Rinnai Tankless starts to run when cold tap turned on

[poriordan]

We have just installed a Rinnai RU 199IN tankless water heater. This is replacing an Eternal unit we had for several years. We are using exactly the same plumbing but are experiencing a problem with the Rinnai that we never had with the Eternal. Whenever we turn on any cold tap, the Rinnai unit starts to run. The "In Use" light does not turn on but we can hear a pump (or similar) running. The Rinnai turns off again after about 10-15 seconds, regardless of whether we keep the cold water running or not.

We have isolated the problem to the fact that we have 2 "on-demand" recirc pump units in the house. These are units where you push a button when you want hot water and a pump runs that pumps the water in the hot line back into the cold line until the unit senses that the water is warm. One is a Taco Genie 0011-CF; the other is a Chili Pepper CP9000. If I close the isolation valves to *both* of these units, the problem goes away, but if I leave either one connected, the problem happens.

Our static water pressure is around 65 PSI. It drops only very slightly when a cold tap is turned on (or a toilet is flushed). I should add that if I slowly open a cold tap and gradually increase the flow, the Rinnai unit does not turn on. So it definitely seems to be tied to a transient flow of hot water caused by a cold tap turning on.

So far we've tried the following, all to no avail:
- add a check valve on the cold water inlet to the the Rinnai
- add an expansion tank (pumped to 55 PSI) after the check valve on the cold water inlet to the the Rinnai
- add a check valve to the hot water outlet of the Rinnai
- Change the Rinnai's "Low Activation Mode" setting from On to Off.

The remaining suggestions I've been given are:
- remove the check valve on the cold inlet and set the expansion tank to 65 PSI. (I think the theory here is that the expansion tank might offset the transient drop in water pressure and thus stop the hot water flow).
- replace the check valves on both of the recirc pump units. (It's hard for me to believe that both units would be defective in the same way.)

I'm getting reluctant to keep trying things as this experimenting is getting expensive and frustrating :-( Would really appreciate any suggestions from the experts as to what's happening and what, if anything, I can do to fix it.

 

[Fitter30]

Taco only runs when button is pushed whats turning it on a t stat?

 

[Sponsor]

 

[wwhitney]

How about pictures of both the on demand recirculation pump installations?

Does the unwanted triggering of the Rinnai occur when using the cold water at a sink without either of the recirculation pumps? You referred to isolation valves, not exactly sure what you have, but if you turn off the cold water supply at each sink with an under sink recirculation pump, does that then stop the unwanted triggering of the Rinnai at that other sink?

If so, it seems like the drop in water pressure on the cold side is allowing some water to flow through each of the pumps from the hot side to the cold side. Which to my limited understanding of pumps is what one might expect. I'm not sure if there's something clever that is or can be done at this type of hot to cold pump that would prevent this. A check valve wouldn't help, as the water is flowing in the direction intended while the pump is operating. A solenoid valve on the input or output of the pump, which is closed when the pump is not running, seems like it would work.

Cheers, Wayne

 

[wwhitney]

A couple follow up comments after looking at both installation manuals:

My proposed test of closing the cold water supply is for an installation like on the Chili Pepper where the pump connects downstream of each supply stop. The Taco installation manual shows it connecting upstream of each supply stop (so you have to turn off water to the whole house to install) and that it has its own isolation valves. So obviously for that installation style you'd have to use the isolation valve.

I see that the Chili Pepper has a spring loaded check valve that acts to close off the cold water from the hot when the pump is not running. When the pump runs, it has to build up enough pressure to compress the spring and move the check valve before any water flows. So that is designed to prevent the hot water from bleeding into the cold water when the pump is off, which I did not understand when writing my earlier answer.

However, the Chili Pepper manual has instructions for increasing that spring pressure if you still get hot water bleeding. So perhaps that spring is worn out or misadjusted. And perhaps addressing that would prevent your Rinnai activation via bleed through on the Chili Pepper. I didn't check when the Taco has something similar, but if you get the problem fixed with the Taco isolated and the Chili Pepper in normal working configuration, then hopefully it would be possible to do something similar for the Taco.

Cheers, Wayne

 

[poriordan]

Taco only runs when button is pushed whats turning it on a t stat?

