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Thread: Pressure drop with tankless water heater

  1. #1

    Default Pressure drop with tankless water heater

    We got a tankless water heater installed a few weeks ago. It is a 199,000 BTU model (don't know brand). Our gas line is 1.25" to the house and the plumber ran 1" to the water heater and .75" to the gas stove. 199K BTU is in excess of what is needed to run all of our hot water using items at the same time.

    When you turn on the hot water from a fast-flowing faucet (e.g., our shower) the pressure drops sharply. The plumber has been back with a representitive from the water heater company. They used a manometer (sp?) to measure the pressure. When no water is turned on, our pressure measures 10 (not sure of the unit--guessing psi) and then when you turn on the shower, the gas pressure drops to 2 which greatly reduces the water pressure. The plumber and tankless rep said it must be the gas regulator. We had the gas company come out and replace our regulator. No change.

    Anyone have any ideas on what could be causing this??

    Thanks in advance.
    Heather

  2. #2
    Retired Defense Industry Engineer jadnashua's Avatar
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    Is the symptom you are seeing show up as the inability of the system to produce enough hot water? Many gas appliances operate internally at less than a pound of pressure...the in-line regulator adjusts that so 2 on the inlet might be fine.

    If what you are seeing is not enough water pressure, or the water not hot enough, there may also be a volume restriction in the tankless unit. It can only supply so much volume while producing a suitable heat gain. On a multi-head shower, the unit you have may not produce enough volume for the shower you want - it should work in many places for a single, restricted (per today's codes) head, though. You may need yet a bigger gas line and a second, in-line tankless to produce the volume you want.

    How many GPM are you trying to achieve, and how cold is the incoming water?
    Jim DeBruycker
    Important note - I'm not a pro
    Retired Defense Industry Engineer; Schluter 2.5-day Workshop Completed 2013, 2014

  3. #3

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    We haven't noticed any issues with our gas applicance but haven't really compared the burner intensity when we turn on hot water either. In the shower, we still have hot water to shower (it is just a single-head but older shower so not efficient with water usage I am sure). However, if one person is showering and you want to run the clothes washer, the washer water comes out in just more than a trickle. The cold water pressure is unaffected.
    Heather

  4. #4
    Retired Defense Industry Engineer jadnashua's Avatar
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    That is a common problem with an undersized tankless system. To heat water 'instantly', you need to do one of two things, either limit how much you pass by the flame, or have a bigger flame. When turning on multiple items at the same time, you are exceeding the ability of that unit to produce sufficient volume of hot water and to keep it hot, it reduces the flow. think moving your hand over a candle, then a blowtorch. Move it slow (low volume) and it'll get hot. But you better move it fast with a blowtorch, or you'll get too hot. You have the equivalent of a candle..maybe a big one, but still a candle.
    Jim DeBruycker
    Important note - I'm not a pro
    Retired Defense Industry Engineer; Schluter 2.5-day Workshop Completed 2013, 2014

  5. #5
    Master Plumber master plumber mark's Avatar
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    Talking this is a water pressure issue

    Quote Originally Posted by hlalicker View Post
    We haven't noticed any issues with our gas applicance but haven't really compared the burner intensity when we turn on hot water either. In the shower, we still have hot water to shower (it is just a single-head but older shower so not efficient with water usage I am sure). However, if one person is showering and you want to run the clothes washer, the washer water comes out in just more than a trickle. The cold water pressure is unaffected.


    IPerhaps I dont know what I am talking about , but

    if your cold water pressure is unaffected, and its only on the hot side of the system, I dont see what this has got to do with the gas line pressure ??
    ..

    It sounds like you have some sort of restriction on the water line going into the tankless water heater.

    most of them have a in --line water filter that gets clogged up.... maybe the plumber simply turned on the water without flushing out the system.. and now it needs a new filter.


    .

    it could be something as simple as teflon
    tape clogging the inlet to the tankless heater or a solder ball..

  6. #6

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    Jim, we know our unit isn't too small since it can handle more BTUs than we would need if all of our faucets and gas stove are on at the same time? Are you saying that it might help to turn up the maximum hot water temperature? How does one get the flame "bigger" or is that possible?


