How to install two water heaters

Two water heaters

Two water heaters
INSTALLING TWO WATER HEATERS

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2024

SERIES: Cold water feeds into 1 heater:
Applies to gas and electric water heaters

1st water heater feeds into 2nd water heater
Advantages: 1st tank can be turned off and serve as tempering tank, Turn 1st water heater off except when peak usage is expected.
Double water heating capacity, avoid buying large expensive water heater, lets you fill oversized bathtub without buying 80 gallon heat pump water heater..

Cold water feeds into 2 heaters
Applies to gas and electric water heaters

Cold water feeds into both water heaters. Use this design when house has water line to two different parts of house. Advantages: Turn 1 water heater off except when needed, avoid buying large expensive water heater, lets you supply different temperature water to areas of home.

SERIES Heaters can be any type (gas electric, solar etc) Heaters can be any size and temperature. Heaters do not have to sit next to each other or be plumbed to exact specification etc.
SERIES option #1 Hot water from water heater #1 feeds into #2. Both water heaters supply whole house. Advantage: Repurpose old heater. Twice as much hot water without buying high-tech electronic water heater that cannot be repaired because parts are not available. How it works: Heater #2 is nearest to taps (faucet, shower, etc) so it supplies hot water first, and heater #1 immediately starts adding hot water to the bottom of heater #2 as hot is consumed. This means heater #1 does most of the reheating, and does most of the work, and wears out sooner. How to control cost: 1) Turn off heater #1 and save $20-40 per month 2) Use heater #1 as tempering tank to passively raise temperature to ambient room temperature 3) Install timer (or home automation or switch etc) on heater #1 or both heaters 4) Insulate pipes, Insulate water heaters, Turn down thermostats and save, or turn up thermostats and install mixing valve to increase amount of hot water. Buy: Water heater installation kit Shark bite water heater connection Resources: 9 ways to save Add another thermostat to water heater Advantages mixing valve How to increase amount of hot water
SERIES option #2 2 separate hot water lines for different parts of house. Purpose: Increase amount of available hot water. Turn up water heater temperature and add mixing valve to moderate temperature on pipe going to bathroom and allow hotter water on pipe going to washing machine. The first heater does most of the work and wears out sooner. Resources: Mixing valve Tempering tank Compare timers
Cold water feeds into 2 heaters: This is just two water heaters sitting next to each other. It is not series or parallel. 2 separate hot water lines for different parts of house. Increase heating capacity without buying large, expensive heater Advantage: Either heater can be any temperature or any size or type heater. Either can be controlled by a timer, switch, or home automation. NOTE.. Do not use circuit breaker as a switch on regular basis ... occasionally turning off breaker is ok. Install 30 amp toggle switch. Read
SERIES with recirculation This is a possible solution for high usage shower. A new home had two 50-gallon AO Smith Vertex condensing gas water heaters, and each was plumbed separately, same as shown in image above. Heater #1 supplied hot for rest of home while heater #2 supplied a single bathroom with high-usage shower. Heater #2 could not keep up with the shower. The final fix was a flow restrictor on shower, but one suggested solution was to connect both heaters in series as illustrated. Because heater #2 had a recirculation pump, then a check valve might be needed between heaters. The check valve could cause pressure issue, requiring expansion tank between check valve and heater #2. If heater #1 also had a recirculation system, then this design wouldn't work. Resource: Problem with Vertex heater
SERIES with recirculation and tempering tank Simply add electric heater #1 to act as passive tempering tank, no need to connect to power,... and the tank passively pre-heats very cold incoming water to room temperature, making it easier for heater #2 to heat water. This is good strategy for pre-heating water in winter months before it enters bottom of heater #2. A tempering tank might not work efficiently with a recirculation pump unless there is a direct return line as illustrated. Why? If the return is on the cold water line, then the returning warm water will travel through the cool water inside the tempering tank before it re-cycles through heater #2, which will waste heat.
Cold water feeds into 2 heaters: This is not series or parallel. This is just 2 heaters next to each other. Both heaters are not turned on at same time .... instead the indirect heater supplies hot water during winter, and heat pump supplies hot during summer. If both heaters are turned on at same time, then this design is an incorrectly plumbed 'reverse' connection that will give unknown result. If home has a recirculation system, then return on cold water pipe, or direct return into both drain valves will probably work fine. How it works: Indirect heater receives hot water from a boiler, or geothermal loop, or solar collector etc. The boiler heats the house in winter and also heats water inside the indirect heater via the spiral heat exchanger, where hot water from the boiler circulates through the heat exchange coil and then back to boiler. This produces large amount (200+ gallons first hour) of DHW or domestic hot water, for faucets, shower etc, available from pipe connection at top of indirect heater. Running the boiler in summer might be expensive, so a home with modest hot water consumption can install 80 gal heat pump (82 gal first hour) for hot water, then turn boiler down or off in summer to save money ... and maybe extend life of boiler ... but the price of a heat pump might be greater than any savings. FYI, older boiler equipment that is turned off in summer, might not start again when needed. This water heater concept could be applied to solar or geothermal water heaters etc.... or you can heat both tanks and use home automation to activate motorized ball valves on each heater to select which to use.

