Two water heaters
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Install two water heaters
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.
Uses first heater more than second, unless first heater is on timer.
Two water heaters
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 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
parallel water heaters
Two water heaters
Cold water feeds into 1 heater: option #1
Hot water from water heater #1 feeds into #2
Both water heaters supply whole house
Purpose: Twice as much hot water available.

How to control cost:
1) Turn off heater #1 and save $40 per month
2) Use heater #1 as tempering tank to passively raise temperature to ambient room temperature 
3) Install timer on heater #1 or both heaters 
4) More ideas: Insulate pipes, Insulate water heaters, Turn down thermostats and save, turn up thermostats and install mixing valve to increase amount of hot water.
Buy:
Water heater installation kit
Shark bite water heater connection
Resources:
Tempering tank
Compare timers
9 ways to save
Add another thermostat to water heater
Advantages mixing valve
How to increase amount of hot water
Two water heaters
Cold water feeds into 1 heater: option #2 
2 separate hot water lines for different parts of house.
Purpose: Twice as much hot water available.
Turn up water heater thermostats and add mixing valve to moderate temperature on pipe going to bathroom and allow hotter water on pipe going to washing machine.

How to control cost:
1) Turn off heater #1 and save $40 per month
2) Use heater #1 as tempering tank to passively raise temperature to ambient room temperature 
3) Install timer on heater #1 or both heaters 

Resources:
Mixing valve
Temperature needed to kill bacteria
Tempering tank
Compare timers
two water heaters Cold water feeds into 2 heaters: Option #1
2 separate hot water lines for different parts of house.
Increase heating capacity without buying large, expensive heater
Advantage from series: one heater can be controlled by a timer, or home automation.

Plumber's note about terminology:
This is not considered parallel piping according to plumbers. This is just two water heaters sitting next to each other, but is labeled 'parallel' on this webpage for orgianizational purposes. Parallel option #1 shown above is an actual parallel system, and requires the piping on hot and cold to be equal for the heaters to draw down equal. Two water heaters sitting next to each other, shown on left, does not require specific lengths of pipe to work correctly.
parallel water heaters Parallel option #2 parallel 
Both water heaters supply whole house
Increase heating capacity without buying large, expensive heater
Disadvantage from series: both heaters must run unless you install shut off on hot water line isolating one of the two heaters. Uses both heaters equally, but requires both heaters to operate, and cannot add timer to one heater.

Caution: 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. The piping on the hot must be equal for the heaters to draw down equal. Ideally you want 50% from each heater, otherwise one heater will run out of hot before the other heater, and begin to add cold water to the line.

Note: Both water heaters must be ON, and set to same temperature. If one water heater is off, then it will add cold water to the line while other water heater supplies hot.

Buy:
Ball valve
Brass gate valve
40 gallon electric water heater
40 gallon gas water heater

Parallel / piping must be equal on Hot pipes
Piping on the hot must be equal for the heaters to draw down equal. Ideally you want 50% from each heater, otherwise one heater will run out of hot before the other heater, and begin to add cold water to the line.

If water heaters are different size (different heights), then measure pipes and install so both pipes are same length to the point where they meet.
softener on two water heatersEffect of water softener on two water heaters
-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.
-You cannot run hot water into softener. Hot water will damage softener.

Effect of water softener on water heaters in general
-Softener can cause odor problems on rural water well.
-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 add hydrogen peroxide frequently.
Buy: Water softener
Resource: How to replace anode rod

Expansion tank and two or more heaters: A single expansion tank will work, depending on valves and other plumbing factors such as check valves. Purpose of expansion tank is to prevent problems created when pressure from heated water inside tank is confined and not allowed to expand due to installation of valves and check valves installed on the hot water line. Too much pressure can damage tank, cause pipes to burst, cause TP valve to drizzle or weep hot water. Put a pressure gauge on the water heater drain valve to determine pressure Anything over 80psi should have expansion tank. Some local codes require pressure tank on all installations. Consult local code. Match expansion tank size, to your water heating system.
Buy:
Expansion tanks
Water pressure gauges
Temperature pressure gauge
Resources:
TP valve leak troubleshoot
Troubleshoot water heater valves

