Tankless Water Heaters (...or my life changed from van to homeowner)

Now we're building a house.  It's on a steep hill in Chattanooga.  More details on that later, but for now I'm contemplating tankless water heaters.  After hours of research, I settled on a single, central unit, instead of three point-of-load units.  The main driving factor was cost, particularly since we'd need large heaters at all three locations; each location has a large water demand, although typically only one location needs that demand at a time... thus, with one large water heater, we can meet large demand at all three locations (but not at the same time).  With three separate heaters, we'd need to support that large demand at each site... hence, one central water heater is three times cheaper.  It's possible we could have slightly undersized the units if installed at each site, but the price difference was negligible.

So we're building one tankless water heater.  After much more research, I ruled out "ECOsmart" brand heaters, because they have long term reliability issues.  Long story short, it looks like Stiebel Eltron units last for 10-20 years, and they've been doing tankless heaters in Europe for more than 50 years.  They are more expensive than units you'd find in big box stores, but they have several advantages:
-all internal piping is copper; cheaper units use plastic, which fails due to insane instantaneous heat load.
-no mechanical relays.  I hate relays.  Cheaper units use relays, whereas Stiebel's units use SSRs.
-True temperature modulation provides constant output temperature.

e-tankless.com sells factory refurbished units for a great discount.  The only difference is the warranty is only one year, but I'd much rather have a better built unit with less warranty, versus a shoddy design with a few more years of free replacements whenever a leak occurs.

So I settled on a refurbished Stiebel Eltron Tempra 29B unit, which consumes a whopping 29,000 watts under full load!  If wattage is a foreign concept to you, think about this: With that much power, you could charge Tesla's largest battery pack in under 3 hours... enough to drive 310 miles.  Put another way, if your pulled this much power 24/7, your power bill would be ~$2600/month.

That sounds expensive!  Well, it's not, because the tankless water heater is only on when you're actually using hot water.  Even when it's on, it's not running full tilt unless you're consuming hot water from all sinks/showers/etc at the same time.  Say you shower once per day for 10 minutes, wash dishes twice a week, run the clothes washing machine twice per week, and wash your hands a dozen times a day (as you should).  Under those conditions, you're only requesting hot water for maybe 30 hours/month, and you're maybe pulling 15 kW on average over that time.  That works out to a much more reasonable $50/month, which is probably still on the high side because I don't do any of the above nearly as often as listed ;), and I've been ultra-liberal on the power/water usage.

Since we're all used to tanked heaters, here are some notable differences a casual user will notice when using a tankless system:

A)-There's a minimum flow rate requirement before the tankless heater will heat the water.  If you pull less than this much water, then the water will pass through the heater unheated (i.e. it'll be cold).  For example, if you are washing dishes with the sink running slowly, the water is going to be cold.  We'll discuss ways around this in a minute.

B)-If you constantly turn the water on and off, the water coming out of the sink will be hot/cold/hot/cold... this is because the tankless heater turns off every time the water flow stops... hence the water in the piping after the heater will remain warm, but the cold water flowing through the heater when you turn the hot water back on won't get heated until the electrical heating elements heat back up.  We'll discuss a solution to this in a minute.

C)-They aren't truly "instant."  Nothing in an ideal world is instant.  From what I've read online, the water takes another 5-15 seconds to heat up compared to traditional water heaters.  Since traditional heaters keep water in a tank hot 24/7, as soon as you turn the faucet on, that hot water starts flowing through the pipes (which have cold water in them).  The time it takes to run the cold water through the pipes won't change, so the 5-15 second figure quoted above is in addition, while the heating elements are heating up.

D)- You probably won't use the cold water taps in the shower/sink/etc... you can set these units to any temperature you want, so why waste that heated water by mixing cold water?  The only reason tank water heaters require mixing is they must superheat the water so that the tank water doesn't rapidly cool below your desired temperature when cold water backfills the hot water that left the tank.  This isn't a problem, but is a noticeable difference.

So what's the solution?  We can solve A, B & C by adding an additional return water line that recirculates the water from the hot line to the water heater's cold input.  Note: This isn't allowed on all heaters!  The Stiebel Eltron Tempra units accept input water up to 131°F, so we're good (most people shower between 105-115°F).

Since we want hot water to get to each hot water zone as fast as possible, we use a hub-and-spoke design to distribute the hot water (pink) in 3/4" lines.  The (green) return lines are intentionally undersized (1/2"), so that the recirculating pump doesn't affect water pressure at any point-of-load (PoL) that's actually being used.  Undersizing the return lines also helps evenly distribute the recirculated water across all three zones.  You could use three separate pumps (one for each zone), but since the pumps are expensive (~$200), you're much better off using a single pump.

We could leave the recirculating pump running 24/7, but that would mean you'd lose tons of energy as the water cooled while traveling through the pipes.  This would easily add $50-100 to your energy bill each month, which is more power than you'd actually use.

Instead, we control the pump based on user demand.  There are a few control strategies:
1: Add a timer... for example, the timer runs from 8am to 11 am, then 5pm through 11pm.  That is still really wasteful, and doesn't fit my erratic schedule; I sometimes shower at 3am.  Of course, you could still get hot water whenever, you issues A,B & C still apply.

2: Add a button, or rather multiple buttons at each sink/shower/appliance.  Pushing any button starts the recirculating pump and keeps it running for 5 minutes (for example).  That solves A, B & C quite tidily, but requires the user to push the button... if they don't push the button, then A,B & C persist, but the user still (eventually) gets hot water.  It also prevent you from wasting all the cold water that would otherwise go down the drain while you waited for the water to heat up... simply push the button 30 seconds before you hop in the shower, start the dishwasher, clothes washer, etc.  It's important to reiterate that you don't need to push the button...

3: Add a water flow sensor and use it to enable the recirculating pump for 5 minutes each time some minimum flow rate occurs.  Note: The flow rate that enables the recirculating pump can be much lower than the flow rate that activates the water heater.  For example, if the water heater enables at 0.75 gpm, we can enable the pump at 0.25 gpm.  Since the pump would flow at 3 gpm (in my case), the 0.25 gpm flow will force 3 gpm through the heater, and thus it will stay on.  This solves A & B, but you'd still have to wait for the water to flow through the pipes, and for the heater to initially heat up.

4:  Implement options 2 & 3.  Then the user only needs to push the button to solve C (instant hot water as soon as you turn on the spout).  Option 2 takes care of A & B, and pushing the button takes care of C.

5: Do nothing.

...

I'm going to implement option 4, as it only requires one additional component (flow rate sensor).  In fact, it might not require the flow sensor at all, as the Stiebel Eltron already has a built-in flow rate sensor... if I can connect an ADC to its output, then no additional parts are required.

This is a perfect application for an Arduino Uno.  And hell, if it breaks there's no harm to the system... check valves always work.

...

So overall, this system only requires two extra components during construction:
-A return line from each heated water zone (kitchen, guest, master).
-A differential data line to each zone.

The other components can be added to an existing system:
-QTY2 3/4" NPT check valves.
-QTY1 1/2" NPT check valve per zone.  In my case, QTY3 total vales.
-QTY1 pump (3 GPM).
-QTY1 splitter 3/4" NPT to 3x 1/2" NPT
-buttons and arduino.