Parallel and Reverse
Heaters must be the identical type, size and temperature
Pipes must be installed correctly to achieve equal draw from each heater

Parallel / piping must be equal on Hot and Cold pipes
''Piping on the hot and cold must be equal for the heaters to draw down equal.'' Ideally you want 50% from each heater, otherwise one heater could run out of hot first, and start adding cold water.

Pipe diameter, elbows and distance must be identical, for the Hot to be drawn equally. Obstructions in the pipe can impact outcome, so new plumbing should be installed on both heaters.

If water heaters are different (not identical), then parallel is not recommended ... but you might try measuring pipes carefully and install so all pipes are same length as illustrated ... then adjusting thermostats on each heater to see if equal draw can be achieved.

Two heaters must be same size for parallel and reverse
Parallel is not best choice if you have two different size heaters. Why? Because even if the heaters have same BTU output, the small heater will run out of hot before the big heater, and then small heater will start adding cold water into the hot water line.

Two heaters must be same type for parallel and reverse
Parallel is not best choice if you have two different types of heaters, for example a gas and electric heater, or gas and indirect heater, even if both are same size. Why? Because the gas will outperform the electric, or indirect outperform the gas etc and then one heater can start adding cold water into the line.
With parallel, the temperature setting should be close to identical on both heaters, except equal draw might require adjusting thermostats so each heater performs the same. Be mindful that thermostat settings are approximate and each thermostat will have a different calibration ... or in some cases a wide difference in calibration, for example with gas heaters, the range of temperature drop before tank fires up again can be 10+° ... and each heater is different from the other. This might be a problem that requires changing a thermostat.

On electric heaters, it's cheap and easy to swap thermostats to solve a calibration issue, but gas valve thermostats are expensive to replace. Do NOT expect manufacturer to supply a new gas valve under warranty over a calibration issue .. they will argue that it's normal.

