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Thread: Fine tuning tankless water heater on well system

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    DIY Junior Member artbob's Avatar
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    Default Fine tuning tankless water heater on well system

    I recently undertook installing a Navien tankless in my home. Its a cr240a. The box itself seems to work. I have checked the gas pressure (11-13 column inches) and the water pressure, on a well, (40-60psi). It works well enough with one shower on although its warmer and cooler with the cycling of the well pump. I also notice that when more than one person takes a shower at a time, the hot water pressure drops significantly, especially in the upstairs bathroom. I think there may be 2 different problems and wondering the best solution. One is the variable water pressure. I wonder if a stop cycle valve is the the way to go. The other problem is the cold water pressure overcoming the hot water when more than one appliance is on. Is a tempering shower valve the best solution for this or is there someother fix? I bought the larger unit believing it would power 2 showers at one time, but apparently there is more complexity to this concept. Thanks, Jim

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    DIY Member ChuckS's Avatar
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    I'm no expert but are you sure you have the 199,000 BTU?

    What is the input wanter temp?

    What do you have the output temp set at?

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    DIY Junior Member artbob's Avatar
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    I aproached it from several angles. It has a 2 stage regulator. 3/4 pipe from the tank to the house at high pressure. Then 3/4 pipe about 8 feet from the 2nd stage regulator to the tankless heater. I maintain and install the gas kilns for our tile business, and according to my piping book, this should supply 329000 btu. I also installed a pressure gauge on the gas line before the heater and it shows the pressure to be 11.5 column inches while the heater is on full bore. This should be more than enough supply, unless I am missing something. The line used to supply a hot water heater tank and a 125000 btu furnace, so the regulator should be able to handle the supply. That is my only uncertainty is the regulator. It is older, but the gauge says the pressure is there! Don't know the inlet temp. I'll check it out. Output is set at 120f. Jim
    Last edited by artbob; 11-29-2009 at 03:43 PM.

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    DIY Junior Member artbob's Avatar
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    cold water temp is 55 f
    I changed the output to 125f. I can now can get 120f at the bath faucet.
    I tested the gas pressure again. With 2 bathtub faucets on, and 3 sinks on the gas pressure is 10.75 column inches. With 5 hot water facets on the water flow is very low. I am going to check how many gpm the cold water is bringing to the house.
    Last edited by artbob; 11-29-2009 at 04:11 PM.

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    DIY Junior Member artbob's Avatar
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    125-55 ambient water temp =70 degree rise.
    From the cr240a specs:
    77 degree rise = 5 gpm
    operating lp pressure is 8-13.5 WC

    here is an interesting note:
    The flow rate of a faucet or shower head is measured in GPM and is a critical factor in sizing a tankless water heater.

    The tub fill flow is about 5 gpm. The shower head is 3 gpm
    Last edited by artbob; 11-29-2009 at 05:48 PM.

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    DIY Member ChuckS's Avatar
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    I'm no expert but the guy who installed mine put it right by the gas regulator and said the length of the gas line makes all the performance difference. For all I know he could have been FOS.

    I do know from testing mine increasing my water pipes to 3/4 made a big difference. I also noticed you can use more showers if you lower the temp to say 105'.

    I do agree with you that yours appears to be limiting somewhere. I would say lower the temp to 105 and if you see more pressure then you know the problem is in the tankless. I'd call the manufacturer to see what they say.

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    DIY Member ChuckS's Avatar
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    Quote Originally Posted by artbob View Post
    125-55 ambient water temp =70 degree rise.
    From the cr240a specs:
    77 degree rise = 5 gpm
    operating lp pressure is 8-13.5 WC

    here is an interesting note:
    The flow rate of a faucet or shower head is measured in GPM and is a critical factor in sizing a tankless water heater.

    The tub fill flow is about 5 gpm. The shower head is 3 gpm
    You say one shower is 3 gpm so two would be 6 gpm. At 77 degree rise you output 5 gpm. That's proving your input water temp is exactly 55 in the morning. From a well???

    Also, the chart I just looked at here says you will get 3.2 gpm with 55 degree ground water and a 2.5 gpm shower head.

    I think you may be operating just about right. I know it sucks but I was noticing that I can't run two showers effectively with the temp at 125 degrees (my normal setting). It was about 40 degrees outside this morning but don't know about the input water temp. Luckily we shower seperately but the good news is I get plenty of hot water now (I'm last after two women) which I didn't always get with the tank.

  8. #8

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    Quote Originally Posted by artbob View Post
    125-55 ambient water temp =70 degree rise.
    From the cr240a specs:
    77 degree rise = 5 gpm
    operating lp pressure is 8-13.5 WC

    here is an interesting note:
    The flow rate of a faucet or shower head is measured in GPM and is a critical factor in sizing a tankless water heater.

    The tub fill flow is about 5 gpm. The shower head is 3 gpm


    Install lower flow shower heads. 3 GPM is kind of steep. I use shower heads that conform to the current national standard. I use about 1 GPM at the normal shower volume setting.

    I measure the hot water temperature at the end of the longest run after pipe warmup. 120 degrees is my stable measurement.
    Samuel James Witwicky

  9. #9

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    Quote Originally Posted by artbob View Post
    cold water temp is 55 f
    I changed the output to 125f. I can now can get 120f at the bath faucet.
    I tested the gas pressure again. With 2 bathtub faucets on, and 3 sinks on the gas pressure is 10.75 column inches. With 5 hot water facets on the water flow is very low. I am going to check how many gpm the cold water is bringing to the house.


    How often do you run all your hot water facets at the same time? That test is fine for the gas pressure test.

    Maybe you should try new aereators and new shower heads before you go too far.


    http://www.energysavers.gov/your_home/water_heating
    Samuel James Witwicky

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    DIY Junior Member jnyost's Avatar
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    What's your well pressure switch set at?

    Ours works very well with our well.

    We added onto our house and didn't have the money to do anything with our how water system yet. I've been very surprised to see our TK-2 keep up. I'm still planning on some changes soon but it seems to be working for the moment.

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