Rheem tankless gas water heater always starts on HIGH!

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toysareforboys

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We had our failed electric water heater replaced with a Rheem RTGH-84DVLN Tankless water heater a few weeks ago. Couple of issues with it but the main one is:

When the hot water heater is at rest and even a tiny amount of hot water is requested the hot water heater starts on its highest setting. This is very loud (the hot water heater is located in a noise sensitive area), and a waste of gas and electricity and wear and tear on the machine. When it switches from the highest mode to the lowest mode there is a big air pressure change inside the tankless cabinet which causes the poor quality/flimsy front cover to do a big flex and give out an all mighty THUMP.

It wouldn't annoy me so much except that for some reason when any toilet in the house gets flushed it triggers the water heater to fire up! IN HIGH mode, THUMP, low mode, off, all in the period of 3 or 4 seconds. What a waste.

I called Rheem tech support at 800-432-8373 and spoke with a "senior" technician and he laughed when I asked if we could make it start in the lowest mode. "But it would take an extra few seconds to get the full hot water out of your taps! Why would you want that?". After that he didn't seem very helpful and just said "Nope, can't change that, always starts at 100%." Yay.

There's eight dip switches in this beast and I only know what one of them is for (DIP1#4 to unlock 140f max temp output setting).

Rheem%20RTGH-84DVLN%20Tankless%20hot%20water%20heater%20dip%20switch%20control%20board%20circuit.jpg


So..... Anyone know what the rest of the dip switches are for? Ideally I would like to DELAY the activation of it by 1 or 2 seconds (to stop the toilet flushes from activating it) as well as make it start always on the lowest mode (i.e. be less aggressive trying to heat the water quickly).

Thank you for any help or advice you can provide.

-Jamie M.
 

Jadnashua

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IF the combustion process caused enough pressure drop inside to flex the cabinet, I'm guessing that you have an installation problem. The burner in the thing is quite large, and there must be enough free air flow available for combustion. I'm guess that that is not the case on yours. If it is in a small room, the doorway to it needs to be vented. If it is closed combustion, the run may be too long (this could be a combination of actual length and/or the number of fittings used or an actual obstruction - ice buildup, a bird's nest, snow, etc). A detailed look at the installation instructions and requirements compared to what was done might be prudent, then report back.
 
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toysareforboys

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IF the combustion process caused enough pressure drop inside to flex the cabinet, I'm guessing that you have an installation problem. The burner in the thing is quite large, and there must be enough free air flow available for combustion. I'm guess that that is not the case on yours. If it is in a small room, the doorway to it needs to be vented. If it is closed combustion, the run may be too long (this could be a combination of actual length and/or the number of fittings used or an actual obstruction - ice buildup, a bird's nest, snow, etc). A detailed look at the installation instructions and requirements compared to what was done might be prudent, then report back.
Wow, thanks for the detailed information!

It's in a room that's completely open on one end and has a massive pass through/bar on the other end, so I don't think there's a room that's more open than this one :)

Here is how they installed it/ran the pipes. The intake pipe is on the left. For elbows I count a 45, 90, 90, then at the end of the 20ft run of the intake pipe outside there's a 90 as well.

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I went outside and inspected the intake pipe and I think you were right!!! Sorry for the dark pictures, it's night time right now.

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Both the intake and the exhaust pipes run straight from the house down to the end of our deck (approx 20ft). The end of the exhaust pipe is just bare and the intake pipe has a 90 degree elbow on the end pointing down. Not really sure why they ran such a long intake pipe, you'd think if they put the exhaust outside of the deck then the intake could have just been 1ft from the house (under the deck), no?

It looks like the board that they attached the straps to that hold up the intake pipe broke. The exhaust pipe is supported by separate straps attached to the end board of the deck. The intake pipe has fallen down so it's barely off the surface of the ground! There was some snow around the intake pipe as well. No telling what kind of garbage has been sucked up but hopefully just a bit of dirt :(

Not sure if this is an installation problem or not, or more specifically who's "fault" it is. The deck is pretty old, could be rusted out nails, rotted wood, etc. I'll call the installers in the morning and see what they say. For the time being we've disabled the water heater.

Thanks again and I'll let you know if raising the intake pipe (or better yet making it way way shorter) solves the problem! Would still be nice if it doesn't start on high though...

-Jamie M.
 

Master Plumber Mark

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This install looks pretty crappy to me with all the sharkbite fittings everywhere....

it says on the front of the panel that it is a rental unit.... Why dont you simply tell
them to take it out and stick it where the sun dont shine..?

It appears you are renting it so at least you did not shell out 3500 for it.... just tell them
you want a tank type POWER VENTED heater and your troubles will go away fast...?
 

Jadnashua

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There are some fairly stringent installation requirements about where the pipe can be and clearances. Without seeing the instructions, I can't say, but I do not think it was installed properly with the minimum required clearances. While they burn fairly efficiently, it is not supposed to have any obstructions above it, so if there's a deck above, it probably is not installed properly.
 
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toysareforboys

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Still no solution for the cover to stop flapping but the toilet flush activating the water heater problem got fixed today! So happy :)

Rheem%20tankless%20water%20heater%20thermal%20expansion%20tank%20stop%20toilet%20flushing%20from%20activating%20the%20water%20heater%20sm.jpg


Rheem%20tankless%20water%20heater%20check%20valve%20backflow%20preventer%20sm.jpg


Now I can build my insane re-circulation pump setup :)

-Jamie M.
 

