Water Pressure Question

I need help in figuring out a water pressure question. I would like to install a pressure tank to store water in my garage. The tank would have an inlet where the water would flow into the tank from the a city water source and would be around 60psi entering the tank. The tank would be completely full of water and the water would flow out an outlet from the bottom of the tank into my water heater and then into my house. The tank would only be used in an emergency situation and would have shut off valves/ bi pass valves before the storage tank and a spigot at the bottom of the tank to use the water if needed. The tank will hold approximately 250 gallons and will be 30" wide by 72" tall. The inlet pipe and outlet pipe are both 3/4".

I am concerned if the water enters the tank at 60 psi it will not leave the tank at 60psi. I think it would increase but I have heard the pressure will decrease due to the size of the tank.

Will the pressure increase or decrease?

Is there anything that can be done to maintain the pressure through the tank and system with out adding anything mechanical?

This is just not a concept I understand and want to make sure the system will work before spending money on a pressurized storage tank.


If the inlet and outlet are at the same level, and the pipes are the same size, the outlet pressure should be the same as the inlet pressure (within a very small margin of error).

Does the manual say that the water will be stored under pressure when bypassed?

Well you specifically said “pressure tank”. I am not a plumber but a pressure tank implies that the tank itself provides the pressure of the water.

Most water pressure tanks are of a bladder design. Inside the tank is a rubber bladder. On the bottom side is water and on the top side of the bladder is pressurized air. The tank is pressurized at the time of the tank installation, and the air pressure can be set at that time. Usually lower than the water pressure at the inlet.

If you have this type of tank then the pressure as you set it, and I am quite sure at less pressure than the inlet pressure.

If you do not have a pressurized tank, then the water pressure is either at city pressure, or at zero if there is no city water available.

I misspoke when I said pressurized tank. The tank will not provide the pressure it is just a tank that can hold the pressure provided by the city water line (60psi).

There is no manual. I am just trying to design a system that will work. When the water is bi passed the tank will just gravity drain. I am only concerned when the water is flowing through the water tank into the home and if it will maintain pressure.

Under static condition (no flow) you will have no pressure drop. Under maximum flow you will have a small pressure drop due to the extra pipe you will be adding when the tank is installed. If this is for a single family dwelling, this may not be noticeable with 3/4 pipe.

Put a soft seat check valve at the inlet. You will have a small head (pressure) loss due to the valve, but the tank will charge to the peak water pressure in the middle of the night, and sustain that as long as it is able.

Without a check valve, your tank will backfeed and supply your neighbors when the pressure drops.

When the city supply pressure drops, the check valve will close, and the tank will supply your house. As you use the tank, the pressure will drop as the air in the tank expands to push the water out.

Pressure tanks have Schrader (tire stem) valve to adjust the air pressure in the tank. If you set this on the low side, the tank will store more water, but the pressure will be rather low as you reach the end of the tank. If you set the pressure a bit higher, it will maintain pressure better, but storage volume will be reduced.

This will work if you have unreliable water supply. If you are needing more pressure than the city supplies, then you are wanting a boost pump as well as a tank(so the pump does not have to run constantly)

What is your existing problem in the first place??
Water stored for emergencys will be non-potable within a relatively short time. Then the water would need to be treated or filtered.
If its just toilet flushing, then a barrel in the garage and a bucket will do it.

If you want pressurized water in the event the city can no longer provide it you would indeed need a tank that can provide pressure. If the tank only holds water and the city pressure brings it to 60psi, when the town stops providing pressure the tank would only be able to provide a gallon or two at pressure before you were limited to what gravity can provide for pressure.

A actual pressure tank in the 250 gallon range is a very specialized item and would be much larger than your dimensions in addition to being cost prohibitive.

If it is a sealed tank and can hold 60 pounds pressure it would not impact your water pressure if it was inline with the city feed.

Without an air compressor attached to the tank to provide pressure at ground level, wouldn’t the tank need to be installed on the roof? Plus, at ground level, the tank with corresponding air compressor for pressure would need electricity, so in case of emergency where water and electric are lost, a gas powered generator would also be needed. Isn’t that correct?

I think the OP really needs a professional to come assess and give an estimate. Usually the estimate process is free, and you can kind of use them for information if you’re not too obvious about it, if you’re really determined to try to DIY.

No, that’s what the bladder does.
City water pressurizes the air in the bladder, which then expands to force the water out when needed.

Think about it this way: If the water in the tank was at 60psi - why would water from the main line flow into it since it is also at 60 psi (the pressure difference is 0)?

A simple workaround will be to have the tank in parallel with a small pump on the discharge side of the tank ( you can even put in a submersible pump). You’ll need to manually (or you can automate it) set the system so that the water from the tank is used out every X number of days and the tank is refilled with fresh water.

Depending on the water hardness, be prepared to clean the tank every couple of months or so since you have a lot of sitting water and it will have deposits.

Without a pump, you cannot maintain pressure, you must lose SOME .
I think with the emergency water tank, you will be happy to lose MOST, just to have the water flowing, even its just a little trickle that takes 30 seconds to fill a cup.

For a water tank to maintain 60 psi, the water that you want to use has to be … 140 feet above the outlet where you want 60 psi… you aren’t going to do that.

