"Losing" hot water

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Old 02-13-20, 01:19 PM
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"Losing" hot water

Hi - I have a Rinnai RUR98i water heater with dedicated return line in a house we recently renovated (about 3 yrs ago). As of a few months ago, we noticed our shower running out of hot water. The water will start hot, but then cool off as the shower progress - not cold, but lukewarm. I've noticed the other showers in the house do the same. I replaced the mixer valves at the one-handle faucets throughout the house, without helping the issue. I've also had a plumber come out and say the water heater needs to be descaled, which I did, and it didn't help. He wasn't much help on his return visit.

Also of note, only one sink in a bathroom now is noted to not have any hot water at all when the shower next to it is on (never a problem in the past, and runs hot when shower is off). We've also noted the hot water fades faster when other appliances/showers are running, which again was never an issue, as we could practically be running the washer and all showers with good hot water in the past.

The hot water heater shows its putting out water at 120*, so it seems like its working, and when I turn the hot water off to the heater, and try turning on a hot water faucet, no water comes out.

I appreciate any help. Thanks much.
 
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Old 02-13-20, 05:23 PM
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when I turn the hot water off to the heater, and try turning on a hot water faucet, no water comes out.
Sorry, what hot water would be running to the hot water heater?
 
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Old 02-13-20, 05:27 PM
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Sorry - I mean turn off the water supplying the water heater
 
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Old 02-14-20, 06:55 AM
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Of course if you turn off the water, either the cold inlet or the hot outlet, at your water heater then the water will stop coming out of an open hot faucet upstairs.

(If there is an expansion tank then water will come out for a short time as the expansion tank discharges.)

Are you overdrawing the gallons per minute capacity of your water heater? Are the burners in the water heater working at full efficiency? If the heating is not up to snuff you can still get the full 120 or whatever degree hot water at low volume. If you try taking a shower with less water flow, does the water stay hot? (To test that, temporarily put a (usually red plastic) flow restrictor or water saver in the shower head if you have a single handle faucet without independent temperature and volume control.)
 

Last edited by AllanJ; 02-14-20 at 07:21 AM.
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Old 02-15-20, 04:26 PM
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I thought if a mixing valve was bad somewhere, cold water would come out of the hot faucet if water to the heater was shut off - which is why I mentioned it.

I doubt I'm overdrawing the capacity - everything worked great up to a few months ago. Now even if only one shower is on the hot water will turn cool (previously we could have multiple showers going on at a time).

I can try limiting the flow over the water and see if that helps, but even faucets at sinks "lose" hot water, no matter how much water is coming out.

Thanks.
 
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Old 02-15-20, 05:44 PM
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I've also had a plumber come out and say the water heater needs to be descaled, which I did, and it didn't help
That doesn't mean what you did helped the problem.
Descaling these units is not easy and sometimes requires the use of acids.

You said the output gauge shows 120.
What happens when you start using hot water...... does it go down ?
If it goes down you need to focus on a heater issue.
 
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Old 02-15-20, 07:08 PM
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I do not understand why but a laundry hookup left "on" with the machine turned off, or a shower left "on" but muted by a sprayer hose, might result in cold water going backwards up the hot pipe or vice versa, resulting in this problem.

(long shot) Debris, or a pebble from the water main, in the hot water line, near the heater since all hot water outlets are affected.

When you start using hot water the debris starts moving and hangs up at a narrower point or bend in the line causing a near stoppage of hot water a minute or so later. When you turn the water off the debris settles back down allowing a little hot water to come out on the next usage before rising and clogging the line again.

Try turning only a hot faucet on medium. Then leave it unattended while you go down and carefully feel the output pipe of the water heater. See if the hot supply pipe is suddenly cooler a short distance away from the heater, notably at an elbow.

Go back upstairs. If the water out of the faucet has become much slower all by itself then wait 5 minutes and go downstairs again. Is any part of the pipe that originally felt hot now much cooler?
 
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Old 02-15-20, 08:23 PM
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with dedicated return line
check valve is bad probably. Turn off pump and valve to that line and test..
 
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Old 02-17-20, 06:42 AM
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Thanks for everyones reply - I apologize for my delay in responding -work kept me busy yesterday.

PJMax- I descaled the heater with vinegar per Rinnai instructions/recommendations - not sure if it helped. And I've watched the output temp of the heater during the duration of a shower and it did stay at 120, and it stayed constant throughout the entire shower.

AllanJ- I will try your suggestion - sounds reasonable. Issue is much of my hot water pipe is in the crawl so it sounds like I may need to get under there.

Iawrosa- I will try that as well. That sounds like a possibility.
 
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Old 02-19-20, 07:55 AM
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Turns out it was in fact a check valve on the recirculating return line (hidden in the crawl). Thanks all for your help!
 
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