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Are you sure that your gas supply pipe is sufficient for that device? That can cause ignition issues, and some noises during firing. Many of those devices an fire at up towards 200K BTU, and that takes a large gas supply line to work right.
 
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toysareforboys

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Are you sure that your gas supply pipe is sufficient for that device? That can cause ignition issues, and some noises during firing. Many of those devices an fire at up towards 200K BTU, and that takes a large gas supply line to work right.
I would assume so. They ran a brand new 1" line directly from the gas meter (nothing else is tapped off it).

-Jamie M.
 

Master Plumber Mark

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all this work, all this trouble.....
are you going to be the guy who gets to de-lime this puppy once a year or
does that come with the rental program for free????


I am Just wondering why a 50 gallon power vent was not an option here...
was there a space issue involved, or did you just have a throbbing hard-on
to be the first guy in the neighborhood with a tankless heater????

I hope that you can tweak this tankless heater and
get that recirc line to work for you.....oh what fun you have
in front of you.......
 
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toysareforboys

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all this work, all this trouble.....
are you going to be the guy who gets to de-lime this puppy once a year or
does that come with the rental program for free????
They gave us a flush kit with the tankless water heater (Rheem RTG20124) and a bottle of Flow-Aide but they said they will come and use that kit once per year which is included in our rental program.

I am Just wondering why a 50 gallon power vent was not an option here...
was there a space issue involved, or did you just have a throbbing hard-on
to be the first guy in the neighborhood with a tankless heater????
Our 50 gallon power vent gas water heater was only 4 years old when they replaced it with this tankless. It never worked properly from day one, even after the manufacturer replaced it, so eventually they replaced it with this tankless one. I can't remember exactly what the error codes were with it, "low pressure intake sensor" or something, but it'd only heat a bit of water in the 50 gallon tank, so half way through your shower it'd run cold then you'd have to get out of your shower, unplug the water heater, then plug it back in, wait 10 minutes then continue your shower. It got progressively worse to the point that it'd take two people to have a shower. One to stand by the water heater to power cycle it when it shut off and the other one having the shower :( If you kept power cycling it for 30 to 40 minutes BEFORE your shower then usually you'd have enough hot water to last.

After they replaced it once and we continued to have the same problems we didn't hesitate to jump at the chance when they recommended tankless! We love that it's much quieter than our 50 gallon power vent, that you can have a shower for an hour and the temperature doesn't change one degree (with our 50 gallon every 30 seconds in the shower you were turning down the cold a bit, and again, and again, until the point you were on full hot then you would get out and power cycle it.). We don't mind that it takes a bit longer for the hot water to come out of the tap, and don't mind that it tricks you into thinking the warm water is almost here so start washing your hands then BOOM, cold water sandwich! :( lol.

I have an idea on how to totally prevent cold water sandwiches but if a sensor or valve went bad with the setup it could be a run away problem. Basically you put an analog or digital temperature sensor and an analog flow sensor (or could tap into the flow sensor inside the heater or run a relay activation setup from the fans) on the hot water output line right were it comes out of the water heater. There would be an electrical diverter valve and re-circulation pump also right under the heater. How it would work is when someone requests hot water, it engages the diverter valve so that the hot water line now goes right back into the cold water input line and the re-circulation pump will continues to circulate the water through the water heater until it reaches full temperature at which time the diverter valve would change back to normal and the re-circulation pump would shut off. This would guarantee that ZERO cold water would EVER came out of the hot water heater, banishing cold water sandwiches FOREVER! :)

I hope that you can tweak this tankless heater and
get that recirc line to work for you.....oh what fun you have
in front of you.......
Now that it doesn't fire up every time we flush a toilet we are much happier with it. The installer came and chopped off the 20ft of intake line and removed the 90 degree elbow from the end so now our intake line is only 2ft long from where it exists the wall and the cover flexes WAY less when the burners fire up now. He said that if we still get annoyed with the cover flexing that he would come back and put a one way air valve in the cover or bottom so when the burners fire up it can suck a bit of fresh air from the room instead of creating a huge vacuum inside the tankless cabinet.

I'll be using this digital temperature sensor/controller for the under sink re-circulation pump so that it's only able to activate when the water temperature drops below the set temperature and of course shuts off when the incoming water has reached your set temperature.

12v%20temp%20controller.JPG


The temperature sensor will be activated by an analog flow sensor and even if the flow stops the re-circulation pump will continue to run until the hot water has reached the pump (so basically you just open the tap for a split second, shut it off, twiddle your thumbs while it's moving the cold water from the hot water line and pushing it back down the cold water line, then when you hear the pump/relays click off, then you can start using the hot water). Should be super sweet :)

-Jamie M.
 

Dana

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The only thing that can cause the unit to fire up is flow. If it's firing when a toilet flushes it means you have some how-water cross-over between the hot and cold so that when the pressure on the cold line drops, hot water flows into the cold plumbing. That's probably going to be at shower or sink mixer valve. Hacking in some recirc plumbing may have made the symptom subside, but it may be just masking another issue.

With a long run on the vent pipe outside the warmth of the house there's a good chance you may end up with a frost-constricted vent during an extended cold snap. Short hot water draws put plenty of moisture in the vent pipe, but not necessarily enough heat to keep it from freezing inside the pipe. Even on long draws condensing burners put out cool enough that 20' of pipe could present a frost-up risk during a week of cold weather, even in balmy Toronto. It's a fairly common problem with condensing boilers even with short exterior runs of vent pipe in climates colder than yours.
 
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