Tankless WH questions

[PeteP]

Hello all, and thanks in advance for your time and expertise. I recently moved into a 640sqft 1BR condo that has a Bosch AquaStar (GWH 2400 ES NG). I know that the building was renovated in 2007, and I have no reason to believe that the heater has been replaced (or maintained) since then. It has been acting up since I moved in, and I am hoping to solve the problem correctly the first time as to avoid wasting time and money with multiple plumber's appointments.

I am experiencing pretty extreme temperature fluctuations out of all of my taps, most noticeably while using the shower. Depending on the flow rate and how hot I set the faucet, the temperature can swing from room temperature or colder (possibly the ground water temp? I'm not sure.) to 140+ degrees. (With heater set to 108F.)

I had a plumber come look at the system. Based on his suggestions, I have done a few things.

- Lowered the temperature setting from 120 to 108F.
- Used a circulator pump to attempt to descale the unit. I did this twice.
- Removed and cleaned the inlet water filter.
- Cleaned and eventually replaced an old shower head.
- Turn on multiple taps at once to increase the overall flow.

The only thing that has seemed to consistently work to stop the fluctuations is the last one. When there is a higher flow rate, the temperature evens out... mostly.

I also contacted Bosch, who did not give me any information but gave me the name of a Bosch approved technician in my area. I have not called them yet, as I don't want to pay somebody a couple hundred bucks to tell me I need to replace the unit.

Occasionally, the system will shut down with an error code. The manual says this code means "Overheat sensor (ECO) open circuit (should resent when cooler temperatures are detected)." I have always simply reset the heater after getting this code. Sometimes, the heater will work normally (normal temperature shower without opening multiple taps) for a day or two after a reset. This really confuses me!

These are the specs for my unit:

Bosch AquaStar GWH 2400 ES NG
6.4GPM @ 45F rise
Min water flow: 0.65gpm. Though the manual says: "Activation varies with inlet water temp from 0.65-1.6gpm"

Shower head flow: 1.8gpm
Not sure about flow rates of kitchen and bathroom sinks, but they seem average and both have aerators.



I'm wondering if it is just a matter of replacing a temperature sensor, but the initial plumber did not seem to think so. If I end up replacing the unit, what GPM is really necessary? It would be great to be able to run two things at once. Nothing crazy, just being able to take a shower while doing dishes or laundry.
My bigger question is how the minimum flow works. When using my kitchen sink, I need to open the tap most of the way to get the heater to turn on, unless I'm at the hottest setting. Do you have to have 0.65gpm of total water flow to get the heater on? Or do you have to be trying to draw 0.65gpm of hot water? (As in - a 1gpm fixture would need to be set so that at least 65% of its water was being requested from the heater.)

Any advice or input would be greatly appreciated. I am leaning toward replacing the unit rather than sinking money into an old unit I don't know much about, but new heaters are pricey and the installation is even pricier.

Thank you for your assistance!

 

[Jadnashua]

The flow sensor is for the hot water component. Kitchen faucets, vanity faucets, and shower heads are flow restricted. SO, depending on your incoming water temperature of the cold and how hot you are trying to make the outlet, you might be on the edge of turning the unit on, causing it to cycle on/off after hitting the high temperature limit. One of the known hassles with a tankless system is trying to get a low flow, warm outlet...often, doesn't work. Some of them have a lower gpm turn on limit, but those also require a variable output burner so that it doesn't overheat. That's one reason for the minimum flow. Think moving your hand through a candle flame. Move it fast, it never gets warm. The slower (less volume) you do it, the hotter things get unless you can adjust the flame's intensity.

To descale the heat exchanger, you need to circulate an acid through it. That could be vinegar, or something a bit stronger. That also requires isolation valves to enable it to circulate. Is that what you did? If so, how long did you run it?

The flow sensor could be gunked up or defective. There are likely inlet filters that you should check to verify that the flow isn't restricted.

 

[Sponsor]

 

[PeteP]

Hi there, thanks for your reply. If I am understanding you correctly, the answer to my minimum flow question is that the faucet must be drawing X gpm of exclusively hot water in order to activate the unit. That makes sense.

I descaled the system twice, once with a commercially made solution designed for tankless water heaters, and once with plain white vinegar. As for the time, I followed the given instructions for both, but I seem to remember running the circulation pump for 30-60 minutes.

I don't know how to tell if the flow sensor is the culprit, but I did attempt to address the flow issue (since the heater seems to work more or less normally with sufficient flow) with the steps I have already taken. Are there other things I can try? You mentioned inlet filters, and I was able to clean the inlet filter going into the heater itself, but that is the only one I'm aware of. Another thing I have tried is to remove flow restrictions (remove shower head, aerators on faucets) and open the faucet and check the temperature, but the same issue happens.

I think that my usage is average - that is, I think that the temperature that I am trying to get at each tap is average. I understand that unless there is already hot water in the pipes, I won't be able to get lukewarm water at the vanity, but I would like to get the heater to function normally with just one fixture open, and that has not worked for me yet, even with a fixture set all the way to hot. With the heater set to 108F, I still get swings from cold/room temp to up to 140F and higher.

Is the flow sensor or the temperature sensor (or the overheat sensor, as designated in the error code?) something that can be easily addressed by a technician?

Thank you so much for your input.

 

[Jadnashua]

Depending on how long the unit went before you descaled it, it could have a bit more deposits in it than the normal cycle will clean out. Depending on your mineral content levels, it may be required annually, or maybe even sooner. If you have soft water, you might be able to go longer in between those services.