Yes, the Taco (and the Chili Pepper) only run when a button is pushed. That turns on the pump that drives the water from the hot line down the cold line. The pump shuts off when the Taco senses the temperature of the water in the hot pipe getting warn.

 

[poriordan]

A couple follow up comments after looking at both installation manuals:

My proposed test of closing the cold water supply is for an installation like on the Chili Pepper where the pump connects downstream of each supply stop. The Taco installation manual shows it connecting upstream of each supply stop (so you have to turn off water to the whole house to install) and that it has its own isolation valves. So obviously for that installation style you'd have to use the isolation valve.

I see that the Chili Pepper has a spring loaded check valve that acts to close off the cold water from the hot when the pump is not running. When the pump runs, it has to build up enough pressure to compress the spring and move the check valve before any water flows. So that is designed to prevent the hot water from bleeding into the cold water when the pump is off, which I did not understand when writing my earlier answer.

However, the Chili Pepper manual has instructions for increasing that spring pressure if you still get hot water bleeding. So perhaps that spring is worn out or misadjusted. And perhaps addressing that would prevent your Rinnai activation via bleed through on the Chili Pepper. I didn't check when the Taco has something similar, but if you get the problem fixed with the Taco isolated and the Chili Pepper in normal working configuration, then hopefully it would be possible to do something similar for the Taco.

Cheers, Wayne

Thanks so much for both posts, Wayne. To answers the question in your first post, the unwanted triggering occurs when any cold tap or toilet is used; not just the ones where the recirc pumps are located. And yes, we have isolation valves on both the cold and hot sides of each recirc pump; closing either of those valves on each of the recirc pumps makes the problem go away as it prevents any water flow through the unit.

You are right that the check valves in the recirc pumps can't be the issue, as they prevent cold-to-hot flow which isn't the problem. I'll take the Chili Pepper out and see if I can tighten the spring loaded check valve you mention.

 

[poriordan]

Wayne - you fixed it! I isolated the Taco unit and serviced the Chili Pepper as you suggested. There was no dirt inside the spring-loaded check valve so I just tightened it a turn. With the Taco unit isolated and the Chili Pepper "enabled", I can turn on water anywhere and not have the Rinnai trigger. I will do some digging to see if there's an equivalent adjustment that can be made on the Taco unit.

Sincere thanks for your advice. This has been driving us crazy ever since we put in the unit!

 

[wwhitney]

Glad that it worked out! Score 1 for reading the manual.

The Taco supposedly has "Integral Flow Check" but I couldn't find any details on it. If it's non-adjustable, then if it's a two way flow check valve, the spring pressure is too low. You could try adding a spring loaded check valve on the Taco outlet (maybe someone can suggest an adjustable one?). Otherwise, I don't have any suggestions beyond swapping it for a pump with an adjustable flow check.

Cheers, Wayne

 

[Bannerman]

Not aware of any adjustable check valves, but the example shown at the link below has a spring closure which will open with only 4.5 psi differential pressure. Other check valves may incorporate heavier springs but you will need to consider the pressure increase capacity of your Taco circulator pump since it will need to overcome the resistance of 2 check valves.

To utilize the example below, since that spring check is equipped with female connections, you will need to obtain a suitable brass nipple to adapt the 1/2" female braided hose fitting to the check valve inlet.

https://www.amazon.com/DERPIPE-Vert...af0cd&pd_rd_wg=YlJT2&pd_rd_i=B08L9KS32P&psc=1

 

[poriordan]

Thank you both for your suggestions. I've been banned from any further work on this until after Thanksgiving ;-) I will report back next week

 

[wwhitney]

A couple speculative questions:

1) For the OP, do you have a pressure reducing valve on your city water supply?

2) For the knowledgeable, does a PRV have an equivalent of a cracking pressure? Seems like it would, since it has a spring loaded valve that wouldn't be able to open instantaneously when the downstream pressure drops.

If the answer to both question is yes, then the OP at least needs a spring check valve whose cracking pressure is bigger than the PRV cracking pressure. As the Taco presumably has a spring check valve in it, does cracking pressure add for spring check valves in series?

