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Notes about a Vertex water heater
Home in northern latitude of US has 2 Vertex 50 gallon heaters.
The Vertex that supplies hot to high-volume shower cannot keep up.

We confirmed there was a shortage of hot from unknown cause.
The Vertex might be undersized, or incoming water too cold, or water heater malfunctioning, or combination.
Troubleshoot, solutions and work arounds are offered.
Resource: Vertex service manual
 
Use 6 wraps teflon tape
White for water lines
Yellow for gas lines

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Teflon tape at Amazon

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Pipe wrench at Amazon
Channel locks

Resources:
How to remove rusted element


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Knipex channel locks
Irwin pliers
replace anode rod
Inspect anode each 2-5 years
Anode rod prevents tank from rusting
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Anode rods at Amazon

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Replace anode rod
How to maintain water heater

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Impact wrench at Amazon

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Replace anode rod
How to remove short water heater nipple
Water pressure gauge

Pressure gauge/ attach to water heater drain valve
Pressure over 80 psi can damage water heater and household plumbing
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Water pressure gauge
Temperature-pressure gauge

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Identify water heater valves
Troubleshoot water pressure
Automatic water heater shut off valve
Prevent flood
Automatic water shut off valve
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Water detectors at Amazon
Automatic water heater leak shut off at Amazon
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Leaking water heater/ steps
Pressure reducing valve
Pressure reducing valve
Pressure over 80 psi can damage water heater and household plumbing
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Pressure reducing valve

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Identify water heater valves
Troubleshoot water pressure
Sharkbite pressure gauge
Sharkbite pressure gauge

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Sharkbite pressure gauge

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Water heater valves

Water heater circulation system

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Grundfos at Amazon

Resources:
How to install recirculation system
COLD side Sharkbite flexible water heater connector
COLD side Shark bite flexible water heater connector

Buy:
Sharkbite water heater connection
Braided water heater line
Stainless steel flexible connection

Resources:
How to install electric water heater
How to install gas water heater
Leaking water heater
HOT side Sharkbite flexible water heater connector
HOT side Sharkbite flexible water heater connector

Buy:
Sharkbite water heater connection
Braided water heater line
Stainless steel flexible connection
Braided water heater line
Flexible water heater connection

Resources:
How to install electric water heater
How to install gas water heater
Leaking water heater

Vertex is in a class of heaters called 'condensing water heaters' due to the high efficiency design of the heater ... where a large propane or natural gas burner produces hot combustion gasses that are recycled through an helical shaped heat-exchanger located inside tank ... resulting in large output of hot water, rapid recovery ... and a flow of condensed acidic water vapor that must be routed into floor drain.

Condensing heaters of all types and brands are relatively compact, high powered machines that generally produce a LOT OF HOT WATER with 94% thermal efficiency (the percentage of heat from burner that is transfered into water). But they are expensive, complicated electronically controlled, and sometimes temperamental ... often needing considerable maintenance.

AO Smith will refer local technician. Parts and repairs are expensive. Vertex connects to 120 volt household power, is rated for residential use only, has 2 hex head anode rods (one aluminum and one magnesium) to prevent tank and heat exchanger from rusting, cannot be exposed to hard water, and comes with 6 year tank and parts warranty.

Vertex has 3 models: one power vent model and two power direct vent models.
Power Direct Vent: the incoming air supply comes from outdoors, and vent terminates outdoors
Power Vent: incoming air supply comes from inside room, and combustion vent pipe terminates outdoors.
Not all power vent and power direct vent heaters are condensing heaters.
Resources courtesy AO Smith:
Vertex service manual
Vertex parts
GDHE parts
Installation manual
Spec sheet

Warren asks about problem with Vertex/ w notes added:
I have a 4 bedroom 3 bath house with two independent A.O. Smith Vertex GDHE-50 natural gas water heaters.
Note: GDHE-50 is a 50 gallon ultra low NOX power direct vent with 94% thermal efficiency and 80% overall efficiency when standby is included. 
6 year parts and labor warranty does not cover installation.

