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People often ask me if tiny houses can have holding tanks. I would like to give more information on this, based on the homes we have built and some research I have done.
So, can tiny houses have holding tanks? A tiny house can have holding tanks like RVs. Many tiny houses have composting toilets, if that’s the case they will not have a black water tank only fresh water and a grey water tank.
My husband and I have been living tiny for several years now and we want to share our experience. My husband has built us three homes and has helped build hundreds of tiny houses for other companies.
The Types of Holding Tanks You Put in Tiny Houses
Tiny houses on wheels (THOWs) can be set up similar to RVs. You can have holding tanks in your tiny house. You will need a freshwater tank, a grey water tank, and a black water tank if you have a normal flush toilet. Typically they get RV tanks.
If your THOW has a composting toilet then you will not need a black water tank, only a grey water one. I have written more information on toilet options in tiny houses here. You will still need a grey water tank for showers and sinks.
You will need enough storage in your grey and black water tanks to hold everything from your fresh water tank. If you have a 50-gallon fresh water tank, your grey and black water tanks should equal 50 gallons total.
The used water tanks should equal out to be 70% for the grey water and 30% for the black. For this example, if you have 50 gallons of fresh water you should have a 35-gallon tank for grey water and a 15-gallon tank for black.
That would mean you should have room for 100 gallons of water storage.
What Size Holding Tanks Should You Get
Most people go with 40 gallons of fresh water storage. This is great for backup. If you are living off-grid, this is not a lot of water. We have found that 40 gallons will only last us a day or two.
A better option would be 70-100 gallons of fresh water storage. If this will be your main source of water than a bigger tank will last you longer and cause less hassle than having to fill up every two days.
When buying an RV water tank you should calculate the actual water storage. On average the tank will hold 7 gallons less than the actual size because you will need an air pocket to keep it from pressurizing or running out of the burp tube.
We bought an RV tank that was advertised as 42 gallons. And it does hold 42 gallons of air. But we can only put 36 gallons of water in it. RV tanks are the easiest options to use because they are already set up for the plumbing in your house.
There are other tanks you can buy that actually advertise the amount of water you can put in them. For example, we bought another tank that said it was 100 gallons of water storage. The tank size was about 115 gallons, but you could only put about 100 gallons of water in it.
However, these tanks are not set up for plumbing, only general storage. It would take someone skilled to tie one of these tanks into a normal plumbing system and do it correctly. That’s why it is usually best to go with an RV water tank.
The reason most people go with a 40-gallon tank is to save space. If you are going to be living full-time in your home you need lots of storage. However, we have found that it is much more beneficial to have sufficient water storage and lose some space when living off-grid.
This also varies depending on how good your shower head and water faucet are at saving water. We try to get the most water-saving options possible. If you can find a shower head that is a gallon or less per minute that’s your best option!
Please note that if your tiny house has a drop axle trailer with black and grey water tanks, you will need to be careful when traveling. Given how low these trailers sit, your tanks are very close to the ground and you need to be cautious when pulling in and out of stops.
Hauling Water
If possible, it is best to have an additional outside tank, or two to haul water to your house instead of always moving your house to fill up. If you can keep a lot of water storage outside of your house and fill that up, it makes life much easier.
We lived in Utah for a while in our tiny house and we had an extra tank to fill up our holding tank. Be careful of winter. In winter the water in our tank froze and we couldn’t use that tank until the weather warmed up. There are ways you can keep an outside tank warm too.
Currently, we are living in our 112 sq ft converted cargo trailer and we have a 42-gallon RV tank, holding about 36 gallons of water. We bought a 275-gallon ICB tank where we can haul water.
Keep in mind that when you fill up 275 gallons of water it weighs over 2 thousand ponds! You will need a heavy-duty truck that can haul this, and you will not be able to move the tank or unload it until it is almost empty.
It is also a very bad idea to haul a tank like this when it is half full. You would have a thousand pounds of water sloshing from one side of the truck to the next. This is very dangerous and not easy.
Ideally, you should have two of these tanks. You can fill up the one and come back to pour that into the other one on the ground. Then you can remove the empty tank from your truck. Although it is best to have a gravity-fed method to fill your water tank since you will not have pressurized city water filling it. So you should still try to have the second tank elevated.
Can I Use Water While I am Traveling?
This depends on the type of electric system you have in your house. If you do not have solar and a battery system, then your house will not run unless you are plugged in. So you will not be able to use water while traveling down the road, because you will not have electricity to run the water pump.
RVs can still work when they pull over on the side of the road because they are running off of the battery tied to the motor. Tiny houses do not do this. If you will be traveling a lot with your tiny house then it would be ideal to get a solar system.
Otherwise, if your house is going to be stationary and you can plug into a power source, you would not need solar. If this is the case then anytime it is not plugged in you cannot use any power, which includes the water since you need a pump if you have a holding tank for it.
Where to Empty Your Tanks
You can empty your tanks at any RV dump station. These can be found at RV sites, some gas stations, and even some truck stops or outdoor sports shops. There is a small fee to dump, usually around $5-$10.
If you have your own land, some people set up a french drain for their grey water. But you would still need a sewage system or to take your house to a dump if you have a regular flush toilet.
Related Questions
What size water pump should I get for my tiny house? Most tiny houses use a 3.1 gallon per minute water pump. They can come in 12-volt or 120 VAC. If you have solar you should use the 12-volt option, or you can also use a 24 or 48-volt water pump, although they are harder to find and more expensive.
What plumping pipe size should I use in my tiny house? Typically tiny houses use a 1/2 inch pex plumbing pipe. Some people use a 3/4 inch main trunk and branch off to 1/2 inch, this will help maintain water pressure when multiple fixtures are on due to a higher volume of water in the pipe.