Pressure reducing valve and two or more heaters: A pressure reducing valve is used when incoming water pressure exceeds 80psi. Too much pressure can shorten life of plumbing fixtures, cause leaks, rupture pipes, and damage heater. A pressure reducing valve can create a closed system that results in water heater causing too much pressure on the hot water line, thus requiring need for expansion tank with every pressure reducing valve installation.
Buy:
Pressure reducing valves
Pressure regulating valves at Amazon
Resource: Closed system / pdf
Two water heaters 20 gallon point of use/ Total 40 gallon
Applies to electric water heaters only
20 gallon point of use water heaters are electric

Typically 120 volt water heaters arrive with one element rated 1500 watt.
Element voltage and wattage shows on end of each element.
Watts divided by volts equal amps
1500 watts divided by 120 volt = 12.5 amps ... use 20 amp breaker
1500 watt will raise 6.9 gallons from 60 to 120 each hour.
Buy:
20 gallon water heater

Resources:
Figure volts amps watts for water heater
How to install point of use water heater
How to install electric water heater
gas meterGas supply and two water heaters:
When adding second gas water heater, total BTU of both heaters must be calculated. Call local gas company. Read label on side of each tank for BTU rating. For example both heaters together 80-100,000 BTU, and gas pipe length is 80 feet, then 3/4" pipe is needed.
Resources:
Gas pipe chart
How to install gas water heater
two water heaters with circulating pumpParallel Two water heaters with circulating pump/ applies to gas or electric heaters

Recirculating system is handy for folks wanting faster hot water supply at distant faucets. Ideal home construction clusters usage points near water heater to increase efficiency and reduce costs, but many oversized homes and multi-dwellings are not able to meet that level of efficiency and a recirculating pump is chosen for convenience.
Note: Recirculating pumps have different installations, and piping configurations. Some pumps must be installed on vertical section of pipe, and some systems have a direct return line entering the drain valve port.
Resource:

Recirculating systems
 Recirculating system for Tankless

The problem of distant faucets can be solved with a small point of use heater, plumbed in different ways to meet different needs. Resource: How to install under counter heater

Easiest choice for recirculating system is to have Parallel heaters as illustrated on left.
Recirculating system can also be used with Series heaters. However, in a series system, if Timer is used on heater #1, or heater #1 is turned off except when demand requires more hot water, then best recirculating plan is direct return line that goes into drain valve port on heater #2 and does not enter both water heaters on the common cold water line.
.
Resources
How to install gas water heater
How to install electric water heater
Recirculating systems

vent for 2 heatersNote about installing 2 gas water heaters.
If current installation has 1 gas water heater, then venting and air requirements for 2 heaters is different.

Vent: Illustration shows basic connection for 2-3 water heaters on same vent. DO NOT ASSUME the numbers apply to every installation or your installation.
Both heaters can be connected to 1 vent, but the vent must meet specifications to meet BTU per hour operation, number of elbows, horizontal distances, vertical distances.

Air supply: New heaters cannot be installed in a dusty, dirty environment or the air intake screen can get clogged and cause FV failure. Two water heaters mean more air intake into house. The mechanical space might need a pass-thru vent fan to draw air from another part of house.

Gas supply: The gas meter is rated for certain level of BTU consumption. Gas supply line that is designed for 1 heater will not supply enough gas for a second heater without running larger gas line.. unless total BTU of 2 new heaters is equal to the BTU per hour rating of old heater. Too much BTU draw by one heater or two heaters can starve other appliances. Call gas company for specifications, or refer to gas chart pdf. Low gas pressure can be tested using manometer, or more easily, observe other appliances.

