With the tempering valve set incorrectly, there was no cold water mixing at the faucet, only hot side. My main issue is lack of flow, not quantity of water. Sounds Contradictory but that's how I see you. Thanks for any advice and help!
I think you have both problems, not just one. Not enough flow, and not enough stored heat in the tank.
Bucket-test the total flow rate again, and time the fill to get a realistic handle on the total volume. There are only so many stored BTUs in a 50 gallon tank, and even with a special controls to raise the storage temperature to 180F, filling a 125 gallon tub with 40 gallons of 180F water and 85 gallons of 55F cold would still end up at a warmish 95F, even if it filled in 2.7 minutes instead of 27.
Play around with this
online mixing calculator a bit, you'll see what I mean. A typical spa temp is 104-105F, so that's your target.
Don't count on getting more than 4/5 of the tank volume out at the storage temperature- it'll often only be 2/3. In most of GA the average incoming water temp over the course of a year will be in the low to mid 60sF, but if you're on a city water main it'll be at least 10F colder during the coldest weeks of winter, which is why I'm using 55F. Run a cold tap for a full 3 minutes and take the temperature of the water to get a handle on what your actual early January incoming water temp is. It's usually a bit colder by early February, but not another 10F colder.
Most private well systems run between 30-50 psi, which would deliver about half the rated flow of showerhead specified at 80 psi. Terry's first response indicates that the faucet should be good for 15gpm at 40 psi.
A static pressure of 80psi or higher would be the starting point, then you'd have to calculate the pressure drops with flow across the PEX and look up the specs on the tempering valve. (A 3/4" thermostatic mixing valve might be a better option than a half-inch tempering valve if that turns out to be the bottleneck.)
A crude rule of thumb, for half-inch PEX , for every gpm of flow, for every 10' of length there is about a 1 psi drop (but it's not really that linear) If the goal is 10 gpm through the hot side mixing with 5 gpm of cold, and there is 50' of PEX between the tank and tempering valve it's losing about 5 x 15= 75 psi. The faucet needs to see 40 psi to deliver the 15gpm total which means you'd need a static pressure of about 110 psi (which is on the high side.) If there is only 30' of pipe, the pressure drop would be roughly 45 psi at 10gpm, and you'd need a static pressure of 45 +40= 85 psi. All pipe lengths have to include the "equivalent lengths" of all ells & tees & valves, etc, which add on to the straight lengths. If the cold feed to the water heater is also half-inch pex, that has to be factored in too.
So if your static pressure is only 50 psi, you're never going to get to 15 gpm unless the pipe lengths (including the cold feed to the water heater) are very short.
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