One Vertex unit services 2 full bathrooms and the other unit services a full bathroom, ½ bath, laundry and kitchen.
Both water heaters have continuous (for the moment) recirc pumps for the fixtures in the two independent “circuits” serviced by each water heater.
Note: independent means the heaters share incoming cold water line per usual, but have no other pipes in common.

The issue is that the Vertex servicing the two bathrooms is running out of hot water after one shower. The temp is set at 160°, with actual temperature inside tank at 150° (which is how the heater is designed, and is not a problem).
The following settings appear on the LCD display:
· Operational Setpoint:  160°
· Differential: 8°
· Tank Probe Offset: 0°
· No faults are indicated currently or in history

I am not sure if it’s the “too much” water volume of the European fixtures, or if the tank was installed improperly or I am just expecting the tank should deliver the equivalent 75 gallons worth of hot water and continuous hot water per the AO Smith marketing. 
It should be noted that there is difference in the delivery if the recirc pump is operating or off. 
Short of incorrect settings or a faulty installation, I am considering connecting the two tanks together in a series.  The tanks are about 40 feet apart in two separate basement mechanical rooms. Resource: Two water heaters
I am speculating that I could redirect the recirc pump(s) to provide supply side water of both heaters.
I would appreciate your thoughts and insight.

Response edited for clarity:

Water turns cold after 1 shower but no error codes are observed.
If heater was working fine in January, but not working in April, then something changed recently, and it's either the water heater or plumbing.
Note: This problem was about a newly installed water heater which points to a plumbing problem or overconsumption. 
-A plumbing problem could include installation, faulty plumbing fixtures, high water pressure, non functioning check valve etc.
-Overconsumption would be a shower that consumes more than the 100 gallon first hour rating of heater.
Why not a water heater problem? No error code. If Vertex was malfunctioning, the control board would likely issue an error code, unless entire control board was faulty, but then why would heater fire up on next cycle?

100 gallon first hour delivery
According to spec sheet, the 50 gallon Vertex GDHE-50 has 100,000 BTU burner that should deliver 100 gallon first hour.
More detail: First hour is an industry term that estimates the amount of usable hot water from single continuous draw. It does not always mean 100 gallons, does not mean continuous hot water flow no matter what, or that hot water will last an hour. Resource: Spec sheet
Comparison: Ordinary 40,000 BTU, 50 gallon heater has a first hour delivery of 80 gallon. Numbers based on 60 degree incoming water temperature, and 120 thermostat setting.
More detail: First hour represents a relative measure when comparing heaters before you buy. Manufacturers might fudge the numbers a bit to get leg up on competitor. We can use the 100 gallon number as a baseline, but realistically, the 100,000 BTU burner, 94% thermal efficiency of condensing heaters (percentage of combustion heat that is transferred into water when burner is running), and 150° water temperature, means the heater should be able to keep up. Resource: Spec sheet

All water heaters deliver less when incoming water is cold (in winter) because each gallon of incoming cold water is added to the hot water inside tank, with a cooling effect on overall temps inside tank.

Colder incoming water can be partially mitigated by increasing temperature setting to 150° as you report ... but of course it would take longer to reheat tank (recovery) to 150° than 120°.
Once heated, the 150° tank holds more hot water energy that should yield more usable hot water because less hot is drawn out of tank for each gallon of 104° hot used at shower ... meaning that less cold water enters tank, so tank should cool slower ... meaning a tank full of 150° water would last longer than tank heated to 120°, and the result is higher first hour delivery.

Action:
1) Using a timer and bucket etc, gauge the amount of hot consumed during shower. This will let you compare usage with 100 gallon first hour delivery for the GDHE-50.

2) If shower exceeds the 100 gallon first hour tank capacity, then a flow restrictor could be installed on shower, incoming water pressure reduced with a pressure reducing valve and expansion tank, a tempering tank installed, and maybe plumb both heaters in series.

