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Will recirculation pump work on Tankless heater?
Tankless recirculation system
Larger image
Best design
20 gallon tank will heat recirculation water
Tankless manufacturers suggest installing 20 gallon electric tank heater. Water must circulate through small water heater to ensure there is hot water in the recirculation loop. Reason: small pump cannot reliably activate tankless burner or element.

Three Illustrations show:
-Top two: direct return line/
-Bottom: without direct return line
-Direct return line is better installation
-System can have faucets that are not connected to recirculation loop... add a 3 gallon under counter water heater to this faucet

Water flow causes pressure
-Check valve can be added on direct return hot water line as shown.
-Install check valve to cold water line on loop without direct return.
The small pump creates pressure. The check valve between tankless and recirculation loop (on top illustration) will keep recirculation pump from pushing water into tankless. Result: the pump is recirculating hot water in the loop.
Using hot water during a shower also causes pressure... and when tankless is being operated, the other check valve will keep water from being pushed into small tank and backwards through recirculation loop.

Water flow can cause suction.
For example if end of garden hose is left inside a pond after filling pond.... and you turn on water tap inside house... the pull of water can draw water from the pond into the house if the outdoor spigot has a small seep.
To prevent problem, add anti-siphon connection on outdoor spigots.

In the case of recirculation pumps, the water flow is not very strong, and the system is a closed loop... a closed loop is not drawing water like an open spigot... and the 1/25 Hp recirculation pump is not moving the water very fast, so drawing cold water into the loop is not an issue.

Which 20 gallon water heater?
For whole house, use water heater with 3/4" pipe connections.
Under counter water heater with 1/2" connections is too small for household size recirculation system. 1/2" is for single faucet
Compare small water heaters

Water heater valves

Which recirculation pump?
Each plumbing system is different... distance, diameter of pipe, condition of pipe, number of elbows, ...  effect flow of water.
Compare recirculation pumps
Why isn't there a stronger recirculation pump?
Faster pump on residential pipe might cause water bubbles, and no water movement,. Result: resistance would overheat pump?
Tankless recirculation system
Larger image
Connect Hot from tankless to cold water inlet on small hot water heater
Tankless recirculation system
Larger image
No return line/ not best design
Check valve
Swing check valve must be installed in horizontal position ... grunfos series household series pump must be installed in vertical position as shown ... consult product manuals ... the illustation shows relative location of parts.
Swing check valve
Swing check valve
Look at directional arrow located on check valve
Make sure arrow is same direction as flow of water.
Most check valves depend on gravity to close the flapper and must be installed in horizontal position. Installing in vertical position stops function.
Swing check valve at Amazon

Low pressure
Does check valve reduce pressure?
Check valve would have minimal affect on water pressure, Unless check valve is stuck with sediment build-up, or lost its seal so water is flowing backwards.

Resources on low pressure
Swing check valve
Check valve swings open when water passes one direction. And swings closed when water tries to pass other direction: Lets water pass one direction, but not other direction.
Why is check valve important?
With some installations, recirculation system loop can draw cold water into hot pipe causing cold flashes during shower. This occurs during heavy use periods.
Or with the direct return loop type installation, the recirculation loop can lose pressure or lose recirculation effect when cold or hot is drawn elsewhere in the plumbing system.

If check valve is used elsewhere in plumbing system, and check valve goes bad, it can cause crossover, where cold enters hot pipe and hot water never arrives at faucet:
Or a bad check valve can cause high or low pressure
Swimming pool, or outdoor ponds: If water hose is left in pool or pond or puddle of water, then using water inside home can draw this unsanitary water back through water hose and into home water pipes. Install swing check valve. Do not leave hoses on ground. Or in pool.
Swing check valve at Amazon

