Apologies in advance for a veeeeerrrry looooooooooong post. But I'm stumped. Just before Christmas Day, I had a new rental water heater installed to service a WALK-IN tub that we had just added to our house about a month earlier. The new 50 U.S. Gal tank that was installed was a -> Bradford White model RG1PV50S6N = Gas, Power Vent, 50 U.S. Gal that has a spec'd "First Hour Rating (Gal.) = 78 U.S.Gal" Previously we had been using a similar ~7 year old rental RHEEM 40 U.S. Gal tank that had been servicing our entire house at least adequately for the previous 7 years (i.e. with a conventional 5' tub, 2 standalone showers, a clothes & dish washer, miscellaneous sinks etc.) When we replaced the conventional tub with the WALK-IN, we realized that the WALK-IN (for whatever reason … still to be determined (the reason for this post)) was basically unusable because the water temperature in the tub when it was full is not even close to 130F. As well, if you wanted to wash your hair at the end of the bath, you'd only be getting "cold" water from the tub's taps (much much less than the maybe even 100F you'd be reasonably expecting). So I made the simple decision that the 40 U.S. Gal tank was not cutting the mustard for the new WALK-IN tub (larger volume - nominally contractor told me it's capacity is 61 U.S. Gal (but obviously when a person is sitting in a WALK-IN tub they displace some of that volume, so it really uses less HOT water than when it's empty. (According to the web, an approximation for the volume a body would displace is ~10 U.S. Gal). So now I step back to try to figure out the cause of my "not enough hot water" problem and tell myself: 1) the Bradford White 50 U.S. Gal tub has a "First Hour Rating (Gal.) = 78 U.S.Gal" … does that mean I can expect the tank to deliver ~78 U.S. Gal of HOT water (if it's thermostat is set at 130F) in it's 1st hour of use? 2) when my wife is sitting in the WALK-IN tub, once it's full, she should only need around ~51 U.S. Gal of HOT water (61 tub's volume - 10 for a body) @ 130F (if that's what the tank is set to) … and after she's "soaked" she should still be able to wash her hair with at least reasonably HOT water (understanding that the tank's "recovery time" is also a consideration) So now I did a test. 1. the WALK-IN tub is empty. I connect a hose from the TANK's drain valve and drop it into the WALK-IN tub and start filling the tub with ONLY HOT water directly from the TANK itself (no COLD is being mixed in). (I wanted to use a hose connection to take my HOUSE plumbing and the WALK-IN tub itself totally out of the picture). 2. the tub starts to fill with HOT water at ~130F (I was using a Hot Tub floating thermometer, whose scale is set at 120F Max for safe Hot Tub use). It continues to fill the foot well area and starts to cover the seat area with HOT water. However once it reaches the seat level, you can feel the temperature coming out of the hose noticeably starting to drop. The tub continues to fill the area above the seat, the 1st 2 seat back jets and finally the top 2 seat back jets. The temperature of the water in the tub continues to drop as the colder water (coming directly from the drain valve on the HOT WATER tank) is mixing in and it drops from it's original 130F to less than 100F by the time it's full. Additionally, the temperature actually coming out of the hose itself, after the tub itself is full, is close to 70F and is not even close to being reasonably warm enough to wash your hair. My Question: Can anybody explain why this is happening? Is there something I don't understand about this kind of plumbing set up? Do I have a brand new Bradford White water tank that's a lemon / defective in some way?