Parallel: Cold water feeds 2 heaters: Parallel is defined as: ''Equally manifolding the inlet water pipe and outlet water pipe in multiple heater installations. Having equal number of fittings; as well as lengths of pipe.'' Both heaters are used equally. Advantages Both water heaters work together to supply whole house. Recirculating pump can be added to the system. Increased hot water capacity without paying premium price for high tech electronic heater or less efficient 75-100 gallon commercial grade heater. Review Polaris condensing heater Problem with Vertex condensing heater Review Marathon electric heater Commercial 100 gallon gas heater specs pdf Disadvantages: Both water heaters must be identical. Both heaters must be functioning. If one heater malfunctions, it will add cold water. You cannot turn off one heater to save energy. To solve some disadvantages, install shut off valves (gate valve, ball valve etc) so each heater can be isolated. Caution: 'Balancing in parallel is tricky and often one heater does all the work and the other just sits there.' If the heaters are not piped properly, it will NOT double the capacity and the system will not be efficient. One heater will supply more than the other heater.'' Buy: Ball valve Brass gate valve
Parallel Two water heaters with circulating pump/ applies to gas or electric heaters This example shows circulation pump on hot water line. At farthest faucet, there is a one-way 'crossover valve,' and then the cold water line is the return line. When cold water is turned on at any faucet, the crossover valve closes off and the faucet works normally, which stops the flow of water from the recirculation system. The crossover valve makes it possible for each tap to work correctly, and when not is use, the recirculation system supplies 'instant' hot at each tap. Ideal home construction clusters usage points near water heater to increase efficiency and reduce costs, but not all homes and multi-dwellings can meet that objective and so a recirculating pump is installed to reduce wait-time for hot water. Note: There are different piping configurations for recirculating systems. Some pumps must be installed vertically, others horizontal, some systems have a direct return line entering the drain valve port. Resource: Recirculating systems Recirculating system for Tankless The problem of hot water at distant faucets can be solved with a small point of use, or under counter heater instead of recirculation pump. Installation is easiest using electric heater since it does not require vent pipe through roof ... and the heater can plumbed different ways to meet different needs. Resource: How to install under counter heater . Resources How to install gas water heater How to install electric water heater Recirculating systems
Parallel Two water heaters and circulating system with direct return line Shows return line installation, aquastat (thermostat) on hot water line, and two 80 gallon AO Smith heat pumps. This illustration shows the drain valve used for both functions ... to add cold water entering tank, and as direct return for recirculated water. Buy: Grundfos circulating pumps Grundfos clip-on aquastat Resources: Full image High volume system Recirculating system designs Aquastat manual Plumbing for 2 water heaters and Storage tank pdf 2 commercial HP heat pumps and storage tank pipe diagram /pdf
Footnote on recirculating pumps According to AO Smith, on commercial sized tanks, '' Field-installed circulating pumps should be of all bronze constructions .... it is recommended to utilize a pump and recirculation line sized to turn the entire storage capacity of the tank once each hour (i.e., a 600 gallon tank would require a 10 gpm pump)''
Four heaters plumbed in 'reverse' Plumbed in reverse is a type of parallel requiring identical heaters, and identical plumbing as shown, with each heater supplying equal amount of hot water. When heaters are plumbed in reverse, then the first heater to receive cold water is the last heater to supply hot water. The American Standard image shows heaters 1234 .... heater 4 receives cold water first .... so 4 supplies hot water last. Heater 1 receives cold water last, so 1 supplies hot water first. Note: image shows high efficiency and high volume gas condensing heaters from American Standard, with the vertical vent and condensation drain located at bottom of heater. Resource High-Efficiency-Models.pdf Bradford White plumbing for 3 water heaters in reverse pdf Plumbing for 2 water heaters in reverse and Recirculating system pdf AO Smith 2 water heaters in reverse pipe diagram /pdf AO Smith 3 boilers in reverse pipe diagram /pdf

How to Balance two parallel or reverse water heaters

Typically, precise balancing is hard to do ...
One strategy might be to set the thermostat different on each water heater.
Set container under TP overflow tube, open TP valve on each heater and get baseline temperature for each using cooking thermometer, then average two temps.
Run Hot tap at bathtub and measure temperature... if the bath temp equals the average temp on both heaters, then the heaters are balanced.
How you would correct the difference, and possibly balance heaters better, is to keep adjusting thermostats until average is reached

Which is the best design, Series or Parallel?

First off, you might NOT have a choice except series unless you have two identical heaters for parallel.
Secondly, Parallel is a challenge getting each water heater to balance into equal draw. It might be impossible unless heaters have identical BTU rating, size, age, etc, and there is still no guarantee that the pair will stay balanced over time.

Does this mean Series is better?
Maybe yes, especially if heater #1 can be turned off except during peak usage times. This gives Series an advantage for installations where higher usage is occasional, such as weekends or filling a large bathtub occasionally. Also, series is an easy installation.

Sales manager from Lochinvar emailed a disadvantage for Series.
Lochinvar manager says 'if tanks in series are high usage, then tank #1 is doing most the work, and will wear out sooner than tank #2, possibly twice as fast if usage is very heavy. The continual influx of cold water into heater #1 will produce more moisture and cause more stress on the steel tank.'

