I've looked through posts to see if my problem was addressed, and this comes close enough not to start a new topic, but it's not quite exactly the same. Please advise if this should have been a new topic.
Just like the OP, my water is HOT for 10-20 seconds, and then goes to unheated. Makes for very fast dishwashing or showers though.
We discovered this on our trip this weekend.
Everything was fine on last trip last year. This time is the first time this year, with a winterizing in between. We are just north of Metro Atlanta. No severe winters here. Lowest temp the trailer experienced was 14 degrees. Winter was mostly in the 30-38 range.
These below are all the relevant facts I can think of. Can anyone advise what else to look at?
It's a Northtrail 31RED, 2009 model year. We've used it since June 2008. This is coming off its third winterization. The heater is gas. There seems to be no problem with gas supply. I see the heater running flame when I check. The flame is on periodically. When I check multiple times, flame is randomly on or off. It looks like normal operation there. I triple checked the by-pass valves. I turned the hot valve off and on. I turned the by-pass valve on and off. I turned on the hot water at kitchen sink (right over heater tank) and turned the hot valve off to check that water flow stops. I verified the shower faucet to be off. I verified the outside shower faucet to be off. Unlike the OP, there isn't any sort of backflow valve on the water tank outlet. Outlet (hot label) looks just like the inlet (cold label).
The symptom consistently repeated itself over the weekend. I thought maybe first time was just unheated tank or maybe wife used up hot water on dishes or something. But it kept repeating for each need of hotwater. I observed the symptom by running water until it's HOT (to check that it's working) and then turned it down and added cold water according to my usage, shower, wash hands, etc. Water gets colder until all hot faucet and no cold faucet is still cold water, taking place over less than a minute or two.
Is there anything else to look at? It's a very strange symptom. (Note: I'm not very mechanically savvy, so extra explanation of terms will be much appreciated.)