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
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
