Navien NPE-210A fan runs on toilet flush

What's your inlet water pressure PSI

In humid places it used to be somewhat common to put a mix valve between hot and cold on the supply for the toilet to keep the tank from sweating. It was called an "anti-sweat valve" or a "tempering valve." The last one I saw was in a house built in 1976, as air conditioning has become more common they've gone out of fashion. But they're still available.

Is it possible your house has one?

As Hot_rod mentioned: it’s probably from a cross connection.

Modern pressure balanced shower valves have internal check valves to prevent cross flow between hot and cold. If one of those is not sealing, it could be pulling water through there.

Tanklesses are very flow-sensitve. Can be a real P.I.A. I had a Rinnai that would not fire the burner when the hot was opened on a Lav faucet with a Trough type spout...Mad Dog

My sister's house had two cross connections via two "touchless" faucets that I guess the plumber didn't install checkvalves on. Since the faucets are left in the on position and a solenoid turns it on and off, there's a connection between hot and cold all of the time unless there's checkvalves installed on the lines.

The crossover everyone is talking about has absolutely positively nothing to do with the actual piping the plumber ran.

If you have a single handle valve of any kind in the house you have the potential for a cross connection. Or as @ChrisJ said the automatic valves can have issues if not properly installed. To be very, very clear. All the piping can be perfect and you can still have a crossover because it happens inside the valves.

Flushing the toilet is putting a draw on the hot water, the heater is then sensing flow and turning on, but the draw isn't long enough for it to fire all the way so it shuts off.

The title of the thread is "Navien NPE-210A fan runs on toilet flush." In your original post you write: "the fan still runs intermittently on a faucet draw and every time the toilet flushes."

You have a device that is triggered by demand for hot water, and devices that use only cold water triggering it. The simplest explanation is that those cold water devices are in fact using some hot water.

Have you tried this: when you're in a period when flushing the toilet causes the fan to run, turn off the hot water at the water heater. Then flush a toilet. If the problem is being caused by hot water flowing when there is demand for cold water, turning off that valve should prevent hot water from flowing and prevent the problem.

I saw tankless start the fan on cold water draw on more than one job. With pex piping there is a little give, so when a valve opens there is more movement in the whole plumbing system than we may think. Just enough movement for the tankless to register flow even tho it may only be a momentary "wiggle". Seems a properly situated check valve could take care of it but I never worried about it as I never saw a case where it happened with every cold water run.

Has any looked for an anti-sweat valve supplying a toilet?

"If there was a crossover it would do it constantly and it does not. I don't know how many times I can say this."

So far two reasonable explanations have been put forward: one is crossover between the hot and cold somewhere in the system, the other is water hammer on the cold pipe. Until those two have been eliminated there's not point in looking further at more exotic solutions or firing up the parts cannon and shooting new parts into the water heater. And saying, "I don't think that could happen," or even "I really don't think that could happen" isn't eliminating them. I don't think anyone who has suggested crossover is at all dissuaded by the fact that it's intermittent, crossover problems are sneaky like that.

There is an easy way to eliminate both of them, if you are in a state where you can reliably get the fan to run on a toilet flush. There is a valve that provides cold water to the water heater. Shut that valve and flush the toilet. Does the fan run? Open the valve. There may or may not be a valve downstream of the water heater so it can be serviced. If not you should install one, it's a good idea. Shut that valve, and flush the toilet again. Does the fan run now?

If it was water hammer, closing the input valve would keep the fan from running, the water heater is isolated from the water hammer with the valve closed. Closing the valve on the output doesn't isolate it. If it was cross connection, closing the downstream valve will keep the fan from running. Closing the input valve may or may not isolate it, you can get some water motion downstream.

I had the same problem. After I flushed my unit the Navien fan would run anytime I used cold water. Everyone wanted to tell me I had a cross connection somewhere in my house. Water hammer is also discussed on this thread as a possible cause. But the problem only began after I flushed my unit. So logic suggests that the problem is in the unit not in my home's plumbing. Navien's technical support would not talk to me because I'm not a licensed plumber. (That is not what I consider acceptable suport.) I called a Navien authorized technician to address the issue, and he spent a good deal of time on the phone with Navien's level 3 support. But nobody had a solution and the problem persisted. We did some diagnostic testing including turning off the hot water outlet valve, which suggests the problem is not crossover. I also turned off the cold water valves very slowly, which minimized the effect of any water hammer. I finally called my brilliant engineer son, who discussed the problem with me. He correctly pointed out that water does not compress, but air does. We were of the opinion that air was trapped somewhere in the unit (likely in the buffer tank) after the flush. I ran my hot water wide open for awhile and let the internal recirculation (previously turned off) run. The result is I'm still getting a little flow across the flow sensor when I use cold water but the problem is much less severe than before and the unit now only occasionally cycles when I use cold water. Presumably I purged most of the air out of the system, but there is still a little left.

I will try and relay this information to Navien, but that might not do any good since their technical assistance does not talk to homeowners. Navien could also solve this problem by changing the programing of the unit to make it less sensitive to minor flows across the flow sensor, but I'm not going to hold my breath waiting for that to happen.

i personally have seen this happen on one or two occasions and both times it was related to a bad fluid master which was causing migration from the combi and causing issues . And yes a lic plumber buddy of mine was baffled as to what was the issue being he had never seen this occur. It ended up being a combination of things including no portable expansion tank and high water pressure over 125 psi which as strange as it seems caused hot water to be feed to the toilet even though it was clearly piped w a cold water line but i gather there was less of a pressure drop through the hot water line then the cold . Again this was quite a while ago and i haven't ran into this issue again since then . You never know what you ll run into each day the best you can do is start fresh narrow down the issue and use a little non rational thinking to sort it out . i would start by isolating the toilet ,start where the issue occurs and fiqure out why . Remember the cause and effect usually do not happen in the same place .

peace and good luck clammy

Take a look at the install diagram and you will see that they show a check valve on the cold water supply. If you install a check on the cold water supply, you are going to need to install an expansion tank. My older generation NR240A will turn on randomly, most consistently when the irrigation system draws (the pipe that feeds the irrigation system is near the cold water suppoy of the Navien). I have been living with this since 2010 but I really should of taken care of it.