I have a rinnai tankless closed loope hot water heater. it has a recirculating pump and a 6 gallon hotwater heater installed per rinnai's requirements. the 6 gallon h/w heater is electric and pulls alot of amps/watts costing nearly $100/mo for the gas rinnai. it was recommended to unplug the 6 gal water heater, and install an aquastat. Is this a sound solution. Does the aquastat pull alot of electricity. Is there another solution

Check the circuit breaker?

http://waterheatertimer.org/How-to-install-electric-water-heater.html
http://waterheatertimer.org/How-to-install-gas-water-heater.html

Gene
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If you need further help, I’m available over the phone at https://www.6ya.com/expert/gene_9f0ef4df2f9897e7

Rinnai RU98e is gas water heater with .4 gallon per minute activation.
This tankless gas water heater plugs into ordinary 120Volt 60 Hz outlet and consumes 63 watts when running.

For electric tankless:
Copy following links for illustrated walk-thru and wire-breaker size calculation for electric:
http://waterheatertimer.org/How-to-wire-Tankless-electric-water-heater.html

If you need further help, I’m available over the phone at https://www.6ya.com/expert/gene_9f0ef4df2f9897e7

Bad PC board.
Bad sensor.
Don't buy a tankless because the problems can be endless.

Before blaming the tankless however.
Perform the yearly required maintenance on burner
Delime the unit.
http://waterheatertimer.org/pdf/Bosch_Descaling-heat-exchanger.pdf

Remove and clean aerators, replace low-flow shower heads with large water-wasting shower heads.

IMPORTANT check all your single-handle faucets for crossover.
Water getting cool after 15 minutes could be bad cartridge on another faucet.
http://waterheatertimer.org/Crossover.html
http://waterheatertimer.org/pdf/No-Error-Code.pdf

If you need further help, I’m available over the phone at https://www.6ya.com/expert/gene_9f0ef4df2f9897e7

If you have tankless electric, it can simply be insulated... but tankless electric are seldom mounted outdoor.
Service bulletin says: "the only way of assuring protection is to completely remove the heater during those times the area may experience freezing temperatures.

There are tankless with built-in heater for exterior installation.
Check your manual, or add a comment with specific model number and maybe I can find the information.

Here's why they install tankless outdoors: Because tankless burn more fuel per gallon of hot water. This requires larger gas line and larger vent pipe than typical tank-type gas water heater that is usually installed indoors.
They install tankless outdoors, so there is no need for expensive vent pipe, and because there is ample incoming air to support combustion.
So if tankless does not have factory-installed internal heater, then the unit can freeze easily when the combustion draws in cold air.
Of course you can put heat-tapes on the pipes but the heat exchanger inside will quickly freeze during periods when hot water is not being used, and combustion is not taking place.

You can move the tankless installation inside and use correctly-sized vent and intake pipes directly through the wall. This will generally protect tanklesss from freezing if room is heated adequately.

If you need further help, I’m available over the phone at https://www.6ya.com/expert/gene_9f0ef4df2f9897e7

1) Never, ever install tankless water heater, unless your wallet is 2" thick and you enjoy nuisance problems and recommended continual maintenance.
2) Add in-line air vent.
http://waterheatertimer.org/images/Air-vent-added-to-return-22.jpg
3) Look for check valve along the line. Look for mixing valve, pressure regulating valve, etc.
4) If you have recirculation system, remove cross-over check valve and see if problem is located there.
5) Contact Rheem. 800-432-8373 They will want model number, serial number, and name of installer. And might recommend contacting installer or plumber.
6) Increase water flow. Check water flow requirement of tankless (.5 gallons per minute activation) with actual flow at faucet.
7) Clean water filter at tankless unit.
http://waterheatertimer.org/pdf/Tankless-maintenance.pdf
8) Crossover at single-handled faucet.
http://waterheatertimer.org/Crossover.html
9) Fix all leaking faucets indoor and outdoor so no air is being sucked in.
10) Bad TP valve on water heater sucking in air each time water pressure pulls on the valve?

Additional resources: No information about air-in-line
http://waterheatertimer.org/Troubleshoot-Rheem-Tankless-water-heater.html

Maybe I am not understanding your problem exactly, but if you have the instant hotwater system installed it should be measuring the temperature of the water in the hot water line and if it drops below the set level it should open the hot water side until the temperature is reached. If it was installed correctly it puts the hot water back into the cold water line so it isn't wasted.

large water heater, turn p the themo stat to a higher setting ( watch for burns with young childern and older folks)

or the best install a tempering valve. this valve mixs hot water with cold water before it goes into the building.

Again if you have young ones or old people they can burn faster than any other group.the best is a larger heater for the DIY person.

Thanks Tom

Laing UCT-909-TANKLESS UltraCirc Tankless Water Recirculating Pump

This is designed to maintain the proper flow to fire the heater. Please be sure the pressure in the house is at least 5 psi over the minimum required for your unit.

You need a recirculating line. You will have to hire a plumber to run a water line from the farthest fixture or to the fixture you are concerned about on the hot side and return it back to the water heater. this will cause hot water to constantly run throught the hot water line (via the recirculating line) and when you want to draw water from the fixture, you will already have hot water right at the line.
I know this works with standard tank water heaters, but it is a lil trickier with rinnai units(but very possible) if your plumber pleads the 5th and tries to sell you another unit call the Rinnai manufacturerand ask them how.