New 75 Gal - Not Good For The Big Tub

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Hogan

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As I noted in earlier posts, I replaced 2x 50gal gas heaters with a new single 75

Wife just used the big tub for the first time and said the water got cool quickly. I had measured it as about 65 gal (total water, not just hot) needed to fill to typical level we use (don't use it much!)

For some reason I had gotten myself comfortable that this would be no sweat to fill. I had done some back of the envelope calcs, thinking it doesn't use 65 gallons of pure hot water, etc. WHAT I DONT THINK I ESTIMATED is the quick drop in temp for such a quick outflow. Yes, my Bradford White has a 121 first hour rating etc. But think about this....at about 10gal per minute, if I am starting with 120 degree water and mixing in 50 degree, the progression goes like this....

Minute 0 - 75 gal hot = 120 deg
Minute 1 - 65 gal hot + 10 gal cold = 110.6 deg
Minute 2 - 55 gal hot + 20 gal cold = 101 deg
Minute 3 - 45 gal hot + 30 gal cold = 92 deg
Minute 4 - 35 gal hot + 40 gal cold = 82.6 deg

Wow, I never did the calc this way before

With 2x 50s, I had much more USEABLE hot that wouldn't fall so quickly. They were piped in Series and so the 50 gal was being fed with hot water (which would slowly decline like the above too).

I had seen some citations that the "usable capacity of a water heater is 70-80% of nominal" but here when we get just below 50% capacity with such a quick drawdown, yes we have "warm" water if one was washing hands but it would definitely start to feel cool in a bath.

Even though I did install a tempering valve at the tank, I was initially running the tank at around 120 and just leaving the valve on "max" to see how we go as if this was a typical hot water tank. Never had a problem with normal usage because a shower draws down 2.5 gpm and the recovery is kicking in to help too. But draw at 4X that rate for this jacuzzi tub and wow. Unfortunately I may need to turn up the tank some and actually use the tempering valve as a capacity extender especially if she is gonna use that big tub. Was trying to minimize that "overheat" condition for best tank life and best efficiency.

I probably would have made the same choice again of a 75 rather than buying 2x 50s but just surprised at the temp drop here, but the math is simple
 

Fitter30

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Here's a hot cold mix calculator just change setting to gpm. The hotter the water the more minerals are driven out of the water.
 

Hogan

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Here's a hot cold mix calculator just change setting to gpm. The hotter the water the more minerals are driven out of the water.

Ok thanks

Is there a material difference between running at 120 and 130?

What I struggle with is that Jacuzzi tub gets used like twice a year on average, so it feels dumb to run the tank hot 365 days a year just in case. I guess I could tell the wife to give me a heads up if she wants to use it and I can go turn up the heater and also turn the tempering valve down to flow 120. Sort of need 30 + minutes heads up I would think.
 

Hogan

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Here's a hot cold mix calculator just change setting to gpm. The hotter the water the more minerals are driven out of the water.

This calculator is fine but doesn't really highlight the effect I am noting.....the very quick influx of cold water that will cool down the whole tank within mere minutes at a strong pullout rate of like 8-10 gpm. Unfortunately I think the only option is to start with a higher tank temperature to give a little more water that can get into the tub before the blended tank feels "lukewarm" and then unfortunately one needs to wait probably at least 20 mins till the 76,000 BTU burner can raise the temperature of the water. Maybe more. Im sure there is a calculator on that topic that I will search for
 

Tuttles Revenge

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What about using a water heater booster?

"AutoBooster™ tankless electric water heater boosts tank water heaters by adding up to 45% delivery capacity to any tank. AutoBooster works with the tank to deliver more hot water. An 80-gallon tank combined with an AutoBooster delivers the hot water capacity of a 120-gallon tank. Install the unit directly on the tank outlet or on a nearby wall. No need for additional electrical infrastructure when used with an existing 30-amp electric water heater."

 
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John Gayewski

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If you had the space and felt the need you could buy a small tankless to fill the tub. It will not heat the water at 8-10 gpm, but neither will the most powerful tankless heater (in most cases). To heat water that fast you need a lot of btu's like a boiler can supply.

