Rheem NG tankless water heater craps out at 100F for hot tub. We want 108F.

nibs

Brand new Reem 200K BTU tankless for heating hot tub,
Have a new recirculating pump pulling from the hot tub with sufficient flow. Have been playing with it for a week, once the tub water reaches 100F it gives false insufficient water flow error. Manual says it should work with return water temp up to 15F below the output setting, we have tried the heater at 120, 130 & 140 & it still craps out at about 100F.
I suspect we have a bad inlet water thermister & wonder if there are ways to disable it, or dip switch settings to adjust it. there are two little white boxes clipped to the heater inlet tube which I suspect are thermisters, tried unclipping them from the pipe but no joy.
Just turned 79 last week & would really like to get a hot soak before I croak.
Got so much help here, with my hydronic build, and learned so much that I am a little embarrassed to come back with this sort of problem

mattmia2

So you are pumping water out of the tub and in to a tankless water heater?

Is this the only thing this water heater is used for if that is the case?


If you have 100 degree water entering the tankless water heater, it probably isn't designed for that, it probably either has flue gas temps that are too high, can't modulate the burner low enough or something else is out side the sage design parameters as a result of the supply water that is much hotter than it is designed for as a tankless water heater. You would probably need a boiler with a heat exchanger to make this work.


Usually the recirculation pump has a thermostat that stops recirculation when hot water reaches the fixture or comes back to the tank.

kcopp

Another wrong application for a tankless water heater. We see this all too often here. As stated above you need a boiler w/ a heat exchanger to properly do this.

nibs

The manual clearly sets out that in a recirc situation the returned water will be maintained at 15 degrees below the set temp on the thermostat.
The burner shuts down before it can even raise the flue gas temperature.
Lots of articles written by folks who have used tankless gas water heaters to do the job.

nibs

Also each of mattmia2's items if a problem would be giving the appropriate error code.
The manual sets out a description for using the heater in recirc mode as a supply for air handler space heating.

Hot_water_fan

A large tank would have been much better for this application. Tankless units are limited flow wise and aren’t more efficient. 

mattmia2

nibs said:

Also each of mattmia2's items if a problem would be giving the appropriate error code.
The manual sets out a description for using the heater in recirc mode as a supply for air handler space heating.

It may be interpreting seeing a temp higher than it expects as too little flow. May be time related as in i haven't fired they burner yet but the hx is hot or something like that.

This isn't the right product for this application. That being said you may be able to make it work if you think carefully about how it makes decisions. there is good information about that in the service manual.

Larry Weingarten

Hi @nibs , Is there any way you can pull only the coldest water off the bottom of the tub for heating? Also, can any jet pump remain off until the tub is hot? This would prevent mixing and keep temps at the bottom of the tub cooler, giving the tankless more opportunity to behave.

Yours, Larry

nibs

Thanks Fan for the positive input. Interestingly when we bought the property there was a propane fired HW tank which we used for a year until we built the wood boiler and got rid of the tank.
We appear to have found and hopefully cured the problem this morning, ran the system until we got a nice toasty tub.

Larry Weingarten

"We appear to have found and hopefully cured the problem this morning, ran the system until we got a nice toasty tub." I'm curious. Please do tell more about what you found!

Yours, Larry

nibs

Larry Weingarten said:

Hi @nibs , Is there any way you can pull only the coldest water off the bottom of the tub for heating? Also, can any jet pump remain off until the tub is hot? This would prevent mixing and keep temps at the bottom of the tub cooler, giving the tankless more opportunity to behave.

Yours, Larry

Thanks Larry, we use a 6'diameter galvanized stock tank about 300 gallons, no jets. Also we do pull the water from a bulkhead installed at the bottom of the tank. Next time we replace the tank will move the supply and return lines to opposite sides of the tank.
Problem has been solved, wife is claiming the diagnosis and cure, I of course disagree.

Larry Weingarten

Hi @nibs , A bit of wisdom I've seen demonstrated here at HH is that disagreeing is fine as long as you don't let on in any way, but just smile and nod.
Yours, Larry
ps, glad it got fixed!

nibs

Larry Weingarten said:

"We appear to have found and hopefully cured the problem this morning, ran the system until we got a nice toasty tub." I'm curious. Please do tell more about what you found!

Yours, Larry

OK I will tell you about our steps to tame the beast as long as I can leave out the embarrassing part where she insisted that we clean the teeny tiny strainer on the input water line.

Started out with the pump powered online and for the first day it worked ok.
We have a gravity flow sand filter which shares the intake but otherwise operates separately.

So the cold side intake was a short length of 3/8 pipe connected to the 3/4 pex lines, figured that had to be the problem. replaced it with 3/4".
That worked for another day.

Noticed that the heater pump was getting hot, so put a timer on 15 mins on, 15 mins off, got another day or two after that.

Yesterday reading about recirc possibilities discovered that the heater could manage and power the pump directly, so rewired the pump and set the appropriate dip switches. Still no joy.
This morning set the altitude dip switch as we live about 500 feet above the factory setting.
Measured the water flow at 1 gal/min, which should be good, as .4 GPM is minimum.
SWMBO storms up to the engineering dept & says OK we are going to clean the damn inlet strainer, So had the engineering dept, remove same and turn it over to the sanitation dept.
re installed and hid in shame while the tub temp came up to 110 deg.

Am quite convinced that it was my careful setting of dip switches and wiring the pump to the heater, She of course has her own opinion, which will be repeated especially at the mothers day dinner with the family.
Cheers and thanks for the positive attitude.

PS am installing a larger strainer on the heater intake as soon as I post this.

nibs

Just a brief update, now that we have the Rheem tankless working as desired, we find that by setting the thermostat to give us 125F water, the system shuts down automatically at the perfect temp for us (110-F). perhaps later in the summer we will drop the unit down to 120F.
So we have gone from the hit and miss wood boiler I built about 10 years ago to a virtually automatic gas powered hot tub which suits our needs and requires little daily caring and no daily firewood feeding.
Thanks again for looking over my shoulder.

nibs

Just to follow up incase anyone decides to use a tankless as a hot tub heater thought I would mention that after using the tankless for more than a month we could not be happier, just got out of the tub & thought I would pass on a couple of things that we had to learn.
We tried a few pumps because we kept getting a low flow shutdown, the third pump a 1/6th hp Grundfos was meant to fix the problem and it helped. Also the Rheem has a thimble sized strainer (50 mesh) in the water inlet to the heater, it is totally inadequate and kept plugging up reducing the flow. I was amazed to find after about 5 phone calls that no one knew the mesh of the screen, it took them a week to contact their supplier before they could tell me 50 mesh. To assist the teeny tiny installed screen, I put a 100 mesh spin down strainer between the circ pump and the heater. went 20 days before cleaning it and both strainers had detritus but nothing serious. Our grand kids 5 &7 yo girls use the tub in most afternoons and they stir up the water and leave some hair etc behind, hence the detritus.
But the aha moment came when I discovered that the heater needed a little pressure to activate the gas switch, so I simply put a 1/4 turn ball valve after the heater on the return line and closed it by about 1/3, that raised the pressure and the system seems to be working flawlessly since.
There are cheaper ways to do it but you guys taught me the value of quality when heating water. Thank you.