I'm just a DIY homeowner with limited plumbing skills. Swap out a water heater, shallow well pump install, change taps, install yard irrigation system, sweat copper joints, glue PVC/ABS, crimp some PEX etc .... plenty of experience. But I'm not sure I have every fully understood the recirculation system in my house. I assume it returns ambient water from the farthest point in the house, via the pump that is about two feet from the HW tank, and returns into a fitting on the bottom drain (check valve between pump and bottom of tank). I may be confused about direction of flow, but that's customary, correct? Pump pressure in the drain, and circulation out the standard hot water port?
Anyhow, here's what's going on. Last night I swapped in water heater #3 (electric) since the house was built in 2000. It's a temporary used unit (Craigslist find) since we're going tankless as soon as I can get the electrical and plumbing in order (relocating HW supply, so it will take some work). I configured plumping and electric as it always has been. It was literally plug-and-play, not even a coupling change, even recycled my brass drain valve in lieu of the plastic crap that was on the replacement tank. Here's how the replacement differs from two water heaters I've previously utilized: it's 50 gallons with two 5500W elements vs. the old 80 gallon with two 4500W elements. It was a "smart" unit that had a failed control board. The smart controller was deleted, the heater was converted to conventional operation with non-simultaneous thermostats. Due to the higher wattage, I noticed a remarkable recovery time compared to the old one. I also increased the thermostat setting up to 135 from our traditional 125, in hopes of not running out of HW given the 30 gallon difference. My impression is it will support just as much shower use as the old one. We used to be almost wide open for a comfortable shower on a single handle valve. Now it's 50% or less to get the same mix temperature.
Problem is, zero recirculation. In our two story house, that means there minutes to feel a bit of warmth in the primary bathroom, and about five minutes for a good shower. SO working record is crucial. I noticed the pipe is warm on the inlet side the pump, but not downstream (well, just a bit of warmth on the outflow side for a few inches; I assume it's just basic thermodynamics at work, not really water flow - it's stone cold close to the water tank). I considered the possibility of an air lock somewhere, and thoroughly ran the system. Every HW tap was exercised, and indeed some air was purged. No change. This morning I decided to crack open the line between the pump and tank. Using a bucket, I demonstrated good flow from the pump to the tank. I opened the drain drain valve, and noticed ultra strong flow, more than I have ever observed. But honestly, in the past I have already turned off city water into the tank when I open the drain, so I have no basis for comparison. But it was really strong flow, seemingly more than a household tap.
So as the wheels in my mind spin, I am assuming the tank pressure is somehow overriding the pressure of the recirc pump, or at least closing the check valve. Due that sound like the most likely scenario? If so, why? The obvious changes are tank volume, element wattage, and temperature. Does the higher operating temperature or smaller tank size create the potential for greater tank pressure? Any issues with running a 5500W element on full power? I assume the original smart controller would dole out power in some sort of metered fashion, but at times would ramp up to full wattage. This set up runs 5500W or zero, nothing in between.
My next try will be turning down the temp. Meanwhile, I'd appreciate any thoughts and suggestions.