Low hot water pressure
Why air vent?
Recirculation pumps can cause air bubbles as the impeller spins.
Air bubbles cause noise in pump and plumbing pipes.
Air bubbles can cause water to stop circulating, and cause overheated pump.
Air vent releases air trapped in the water line. Air vent has 1/4" pipe thread/ install ordinary plumbing tee and use 1/4" reducing bushing / use teflon tape on 1/4" pipe thread
Install at highest point in recirculation loop.
Buy air vent:
Air vent/ Float vent
expansion tank
Installed on cold water line
Expansion tanks at Amazon

expansion tank
Why expansion tank on cold water line?
Expansion tank is installed anywhere on the cold water line.
In some tankless recirculation drawings, expansion tank is located on hot water line of recirculation loop.
Generally, however, the recirculation loop will not need expansion tank.
However, if the small water heater is set to very high temperatures (135+), then it can cause thermal expansion on the 'closed' recirculation loop.
Remember, the recirculation loop is 'closed' because of the check valve between tankless and recirculation loop. Any closed system is risk for thermal expansion, especially if water heater is set for very high temperature. Tank temperature for the 20 gallon should be 120F or less since typical hand wash would be 110. Temperatures over 140F pose serious risk of scald and should never be present in a residential water heating system unless sterilization is needed. Use a mixing valve and separate hot water lines for safety.
Most cases, the expansion tank on cold water line will ensure that plumbing lines are not vulnerable to thermal expansion, or high pressure that can cause damage to pipes and fixtures and water heaters.
Expansion tanks at Amazon

Thermal expansion and closed system
Expansion Tank Installation pdf
Advantages mixing valve 

Temporary pressure gauge
Avoid water pressure over 80psi
Save cost of water heater replacement and plumbing repairs
Water pressure: Pressure over 80 psi can damage water heater
High pressure will also shorten life of plumbing fixtures, and cause washing machine hoses to rupture, etc.
Symptoms: TP valve goes bad frequently. Cause: Closed system because of check valve or faulty pressure valve, or community water problem or water well pressure set too high.
Test and diagnose TP valve problems
Connect pressure gauge to water heater drain valve to accurately check pressure.
Install pressure reducing valve. Replace check valve. People with water well > turn down pressure on water well > save both hot and cold water, save electricity, and well equipment lasts longer.

Water pressure gauge
Pressure reducing valve
Temperature-pressure gauge
Pressure gages at Amazon

How to test pressure
Test and diagnose TP valve problems
Crossover valve

Replacement crossover valve
Watts sensor valve kit
Applies to Taco, Grundfos and Watts. Does not apply to Laing
Crossover valve installs under sink, and allows hot water to circulate from hot side to cold side. Purpose of crossover valve: When recirculation pump is ON, the crossover valve opens and lets hot water circulate back to water heater through the cold water line. Immediate hot water is available at the faucet. When you turn on cold water at the faucet, the crossover valve closes, allowing cold water to enter the faucet.
Note: Cold is not available immediately at the faucet.
Taco recirculation systems  
Install pump at water heater
Install crossover under sink
Choose timed or thermostat operation
Plug into 120V outlet
1 year warranty with receipt
Taco recirculation systems
General Taco manual
General manual
Figure flow rates for recirculation pumps
Basic pump science/ pdf
Read more about recirculation
Grundfos recirculation systems
Install pump at water heater
Illustration shows typical water heater model
Install crossover under sink
Choose timed or thermostat operation
Plug into 120V outlet/ draws 1/4 amp
2 year warranty with receipt
Grundfos recirculation systems at Amazon
Replacement crossover valve
Clip-on thermostat control
Cast iron flange set for flanged pumps
General Grundfos product manual
Can use on tankless 0-9 GPM
How to choose proper circulator pump
watts recirculation systems
Install pump at water heater
Install crossover under sink
Choose timed or thermostat operation
Plug into 120V outlet
1 year warranty with receipt
Watts recirculation systems
Replacement crossover valve
Watts sensor valve kit
Watts recirculation system manual
Do not install on tankless
Tankless require .5 GPM
Laing recirculation systems
Laing Thermotech
Install under sink/ built- in crossover valve
Pump and crossover built into same unit
Choose timed or thermostat operation
Plug into 120V outlet
2 year warranty with receipt
Laing recirculation systems
Autocurc manual
ACT 909 tankless recirculation pump manual
E10 autocirc manual
Laing thermotech
Read more about recirculation
Reliance water heater
6-10 gallon
Side connections/
6 year warranty
Reliance water heaters
12-3 extension cords
12-3 Southwire cable
Reliance manual
State manual
American tiny titan water heater
6-12 gal
Top connections/
6 year warranty
American tiny at Amazon
12-3 extension cords
12-3 Southwire cable
American manual
State point of use water heater
2-6-10-20 gallon
Side connections/
6 year warranty
Rheem point of use
12-3 extension cords
12-3 Southwire cable
Rheem manual
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