Tankless: Some folks imagine that tankless with solve all issues of hot water. And there are applications where tankless is suitable substitute. But the same problems of vent, air supply, and gas supply also apply to installing tankless to replace tank-type heater. Why? The tankless must quickly heat water as is passes in a pipe, consuming more fuel per gallon of heated water than tank-type heaters.
-The need for greater fuel consumption also produces hotter and more vent by-product, requiring larger stainless steel or PVC vent pipe and not the typical galvanized pipe used for tank type heaters. The tankless will fail to turn on if incoming cold water exceeds demand. This is especially true in winter.
-Other issues include failure of parts if water hardness exceeds 11 grains. A floor drain is needed for acidic condensation that drips down from the vent line.
-Maintenance for tankless exceeds tank-type heaters, and includes annual deliming and cleaning water filter. Unlike tank-type heaters, the tankless cannot be repaired DIY, and local technician might not be qualified to repair your model. I do not recommend tankless.
Resources:
Troubleshoot water heater vent
Troubleshoot water heater air supply
Read about FV failure
Gas pipe sizing .pdf

Full image
High volume system
Two water heaters and circulating system
Shows return line installation, aquastat (thermostat) on hot water line, and 2-80 gallon heat pumps.
This illustration shows the drain vale used as entry point for both cold water and recirculated water.
The advantage might be using the open cold water port to add second anode rod and extend life of tank, or at least extend the length of time between anode rod inspections.

Buy:
Grundfos circulating pumps
Grundfos clip-on aquastats

Resource:
Recirculating system designs
Aquastat manual
Calculate first hour delivery
Calculate recovery
Replace anode rod
Plumbing for 2 water heaters and Storage tank pdf
Plumbing for 3 water heaters pdf
Plumbing for 2 water heaters and Recirculating system pdf
Larger image
Super high volume system
Storage tank and heat pump
Purpose to increase storage capacity to meet high volume demand.
Typically, a gas water heater is used for high volume systems, and not heat pump. Storage tanks are available in 150-200 gallon etc and rated for high temperature applications.

Tankless will stall and quit heating when demand exceeds parameters of unit. Even 3 tankless heaters can stall.

Heat pump supplies hot water that circulates into the storage tank.
Storage tank thermostat controls circulating pump, so when water drops below set point inside storage tank, the heat pump replenishes the hot water.
Storage tank can contain super-dangerous 140+F water. The mixing valve located on top of storage tank, adds cold water to the line, so end user receives 115F water.
This strategy increases the amount of hot water available to meet high volume demand. Typical bath-shower is 104-110F.
Buy:
AO Smith Storage tank
Rheem thermal storage tank
Resources:
Advantages disadvantages mixing valve
Plumbing for 2 water heaters and Storage tank pdf
Gas water heater timer
Larger image
Timer or zwave or wifi for gas water heater
Can be installed on Honeywell gas control valves, and on mechanical valve as shown.
Each water heater must have separate control if desired.
Use ordinary 24-hour timer as shown, or use programmable 7-day timer, or use wifi-zwave-Insteon house automation module. Simply plug timer module into gas water heater timer, and choose when to dial temperature down to 50 vacation setting, and then return to Hot setting.

Gas supply and two water heaters:

When adding second gas water heater, total BTU of both heaters must be calculated. Call local gas company Read label on side of each tank for BTU rating. For example both heaters together 80-100,000 BTU, and gas pipe length is 80 feet, then 3/4" pipe is needed.

Buy:
Gas water heater timer at American Pacific
Enter coupon code: GENO to save $5
Resources:
Gas pipe chart
Instructions for gas water heater timer
Plug-in timers
Control 2 water heaters
Larger image
Timer or zwave or wifi for electric water heater
Each water heater can have different schedule

Resources:
Water heater timers
EH40 water heater timer
Zwave controller

WHQ4 Water heater timer  WHQ water heater timer One timer for 2 water heater
Use 40 amp circuit breaker and 8 gauge wire from breaker box
Use 8 gauge wire between timer terminals
Use 8 gauge or smaller 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.

