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Mystery hot water capacity decrease
I have a 50 gal Lochinvar Squire indirect tank heated by a Viessmann 100W. My informal capacity benchmark is that I could fill a large'ish bathtub with only hot spigot on and had a very hot bath (intolerable temp to me, but perfect for my wife, who apparently has thick skin). Once tub was full, the water coming from the spigot was lukewarm and hot water was pretty tapped out.
Unrelated to this, we had lousy pressure. Static pressure about 45-50 psi, but with an open tap the dynamic pressure dropped to 25-30 psi. I replaced defective pressure regulator, and now all is good. Regulator now set to ~60 psi, and with an open tap pressure remains in the mid 50's. Supply pressure is 150psi.
About the same time I replaced regulator, the bath behavior changed. Now with the tub hot spigot wide open the water goes lukewarm by the time the tub is half full (vs. all full before). In fact, with hot spigot wide open, the bath now fills a lot faster, but isn't hot by the time full. If I run tap at half open, it's still hot by time full.
Is there a theory to explain this? Could it be that with the improved pressure and the spigot wide open, the flow of cold water into the hot water tank is more turbulent and mixing cools the tank water, whereas before with low pressure there was less tank mixing and I more gradually siphoned off off the stratified layers of hot water more evenly?