3) If shower is less than 100 gallon first hour capacity of heater, then next time you run out of hot at the shower, check if the tank still contains hot water.
Open TP valve and let some water escape ... if thermostat temp is set above 120°F then be careful the hot water does not spew onto your person... use kitchen thermometer to get most accurate reading.
Buy: Kitchen thermometer   Temperature gauges

4) Is the water from TP valve hot when shower is cold?
If consumption is less than 100 gallon, and both shower and water heater are cold, then heater is suspect. Read (5).
If consumption is less than 100 gallon and shower is cold, but water heater is hot, then plumbing is the problem. Read (6-10)
If consumption exceeds 100 gallon, then some things can be done to 'improve' situation, and perhaps solve problem. Read (11-12)

More detail:
If TP continues to leak, raise lever again and gently lower into place.
If TP ever becomes a problem ... the TP on 100,000 BTU heater is typical off-the-shelf-TP-from-hardware-store replacement that rated for maximum 100,000 BTU ... the larger Vertex or any heater rated more than 100,000 BTU requires TP valve rated for that BTU.
Resource: TP valve code


5) Service call:
If problem is heater: I suggest calling AO Smith for a service technician.
Have them confirm BTU rating of heater ... should be on label on side of heater.
Have them confirm that temperature sensors are in calibration, and that burner is firing up correctly as usage takes place.
More detail: I have resources, including service manual that matches the GDHE-50, except Vertex is a complex machine, like a jet fighter ... made to run big and fast with a large burner controlled by electronic components that are expensive to replace.
Resources:
Vertex service manual
Vertex parts
GDHE parts
Installation manual

6-11)   If plumbing is the problem, then following information will help narrow possible suspects.

6) Are the two water heaters connected in any way, other than both receiving same incoming cold water line? (We confirmed later there was no common hot pipe)
More detail: One of the problems that might cause a shortage is if the two hot lines leaving heaters are somehow connected together ... except you report each heater is connected to separate 'circuit.'

The reason I say this, it that a 'common hot line' when house has 2 water heaters, and each heater services different areas... this can draw cold water and stop the flow of hot to one of the areas ... which is what you are experiencing. The solution would be break apart that common hot line.

Now if each heater is designed as a dedicated unit with no shared water lines, except incoming cold water, then that is best. And you would not combine the two heaters, unless there was a genuine shortage of hot water. (We confirmed a shortage from unknown cause, but the conversation took place in late winter from same latitude as Chicago which would have cold incoming water temps that would lower the expected first hour delivery).

Honeywell mixing device7) Mixing valve, or tempering valve? If you have one located on hot pipe where it leaves tank, it might be defective. If you don't have one, then install one since temps are set at 150°F.
More detail: As a general rule, you can gain more hot from a fully heated tank by setting max temperature at 150°, which you have done already.
When water is 150°, then more cold is mixed into the hot at your shower valve, to temper the bath-shower down to the typical 104-110°F°.
Common safety standard: if water heater temps are set to a potentially fatal hot water source (anything above 130°-140°F), then a mixing valve is installed on the hot water nipple as it leaves tank.

A mixing valve is connected to hot water where it leaves tank, and then connected to cold water line. The mixing valve automatically adds or mixes cold water to the outgoing hot water, to temper it down to safe levels.
Mixing valves are adjustable, and normally set to 120° ... or 110°-125°F. Typical bath-shower is 100-104°F
A malfunctioning mixing valve can allow too much cold water into the line, and can be a suspect in the plumbing problem that you describe.
Buy:
Water heater mixing valves
ResourceAdvantages/ disadvantages mixing valve

8) Since your home is new, then your recirculation system probably has a direct return line. Resource: Water heater recirculation systems
What we're looking for is a check valve on the direct return line.
A check valve is a one-way valve that lets the water flow one direction but not the other.
A faulty check valve on a recirculation return line (a spring-powered check valve where the spring failed, or a swing check valve installed vertically instead of horizontally) can cause cold water to enter hot water line.

9) Another suspect is crossover in one of the faucets (a bad single handle faucet cartridge that lets cold water enter line instead of hot).
Resource: Crossover

10) Overall
-If you combine the two heaters in series so both supply hot together, then your recirculating systems could become an issue ... not sure exactly what will happen without knowing if heaters have a common hot line and other factors ... but I think combining the heaters might lead to endless problems with recirculation ...a local plumber could debunk that notion.
Resource: Two water heaters
11) More detail: The recirculation system was barely mentioned among the answers, yet Warren reported there was a difference in first hour when it was running.
Apparently Warren experimented with turning recirculation system off since he knew there was a difference, but he did not report significant difference.
A recirculation system would reduce the amount of hot from a tank, so it is advisable to check the output with recirculation turned off.