Resource
How to wire GM40 series timers
Wiring for two water heaters 2 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

Resources:
How to install water heater thermostats
Add another thermostat to water heater
How electric water heaters work
Two water heaters
Actual installation photo.
Larger image
Two water heaters
Where to connect wires going to second tank. Actual installation photo.
Larger image
Two water heaters
Larger image
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
Wiring for two water heaters 1 ordinary non-simultaneous water heater and 1 off-peak water heater on one 30 amp wire and breaker
Applies to electric water heaters



Resources:
How to install water heater thermostats
Add another thermostat to water heater
electric thermostat on gas water heater  electric thermostat on gas water heater
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
Upper and lower thermostats
thermostat temperature range
Buy residential upper and lower thermostats
Thermostats are interchangeable/ each works with your water heater
Temperature range 90-150 Therm-o-disc/ reset trip @ 170
Temperature range 110-160 Aprom/ reset trip @ 180
Local YMCA hot tub = temperature 105/...  Typical shower-bath = 110F
Rated 120, 208, 277, 480 volt AC

Buy 90-150 degree F for residential tank:
Water heater thermostats at Amazon
Rheem thermostats
More thermostats<
89T short/ dual-element thermostat
All therm-o-disc products

Resources:
Therm-o-disc 59T-66T sell sheet/ pdf
Therm-o-disc thermostats
How to install water heater thermostats
Difference between Rheem thermostats
Make lower thermostat from upper

Best to replace both thermostats.
There is no guaranteed way to test thermostats.
If thermostat is sticking, then test includes rotating temperature knob which can temporarily unstick thermostat. So bad thermostat appears to work correctly.
120 volt or 240 volt
If you have 240 volt line, you can change the breaker wiring in the panel into 120 volt. To change 240V line into 120V line... the 240V has 2 hot wires. Move one hot wire to neutral busbar. If you connect 240 volt to 120 volt element, the element will burn 4 times hotter/ and burn out.
If you want to convert 120 volt water heater to 240 volt, then element must be changed. Buy elements rated for 240 volt.
Thermostats are single-element type with 2 screws on right side of thermostat Thermostats do not need to be changed if each tank is wired for 120 or 240 volt.
If heaters are wired together, then change thermostat to dual-element type with 3 screws on right. 120 volt and 240 volt wire the same way at top of tank... either wire to either wire unless specified
Wiring diagrams below can apply to electric water heaters.

Resources:
How to wire thermostats and elements
How to troubleshoot electric water heater
How-to-wire-water-heater-with-switches-and-timers
How to wire off-peak water heater thermostats
Figure volts amps watts for water heater
Convert 240 volt water heater to 120 volts
Match wire and breaker size
How to change thermostats
How to install electric water heater
Chart for figuring heating time
Kwh needed to heat water
Things you can do to increase amount of hot water
9 ways to save with water heater
-Pressure should not drop when two water heaters are installed. Entire household water system is pressurized by incoming water pressure from city supply, mudd district or water well. There is no air inside the tank... or water lines... so all pressure remains the same assuming no restrictions in pipe, no leaks, etc
-You can check pressure using pressure gauge on water heater drain valve or outdoor faucet
Buy:
20 gallon water heater
Water softener
Pressure gauge

Pressure depends partly on temperature setting of each water heater... Over 140 can cause high pressure that expands into the pipes located above water heater. HIGH pressure over 80 PSI can damage tank and plumbing over period of time
- Locating 2 water heaters next to each other 'might' cause a closed system if both water heaters are set to fairly high temperatures.
-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
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

How to install tankless recirculation


Install point of use water heater
Titan tankless

Contactors/ relays
Install and troubleshoot garbage disposal Waste King disposal
Programmable timers

Woods timers
surge protection
Anode rods anode rods
Recirculation system

Tankless recirculation
Point of use water heater
Type 1 and type 2 surge protection surge protection
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Electronic parts

Industrial supplies

Tools and home improvement

Home and garden

Hayward pool
Pentair pool

Southwire electric
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Tools

DEWALT
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SKIL
Makita
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Bosch

Irwin Hand tools
Klein hand tools
Fluke electrical testing
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Campbell Hausfeld air powered
Tekton automotive

Powerbuilt automotive
Saws and saw blades
box timers Resource:
Compare box timers
DPDT timers
Intermatic
Control water heater w/ Z-wave
Intermatic control centers Resource:
Intermatic control centers: and parts
Digital control centers and parts
Tork control centers and parts
Subpanels
All control centers and parts
Enclosures
programmable timer Resource:
Programmable timers
Countdown timers
Off delay timers
On delay timers
Timer modules
Din rail
Twin & one-shot
Woods
Intermatic
Tork
Low voltage power packs Recirculation systems
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