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Description: Navien tankless hot water heaters are white with “T-Creator” and “NAVIEN” on the front. Recalled model numbers are CR-180(A), CR-210(A), CR-240(A), CC-180(A), CC-210(A) and CC-240(A) manufactured in 2008. A label on the side of the water heater lists the model number along with the manufacturing year in YYYY format.
Hi guys. When I moved into my house a couple of years ago, I was delighted to discover that it came with TWO Navien CR-210A units. They are hooked up master/slave, supplying both hydronic heat and hot water to the tap with circulation.
While I like the units in principle, the day to day reality has been disappointing. Here's a list of some of the problems I've had:
1) Both units had ignition problems (error 3). We replaced the circuit boards.
2) One unit had an intermittent problem with the flow adjustment valve. We replaced the entire unit.
3) I've had two flow adjustment valves that failed (error 34), requiring new valves.
4) Both units have been through a handful of flow sensors (error 36). They seem to last 3-6 months.
5) One unit gets intermittent air flow faults (error 10).
With the recirculation system, the units short cycle constantly. Each new ignition causes a nasty belch of exhaust as the units clear themselves. My elderly neighbor complained about the fumes. The gas company showed up and red tagged the units. Testing revealed that the units are venting "as designed" and we simply turned them back on after a chat with the neighbor.
I've finally had enough. I'm not a plumber and I'm tired of dealing with this mess, not to mention the expense of constant repairs.
So, what should I use to replace my broken heaters? I need hydronic heat, hot water, and (ideally) a new circulation system. It has to fit into the existing (smallish) utility room. I've got PVC venting at the moment. I know very little about plumbing, so please use small words and be specific. :)
The biggest problem was the domestic tankless water heaters were applied as a combi system. The wearing of valves and flow sensors indicates excessive flow to satisfy the heating side, common installation mistake.
There are numerous wall-hung condensing boilers that could be installed which don't take up much space that use pvc for the air intake and exhaust. The bigger space consumer would be an indirect water heater tank. But, these typically can be smaller than a normal stand-alone tank because the boiler is typically a lot more efficient and usually larger than the burner of a standalone. WHen I had my system updated, I had a Buderus unit installed that has been working well for about 5-years now, and I expect it to continue to work well for a lot longer.
The brand isn't super critical, but the installer is so that it is installed properly and adjusted correctly. Buderus US headquarters is 1/2-hour ride away from me, so that was part of my decision to go with their unit, the other part was what my installer was comfortable with.
What I'm trying to wrap my head around is what place in Seattle would have a heating load OR hot water load that would call for TWO 175KBTU burners???
Even running the heating system at high temp and only pulling ~85% efficiency that's still 280KBTU/hr output, which is a bit more than 10x the heat load of my not-superinsulated house at Seattle's outside design temps, and enough to supply a good half-dozen 2gallons per minute showers.
Turn off the recirculation pumps- they're just a waste of energy designed to wearing out the burners by short cycling, especially if the plumbing on the loops isn't insulated.
Does this place have a dual gusher-head master shower with 8 sidesprays or something?
Assuming more conventional bathing facilities, how much gas did you go through last winter? (to come up with a reasonable estimate of the whole-house heat load at design temp.)
What type/how much radiator/baseboard/radiant-floor/whatever do you have for heat emitters?
zl700: "The wearing of valves and flow sensors indicates excessive flow to satisfy the heating side, common installation mistake."
Tankless heaters are designed to tolerate fairly large delta-T- larger than most radiation is set up for. But rather than running a primary/secondary to be able to run a high delta-T/low flow on the tankless (and tweak flow & delta for the actual design heat loads) it seems some people just pump like hell at a 10-15F delta, often at 2-3x the design condition load to limit the output temp to something less scalding for DHW draws. To get 40KBTU/hr at a 15F delta takes an already hefty 5.3gpm, and many systems are probably running 2x that with some monster pump driving it.
"Reasonable" tankless based systems would rarely need more than 2-3gpm with a higher delta-T. Deltas of 80F (or even a bit higher) are not a problem for a tankless- it's exactly how they were designed to operate (40F in, 120F out), even if that large a delta would stress some mod-cons. At 2gpm @ 80F you have ~80KBTU/hr, which is more than 2x my 0F outdoor temp heat load. At 2-3gpm I'd expect flow sensors to last a very long time,since that's well within the bounds of the peak flow ratings, and very much like shower flow rates, lower than tub-fill rates, which a tankless should be able to sustain almost indefinitely.
Higher output temp required for high delta-T combis has negligible effect on combustion efficiency- condensing is all about the return water temp first, followed by firing rate. The sweet spot on firing rate for most condensing burners is between 1/4 & 1/3 of full fire, but if the return water is much above 120F you can forget about condensing efficiency on a tankless, even if firing at the optimal rate. Tankless designs can usually count on very low entering water temps for getting the condensing efficiency, and can be designed for higher excess combustion air than mod-cons, which results in a lower dew point on the exhaust gases. (Mod-cons can be condensing at min-modulation even at 130F return water temps, but I doubt that's the case for most tankless HW heaters. Cood be rong, offen am.)
I have a kludgy tankless based combi running at fixed ~2.5gpm on the tankless, with a min-delta of ~30F, but it can hit ~60F or more during heavy DHW draws. To boost shower performance ther is a drainwater heat recovery unit kicking back 20K+, but by limiting it the tub fills are slower, and count on the buffer tank a bit. By having a buffer tank never short cycles, the tank has a fraction of the standby loss of a recirculation pump. It's not perfect (not even close), but it's been tweaked for the actual loads, and I expect the tankless to hang in for quite awhile since it's never being abused, not on flow, not on firing rate. (It's max input is 185K, and it's usually firing between 1/4-1/2 full fire under all uses.)
Cold inlet water and high flow needs (desires) could easily dictate a couple in series. In a multi-bathroom house, 2-3 children and maybe an adult all showering at the same time, for instance. My sister's house has 4.5 baths, and it's not uncommon to have all of them in operation at once in the morning. Nice to not be the last one with a tank that's now cold...
That would work just as well or better in parallel for the DHW load, using just one as the combi-heater. It takes some pretty fat distribution plumbing to deliver 4 tub fills and a shower at the same time. Even if the firing rate of the tankless(es) would keep up the flow might not when plumbed in series. Operating them master/slave and running them in series would guarantee the wear problems he's seeing.