If thermostat is set higher than 120°, a mixing valve might reduce energy waste from running the recirculation system.
Why? The recirculation system is a loop, so it does not cause new cold water to enter the tank. But the 150° hot that leaves tank cools considerably while it travels the loop and then re-enters tank where it starts to cools overall water temperature. The thermostat does not trigger a heating cycle until tank temperature drops 6-8°F (wider range on some water heaters), which means water inside the Vertex can cool down to 140°F for example, depending on actual calibration of electronic controls. Using a recirculation system means heater will waste energy by running more to reheat water, but might also reduce first hour delivery if the tank was cooler due to wider-than-expected calibration (which is a good reason to have service tech check actual calibration range, or check it yourself using thermostat reading from TP valve and drain valve).
Buy: mixing valves
Resources:
Advantages mixing valve
Recirculation system

12) Warren answers back/ edited and reworded to shorten conversation
His findings are summarized.

Warren says: My shower consumes 8 gallons per minute but I only get 8-1/2 minutes of shower before hot water runs out.
Answer:  
Your measurements show that your heater is delivering 68 gallon first hour, not 100. Something is not right because spec sheet shows 100 gallon, but you're getting less than an ordinary 40,000 BTU heater. Spec sheet for ordinary 50 gallon gas water heater with thermal efficiency of 70% and 40,000 BTU burner shows 80 gal first hour. Ordinary spec sheet
Vertex GDHE-50 should deliver more based on the 100,000 BTU and 20% higher thermal efficiency. Of course, cold winter in Northern latitudes of US means water entering tank is probably 45-50°F which will reduce first hour delivery.

Is there a leak somewhere? Turn off all taps and put ear against pipe and listen for water leak. Water leak can run hot out of tank.
Check if any toilets are connected to hot line ... turn off all taps, flush and feel hot pipe top of heater to see if hot is leaving the tank ... sounds strange, but some guy bought two new heaters before he discovered the remodeler hooked toilet to wrong pipe.

Calculations: The first hour delivery calculation is as follows: Tank capacity x .7 (standard formula) + 129 gallon recovery (as shown in Vertex spec sheet) = 164 gallons first hour (The GDHE-50 spec sheet shows 100 gallon first hour)
If 164 is true... at 8 gallons a minute, that should give roughly 20 minutes ... of course it would be less with cold incoming water, maybe down to 15 minutes of hot ... but up because tank is fully heated to 150° and contains more heat energy.
If 100 gallon first hour is true as shown in Vertex spec sheet ... at 8 gallons a minute ... that should give 12 minutes ...
Which number is right? Or maybe typical calculation only holds true for small 34-40-50 gallon 40,000 BTU heaters?
Water-heater-Formulas-and-terminology.pdf

Condensing heaters comparedWarren says: Before running the shower, the water at TP valve is correct 150°F. So tank is heated to thermostat set point.
But after 8-1/2 minute shower, the shower and tank are same cool temperature.
The heater is heating at this point and water starts getting hot pretty fast.
Answer: If overconsumption is the issue, I've had limited success solving problem for other folks without changing the heating equipment in some way because it takes a lot of BTUs from the burner, plus a large tank, to supply 'endless' hot water.
Folks with similar challenge should spec out the numbers beforehand, and install 2 Vertex units  plumbed in series or parallel, add a storage tank to the Vertex, add a tempering tank, or choose the largest 75-gallon Vertex GDHE-75, or install AO Smith commercial grade Cyclone 3-year warranty 250 gallon, 150,000-499,000 BTU condensing heater, or install a residential grade Polaris 1 year warranty 50 gallon with 199,000 BTU burner.
Large heater and large BTU rating, mean more first hour delivery.

