Jetted or Whirlpool Bathtubs

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Add A Second Hot Water Tank or Upgrade The Existing Hot Water Tank?


Okay, so you've decided you want a bigger shower with multiple heads, a bigger bathtub or a jetted tub, and you suspect your old hot water heater might not be up to the task of providing enough hot water for you. I get asked these two questions all the time:


 Should we upgrade our existing hot water heater?


 Should we add another hot water heater?

Well, first off you have to figure out if you are going to need more hot water then you current system is going to be able to supply. There is always the wait and see method and there is the do some simple math and make a guestimate method.

First you need to figure out how much water you need then figure out how much water your tank is going to supply.

For shower heads, they all will deliver a maximum of 2.5 gallons per minute or less. So 4 heads equals 10 gallons per minute. Multiply that by the length of time you plan on showering, let's say 20 minutes. That would equal 200 gallons. 

Now there are a lot of variables that go into figuring out how much hot water flow you are going to get. The temperature of the ground water feeding the tank is an element of the equation. Average ground water temperature is 50 degrees, and most people like to take showers with water somewhere between 105 and 120 degrees. The age of your tank, whether it is gas or electric, the recovery time of your tank and even the insulation or lack of on your pipes! 

So using our example of our 4 head shower we need 200 gallons of water, now not all of that amount will be coming out of the hot water tank, you will be mixing cold water with it. So a good rule of thumb is figure somewhere around 60-70% of the water will be coming from the hot water tank on average. Then you have to figure in that as hot water comes out of the tank, fresh water will be coming in and it will be heated so you should end up somewhere around a figure of 40% of the water needed (200 gallons) * 40% = 80 gallons. So your 40 gallon tank probably won't be doing what you want.

For a jetted or larger soaking tub figure about a 70% number to figure what your tank can deliver. So a 40 gallon tank can deliver about 28 gallons of hot-hot water then you are also mixing in some cold so a 40 gallon hot water heater should be able to deliver close to 40 gallons of hot water give or take some. If your tub is a 65 gallon tub, which is about average for a good sized one, your 40 gallon hot water heater probably won't be doing what you want.

So what to do? Upgrade the existing tank to a bigger one or add a second one?

The definitive answer is - It depends.

Do you have room for a second tank? If not then upgrading the existing tank is the only answer.

If you have room there are other issues such as feeding power to the tank such as gas, there could be a slight chance you might be maxing out your gas flow with your current sized gas line, it's a possibility, a slight one, but a possibility depending on a lot of factors. Have you added a gas fireplace? Converted an electric stove to gas? This could cause you a problem now with adding another gas hot water heater. An upgrade to the gas line from your meter might be required.

There is the flue issue. You might now be required to upgrade the size of the flue to accommodate the second heater.

How old is your tank? If it's on it's last legs then adding a 2nd tank just means you will be replacing the first tank pretty soon too.

What would I do?

I would always try for the 2nd tank if it's possible. Some of the reasons are because two tanks are always going to give you more hot water then one large one. A 40 and a 50 is bigger than a single 60. A second tank if we have the plumber do it right means you are never going to be out of hot water due to a tank dieing on you. You simply turn a couple of valves and temporarily take the dead hot water heater out of the loop and by pass it, you run on the single tank until at your leisure you replace the bad tank. 

What we like to do is hook them up in series, not parallel, you use the old tank to feed the new tank. The new tank would be a quick recovery hot water heater, somewhere around 50 gallons. You turn the old tank temperature down, and it feeds preheated water to the new hot water heater, the old tank's life gets extended by a large amount since it isn't working so hard anymore and the new tanks life gets extended since it isn't either since the water being fed to it is already pretty warm, it might only be raising the water being fed to it by 40 degrees now instead of 60 or 70 as before.

You can dial them in over time, fooling about with the temperature settings on the feeder tank and seeing what works the best. If you want to take a hour long shower some time, you might need to dial up the first tank. Regardless you are going to get a ridiculous amount of hot water using the two hot water heaters in series method, a very long life to your heaters and peace of mind of never being out of hot water if one dies on you.

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