The reason the town water supply is at 60 psi is so there is pressure to run to the 5th floor of a building - taller buildings need their own pumps.
What works for billions of folk around the world is to have a water tank in the roof space or above the roof.

The drop of a couple of feet is enough pressure to get a regular flow out of it, it gives a reasonable flow even on the floor immediately below the tank.

What happens with a tank at ground level is that your plumbing in the house is above a lot of the water , so that water won’t get to the house, you’d have to go out to get the water. (in a bucket).

If the tank is completely full of water as soon as the city quits supplying the pressure the pressure will drop to just the height of the water in the tank. You are going to need a bladder or expansion tank. And the pressure will drop as you use the water.

Without an external pressure source, the pressure at the outlet will be the weight of the water above the outlet.

This is why municipal water tanks are elevated - they use pumps at night (when electricity is cheap) to lift the water. During the day, the weight of the (lifted) water provides the town’s water pressure.

Run your filler pipe straight up - see how far up the water comes.
That is the max usable height of a tank without inlet pumps.

Huh?? Why would the water no longer be potable? It was clean and is stored in a clean tank. Since the tank’s flow keeps the water in it clean and fresh, why would it get contaminated very quickly?

Also my water meter has a back-flow prevent-er valve in it. My water can not flow backwards out of my meter into my neighbors house. Thus, if I had an installed water storage tank in the water line to the house, it can not drain out into the neighbors piping through my meter. This is a requirement here.

There seems to be some confusion as to how pressure tanks work, though it has been stated correctly. The water pressure 60psi, compresses air in the tank at a lower PSI till the air in the tank reaches 60psi, then no more water can enter the tank. If the city water supply goes out and you have a working check valve preventing your water supply from backdraining, you have 60PSI waterpressure. As you use your water the compressed air will provide the pressure, but will drop in pressure. So after flushing a toilet you may now only have 55PSI available This would continue till the pressure tank is empty of water and then you will suddenly lose all water pressure (so it may go from 20PSI to 0PSI right away)

As mentioned there is a schrader valve so you could add more air to boost the pressure back up again. In your application you might not want a air bladder at all, or have practically no air in the system. This way your tank is totally full of water, when you want to use the tank add air then (you will need air several times, but it will get you full capacity of the tank. The down side to this is even greater stagnation of the water in your pressure tank (see below)

It’s a big dead end in the line, lots of water with no where to flow. There is no separate outlet and inlet in most tanks, just one pipe that serves both directions. Small variations should provide some exchange with fresh water if you have air in the system, not so much if you don’t.

Additionally if the water does go bad in this pressure tank you need to consider that this bad water may also gently constantly feed into your piping system.

A better design may be a large water holding tank, a small pressure pump and a small pressure tank in a system isolated from your main water supply by a valve that can be turned on and connected in that type of situation.

This assumes the valve works 100%

That is all the OP is looking for. All this talk about pressure tanks, etc, is ignoring that the OP was never looking to run his whole house off this tank.

If the tank is not pressurized and the city water pressure quits, you will have no pressure after about the first toilet flush. It might not take that long if there’s a water main break and you don’t have a check valve in the system as any stored pressure would flow out the tank.

Water, like most fluids, does not compress at anything as low as 60 psi. Heck aircraft hydraulic fluid doesn’t compress at 3000 psi. So the second any of that fluid is used, such as flushing a toiler or getting a drink of water, the pressure drops dramatically.

What you could do is have an air bubble at the top of your tank/reservoir which would maintain some pressure as gasses do compress. If you wanted to push out all the water, you’d probably need about one third of the tank to be air. Even then the pressure would drop greatly towards the end and might not get water to a second floor bathroom.

But here’s what you can do: put the storage tank in the attic! Let gravity do the work. This is very common in England as they don’t have many water towers. Each home keeps a reservoir of water for when the pressure drop. Some issues.

  1. Water weighs about 7 pounds a gallon. At 60 gallons you’d be talking around 500 lbs with the water, tank, and plumbing.

  2. Does your attic get below freezing in the winter. Is so, that’s an issue.

  3. In England you are only suppose to drink water from the cold water tap in the kitchen. It’s hooked directly to the water mains and shouldn’t be getting water from the tank in the ceiling. They find things like dead pigeons in those tanks all the time as they are usually something similar to a plastic swimming pool with a plastic cover on it.

To clarify the tank will be inline so there will be a constant pressure of 60psi going into the tank. The tank will only be used in an emergency. Living in a desert there is always a concern during the summer that if there is an issue with the city supplied water that we would not have any water, which is an issue when it is 110 degrees outside. The water will not be used for showers, toilets, etc, it would only be used to drink, make food etc.

I believe the water would continue to circulate because it is going in on the top and exiting on the bottom of the tank.

Thanks for all you ideas and opinions!

If you are only looking for water for cooking and drinking this seems like it’s more complex and expensive than it needs to be. Why not just put in some shelves and fill them with bottled water.

I wouldn’t want a tank in the water supply if it’s not absolutely needed. It’s an extra point of failure and provides a home for bacteria to grow in. If you do go with a tank I’d make sure you plumb in in such a way you can introduce bleach to the system and shock it once a year or so.