The moisture issue would be true for gas heaters more than electric.
AO Smith says in their literature that undersizing a gas water heater can cause 'excessive condensation that will corrode the heater faster. The flue tubes, baffles, and burners are susceptible to hydrocarbons and carbonic acid contained in condensate (moisture created inside the vent). Continual exposure will weaken the flue tubes. Condensation can also spoil combustion [by corroding the burner assembly] and produce a carbon monoxide hazard. Read about CO detector and vent test
This means an overworked heater #1 in Series will fail sooner, and need more maintenance.
Meanwhile, Parallel heaters are supposed to do equal work and each heater should last equally long ... IF the Parallel heaters are balanced.

Actions for a series installation: 1) reducing consumption is best strategy.
2) Try turning down thermostat on heater #1 to mitigate some issues.
3) If you are using an old heater and a new heater, then install newest heater for #1 in series and old heater as #2, and the old one can last a bit longer, or use old heater for heater #1 and replace it soon, when it gives out.
Conclusion: 4) The information is helpful for making a decision, but not an answer for which is best ... series or parallel.
6) If two heaters are in Series, then expect that heater #1 will fail sooner, and need more maintenance.
Resources
How to maintain water heater
How to troubleshoot gas water heater

Different sizes or types ... If heaters are not identical to each other, then Series is best choice. This means a 50 gallon gas and 30 gallon electric, or two different size gas, or 2 different size electric can be plumbed together without problem. Two different tanks plumbed in parallel would be a problem. Which size tank should come first? There are no rules except what works. For example if home uses 65 gallon of hot, then install the 50 gallon heater nearest to taps (faucet, shower etc), and 30 gallon farthest from faucets. This might maximize usage out of the 30 gallon, causing less cold water to enter big tank so that big tank re-heats using less energy. Also smaller heaters are slightly more efficient than large heater since the tank has less surface area, and less heat loss. Water heater formulas and terminology Temperature for series Which tank should have higher temperature? There are no hard rules for correct temperature when tanks are in series. Each tank can have different temperature. If tanks are in parallel (shown below), the temperatures should be the same. How-to-adjust-water-heater-temperature There are three ways to look at temperatures. 1) Set both heaters to same temperature and see if it supplies enough hot water. The higher the thermostat setting, the more heat energy is stored inside the tank, so you get more hot water per tank when thermostats are set higher. Ways to increase hot water 2) If you need very hot water right away then closer tank should be set higher and the farther tank can be set lower. 3) Temps over 135° pose risk of scalding ... so set close tank to safe 125-130°, and farthest tank to 150-160° will supply more hot water with lower risk of scalding. 4) Tanks set to different temperatures might cause uneven temperature, or temperature fluctuation at tap. Thermostat settings are approximate, with accuracy +-5% approximately. Water temperature can drop 10-15° at bottom of tank before heater turns on. Manufacturer will argue this is normal, but might be a problem for you. Calibration issues require thermostat replacement. Check temperature at top of tank via the TP valve. Check temperature at bottom of tank via the drain valve. Then compare before deciding if calibration is the problems vs crossover where a single-handle faucet cartridge is bad Read about crossover If TP continues to drip afterwards, then Do NOT cap off TP valve; gently raise and lower the lever, or replace TP. If drain valve continues to drip afterwards, install a brass hose cap. Buy: Kitchen thermometer Temperature gauges Buy: Buy replacement brass drain valve Brass hose caps
Water softener and expansion tank for two water heaters -Applies to gas or electric, parallel or series etc. -One softener can supply soft water for two heaters, any configuration. -Softener cannot be put on hot water. It must be installed on cold water. -Hot water will damage softener. -Use pressure reducing valve if water pressure is over 80psi .... target is 50-60 psi ... but less 40psi and plumbing lasts longer. -Use expansion tank if cold water line has check valve or pressure reducing valve. Expansion tank is installed on cold water line anywhere between water heater and pressure reducing valve or check valve. Identify and repair water heater valves Effect of water softener on water heaters in general -Softened water will deplete the anode rod, causing rusted tank - If you have odor problems, or tanks rust out frequently, the softener is suspect. Turn down the softening, or remove softener, or replace anode rod frequently, or install power anode rod, or deodorize by adding hydrogen peroxide frequently.
-Pressure should not drop when two water heaters are installed. Household water system is pressurized by incoming water pressure from city supply, mudd district or water well. There is no air inside the water heater... or inside water lines... and water will not compress ....so pressure remains the same throughout house assuming no restrictions in pipe, no leaks, etc -Exception: Some city and community water supplies will cause pressure variations ... with high pressure causing potential damage to heater and plumbing. -Check pressure using pressure gauge on water heater drain valve or outdoor faucet. Check outdoor spigot and compare with water heater. Check with neighbor etc to make decision if pressure reducing valve is needed. Expansion tank is easy first-step solution. Buy: Pressure gauge Pressure depends partly on temperature setting of both water heaters ... Over 140° can cause high pressure that expands back into the pipes located above water heater.If you have check valve between heaters, then the pressure can go over 80 PSI and damage tank and plumbing ove time. -To stabilize pressure, use expansion tank on incoming cold water line Buy: Expansion tank Pressure reducing valve Resources Low-hot-water-pressure Troubleshoot water-heater-valves

