No water on the hot side of line. Have rinnai gas tankless water heater. 22 degrees last night but had pipes wrapped. Have cold water pressure but not pressure on hot side. Tried to thaw pipes

SOURCE: No water flow from hot water tap with a rinnai tankless water hea

Its A Possibility The Left Over Water In The Hot Is Frozen. If Your Line Is Exposed Outside Somewhere It Can Easily Freeze Up. Turn On The Hot Water Everywhere In The House And If Accessible Check The Lines To See Where The Warmth Ends. You Can Leave One Off Over Night In Hopes That It Will Eventually Defrost.

SOURCE: Tankless Water Heater - Frozen???

I had them same problem with my AquaStar 125 back in 2005......it hasn't been that could around here since then until last night and it froze up again. The problem is the negative draft. This can be solved with the Power Vent Module and the AQ1 flapper....at least with the 125. It prevents the back drafts.

SOURCE: No hot water

The code # 11 is a code that warns of "no ignition" and the causes of this error code are too many to list. I would suggest that a qualified tech. perform a diagnostic test and repair.

This is not a task for the "do it yourselfer"

SOURCE: My Rinnai tankless water heater cycles up and down

You need to first make sure is the tankless who do the “up and down”! . If the problem manifest on more then one faucet then the problem is the unit. Have a thermometer (we use a infrared gun wich we train on Hot water pipe !) and see if actually the unit is the culprit.
If the problem is only on one shower It is possible also to be a “antyscalding valve” If the trigger valve on your shower is being adjusted too low then it trigger a splash of cold water now and then on regulate intervals. Try to adjust the temperature lower at the unit (say 110F ) and see if the problem change. If it does, then the problem is for sure in the faucet. If it does not change , then you have a problem with the water flow sensor . It is simply getting stuck. Bad bearing or even a piece of Teflon tape. I believe you will need a certified technician for that so no issues with your warranty will appear.

SOURCE: Our Rinnai tankless water heater is producing water as hot as pre

Power outage will reset Rinnai unit to that temperature. Read manual for more info.