Over condensation in a condensing tank-less?

Teemok

Tank-less brand aside, I'm dealing with a fully condensing new tank-less water heater. I was informed by tech support that "over condensation" can cause flame failure. Not via a drain blockage. They couldn't explain how it happens but said it had to do with short cycles. It took some self control to not call BS. I thanked them for their help and ideas. After rolling it around in my head for a day the only thing that makes sense to me is that the post burn fan cycle doesn't fully dry the secondary HX and warm vapor rises up into the combustion chamber where it condenses. When the next ignition cycle starts a flame is established but in the first moments water droplets move and short the flame sensor and the control see it as a flame loss. Anyone have any experiences with or facts on over condensation and flame failure? (flame drop out not ignition problems)

hot_rod

So what was their suggested solution. Seems all tankless work like that when you do a short draw like washing your hands at a .35 gpm lav faucet?

Makes me wonder if that is why all the brands are adding small tanks, inside their tankless??

Teemok

I'm going to talk to a few more manufacture techs before I form an opinion. Maybe the person I got just couldn't explain. I haven't seen it fault ever. Could be someone shut of gas while in use, thinking it was a water ball valve where gas enters the house. All I could find was a fairly large gas P. drop problem. 11.7" static, I'm seeing a 2.4" drop on full fire with all 3/4" less than 35' equivalent pipe on Propane. Maybe it's a tank regulator issue. The last unit was 16 years old. Not bad for doing both Space (fan coil) and DHW heating. The new one ran fine while start up testing and Gas pressure was well above the min. The customer had me preemptively replace the old heater. (vacation rental) I still have it, cause it worked great and I had just done a full burner and HX cleaning. They were having DHW temperature fluctuations. I insisted the old unit was fine because I measured good stable outlet temps. I kept asking about an unseen mixing valve somewhere and suggested we test the fan coil pump 009 check valve before a swap but they made the call to just replace it, not taking the time to understand what I was telling them. Oh well, I have all my recommendations in writing. Heater replacement was not one of them. I found the failed hidden mix valve in a 12" under floor space behind a panel under the deck while on this flame failure call back visit. Grrrr! The job is a 1 hr drive 1 way. I will remain a friendly and helpful service person.
I believe the small tanks are about cold water sandwich mitigation and recirc. optimization not "over-condensation" prevention. I can't believe I'm repeating that term, it's so ridiculous. The same tech told me the appliance that advertises endless hot water can run too long. I asked if they understood that was a contradiction and I got, ( what? ) as an answer. Some tech support calls are better than others.

GGross

Never heard of it, except in cases where the condensate can't drain freely, but that is not what the tech was talking about then I don't know. I am interested to hear more though if you get any better answers.

that pressure drop would also have me suspicious of the regulator, the unit doesn't have an appliance reg right next to the heater right? It's about 35' of pipe from the unit to the reg?

Teemok

No close reg. Talked with different manufactures tech this morning. Thankfully there's no such thing as an over-condensing, condensing appliance. There are conditions that can create a wet combustion chamber but it should not be referred to as over condensation.
There is what I think is a condensate leak coming from behind the secondary HX or from the exterior units exhaust duct (parascope, snorkel). Staining and water running down the back of the case. Could be a temperature dependent exhaust re-breathing with the cover on kind of deal. Warranty parts on the way.
I opened a wall to find 3/4 csst gas pipe. It runs through a place I can't get to or see. 10-12 ft. under floor. I had 3-90's and that length in my estimate. I don't know if they cancel out. There's a non full port ball valve as the shut off. 12" 3/4 flex connector. The csst should flow 200kbtu with with under 1" drop. The rest of the pipe and valve might add .5" It's still odd. The 3/4" galv. goes underground, taped up as far as I can tell for 2-90's and 4-5' under a path way. It's a moisture trap. Inside corrosion or water? There's a union as It rises up. I'll blow some compressed air through it and see what comes out. I've suspected a few reg.'s before but very rarely have they proven to be the fault. It hasn't coded out since the neighbor said they heard the beeping of the original complaint 4 days ago. Some jobs end up being less profitable but they prove who you are. Here's the thing: No one was home when it coded, no DHW use. That leaves the fan coil call. It doesn't cycle at all and runs at low fire so the gas P drop issue would be cold start only. That I couldn't replicate. I'll up date when I go back with parts.