Storage Tanks
AO-Smith-commercial-storage tank-manual.pdf

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High volume system
Storage tank and heat pump/ DIY
This design is good for homes that encounter high usage at peak times. For example apartment building where people take baths, showers etc at approximately same time.

Typically, a gas water heater is used for storage systems, and not electric heat pump, since gas heated will produce more hot water faster than electric. Storage tanks are available in 150-2000+ gallon etc and rated for high temperature storage.

How it works:
Storage tanks store hot water. They do not heat water.
Storage tank has a thermostat (aquastat) that controls the circulating pump, so when water inside storage tank drops below set point, the circulating pump turns on, and hot water from heat pump replenishes the hot water inside storage tank.

Storage tank can contain dangerous 140+°F water. A mixing valve located on top of storage tank automatically adds cold water to the hot line, which tempers or cools the extremely hot water to a safer 115-120°F.
Always follow piping instructions given with mixing valve, one-way check valves, expansion tank, pump etc for best result. For example, typical check valve must be installed on horizontal line (not verticallY), while a mixing valve might require at least 12" of vertical hot pipe to work correctly.
Buy:
AO Smith Storage tank
ao-smith-commercial-storage-tanks-specs-t.pdf
Resources:
Advantages disadvantages mixing valve
Plumbing for 2 water heaters and Storage tank pdf

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This is a very high volume system with a 50 or 75 gallon AO Smith Vertex condensing heater and large storage tank.
"The Aquastat or temperature control on storage tank should be installed no higher than bottom 1/3rd of tank.
Storage tank temperature control is wired to the pump located between heater and storage tank.
Set water temperature of Vertex heater 5°F higher than storage tank temperature control.
Most mixing valve manufacturers furnish detail piping instructions for properly installing their mixing valves."

Expansion tank must be sized correctly to match the pressure caused by heating water.
The design has a check valve on the incoming cold water line. Check valve will stop the pressure from expanding back out the cold water line, creating a 'closed system' that leads to dangerously high pressures inside heater and in the pipes, leaking TP valve etc: Expansion tank absorbs the dangerous pressure buildup that would otherwise damage heater etc.
Resource: AO Smith .pdf

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Illustration shows hot water system with two Bradford White atmospheric gas water heaters, plumbed in reverse (parallel), along with a storage tank.
Illustration shows pump for circulating hot water from the heaters into the storage tank. Arrows show direction of water flow, and show installation of check valves (back-flow preventers), expansion tank, shut off valves, TP valve for each heater along with discharge tubes, TP valve and discharge tube for storage tank, floor drains etc.
All parts shown are required for a successful installation.
Resources:
Storage tank and booster-heater.pdf
Tank plus booster heater

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Commercial HPWH (electric) Heat Pump Water Heater and storage tank
Purpose: convert wasted heat into hot water.
60,000 to 259,000 BTU to 2 million BTU

How it works:
Wasted heat can come from any source (like exhasust from generator or factory operation etc) .... it can be hot or warm, and HPWH models are made for either air or water.