Cyclone large volume commercial heaters are nearly 8' tall, and are the commercial version of the Vertex.
Resources:
Polaris spec sheet
Large volume Cyclone spec sheet
Storage tank with Vertex design sheet
Two water heaters

Warren says There is no crossover (bad single handle faucet cartridge that lets cold into hot water line).
Resource: Crossover

Warren says: I am going to install a mixing valve on top of heater.
Answer: A mixing valve will help increase first hour delivery, plus Insulating the copper pipe will also help.
Resource: How to solder pipe

Warren says: No hot water is entering the recirculation return line because it does not change temperature when shower is running. The recirculation check valve is installed horizontally as you described. These two things indicate the check valve is installed correctly and working fine.

Warren says: My recirculation system runs full time. If I install a timer, will that help?
Answer: Yes. And you reported that there was a difference in first hour delivery when recirc pump was operating.

Warren says Because tank and installation are new and still under warranty, AO Smith has scheduled a service call.
Answer: That is good idea.


Response back:
You did the crossover test and considered the check valve and found no problem.
You're going to add timer to recirc pump, and install mixing valve.
The pipes are insulated already. No hot water leak was found.
These are good things.

You ordered an adjustable flow restrictor for the 8 GPM showerhead, which will improve first hour capacity because it slows the amount of hot taken from tank.

Some tweaks not discussed in detail:
1) You're considering re-plumbing both heaters in series, so Vertex A will bolster the first hour capacity of Vertex B that is running short.
This might work well, if recirculation systems do not cause problems.

You can connect the hot line from Vertex A to the incoming cold line on Vertex B that supplies water to 8-gallon-per-minute shower.
This would put both heaters in series, so when hot at Vertex B runs short, the hot from Vertex A would supply the hot water. Two water heaters
Be careful. You cannot have both ... it has to be either the ordinary incoming cold water supplies Vertex B, or the hot line from Vertex A supplies Vertex B, or you get cold water flowing into the hot line.
A double shut off valve could be added to give a choice between the cold water line and hot from Vertex A .... and that would give options if the shortage was seasonal or occasional.
Two 24 volt AC motorized ball valves (or use 24 volt sprinkler valves) could be used, and wired to home automation module (wifi, zwave). Automated valve might be reliable depending if signal reached the mechanical room, but the valves could also be hardwired to a delay timer that would be more reliable. A small 24 volt doorbell transformer, 2 valves, a spring-wound timer and doorbell wire required. General wiring

2) You can add a 50 gallon 'tempering tank' installed on incoming cold water line before water enters the Vertex.
A tempering tank (get 40-50 gallon electric from local store) passively preheats water ... it uses ambient room temp inside home to preheat incoming cold water ... the tempering tank can be energized or not.
This is a low cost, low impact solution that would add '35 more gallons' onto first hour delivery if tank was energized.
Resource: Tempering-tank.html
It would be useful to know what temp incoming cold water is ... because my informal calculations show that a passive 50 gal tempering tank would help the Vertex deliver more hot water in winter, and perhaps year round ,.... probably your mechanical room is reasonably warm ... warmer than incoming water ....  50 gallon electric in Houston costs $420 (year 2020) ...making it a fairly cheap alternative. Install two 50 gallon electric in series with the Vertex and wire them up with a couple 10 gauge circuits, and you suddenly have 120 gallons more first hour. This is a genuine solution, and cheaper that buying new condensing heater that might require complete re-work of vent line etc, and cheaper than adding a storage tank plus additional circulating pump etc.
A 50 gallon gas heater could be used, and if it was fired up would deliver 80 gallons, except the gas heater can't be integrated into the Vertex vent line, and it would need it's own vent to the roof, plus it would require additional gas line ... lot of work. Electric heater is cheaper, and easier to turn on-off if usage is only needed in winter.

3) Misc stuff for conserving resources:
Resource: 9-ways-to-save-with-water-heater.html

Tank pressure4) Check your incoming water pressure. High pressure coming in from city water supply can diminish lifespan of plumbing pipes and fixtures, cause a leaking TP valve ... but it can also run your hot water out of tank much faster.
Use a pressure gauge to check pressure at drain valve on heater, and on outdoor spigot. Pressure at tank is typically higher than outdoor spigot because heating water causes it to expand, and put pressure on the tank.
80 psi is considered max for household plumbing, while 40-60 psi is best.
If pressure is high (anything over 60 psi), then add a pressure reducing valve on incoming cold water line.
If you install pressure reducing valve, then a expansion tank is mandatory ... the pressure reducer goes first and then expansion tank between that and the water heater.
Set pressure valve to 50 psi. I keep my pressure at 40 psi, so my plumbing installation lasts longer.
If pressure valve is added ... an expansion tank is mandatory.
Buy:
Pressure gauges at Amazon
Watts telltale Water Pressure Test Gauge, 276H300
Temperature pressure gauge