Cheers, Wayne

 

[poriordan]

Back at it ;-)
To answer Wayne's question, I do not believe there's a pressure reducing valve on our water supply.

I called and spoke with Tech Support at Taco. Mario told me that the Taco unit's internal valve's cracking pressure is 18" of water (or about 0.6 PSI) and that's it's not adjustable. (As an aside, he proposed using a Mixing Valve to solve the problem but that won't work at all as the hot water is cold when the problem happens. He sent me a file but I can't upload it as I keep getting a security error.)

So I bought a 3/4" valve with a 4.5 PSI cracking pressure on Amazon (https://www.amazon.com/dp/B0885T6M93?psc=1&ref=ppx_yo2_dt_b_product_details) but the problem still happens with that. I confirmed that the Taco can easily drive water with this in place.

I would guess that the cracking pressures add in series so the pressure drop that's happening must be at least 4.5 + 0.6 PSI to allow water to flow from hot to cold through the Taco.

Next step for me is to return this valve and continue to experiment with higher cracking pressures until I find one that works while still allowing the Taco to pump water. If anyone knows of a good source (or an adjustable one), please let me know; I'm running out of options on Google. I did find an adjustable one but it's for a much smaller diameter pipe.

 

[wwhitney]

Not sure why this website doesn't use a domain name, and the parts are plastic, but maybe one of these would be suitable:

http://63.156.201.111/SMC/VALVES_CV-1.htm#D_ADJSCV

It says it's adjustable from 2 - 14 psi.

Cheers, Wayne

 

[poriordan]

Thanks Wayne. Apparently the site you've found is spearsmfg.com. I will call them tomorrow to see if I can buy the part.

I also found an adjustable part (also plastic) at https://www.ndspro.com/PDFs/Tech-Sp...In-line-Spring-Check-Valve-CV,-UCV,-WCVxx.pdf but it's IPS and I don't know of any easy and reliable way to connect IPS to NPT.

It also looks like Check All Valves at https://www.checkall.com/product/male-threaded-check-valve-style-cn-spring-check-valve/ will make one at any cracking pressure you want (if I'm reading the site correctly at https://www.checkall.com/PDFfiles/CN.pdf) but I'm sure that doesn't come cheap - especially since I don't know what pressure I need ;-)

Peter

 

[wwhitney]

but it's IPS and I don't know of any easy and reliable way to connect IPS to NPT

I don't follow? IPS threads are generally NPT.

Cheers, Wayne

 

[poriordan]

I came across this article that seemed to imply that they could not be connected without leaking:
https://diy.stackexchange.com/questions/11582/is-ips-threading-compatible-with-npt-threading/11590

They are close, and both have 14 TPI. Can they be mated without leaking given the slight difference in diameter?

 

[wwhitney]

Before today I had never heard of the idea that the S in IPS might be "Straight" rather than "Size". Regardless, the document you quoted says "IPS—IRON PIPE SIZE (SAME AS SCH. 40)" So I'm pretty sure they are using the S to be mean Size.

I would expect the threads to be tapered as normal, and just order one and confirm that.

Cheers, Wayne

 

[Fitter30]

Ips iron pipe straight thread made to seal with a gasket. Have had good luck with monster teflon tape with anaerobic pipe dope for sealing threads.

 

[wwhitney]

See PDF pages 11, 12, and 29 of this catalog:

https://www.ndspro.com/PDFs/Catalogs/NDS-Flow-Management-Catalog.pdf

The parts are described on pages 11 and 12 as FPT x MPT or FPT x FPT, and the legend on page 29 says:

MPT MALE PIPE TAPER THREAD
FPT FEMALE PIPE TAPER THREAD

So tapered threads, not straight. In this case, the S of IPS is Size.

Cheers, Wayne

 

[poriordan]

To close this thread out, I bought an NDS part from our local irrigation supply wholesaler. With plenty of PTFE tape, it seals perfectly with the existing 3/4" fittings. After 3 attempts, I've gotten the check valve to the point where the pump can still recirculate water but the Rinnai no longer turns on when a cold water faucet is opened. Success!

Sincere thanks to Wayne (and to the others who contributed) for all his help and guidance on this. Happy holidays to you all.