When I called our plumber a couple years ago to ask about increasing our hot water, he recommended we start by adding a thermostatic mixing valve (TMV) like the Cash ACME Tank Booster. So we did. $300 later (parts & labor) we experienced virtually no increase in hot water capacity. This video describes--in DIY terms--how TMVs work and why the claims they "increase hot water capacity" are basically a scam.