The Heat Pump uses the refrigeration cycle to pull heat out of the waste heat ... causing the heat pump refrigerant line to get hot ... then the hot refrigerant is cycled through a heat exchanger to heat water.

The heated water is circulated through bottom of storage tank, then back into the heat pump HPWP.
The circulation is activated by a thermostat located near bottom of storage tank. Circulation pump is inside the HPWH enclosure.
The storage tank does not have a sealed circulation loop like an indirect heater, and instead all water from HPWH is potable and is mixed with incoming cold and outgoing hot.

Resources
AO-Smith-Heat-pump-storage-tank.pdf
HPWH with storage tank and additional heater .jpg
HPWH with storage tank jpg
Two HPWH with two storage tanks .jpg
Storage tank manual
Air source manual
Water source manual

Booster Heater

A booster heater is used when commercial facility needs very hot water for washing dishes, laundry, industrial process etc.

How it works:
The 1st heater performs two jobs. 1) It supplies 120°F hot water for faucets etc, and 2) it adds 120°F hot to the booster heater either though top or side connection.
The booster heater receives no cold water. Instead it receives only the 120°F hot water from the 1st heater.
Booster heater then boosts, or heats, the 120°F water up another 40-60° to reach 160-180°F, with max temperature usually at 180°F.
Maximum water temperature for any water heater is 210°F to prevent steam explosion. TP valve releases water at 210°F. Never cap off a TP valve.

Commercial facility with booster heater will have 2 hot water lines. One line with 120°F water for faucets etc, and another line with 180°F degree water for laundry, dish washing etc.
180° poses very high risk of scalding if put into water lines going to faucets, showers where is comes in contact with people.
If there is remodel work to facility, it is important to understand the booster heater, and recognize danger of accidentally co-mingling 120° and 180° water sources.

Resources:
Source
booster-heater-with-two-water-heaters.pdf
booster-heater-and-two-water-heaters-diagram.pdf
Booster-heater-with-1-water-heater-diagram.pdf

Indirect heater and Boiler

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Another image
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Indirect Heater and Boiler
Fig 1 shows boiler and indirect heater operation. Purpose is for home heating plus large amount of potable hot water for faucets, shower, drinking etc.

Indirect heater can supply 200-400+ gallons first hour delivery of potable (drinkable) hot water for high usage homes and business.

High heat
Boilers are not DIY. Boiler MUST be installed and maintained by licensed plumber with boiler experience due to high pressures and temperatures from boiler.

A boiler super-heats hot water using gas or oil at a rate of 150,000- 300,000+ BTU per hour producing near-boiling 200°F water temperature (a fatal temperature).

"By code, the maximum outlet water temperature for any type water heater is 210°F (98.5°C)," including boilers, household heaters, commercial heaters, industrial heaters, indirect heaters, storage tank, booster heater etc.

Above 210°F risks steam explosion that will destroy building. The boiler thermostat limits temperature to less than 210°F, and in event of malfunction, then a correctly-rated TP pressure-temperature release valve will release more hot water than the maximum BTU output produced by that specific water heater.
To prevent explosions, all water heaters, of any type or size, must have a TP valve with a BTU rating that exceeds BTU rating of heater. The TP must be installed within 6" inches of top of heater, and TP cannot be capped off.

The boiler performs two functions.
1) Heat living space by circulating 200°F hot water in a continuous loop (boiler loop) from the boiler, through a series of pipes and radiators located in each room, and back to boiler.
2) Heat water inside the indirect heater. It does this by circulating 200°F water in a loop from the boiler, through the coiled heat exchanger located inside indirect heater, and back to boiler.
Water from boiler is confined inside the loop, and contains rust inhibitor, and possibly antifreeze etc. The chemical-laden boiler water is never mixed with the potable (drinkable) water inside the indirect heater. Resource: Greestar boiler chemistry pdf

