter (eBay)
  • If the water in your tanks has been sitting unused for a while, empty them out, flush the tanks and refill
Finding Water on the Road →
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Should You Store Your Caravan Water Tanks FULL or EMPTY?

Storing your caravan water tanks full or empty comes down to personal preference. There is no scientific evidence to support one over the other because bacteria and algae can grow even without sunlight and oxygen, especially if moisture is present.

Here are some general take-away points:

  • For warm & dry climates – either empty tanks and allow to air dry OR keep full (replacing with fresh water before use)
  • For humid climates – either empty tanks and allow to air dry OR keep full (replacing with fresh water before use)
  • For sub-zero climates – empty water tanks so pipes don’t freeze & burst

Wise words from a reader…

Bacteria cannot grow without nutrient. If you use clean, disinfected water (town supply – no nutrient), there is no chance of algal growth. Town water should have 1 to 3 ppm residual chlorine (you may smell it), and this keeps the bugs out! Also light… Make sure all your pipes etc. are lightproof. Fill your tanks.

– Ken
Towing with Full vs. Empty Water Tanks →
Caravan Storage Checklist

Caravan Storage Checklist

Prepare your van for long-term & short-term storage with the Caravan Storage Checklist.

  • 100 pre-filled tasks to complete
  • Checkboxes to tick off
  • Add in extra tasks
  • Download once, use over-and-over again
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14 thoughts on “💦 Caravan Water Tanks FULL or EMPTY when Stored?”

  1. A couple of points, I agree with Kens comment that the residual chlorine will take care of the bugs. This is true but remember the residual chlorine will disipate over time.
    I store my van with fresh unfiltered town water to put fresh chlorinated water in the tanks. When going on a trip I refill the tanks with fresh chlorinated water. I do not use my filter for these actions as it filters out chlorine.
    I do use my filter on the road as you generally don’t know the quality of the water.

    1. That’s actually a really good point. The chlorine in the town water should act to your advantage when storing water in tanks. I hadn’t thought of that – thanks!

  2. So much of biology but it was helpful to understand. One has to really take good precautions for keeping it empty for long periods. I agree that is best to fill with good quality water always!

  3. I would definitely prefer the natural cleaning products for the tank. Vinegar and bicarb are good for so many cleaning jobs. Filtering the water is always best so there are no nasties in your drinking water, or you could boil the water and cool it before drinking.

  4. I am sure this is a question that many people ask. So good to have it covered in such a thorough manner. I guess I would have thought that leaving the tanks would be good for a short period. But longer periods I would want to consider the things you have covered. Good idea to clean the tanks with natural products.

  5. It’s good that you’ve never cleaned your caravan’s water tank with chemical products. Cleaning it with a homemade “natural” mixture of bicarb soda and vinegar is definitely less dangerous for health. Even when cleaning our home, we always use vinegar instead of chemical stuff.

    1. Same here. I don’t like chemicals, so I’d never want to introduce them (beyond what’s already in the water) into our drinking water.

  6. This was really helpful! My partner and I purchased an antique 1965 cree 16ft/5m a few years back. We are looking into different mobile home options, this isn’t something we even thought of until now, so thank you for the tip!

    1. Not a worry. It’s definitely one of those useless facts that wouldn’t cross your mind in a million years… unless you need to know it.

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