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Mark, Union Plumber
Category: Plumbing
Satisfied Customers: 292
Experience:  32 Years Experience in Commercial, Hi-Rise, Residential Construction, 4 years Project Management
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We have a tankless R85 Rinnai water heater. We love it. However,

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We have a tankless R85 Rinnai water heater. We love it. However, it seems that the heater shuts off if the water flow isn't high. Is there any way we can adjust it so that the heater keeps heating even if the water is on a lower flow rate?
Welcome to Just Answer, I will do my best to Answer your question to the best of my professional ability. Do you have a R85 Plus? I , Ext. their are several different models, Please tell me your model so i can answer your question.
Customer: replied 6 years ago.

The model is R 85i-0. After the model number is XXXXX string of letters and numbers:

REU - V2532FFU - US

These heaters have a flow switch , the flow switch tells the heater to turn on and heat the water when a fixture is turned on, if there is not enough water flowing threw the switch the heater will not turn on. So if you only turn the hot water on a trickle or low flow the heater will not turn on. If you are halving the problem of having the hot water open all the way and you are still not getting hot water then here are some common problems, one cause of this problem can be mineral build up in the shower heads, faucet screen aerators, when this happens it reduces the flow and the switch doesn't know to turn on. The other thing can cause this is the switch it self can be clogged up with mineral build up and will need to be serviced and cleaned out or replaced. The first thing to do would be take all your shower heads off and clean all the screens , then take off all the faucet screen aerators and clean them out as well. Now you reducing the amount of restriction. If that doesn't help or solve the issue then you will have to call a certified trained and licensed professional to service and repair.
Customer: replied 6 years ago.
What I need to know is how to adjust the flow switch so that it is more sensitive to flow and turns the heater on at a lower flow rate.

You can not, there is no adjustment for that it comes set from the factory already set, you can only adjust the water temperature .

Initial Settings ONLY IF YOU HAVE THE deluxe CONTOL PANEL-
When you press the ‘Water Smart / Bath Fill’ button, a Shower /
Bath Volume will appear on the indicator. When a deluxe bathroom
controller(s) is first turned on, So when you press the Water smart bath fill button you can tell the heater to turn on automatically this way you can use the water at a low flow and the heater will turn on. IF you do not have the deluxe controller you can have your current controller fitted. The model number you gave me told me that they are sold with the standard controller which does not have this feature.

Q - When I was using the hot water, the water got cold!
A - If you adjusted the flow from the tap to lessen it, you may have gone below the minimum
flow required. The Rinnai Water Heater requires a minimum flow rate to operate). If you mix the water with a tap and attempt to get a temperature well below the temperature being controlled by the unit, it may drop the flow below the desired minimum flow rate. Decrease the temperature supplied by the Rinnai Water Heater at the remote control or increase your total flow. Your unit says that it must have 0.6 gallons a minute flowing threw it en order to turn on.

The previous expert is absolutely correct that the flow cannot be adjusted to less than .6 GPM and below that it will not operate. What you can do is change the aerators on your lavatory faucets from .5 GPM to 1.0 GPM or from 1.0 GPM to 1.5 GPM and this will help or alleviate the problem with minimal cost. Hope this helps.
Mark, Union Plumber
Category: Plumbing
Satisfied Customers: 292
Experience: 32 Years Experience in Commercial, Hi-Rise, Residential Construction, 4 years Project Management
Mark and other Plumbing Specialists are ready to help you

Even if you install different aerators with a higher flow rate, you said that you only have this problem when you only turn the sink on a little , even with new aerators if you still only turn the sink on a Little bit it still will not be enough water flow and you The heater will still not turn on.

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