AquaStar tankless problem

I purchased a house and moved in about 5 days ago to it. It has an AQ 125-BNQ tankless with an install of June 2001 based on writing on the side of the tank.

I've noticed when I shower, it takes about 30 seconds or so to get the hot water flowing. For about 2 minutes, there's no issue, then suddenly all hot water is lost and the water turns ice cold. No adjusting of the shower nozzle helps. Approxmiately 1 minute later, hot water returns just as suddenly as it was lost, and then there's no issue. No other water is being used in the house at the time.

Any idea what the problem might be? The home inspection showed no issues with plumbing or anything. Just wondering why I completely lose all hot water for about a minute.

You need to watch the unit while the shower is running to see if it is producing any error codes. It might be overheating, and shutting itself down until it cools off. There are other reasons, the manual should help here on that. If you don't have soft water, you may need to do a descaling of the heat exchanger. To produce the hot water on demand, it needs an intense burner which, if you don't have soft water, will produce some scale as it is being used. Eventually, this can partially clog the passageways, and lower flow with the same heat input...the water gets too hot. Depending on the shower valve you have, you may not notice until it shuts off.

If the install included the needed valves, it is fairly simple to run a mild acid through the heat exchanger to disolve the mineral scale. You need some hoses, fittings, a pump, and a bucket along with the acid (some use vinegar). The manual should show you how to do this, or a plumber versed in tankless systems could do it for you. nearly 10-years is more than enough time to have scaled up to produce that problem...some manufacturers recommend doing it annually.

Personally, I don't find tankless that great. Most people don't want to do the required maintenance, and they have their oddities, plus generally cost more to install. Depending on use, you may never recover the increased installation costs with operating expense reductions. Plus, finding someone who understands and can service the things can sometimes be tough.

sdjimbob said:

I purchased a house and moved in about 5 days ago to it. It has an AQ 125-BNQ tankless with an install of June 2001 based on writing on the side of the tank.

I've noticed when I shower, it takes about 30 seconds or so to get the hot water flowing. For about 2 minutes, there's no issue, then suddenly all hot water is lost and the water turns ice cold. No adjusting of the shower nozzle helps. Approxmiately 1 minute later, hot water returns just as suddenly as it was lost, and then there's no issue. No other water is being used in the house at the time.

Any idea what the problem might be? The home inspection showed no issues with plumbing or anything. Just wondering why I completely lose all hot water for about a minute.

Click to expand...

There is likely an overtemp condition being sensed when the flow is too slow. This is more prone to happen in the summer months when the incoming water is warmer, and the minimum modulation of the burner is still too much to keep the temp from hitting the designed in scald-safety limits. Some versions had tweakable temperature modulation adjustments, but the details may have varied considrably since the late '80s. IIRC Bosch bought the Aquastar line from E.L.M. at some point in the late '90s, so you may have an E.L.M. version, but the manual for one of the Bosch versions lives here: http://www.tanklesswaterheatersdirect.com/shop/tanklesswaterheaters/manuals/125FX_manual.pdf Bosch played around with ignition schemes a lot, but the basic unit is still pretty much what it was on earlier versions.

The minimum modulated input to these tend to be about 28-35KBTU/hr in, and 2gpm flows are usually enough to keep it from tripping off. To diagnose if it's a minimum-flow issue, try running the hot out of the bathroom sink at a modest flow (not a trickle) while taking the shower, see if it still happens. Alternatively, run the tap at a very low flow, see if it induces the symptom quicker.

If it's right on the edge, running the shower slightly hotter than you usually do from the GET GO (not after it's already flamed out, in which case it'll need to cool off before re-striking), can be enough to keep it from happening.

The fact that the flame-out only occurs once could be a function of the temp of the incoming water varying. If the service pipe in the ground has a section that goes under an asphalt paved driveway, there may be a 1-2 gallon slug of water that's considerbly warmer than the averge due to the solar heated (in summer) chunk of earth under the pavement. Even on city water and deeply buried pipes I measure variations in the temp of incoming water at my place in a range of more than 5F (and a seasonal shift far higher than that between January & July.)

When I was running an old-skool Aquastar (circa 1993) in my house I'd sometimes have to make seasonal adjustments to keep it happy at the flows we were using it. It's been retired for about a year now, but it was retired-working. It definitely had it's quirks, but wasn't terrible. I wouldn't go back though- we're currently heating hot water with an indirect tank heated by a low-mass space heating boiler, which has no flow issues high OR low, and runs with slightly better efficiency overall.

thanks to both of you...gives me some ideas on where to start looking. I'll report back if I find anything!

Dana I think you're on to something. Today I had a 6 min. uninterrupted hot shower! I turned the sink faucet on hot with a modest-to-full flow and let it run while showering. Both sink and shower ran hot with no issues. I'll try the trickle in the sink next time i'm dirty enough!

As far as error codes on the unit, I'm guessing I have to open it to find a digital display or something. I'll try that too.

