Rinnai tankless water heater

[jennakw]

Hello!
I just bought my first house and was happy to see that it has an already installed Rinnai V53 tankless water heater (I believe circa 2011). My first couple of days at the house I had no problems at all. Then this morning, while showering, was a completely different story. About every 1-2 mins my hot water would just disappear and not return until I shut off the shower all the way and turned it right back on again - in which it would come out scorching, then dissipate back to ice cold very shortly thereafter.
I've read some about the "cold water sandwich effect" but unsure if this is similar since I am only getting the hot water back after shutting it off? Not to mention, I've taken 3 perfectly temped showers within the past few days.

I did install a new faucet on my sink, a new head on the shower itself, and my washing machine - all yesterday - unsure if any of those could impact the hot water to my shower.

If anyone has any idea - they would be very appreciated! I am semi hands on and would prefer to just reset something over hiring a repairman

[adnadeau]

Try putting your old shower head back on; your new shower head may have too high a flow for your heater to keep up with it. Also, don't run your dish washer or clothes washer while you're showering. Otherwise you may experience temperature fluctuations when the fill cycles kick in. Does your heater have the optional control panel that is described on the Rinnai web site? Have you changed any settings? Are you showering at the same time each day? Maybe the cold water is colder in the morning than in the evening or you have a long piping run to your shower that gets cold at night. If you turn on your sink hot water faucet, is the temperature constant or does it fluctuate also? If you turn on your shower to your usual settings without getting in and then listening to the heater, does the burner cycle on and off or is it constantly on?

[HayZee518]

heater

I done some reading on the Rinnai site. They mentioned the Rinnai tankless on an outside wall installation or on any inside wall with and adequate outside vent piping. They mentioned zero clearance because their intake & exhaust are the same line.
Also nominal piping for propane or natural gas is 3/4 inch black pipe. Next they say 1/2 inch is tolerable if the run isn't too long. There is no mention of 3/8 inch copper for propane, as I was thinking 11 inches of water for a pressure.
They did say that hot-cold sandwiching could be a problem in some cases because of one or more appliances using hot water at the same time as a shower. They mentioned 7.5 gpm for hot water for one model and 9.5-10 gpm for others.

[adnadeau]

Watts 70A

Something I had forgotten about until it just happened to me. When I took a shower last week, I had to crank the shower control towards hot way up beyond the usual setting to get a normal temperature. When I trouble shot the system, it turned out to be the Watts 70A tempering/extender valve. This is a brass fitting, mandated by some plumbing codes to be on tankless water heaters, that is on the heater output piping. It has hot water in and out ports and a cold water in port and a black knob to set the temperature. If the heater output gets too hot, it mixes in cold water. It has an internal thermostat which has a temperature controlled piston which gets stuck after a few years and lets in too much cold water. It can be taken apart and cleaned with white vinegar, or you can get a 70A RK repair kit from a plumbing supply house.