Overheat is usually caused by restricted flow, where the water dwells in the heat exchanger longer than it should. Another thing (don't know for sure if this model can do it) would be how well the burner can adjust to the flow. Many can adjust the burner output to enable a fairly consistent temperature at various flow rates...hotter as the flow goes up, and cooler if it slows down. Mineral deposits can affect that response both in the heat exchanger and in the temperature sensor.

If the unit is set to a lower outlet temperature, to get the hot water you want, you'd expect it to require a higher proportion of hot versus cold. But, depending on where you live, it may not be able to keep up if the inlet water temperature is too cold. There is a finite amount of heat the thing can provide. In the summer with maybe close to 70-degree water it's one thing to raise it to 110+, it's totally a different load if it has to start with 35-degree water. Even 1gpm is still about 500# of water per hour. It takes a lot of heat to raise that many degrees while passing through at that rate.

Never looked to see what troubleshooting tips are in their manual. First thing, though, would be to read the section on theory of operation and to study the block diagram to get a better idea of how it works. Then, logic should help isolate what's going on after observing what's showing up. There may be a way to test the sensors if you have a meter and maybe an accurate thermometer. Testing first verses slapping in new parts usually ends up less expensive.

One of the hassles with tankless systems is that they are more complex than a conventional tank...not all techs understand, have the tools, or the parts to diagnose and repair the things. They have their place, but they aren't for everyone.

 

[Leon82]

I let mine descale overnight.

But it will turn off and on if someone open and closes a faucet so I put a mini heater buffer tank with recirculation and with an expansion tank. Now there are no cold water sandwiches

 

[PeteP]

Leon82, what were your symptoms before the extensive descaling? I think cold water sandwich is a gentle way of putting what my issue is, as my faucets are getting so much hotter than the set temperature.

 

[Jadnashua]

The cold water sandwich comes from the time it takes for the system to detect flow, turn the burner on, then purge the line of the room temperature water in the supply pipes to the point of use. IOW, you ask for hot, get a bunch of cold before it warms up. Similar to a tank system if you haven't used any hot water for awhile, but the hot, when it gets there, is fully hot at any volume, whereas, that may not be the case with a tankless. Say you turn the faucet on at all hot waiting, get the hot, then turn on some cold to make it useful, you might then turn the tankless off because the flow rate is too small, getting cold again, adjust the temp up a bit, it gets hot again. Lower it, cold...ad infinitum. There's a learning experience to adapt to a tankless system. If you understand how it works, you can live with it. Otherwise, it can be a pain. Given the initial cost and maintenance required, it may or may not end up cheaper in the long term. A buffer tank can help, but it's another cost and complication.

 

[PeteP]

Hi everyone, and thanks for your input so far. I decided to give one more shot to the descaling and ran white vinegar through the heater for several hours. I let it sit overnight (turned off the pump, mine is pretty loud) and then ran the pump some more the next day. The vinegar came out pretty blue, which I am taking as a good sign that I at least put a dent in (potentially) a decade of deposits from city water.

The heater seems to be working properly for now! I even upped the temperature setting back to 120, and measured the output from kitchen sink and shower at 120. I am going to keep monitoring it, but I am cautiously optimistic.

 

[PeteP]

Well, unfortunately, the success didn't last too long. It seems to be *better* but still not holding steady. To clarify, I don't believe I am experiencing what can be called a cold water sandwich. Let's say I have the heater set to 120F. If I turn a faucet on, the temperature will fluctuate between maybe 90F and 140F. So if anything, it's a burning hot water sandwich. It happens pretty quickly, maybe only 5-10 seconds at each end of the spectrum, and getting from hot to warm in about 10 seconds, give or take. If I increase the total flow through the water heater (by opening additional faucets), the temperature stabilizes.

Does the temperature only stabilize because the heater isn't able to "keep up" so it is essentially working at max capacity to raise the temperature of shower + sink (4gpm combined? maybe not even that much)?

The main thing I am trying to deduce is whether or not the unit needs replacing or just a new sensor or two. Flow sensor? Temperature sensor?

Also, why does the heater seem to work fine for a day or two after being reset or powered down for a while? And then it returns to the same issues.

 

[Dana]

The inability to control temperature at low flow is a function of it's minimum firing rate and the response delay in the temperature controls. Even when everything is operating properly some tankless units are much better at temperature control when running near the minimum firing rate than others.

As winter arrives with colder incoming water temperatures it's mis-behavior may moderate, since it'll be further away from the minimum firing rate.

The minimum firing rate output of the GWH 2400 ES NG is about 17,000 BTU/hr, which at a 1 gpm flow has a minimum temperature rise of 34F. If the water in the cold feed to the tankless has stagnated to room temperature the burner would be running very near it's minimum fire range at 1gpm a at a sink tap, but at 1.8gpm in the shower (did you bucket-test that, or is that the shower head rating @ 80psi?) it should be fine.

 

[PeteP]

I did not bucket test my shower head, 1.8gpm is just what it said on the package. So if it should be fine running 1.8gpm even at a low rise, it sounds like a sensor is not working properly? But my biggest question is - if a sensor is shot, why does it seem to work temporarily after a reset, and why does it work when the flow is increased? I would think that the sensor(s) would either work or not.

 

[Dana]

I did not bucket test my shower head, 1.8gpm is just what it said on the package. So if it should be fine running 1.8gpm even at a low rise, it sounds like a sensor is not working properly? But my biggest question is - if a sensor is shot, why does it seem to work temporarily after a reset, and why does it work when the flow is increased? I would think that the sensor(s) would either work or not.

If it's listed at 1.8gpm that will only be the flow if you have 80 psi pressure at the showerhead. If you only have 25-30 psi at the showerhead it delivers about half the flow. The amount flowing through the tankless is less than the showerhead flow, unless the tankless is set to exactly your desired showering temperature.

Bucket test the showerhead to get an idea of your actual flow.