You will be looking for a long time for an error code on that unit. That's the cec unit that was a non-powered hydraulic operated unit that was sold by home depot. The very same undersized, overpriced unit that often gave most quality Tankless units the black mark (reputation)

Yep- no electrical power hookups, just mechanical feedback systems, definitely no microprocessor control & self-diagnosis or error codes to worry about. The Aquastar 125 is a barely-evolved descendant of the crude finned copper water-tube hot water heaters of a century ago. Point-of-use versions of these are still pretty much the standard all over northern Europe- a small one in the kitchen, a larger one in the bath/laundry. Their simplicity is both a blessing and a curse.

The mechanical feedback modulation of these have continued to evolve, as have the ignition systems. The newer-school "look ma, no electricity" hydro-powered igntion systems on water heaters like the Bosch 1600H are nearly ideal for off-grid living, and scores a respectable 0.80 in an EF test (only 2-4 points lower than non-condensing micro-controlled versions.) Look under the hood and it's still pretty much the Aquastar 125- some may even have sub-assembiles in common with the older version. There are similar tankless systems (mostly of Chinese design & manufacture) all over lesser developed parts of Latin America & Asia too, some with ignition bowered by the water flow, others using small batteries, and they all do pretty well...

...considerin'....

Their big drawback in places with large swings in summer/winter supply water temps is the roughly 4:1 turndown ratio between full & minimum fire, which means you run into issues like the one you're experiencing in summer, and may have to cut flow to keep the temps up in winter. But once you understand the limitations, its not a tough workaround. A 120KBTU/80% burner is plenty for supporting a single shower flow even in the coldest-water areas. If you have to boost the flow a bit to keep it from flame-out in the summer, so be it.

From an ease-of use point of view, if you pull the flow restrictor out of the showerhead and put in a "shower shutoff" (a tiny ball-valve) between the showerhead and the plumbing you can typically throttle it between 1.5gpm (winter mode, if the temp isn't keeping up at higher flow), or ~3-4gpm (summer mode, where you need a real gusher to keep the flame from turning off.) It's a band-aid workaround, but effective, and much cheaper than a new hot water heater (or a divorce. )

zl700 said:

You will be looking for a long time for an error code on that unit. That's the cec unit that was a non-powered hydraulic operated unit that was sold by home depot. The very same undersized, overpriced unit that often gave most quality Tankless units the black mark (reputation)

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You are very correct in saying so. I just moved into a house here in Belize that has the AQ 125 in it and it wouldn't heat the water. I found that the springs are calibrated to only heat the water to 122 degrees. So I cut the main pressure spring to lower the tension on it and now I have excellent hot water. It will scald like in the old days but it works like a champ. I may lower the temp setting on it, now that I can.

I've got one of those 125B's as well. Found the best thing for a steady shower is to turn down the knob on the unit so that you pull more from the hot water pipe and less from the cold. It is really not a thermostat (more of a flow control) so you need to adjust it as the incoming water temperature changes throughout the year.

Also probably old enough that the maintenance should be done. Mesh filters to be cleaned and valve rebuild kits are available.

Lots of info on Bosch's site http://www.bosch-climate.us/bosch-hot-water/aquastar-service-bulletins/38b-125-series-bulletins.html

Y'all get that you're responding to a post from 3+ years ago, right?

I have a Bosch Aquastar 250 SX NG. It is approximately 8 years old. I am very happy with the performance even if Bosch's customer support is the worst of any company I have encountered. I actually have three of these in my home, two indoor models and 1 outdoor. This is the oldest one and for the past two years, when I turn on the hot water at a faucet, the water will come out hot but about 20 seconds to a minute into the flow I will get a noise from the heater that sounds like a loud growl for about 5 seconds, it then stops making the noise and works normally. Any suggestions?

Original poster here. Nearly 4 yrs later, I still have the AquaStar and have learned to live with its quirks. I did try everything from moving knob on the unit to low, but getting rid of the low flow shower heads in the summer and putting back in the winter (where the problem didn't occur) was the easiest solution, as Dana noted. The problem with the unit was clearly not being to handle a slight temperature difference from city water temp to low flow shower head temp. I'm in San Diego and don't like hot showers in the summer.

I'm tired of the water bills though, but love the tankless concept. Would a more modern tankless system be able to handle my issue, or should I consider going back to a tank?

I think a modern, micro-processor controlled water heater will work just fine.

Old thread, but new problem for me. And in case there is anyone else out there with a hand-me-down Aquastar 125 BNG...
I was also having a problem with the water flow not activating the burners. Only a bathtub faucet would have any affect, then only got to luke warm. I looked into what Anciano suggested above and found that the water flow diaphram, pushes against a rod in the gas valve above. There is a spring and rod in the gas valve that limit the amount of gas to the burner depending how hard the diaphram is pushing against it. I found the spring to be too much force for the diaphram to act upon. I was hesitant to cut the spring to lessen the tension, so I just rotated/fed about a 5th of it through the top portion of the gas valve housing (hard to start, but once it gets going). It seemed to solve the problem, as soon as I turn the lav sink on hot, the burners kick on. It get scalding hot when the faucet hot is on high, just don't put it on high, or you could feed some of the spring back out (a good reason not to cut it).