Telltale pressure gauge is left in place for a day, and marks the highest pressure so user can tell if there are pressure spikes that are causing TP valve to leak water, or other possible problems

Resources:
Read about pressure gauge
Read about pressure reducing valve

Approximate calculations:
If water temps are 150°, and shower is 100°F ... and average incoming cold water at your latitude near Chicago this time of year (early spring 2020) is 45-50° let's say? Then you are mixing 45-50° with 150° to get 100°F ... but then as hot is consumed out of tank, the water inside tank is getting colder.
Resource:  average-groundwater-temperatures

If you add 50° + 150° divided by 2 = normal 100° shower, you are getting a 50-50 hot-cold mixture.

So for every minute of 8 gallon shower, you are using 4 gallons 150° hot ... or 4 gallons per minute out of the tank ... which would deplete the 70% threshold (since cold water is entering tank at 4 gallons per minute) of the tank in .... 50 x .7 = 35 divided by 4 gallon per minute = 8.75 minutes of hot water for shower.
Except the burner is adding 100,000 BTU per hour ... or 100,000 x 1° per lb of water per hour ... or as shown in spec sheet, about 129 gallons recovery per hour ... which is 2.15 gallons per minute (but less since incoming water temps are low, and thermostat is set to 150° also it takes long to heat to set point) ... let's estimate that recovery is dialed down to 85 gallons per hour or 1.4 gallons for each minute. So for every 4 gallons of hot that are leaving tank per minute, another 1.4 gallons are being added while the tank is diminishing in temperature. In any case, the hot water should exceed the 8.75 by at least 1 gallon per minute, which means you should be getting minimum of 9.75 minutes of hot if the shower is consuming 8 gallons per minute.

Accuracy figuring how many minutes of hot water are available for the shower would take a computer larger than my brain ... but the spec sheet says you get 100 gallon first hour, which would be 12.5 minutes using straight math (100 divided by 8 gal per minute).
With tank temp at 150°, you should get 15-20 gallons more than temp at 120°? = 14-15 minutes?
 
In any case, the water heater might undersized for the job, or a combination of things like very cold incoming water, high water pressure, recirculation system ... or the burner not delivering the full 100,000 BTU, or maybe heater kicking in a bit too slow because temperature sensor is out of calibration.

Warren says I might try to utilize two more recirc pumps to connect the heaters before employing a tempering tanks.
Answer: You want to add a recirc pump between the two heaters ... couple issues strike me, based on previous work on this problem.
It's just as easy to choose the wrong pump as it is the wrong heater ... if pump is too small, it won't move enough water ... too large (like 4 gallons a minute), it could cause a lot of bubbles and won't move water either ... and then how do you switch the pump on off? And how do you balance the flow if water is drawn from a tap vs a shower?
You could install a PID with probe taped to copper pipe that turned on the pump when temps on hot pipe indicate water is flowing on the hot line, except your existing recirc pump is constantly circulating water.
Resource: Add a PID
BuyPID relay and thermocouple/ ITC-100HR/Temperature range :-50~250ºC

An amp switch could be added to a wire on the vertex, maybe on the blower motor, and when motor started, the amp switch would turn on the pump. But amp switch has to be on the hot wire, not both wires because hot and neutral together cancel each other. You could integrate the amp switch with a delay timer so the pump will only turn on after 8 minutes ... which would solve some problems ... I work with these type circuits most of the time, and they function reasonably well... Buy Amp switch/ Current switch/ current transducer

But another problem develops ... you need check valve, and who's to say the pressure of incoming cold water running at 4 gal per minute wouldn't hold the check valve closed ... it would if the pump was wrong. Certainly it holds the check valve closed on existing recirc line.
The whole idea is a long shot at best ... and lot more work than tempering tank(s), which would cause few problems ... but a tempering tank is low-impact and unimaginative, and some folks favor technological solutions.