    Mark, thanks we will check to make sure the line is clear. One thing that makes me think that it isn't the water line is when you first turn on the hot water side it comes out full blast. The water pressure only decreases after the water starts warming up.


    Can anyone think of a reason that the gas pressure would drop like that when we turn on the hot water? We know the pressure drops from 10 to 2 when the hot water gets turned on. So it seems like there is some type of restriction where the tankless unit isn't getting enough gas to be able to warm the strong flow of water as it passes through the unit.
    Heather

  7. #7
    Master Plumber master plumber mark's Avatar
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    Talking What brand is it??????

    you are not explaining your troubles good enough...

    WHAT BRAND IS IT????


    how did the plumber come to the conclusion that it was the gas pressure having an affect on the water pressure in the tankless heater???

    you do actually have hot water, correct???

    but the water is comming out of the tankless heater
    trickeling very slowly?? correct....??


    I dont think that their is a gas flow meter in any tankless water heater that is going to slow the flow of the water down through the unit while it is being heated up.....

    here is another test you can do......

    1. turn off the gas to the tankless water heater...

    2. turn off the power to the tankless heater.....

    3. run the hot water and see if water comes through the unit.....(which it probably still will)...


    again, you probably have something clogged up in the cold water line going to the tankless heater.....

    take the cold line loose to the unit , and my guess is you find your problem as it is going into the unit...

    you also might want to install some sort of flexible connector that can be easily removed and the filter cleaned periodically,



    your gas pressure will natrually drop in the line whenever that amount of gas is
    being used......


  8. #8
    Retired Defense Industry Engineer jadnashua's Avatar
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    A unit that size is probably rated at somewhere around 5gpm. So, anything greater than that flow and it will either slow the water flow through the unit or lower the overall temperature. You'd have to look at the specs. And, few will allow a shower (especially a mult-head one if that's what you have) and a washing machine filling with hot at the same time.

    You never answered how much flow was being called for when you notice the problem.
    Jim DeBruycker
    Important note - I'm not a pro
    Retired Defense Industry Engineer; Schluter 2.5-day Workshop Completed 2013, 2014

  9. #9
    Computer Programmer Bill Arden's Avatar
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    The only thing I can think of that would change the flow rate of the hot water as the unit warms up is a sticking flow sensor valve.

    I think the gas pressure change is a red haring and should be ignored at the moment since that if anything would cause the hot water to turn cold without reducing the flow rate.

    On the unit I got (from a junk pile) there is a small brass slider that senses how fast the water is flowing. This slider could be sticking due to some debris.

    Disclaimer: I don't work on these units professionally and my only experience is a small electric one I took a part for the fun of it.
    Important note – I don’t know man made laws, just laws of physics
    Disclaimer: I'm a big fan of Darwin awards.

  10. #10
    Illinois Licensed Plumber SewerRatz's Avatar
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    OK lets go over a few Tankless water heater basics.

    1. You must have enough gas pressure to meet the demands of the Tankless heater. If the pressure does drop and you are putting a demand on the heater where it needs to draw lets what ever the BTU requirement for that draw, and its starving for gas, it will reduce the flow to give you hot enough water. For example lets say it trying to draw 80K BTU's but its only getting 40K it will cut the flow in half so it still can give you the desired temperature.
    2. Now lets assume the unit is getting the gas it needs. Lets go back to a statement you made, One thing that makes me think that it isn't the water line is when you first turn on the hot water side it comes out full blast., That is called static pressure, when there is no water ran the pressure will build up to normal, and once that is released it will resume back to the low flow pressure. We see this when someone snaps off a gate in a gate valve. People open a tap they get great pressure due to static pressure, then it goes down to a trickle. So as Mark stated it can be a clogged filter or, even a clogged heat exchanger. Some people have experienced a mineral build up in a few short days with a tankless unit.
    3. Next is sizing, depending on the unit installed and your incoming water temperature the unit might be throttling down the flow to give you your desired set temperature. So for example here in Illinois our water temperature is around 50º F the ΔT would be 70º to get the desired 120º water temperature. In most larger units that means no more than 4.5 GPM. If we open two faucets each at 4 GPM the flow would be just fine. But now lets turn on a 2.5 GPM shower now we are demanding 6.5 GPM at 120º so the only way the unit can give you that temperature is to slow the flow down, which will give you a weaker pressure. Only way to solve this is to install a second unit in parallel with a communication cable linking the two units with each other.


