It went down to -30 deg and the rinnai hwh gushes water when it had a demand should that have happened isn't there a safety heater built in ...????? thanks for any input

You will only get power out of the controller when there is a call for hot water. If you have power into the controller, and you still get no power out when there is a demand, then the controller is bad and must be replaced.

It only runs when there is a demand for hot
water.

There is an inlet screen on demand water heaters. Look for that and clean it. The tubes in the heat exchanger could be clogged. The manual will tell how to clean it. This is very involved and uses acid. It can be dangerous, so I recommend you have it done.

It might need cleanning of exaust fan.or system flush of heat exchanger

clean filter on the cold side coming in and clean or replace the aerator at the sink, rinnai works on water flow, common problem if your water hardness to high or if you have iron in your water

Your circulator provides the draw necessary to fire the heater. You are defeating the purpose of this kind of heater by circulating the water. Add a second heater to fix this engineering fault.

i always take them out. they say it voids the warrentee but i have never had any trouble with it removed.

Adjust each thermostat (one under the lower front cover, one under the upper front cover) at the same indication (thermostats may not have degree marks but warm, hot, very hot or A, B, C). Turning the screw clockwise is hotter. For safety sake, turn off the circuit breaker for the water heater at the house electric panel before removing thermostat covers.

I am assuming that this is an electric unit? The heating elements could be bad. I would contact the manufacturer for a service manual for your unit. There you will find instructions on how you might be able to reset the unit if possible. One thing you can try is to shut all power down to the unit and then restart. Also check for any "reset" switches on it as well.

I wonder if you have this problem below caused by a drop in water pressure from someone flushing a toilet or a washing machine. I would call the company to remedy this issue.

17. What is a cold water sandwich effect? The term “cold water sandwich effect” is a term that is used to describe the introduction of cold water into the hot water supply line during frequent on/off operation of an instantaneous water heater. The cold water sandwich effect, when present, appears as a momentary drop in hot water temperature as it is discharged from a hot water supply outlet (i.e. shower, tub, or faucet). This phenomenon is present in the operation of all instantaneous, tankless style, water heaters, but is minimized with the high tech design of Rinnai water heaters.
The technology built into the Rinnai water heaters is designed to minimize the cold water sandwich effect. Rinnai water heaters are microprocessor controlled and when water flow through them ceases, they remain in a “ready to fire state” for approximately 1 minute. If water flow through a Rinnai water heater begins within the first minute following water flow stoppage, the water heater will fire back up within 1 to 2 seconds. This minimizes the cold water sandwich effect that would otherwise be experienced with a low tech tankless water heater. It should be noted that the cold water sandwich effect cannot be removed completely from tankless style water heaters. The safety standards developed to insure the safe operation of water heaters require a delay in the ignition sequence of all gas water heaters.
While the cold water sandwich effect cannot be completely eliminated from standard plumbing systems, it can be eliminated from plumbing systems that have a supply and return hot water circulating system. Rinnai has developed 2 methods to eliminate the cold water sandwich effect in residential hot water circulating systems.
The first (and preferred) method to remove the cold water sandwich from circulating systems utilizes a small electric tank water heater (powered up) that is used with a dual purpose. The small water heater acts as a mixing tank to eliminate the cold water sandwich effect from the Rinnai water heater and it uses its electric heating element to offset the heat losses from the hot water circulating system.
The second method to remove the cold water sandwich from circulating systems utilizes a small 2 to 6 gallon storage tank installed on the hot water outlet of the Rinnai water heater. This tank acts as a mixing tank to blend the cold water sandwich with hot water and eliminating its effect at fixtures.