Bosch GWH 1600 H NG Not getting Hot enough

[carriagehousereno]

So about a week or two ago we started to notice that the hot water temp was a little low. I checked the two dials and adjusted them accordingly, turned the Flame up and the flow rate down as instructed in the manul. Nothing really changed. To give you an idea, we used to have to turn on the cold water to regulate the hot water coming out of the faucet. Now it doesn't even get hot enough to take a comfortable shower. So last night I descaled the heat exchanger by running full strength white vinegar though it for about 20 minutes (again as stated in the manual, I saw elsewhere on here that some folks say to run it through there for at least an hour, so maybe I should try that again). Still maybe only minimal temp increase. Today I took the water valve apart and inspected it, it looks to be operating normally, but i have not purchased the rebuild kit for it yet. I removed and cleaned the burner assembly. Still no improvement. After all that, I called Bosch optimistically hoping to get some troubleshooting help, but they just told me to call a "licensed certified professional" since it is a gas unit they won't discuss tech support with the home owner. (Are you kidding me?) Any help with this would be great otherwise I'm gonna be buying and installing a new one 'cause the fam can't put up with the cool showers like I can.

 

[Reach4]

How hard is your water?

I have never worked on a tankless.

From a troubleshooting point of view, if you run a tap at maybe 3/4 GPM, does the water get plenty hot? If somebody is taking a shower on full hot, but it is not hot enough, does the flame stay at full blast? If both of these things are yes, I would think your theory about the heat exchanger liming up sounds like a reasonable explanation.

Deliming much longer makes sense. I would check that my return vinegar stayed well under 5 pH to make sure the vinegar is not getting depleted. If the solution continues to drop in pH, even after refreshing, I would say you are not done. You can read up on other stronger solutions. https://terrylove.com/forums/index.php?threads/tankless-water-heater-cleaning-solution.69373/

 

[Sponsor]

 

[Dana]

Some of those thermo-mechanical feedback controls won't even fire up the ignition at 3/4 gpm, though the specified "activation rate" on that unit is 0.5 gpm, so maybe.

A fundamental problem with the 1600 H is it's fairly low ~91,500 BTU/hr maximum output, which at a 70F rise (35F in, 105F at the showerhead) is only good for 1300lbs/hr = 156 gallons/hr, or (/60=) 2.6 gpm.

The rated output of a standard low flow showerhead is 2.5gpm, so it doesn't have to be too far out of spec (or very limed up) before a wintertime shower starts feeling pretty tepid in a location with winter incoming water temps as cool as it is in most of PA. The reason it's seeming to get worse over a few weeks is the seasonal cooling of the incoming water. Once it's marginal, every degree F is something you can feel pretty easily in the shower.

Buying a decent 1.5gpm showerhead would probably fix the showering problem.

To have a datapoint, take a food thermometer and test the water temp at a sink tap full hot, very low flow, and see just how high the temp gets before it turns the flame off due to low flow (or overtemp sensed at the water heater.) If it won't muster 110-115F before it shuts down there could be some other issuse with it. Do the flames look nice tall & clean bright blue when running high flow?

An other issue with the 1600 H unit is a fairly high minimum firing level of about 24,000 BTU/hr, which gives it pretty louse temperature control at very low flow. That's a bigger issue during the summer, or in locations with much warmer incoming water temperatures than PA.

Most tankless manufacturers won't do phone-tech with a homeowner- Bosch isn't an outlier, but rather the norm. Takagi is the exception (or at least they used to be), spending quite a bit of time helping people debug their equipment over the phone.

 

[carriagehousereno]

we have city water in Philadelphia, haven't noticed that it's particularly hard, don't get any mineral buildup on fixtures or shower head to speak of. But I can try rescaling again. I'll check out the link you posted first.

 

[carriagehousereno]

That would explain why we noticed it over the past couple weeks, we just got our first cold snap, I didn't realize the incoming water temp would dive so low. Also thought the specs on this unit state a 90 degree rise at the lowest flow rate? Did I read that wrong or is it just inaccurate. I'll test the incoming water temp at the unit after running it several minutes to clear the line of water warmed by the ambient temperature in our crawl space.
We've always felt the hot water was not consistent even during one use, with it getting hotter than colder even when no other faucets or appliances are in use. Could the hi temp shut off be malfunctioning and killing the flame during long uses and even now just keeping it from ever getting hot in the first place.
Starting to think I may just want to upgrade anyway since it sounds like this unit isn't really appropriate for a home with 2 full baths, washing machine, dishwasher and two kitchens(we have a small apt also fed from this water heater). Any recommendations if we do decide to just upgrade?

 

[carriagehousereno]

just tested water temps, incoming is 42 degrees at kitchen sink after letting it run several minutes and it stops dropping. The hottest I can get it even turning the faucet down to 3/4 gpm is 105 degrees, that's a 63 degree rise. Is that the best we expect from this unit. I guess if in the summer time the water coming in was 10 degrees warmer with a 63 degree rise would be 115 degrees at the faucet that might feel pretty hot in comparison. But is that hot enough to really need to add cold water to temper it like we used to do?
And at 3/4gpm, the temp does fluctuate quite a bit, it's not steady at 105.

 

[Dana]

A BTU is the amount of heat it takes to raise one gallon of water by 1F.

A gallon of water weighs 8.34lbs.

The minimum "activation" flow rate is specified to be 0.5 gmp, which is (x 60=) 30 gallons per hour, or (x 8.34=) 250 lbs/hr.

