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Old 06-09-2010, 10:13 PM   #1
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Low hot water pressure, or unequal hot/cold


Hello!

I am not sure whether this problem started before or after the installation of the Rinnai tankless water heater. I have moderate pressure/flow differences between hot and cold. If I just turn on a faucet (any) with just hot, the pressure/flow is fine. However, if I turn on the cold (all the way) instead of getting warm I get cold. I have to turn cold down to a trickle to get warm water. I had to turn the shut off valve on the pictured sink to pictured trickle in order for the faucet to have warm water when handle is fully open.

If I open both the hot and cold wide open at a different sink, its clearly cold. But if I just open the hot on the faucet pictured while the other faucet is still running (hot & cold) there is still good hot water flow. The same thing happens in reverse. Both faucets are two handle. Showers have single handle mixing valves and warm is 3/4 to hot instead of in the middle where it should be.

Anybody ever come across this before? We are on a well but I don't think that has anything to do with it. Cold water temp around 54, hot water 120 degrees. Looking to see if this can be fixed prior to putting in the kitchen faucet.
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Old 06-09-2010, 10:59 PM   #2
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Low hot water pressure, or unequal hot/cold


Are all the faucets the same brand? What brand?

If you get good flow from hot alone, then good flow from cold alone, it sounds like everything is ok until it mixes inside the valve body itself.

Never had that problem before, but that's my initial suspicion...

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Old 06-10-2010, 09:29 AM   #3
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Low hot water pressure, or unequal hot/cold


Sink pictued is Kohler, the other is American Standard. Shower is Moen
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Old 06-10-2010, 04:11 PM   #4
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Low hot water pressure, or unequal hot/cold


Your single handle moen shower valve has a pressure balance cartridge in it, which leads me to think that it ISN'T a pressure issue....

One thing about tankless is the size of pipe inside that is being heated is much smaller than what a normal hot water main in a residential system would have. That = reduced flow rate. Maybe someone else can chime in because I haven't worked with very many tankless heaters yet.
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Old 06-10-2010, 07:53 PM   #5
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Low hot water pressure, or unequal hot/cold


"Cold water temp around 54, hot water 120 degrees" so equal gallons per minute for both should give you 174/2 = 87 degrees; luke warm?
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Old 06-10-2010, 08:45 PM   #6
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Low hot water pressure, or unequal hot/cold


dawktah, did you install the Rinnai tankless water heater yourself, or was it installed by someone else? The reason I ask this is to try to establish what size supply line you have going into the tankless heater. Usually most homes have a 3/4" supply from the meter, and this should be maintained all the way through the tankless heater. If the installer reduced the 3/4" supply to 1/2" when the cold was teed off the supply it would explain why you are having a pressure drop when hot and cold taps are opened simultaneously.
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Old 06-10-2010, 10:09 PM   #7
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Low hot water pressure, or unequal hot/cold


Quote:
Originally Posted by Yoyizit View Post
"Cold water temp around 54, hot water 120 degrees" so equal gallons per minute for both should give you 174/2 = 87 degrees; luke warm?
Thanks for the idea! Pulled out the infrared therm, Yikes! Cold is 60, hot is 110, both full on is 82 instead of 85! Hot water was coming out at 120 back in the winter. Does this tempering valve need seasonal adjustment???

So its actually pretty close. Will be getting single handle faucets from now on!

Thanks All!
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Old 06-14-2010, 06:33 PM   #8
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Low hot water pressure, or unequal hot/cold


Well guys it turns out this isn't so simple. With just the hot water on the faucet in photo flow is 1.0 gpm. However when you turn the cold water on the flow drops through the Rinnai tankless below the 0.5 gpm threshold to keep it producing hot water. Faucet is rated at 1.5 gpm. So I need to make adjustment to pressurizer tank and/or well pump to provide highest flow rate if possible. The whole house is ran in 3/4" copper which can take a flow up to 15 gpm.
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Old 07-14-2010, 10:19 PM   #9
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Low hot water pressure, or unequal hot/cold


Turns out there were some things NOT so obvious that were wrong. The Rinnai tankless water heater lets you display the water flow through it. With the hot water completely on the single faucet pictured the flow was 0.4 gpm the minimum flow to activate the unit. When the cold water was turned on the total flow at the faucet was 0.7 gpm which dropped the hot water flow below the threshold to keep the unit running so the water quickly got cold.

After checking every thing in the system the well switch is set to cut in at 40 psi and cut off at 60 psi. The well pressure tank was pressurized to 46.5 psi??? I let a lot of air out to the recommended 38 psi and the hot water flow rate is now 0.7 gpm and total flow rate is around 1.3 gpm. So the water feels much warmer. Once I make another adjustment to the thermostatic mixing valve it should improve some more. I will have to make a season chart to get the valve tweak right since the well water temp seems to change the output temp a little.

I hope this helps someone that may do a search on the subject.
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Old 07-15-2010, 07:05 PM   #10
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Low hot water pressure, or unequal hot/cold


You checked the pressure in the tank with the system empty, correct?
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Old 07-15-2010, 09:09 PM   #11
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Low hot water pressure, or unequal hot/cold


YES SIR!!!

Pretty much drained the entire system. There are shut offs in-between the old and new half of the house. There is a boiler valve installed right next to the pressure gauge. Completely empty read 46.5 psi.

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