However if you wanted to fill the tub at 2-4 gpm you could get a small remote tankless dedicated to the tub. You might need to pull some wiring depending on how fast you want to fill.

Looks like I was typing as Tuttle posted.
 

Hogan

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Interesting ideas

Given that this tub gets used only a couple times a year it's hard for me to want to invest lots of money just for it
 

Hogan

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So very interesting. My wife used that big tub again tonight and said "did you do something to the water heater? It was much hotter tonight and didn't run cool while I was filling the tub like last night". I had turned it up a little but not that much...maybe from 120 to 125. Surprised it would make that much difference.

I will waste a little water and turn on only Hot into the tub along with my instant thermometer in the stream and see how much it changes as it fills.

One thing I realize in my "math" above is that I am assuming a complete mixing of water and hopefully that doesn't actually happen with a working dip tube....so the cold water should be coming in at the bottom and thus the upper parts of the water should remain fully hot....is that a fair assessment? It's not as if in minute 1 you take out 10 gallons of hot and replace with cold AND that 10 gallons of cold also gets perfectly mixed around in the tank. So it should be that 65 gallons is still pretty hot while the new 10 is getting injected into the bottom via dip tube
 

John Gayewski

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I keep my tank at 135 or so and it makes a very big difference. I can do tub for the boys and two full showers, one after the other after the other. Plus you get nice dish rinsing lava. I don't use mixing valve, but that's mainly because I'm lazy and haven't gotten around to it.

Yeah the dip tube should help keep the cold water at the bottom and leave the hot at the top and it helps with capacity. It's a pretty tough thing to calculate exactly, but at a certain point, after so much hot water has been used, they will mix and the dip tube is just there not really contributing.
 

Hogan

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So yesterday I ran some tests to fill my big tub and found some interesting things. I also got to take a hot bath as a reward haha

First, I figured out that my Hot fills at 6.5gpm while my Cold fills at 10gpm. Is this normal or is the lower fill rate on the hot water heater coming from the fact that I have the tempering valve on the hot water heater? Or does the hot water going in and out of the heater naturally come slower because maybe the inlet and outlet pipes are smaller diameter than the cold water pipe in the house?

Either way, I was pleasantly surprised that the temps held up for a while while filling with Hot only. Started at around 128 and stayed basically there for several minutes, then ticked down slowly. By 12 minutes I had used about 78 gallons (on a 75 gal tank) and by then the temp was around 80. 78 gallons by the way was above the level where once I got in the tub, it was automatically draining down the safety drain at the side of the tub, so normally wouldn't need to fill that much.

So I may have a little latitude to lower the tank temp a few degrees to the extent it has that level of precision, but if I went all the way down to 120 or below then I might have an issue in getting the tub filled with water that feels "hot" vs "warm". If I recall once I had filled the 78 gallons the tub was saying it was around 115-117 degrees which still felt hot for sure. But if we were starting with 120 as the hottest water instead of 128 then everything would be a step cooler and may feel "warm" instead of "hot"
 

John Gayewski

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The hot should not be that much slower (I don't think) it seems like a big difference. There could be a couple issues causing it. The bath valve has buildup on the hot side, the piping had mineral deposits on the hot side and along with those two possibilities the tempering valve will slow out a little. I wouldn't think the tempering valve alone and a little extra pipe should equal 3.5 gpm.
 

Hogan

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The hot should not be that much slower (I don't think) it seems like a big difference. There could be a couple issues causing it. The bath valve has buildup on the hot side, the piping had mineral deposits on the hot side and along with those two possibilities the tempering valve will slow out a little. I wouldn't think the tempering valve alone and a little extra pipe should equal 3.5 gpm.

I just didn't know if the piping was stepped down sizewise going into the water heater or out of it while maybe the cold water is larger diameter piping within the house. I am not home now but will take a look when I get home.
 

Master Plumber Mark

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Just stop trying to be thrifty with the hot water....
if your wife wants hot water to fill up her huge. tub then turn the heater up
to 135-145 and make her happy.... 120 is too wimpy and is not gonna cut it

go out and buy a couple of water heater blankets and wrap the tank to conserve the
heat and you will notice a huge difference too
 
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