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Water heater too small to fill jetted tub


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Last week I found a water heater that was too small to fill the jetted tub in the master bath. I discovered it by sheer dumb luck. I closed the drain, opened the cold water faucet and went off to inspect other areas. When I walked back into the master bath, I noticed high humidity in the air. I felt the water coming out of the faucet and it was cold. The water in the tub hadn't yet covered the jets and was lukewarm. It turned out that the hot and cold were reversed and I inadvertently filled the tub with hot water, which ran out long before the tub filled. The water heater was a 50 gallon direct vent gas unit.

That got me wondering how many times I missed a similar problem in the past. Yesterday I had the same size tub and same size water heater. The seller was tailing us. I usually request that they don't, but the guy was OK and not trying to interfere, so I didn't say anything. Since he heard me explaining the possible problem to the seller, he piped up and claimed that he hadn't used the tub in five years. I wonder why. I should have just gone ahead and filled it with hot water to see if there was enough hot water capacity, but I didn't.

Two within one week made me wonder even more how many I've missed in the past.

Does anyone else compare water heater size to tub capacity? Is there a rule of thumb? I looked online for the capacity for yesterday's tub, but didn't find it. I'd guess it was 75-100 gallons. I did find the installation manual, but there was no recommendation for a minimum water heater size.

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If you had mixed the hot and cold water, as someone normally would when using the tub, it may have been OK at this time of year. However in my neck of the woods, in the winter time when the incoming water temperature is down in the low 50s, a 50 gallon tank won't cut it. Two 50s in parallel is a good setup for these big soaker tubs.

I make mention in my reports about only having one tank with a big soaker tub.

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I don't think that you will find any recommendations on water heater capacity to tub size because there are too many variables (gas vs electric / tank vs tankless vs the new hybrid systems). On conventional gas water heaters a 65 gallon or two 50 gallons in series or parallel would be best. A single tank-type water heater of 50 gallons or under will almost never be enough (unless they have an inline heater on the tub...another one of those variables).

In my opinion this is where an appropriately sized tankless gas water heater shines. Always enough hot water to fill the tub without the static cost of heating a couple of tanks of water.

BTW, the Kohler catalog that I have lists that most soaker tubs that would fit into a standard tub space have a capacity of about 50 to 70 gallons to the overflow. Deck mounted tubs that can potentially fit two people run about 120 gallons to the overflow. They do list one model as having a 170 gallon capacity (83" long by 64 1/2" wide.) called the Super Bath. Looks more like a wading pool to me.

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I always run a tub as if I were using it normally just to see if it will fill hot. Many do not and any large tub with a 40 gallon tank is suspect even if it does. I always let people know that when Mom takes a bath, no one else uses any hot water or you will be in it, "Hot water" I mean

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Always pipe dual hot water tanks in parallel. This way both tanks get the same amount of wear.

Not necessarily true. If you are adding a second tank to an existing system, such as when there's a new addition added, they can be plumbed in series if they are not the same BTU/Capacity, in which case the one with smaller capacity and less Btu/Hr is in second place.

ONE TEAM - ONE FIGHT!!!

Mike

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I note tiny water heaters all the time, especially with these rain head side sprayer turbo showers that blow through about 8 gallons a minute when you have all the heads spraying........it takes about 10 minutes to empty a 50 gallon tank.

I've never understood the series vs. parallel conflict; I've got a couple water heaters that are in series for the last 8 years; I was too lazy and repipe in parallel.

They work fine. In fact, I think they work better, as one is sort of a "pre-heater" for the second tank.

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I respectfully disagree with Terry on never running a dual water heater system in series. My reasoning is that if the house is large enough to require two water heaters then they will be used roughly the same amount with the second tank in the series getting less use, but not enought to make an appreciable amount of difference in the lifespan between the two.

I have also heard that if you run water heaters in parallel and the distance from the outlets of the heaters to where they connect together is not equal distance (or close to it) then water will be drawn unequally from among the tanks. Not being a engineer specializing in hydrodynamics I do not know if this is true or just an old plumber's tale.

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Some time back we had a discussion about this, either at the ASHI board or here. There was an inspector that had found a two water heater system and the first one was trashed while the second one was fine. Turns out the home owner had a big soaker tub.

Here is what happens when the heaters are piped in series: The cold water enters the first heater and then the hot water from the first heater goes into the cold water side of the second heater. The first heater is practically drained before the second heater fires up.

Most folks use these big soaker tubs in the winter when the entering water temperature is at its lowest. As they fill their tub the first water heater runs out of water while the second heater just starts kicking in, the first water heater is seeing all the work. The problem is exacerbated for those of us in the colder climates. As an experiment, in the middle of winter, run your hot water tank out of hot water and then listen as it tries to recover with a fresh load of 50 degree water. You'll hear the noise of water hitting the burners and making the same noise as someone putting a drop of cold water into a hot skillet. The tank is condensating. Do this a few times a week and it's not hard to understand why the first water heater, of a system piped in series, burns out.

I can not speak for our southern states as I have no idea of their entering water temperature during the winter.

Mike, I've never seen a system like you describe ie. one 50 gallon tank and a 30 gallon tank. I would think that a system like that is hacked in by someone that is not skilled in plumbing however.

Last but not least is a diagram on how A.O. Smith wants their heaters piped when using more than one. You can follow this link to view their PDF: http://www.hotwater.com/lit/wiring/315268-000.pdf

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But what about this technical bulletin that someone posted recently?

The .pdf doesn't contain a manufacturer's name, but I thought the poster said the bulletin came from A. O. Smith.

Download Attachment: icon_adobe.gif h2oparallelvseries.pdf

92.22 KB

Which is why I said that it isn't necessarily true. I have a paper here from Rheem that explains how to pipe parallel vs. series and it explains that they should be piped in series when they aren't the same capacity or Btu and that the smaller one should be in second position so it's not overworked.

I just spent some time trying to find it on the Rheem site but they've redone their site and I'm no longer able to access all of the technical bulletins they told us we could use here. That means that I'll have to re-type the article, scan the diagrams and then input the whole thing manually. That's going to take me a day or two, 'cuz I just don't have the time right now.

ONE TEAM - ONE FIGHT!!!

Mike

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But what about this technical bulletin that someone posted recently?

The .pdf doesn't contain a manufacturer's name, but I thought the poster said the bulletin came from A. O. Smith.

Download Attachment: icon_adobe.gif h2oparallelvseries.pdf

92.22 KB

That someone was most likely me.

At least you know someone's paying attention . . .

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So what's the problem with one of them doing more work and/or failing sooner than the other? Are you expecting both of them to fail on exactly the same day? I've found that water heater failures have so many variables as to be impossible to predict. I find failed units less than 1 year old and I've seen ones a quarter of a century old that have no visible signs of problems.

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