The minimum FIRING rate is ~24,000 BTU/hr, so you should be able to get a (/250lbs=) 96F rise out of it at min-fire, min-flow.

A 2.5gpm showerhead is it's rating at 80psi. At more typical house pressures (which can vary by a LOT) ~2gpm is more likely. Thats ~1000lbs/hr.

At the maximum firing rate output of ~90,000 BTU/hr a 2gpm flow should be able to deliver a (90,000/1000=) 90F rise.

Seems like something is out of spec- it may not be hitting it's maximum firing rate if the thermo mechanical linkages are sticky or something, or it could be the gas pressure is out of spec, or the burners are dirty (did you look to see if it's a tall clean blue flame?), or something in the thermal feedback has drifted out of spec and the unit "thinks" it's actually delivering much hotter water.

 

[carriagehousereno]

A BTU is the amount of heat it takes to raise one gallon of water by 1F.

A gallon of water weighs 8.34lbs.

The minimum "activation" flow rate is specified to be 0.5 gmp, which is (x 60=) 30 gallons per hour, or (x 8.34=) 250 lbs/hr.

The minimum FIRING rate is ~24,000 BTU/hr, so you should be able to get a (/250lbs=) 96F rise out of it at min-fire, min-flow.

A 2.5gpm showerhead is it's rating at 80psi. At more typical house pressures (which can vary by a LOT) ~2gpm is more likely. Thats ~1000lbs/hr.

At the maximum firing rate output of ~90,000 BTU/hr a 2gpm flow should be able to deliver a (90,000/1000=) 90F rise.

Seems like something is out of spec- it may not be hitting it's maximum firing rate if the thermo mechanical linkages are sticky or something, or it could be the gas pressure is out of spec, or the burners are dirty (did you look to see if it's a tall clean blue flame?), or something in the thermal feedback has drifted out of spec and the unit "thinks" it's actually delivering much hotter water.

The flames do flicker orange at the tips at times, I did just clean the burner assembly per manufacturer service bulletin, but I would say they are only an inch to inch and a half high at the most. They do modulate slightly when adjusting the flow rate knob on the left. I also was wondering if the problem was with the thermal feedback. Is there a way to test the sensor in the outlet flow pipe of the heat exchanger to see if its working properly? I guess I could remove it and test its response to a bowl of water being heated on the stove and check the leads with a multimeter, but what would I be looking for? Continuity? or resistance? Or something else?
Is there a way to test the gas pressure without a manometer?
If it's not the thermal feedback would it be a problem with the gas valve?

 

[Dana]

Despite the fact that there is a status LED for the burner, the controls of the 1600H are not electronic. The ignition and LED are powered by the flow of the water, as are the pilot and burner main valves, but the temperature feedback for the water temp is controlled by thermo-mechanical means. (Take a look at what the knobs are actually turning inside the- see any wires? )

Walk through the troubleshooting guide in section 6 of the manual, verify what you can. This unit has a higher minimum water pressure requirement that's higher than most (30psi- most only need 15 or 20) which can affect it's ability to control temperature.

A bit of orange at the flame tips is fine, a LOT of orange with or without flickering is not. Verifying the gas pressure and the operation of the regulator is outside of what's safe & sane to do yourself without the training or equipment.

Verifying the gas pipe diameters and total length (including equivalent-lengths of any ells & tees) are within spec is something you can do.

 

[carriagehousereno]

I get that the knobs are adjusting mechanical aspects in the water and gas valves, but there are wires coming in from the flue sensor and another sensor that's inserted in the copper outlet from the heat exchanger. How do they factor in?

 

[Dana]

Take a peek at Figure 25 in the manual. The Temperature limiter, Flue gas safety device, and Overheat limiter are all just make/break switch contacts wired in series, that simply turn the flame off or allow it to be on. They are not modulation feedback devices.

 

[carriagehousereno]

Take a peek at Figure 25 in the manual. The Temperature limiter, Flue gas safety device, and Overheat limiter are all just make/break switch contacts wired in series, that simply turn the flame off or allow it to be on. They are not modulation feedback devices.

I get it. Thanks. Gonna have a plumber come take a look at it this week I guess, since it appears to be a problem above my pay grade.

 

[Dana]

It may be above the plumber's pay grade too, unless they have worked in Europe where these thermo-mechanical feedback tankless water heaters abound. I'm sure there are plenty of plumbers who can figure it out, but it may take them quite a bit more time the first time around than it will the 101st time. I don't know if Bosch has a technician certification for this type of tankless. Anybody licensed to work on gas combustion appliances should be able to figure out the gas pressure/regulator testing, and be able to test the raw combustion efficiency, etc., but the mechanical feedback modulation control stuff isn't found in most US appliances.

 

[carriagehousereno]

Didn't realize these things were that specialized. I've read the service bulletin on testing the pressure at the regulator and manifold. It doesn't seem that complicated, couldn't any reputable pipe fitter/plumber at least manage that part of it safely?

 

[Dana]

It's not that they're specialized, but that they're not the USA paradigm the way they are in Europe. Any plumber who is competent to work on gas can work on these things. Just realize that it may be the first time they got into the internals. There are probably 500 tank water heaters for every tankless in the US, and these old-school type tankless units are but a tiny fraction of the tankless units in the US.

What little I know about them comes from having lived in Europe for a short time back in the 1980s, and having lived with a water heat similar unit to yours from about 1994-2008 when it was retired from service still working, replaced by hot water served up space heating system, with better flow and temperature control when we upgraded the heating system.

 

[Dana]

Hopefully there's no "...gasoline pressure/regulator..." in this setup, only a natural-gas regulator.