How it works:
-Note that an indirect heater does NOT heat water on its own ... instead the indirect heater has a heat exchange coil that receives hot water (or other hot fluid) from a boiler, geothermal, or solar collector etc.
-The indirect heater has it's own thermostat, separate from thermostat on boiler. The indirect thermostat controls the circulation pump, so when water inside the indirect drops below set point, the pump circulates hot water from the boiler, through the heat exchanger, and back to boiler in a continuous loop.
-When water temperature inside indirect heater reaches thermostat set point, for example 120°F, the circulation pump turns off, and water stops circulating from boiler until indirect thermostat calls for heat again. Read more detail about boiler

Resource
Pipe diagram for 3 boilers
Typical boiler-indirect manuals:
Indirect service manual
Boiler instruction manual
Boiler installation / service manual
Commercial boiler install/ service
What size indirect heater is needed

Timers for 2 electric heaters
	Timer or Zwave or Wifi Two 30-40 amp devices of any type Each water heater can have different schedule Resources: Compare Water heater timers EH40 water heater timer 40 amp Zwave controller 40 amp Wifi controller
	One timer for 2 water heaters 10,000 watt, 40 amp GM40 series timer. Use 40 amp circuit breaker and 8 gauge wire from breaker box Use 8 gauge wire between timer terminals Use 8 gauge or 10 gauge wire to each water heater Do not mix different gauge wires under same screw terminal or smaller gauge wire can come loose and become fire hazard. Buy: GM40AV 120-208-240-277V Resource How to wire GM40 series timers

2 electric heaters on 1 circuit breaker

Two ordinary residential non-simultaneous water heaters on one 30 amp wire and breaker
Wiring illustration
Applies to electric water heaters only

Important: install upper thermostat in place of Lower thermostat
This will give ECO protection to lower thermostat
You want to heat tank #1 first... and then tank #2
Or you can wire each one separately using 2 separate circuit breakers and 10 gauge wires
wire water heater thermostat

Buy
36" drill bits
fish bits

Resources:
How to install water heater thermostats
Add another thermostat to water heater
How electric water heaters work

Actual installation photo.
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Where to connect wires going to second tank. Actual installation photo.
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1 ordinary non-simultaneous and 1 redundant water heater on one 30 amp wire and breaker
Applies to electric water heaters

Electric:
Install upper thermostat in place of Lower thermostat
This will give ECO protection to lower thermostat
You want to heat tank #1 first... and then tank #2
Or you can wire each one separately using 2 separate circuit breakers and 10 gauge wires

Resources:
How to install water heater thermostats
How to wire water heater thermostats
Convert 240 volt water heater to 120 volt
Add another thermostat to water heater

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Add another thermostat to gas or electric water heater
Control recirculating pump
Control electric water heater with gas water heater
Control solenoid valve to turn hot water ON

Resource:
Add another thermostat to water heater/ step by step

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This wiring design, both hot wires to element are cut off by contactor.
Do NOT connect 240 volt 30 amp water heater circuit diectly to PID... the PID is rated for 3 amp, not rated for 30 amp, so it will turn contactor on-off ... and 30-40 amp contactor safely switches 240 volt 30 amp on-off

Contactor turn on lower element when water temperature drops below set point.
Advantage: exact temperature reading at probe

Buy:
PID Temperature controllers
Digital temperature controller
30 amp contactors

Solid State Relay Board for Arduino

Resources:
Contactors
Select breaker and wire size
PID manual .pdf
PID manual2 .pdf
Instructions:
Re-wire water heater for PID
Insert probe near lower thermostat.
Position probe against tank wall and cover w insulation

Wire thermostat and element as shown
240 volt line for water heater MUST be routed through ECO located on upper thermostat.
Why? Because ECO is safety device that will cut power to element in event of overheating.

PID is powered by 100-240 volt. Models ITC-106RH for contactor and ITC-106VH for solid state relay. Models vary.
2 hot wires are needed for 240 volt.
Set temperature controller program.
When temperature probe senses cold water, then PID activates contactor
Contactor switches 30 amp water heater circuit, and lower element tuns on.
Lower element keeps tank hot.
Buy:
PID with relay output and thermocouple/ ITC-100HR

Buy
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