Warren says The water heater timer is an interesting idea.  I am not clear how to pull that off with the Vertex unit though.   I suppose a digital timer or a wifi timer would work although I don’t know if turning off/on the electric is a good idea for the Vertex computer.

Answer: Timer on the heater is probably unimportant except as a gizmo .... I think AO Smith sells an interface of some type for the Vertex .... they have building management software for the Cyclone series, which is their commercial version of the Vertex ... call them.
Instead of timer on heater, a timer on recirculation system and mixing valve will have benefit.
Disconnecting power to Vertex every day is usually not good idea ... because it would imitate a power outage, causing a surge event that could easily blow out the Vertex circuit board.

Do you have the Vertex plugged into surge outlet (not a surge module, a true surge outlet) ... the closer the surge protection, the more it protects. Multiple surge protection on one circuit is not a problem. I read that replacement circuit boards cost $1000 just for the board, plus labor.
I also hear that labor to replace parts on Vertex is equally expensive, and labor is not covered by warranty after first year.
Never connect Vertex, or any plug-in water heater to a GFCI or AFCI protected circuit. GFCI and AFCI will cause lockouts and error codes.
Buy: Surge outlet

Possible solutions for high usage shower
Lowest cost option:
Adding a tempering tank on the incoming cold water line is the lowest cost option.
In this design, the Vertex or heater #2 will always do most of the work.
Recommend using 40-50 gallon electric heater, and then running 30 amp 10 gauge wire to heater to fire it up an hour before shower during the cold winter months.

Tempering tank will increase capacity of heater, and as a result, increase first hour delivery.
First hour delivery can increase by around 40-50 gallons if heater is energized.

The recirculation system return line should connect to the Vertex ... and connect at the drain valve. Add check valve between heaters if recirculation is used.
Resource:
Add a wifi or zwave control to turn heater on-off
Connect both Vertex heaters in series
One of the ideas presented by Warren was to connect both of his Vertex heaters in series.
Heater 2 that supplies hot to the shower can be connected to incoming cold water line -or- connected to Heater 1 ... but not both.
My initial reaction was that series would work ... except both heaters have recirculation systems that serve different areas of home ... and that re-circ systems would cause endless problems.
After making this illustration, it because apparent that a solution for the re-circ might be found by installing a check valve on the cold water line that runs between Heater 1 and 2 ... but that would only allow the recirculation system to work on Heater 2.
Heater 1 recirculation pump would push water into Heater 2 and the recirculated hot might never reach other areas that use hot water.
A work around might be possible if a motorized shut off valve was installed on cold water line between the two heaters, and somehow each time hot water was needed in area 2, that the recirculating pumps were turned off and the shut off valve opened... it would take a logic controller and a bunch of remote switches etc... and doesn't seem very plausible.
Larger image
High cost installation and operation:
Adding a storage tank to a 50 or 75 gallon Vertex heater will increase amount of hot water, but cost more than adding a tempering tank.
It will also cost more to operate due to multiple recirculation pumps and need for Vertex to continually heat the storage tank.

Expansion tank must match the build-up of pressure caused by heating water. Because the design has a check valve on the incoming cold water line. Check valve will stop the pressure from going back out the cold water line, creating a 'closed system' that leads to dangerously high pressures, leaking TP valve etc:
Resource: Closed system

Support the economy:
Buy water heaters from my associate links:

50 gallon Gas water heaters at Amazon
40 gallon gas water heaters at Amazon
50 gallon Electric water heaters at Amazon
40 gallon electric water heaters at Amazon
Hybrid / heat pump/ water heaters at Amazon

Resources:
How to install gas water heater
How to install electric water heater
Troubleshoot gas water heater
Troubleshoot electric water heater
Point of use water heaters
Resources:
Troubleshoot Gas valves with manuals
Icon gas valve

How to replace water heater element Waste King disposal
How to wire tankless electric


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Titan tankless

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Type 1 and type 2 surge protection surge protection
Programmable timers

Woods timers
surge protection
Anode rods anode rods
Recirculation system

Tankless recirculation
Point of use water heater
Install and troubleshoot garbage disposal Waste King disposal
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