    So you situation is going to fall into one of them scenarios. It is very hard to do over the internet, with out being there to figure it out for you.

  11. #11
    Master Plumber master plumber mark's Avatar
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    Talking good info

    Quote Originally Posted by SewerRatz View Post
    OK lets go over a few Tankless water heater basics.

    1. You must have enough gas pressure to meet the demands of the Tankless heater. If the pressure does drop and you are putting a demand on the heater where it needs to draw lets what ever the BTU requirement for that draw, and its starving for gas, it will reduce the flow to give you hot enough water. For example lets say it trying to draw 80K BTU's but its only getting 40K it will cut the flow in half so it still can give you the desired temperature.
    2. Now lets assume the unit is getting the gas it needs. Lets go back to a statement you made, One thing that makes me think that it isn't the water line is when you first turn on the hot water side it comes out full blast., That is called static pressure, when there is no water ran the pressure will build up to normal, and once that is released it will resume back to the low flow pressure. We see this when someone snaps off a gate in a gate valve. People open a tap they get great pressure due to static pressure, then it goes down to a trickle. So as Mark stated it can be a clogged filter or, even a clogged heat exchanger. Some people have experienced a mineral build up in a few short days with a tankless unit.
    3. Next is sizing, depending on the unit installed and your incoming water temperature the unit might be throttling down the flow to give you your desired set temperature. So for example here in Illinois our water temperature is around 50º F the ΔT would be 70º to get the desired 120º water temperature. In most larger units that means no more than 4.5 GPM. If we open two faucets each at 4 GPM the flow would be just fine. But now lets turn on a 2.5 GPM shower now we are demanding 6.5 GPM at 120º so the only way the unit can give you that temperature is to slow the flow down, which will give you a weaker pressure. Only way to solve this is to install a second unit in parallel with a communication cable linking the two units with each other.
    So you situation is going to fall into one of them scenarios. It is very hard to do over the internet, with out being there to figure it out for you.


    Great info...

    but I still dont know what brand we are talking about...

    so we are basically still in the dark

  12. #12
    Illinois Licensed Plumber SewerRatz's Avatar
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    Quote Originally Posted by master plumber mark View Post
    Great info...

    but I still dont know what brand we are talking about...

    so we are basically still in the dark
    True true.. do wish people when they ask questions about their issues would be a bit more descriptive, or post a picture.

  13. #13

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    Thanks for all of your comments, and the additional general information SewerRatz. I apologize if what I have posted is not descriptive enough, but I am obviously not a plumber and don't know what all information is pertinent for you guys. I also had to check what brand the unit is and was just able to do so this morning.

    The unit we have is a Takagi TK-3. The Takagi rep has been out to look at it, and he said he has never seen this issue before.

    To answer questions:

    I don't know the exact gpm requirement of our shower head, but it is a single head and is old. From what I have read, that can mean it needs 5-8gpm.


    Thanks again for the help.
    Last edited by Terry; 01-10-2010 at 09:10 AM.
    Heather

  14. #14
    Computer Programmer Bill Arden's Avatar
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    But the real question is IF the unit has some sort of device that would decrease water flow if it can't heat the water?

    I'm not talking about the normal pressure drop that occurs, but one that deliberately slows the water even more.

    http://www.takagi.com/index.php?product_id=1&page_id=2
    "Easy-Link capability, which allows you to connect up to four units(in parallel) without the use of a controller unit. "

    This makes me think it has a solenoid valve connected to the water line and the unit could deliberately reduce water flow by itself.

    If that is true then that means that a low gas pressure could cause low water flow if it thinks there are more units in parallel.
    Important note – I don’t know man made laws, just laws of physics
    Disclaimer: I'm a big fan of Darwin awards.

  15. #15

    Default Possible

    Yes, It can be the gas pressure. And probably is. Also, Check the inlet water filter for clogging. It has to have between 6-10" pressure to operate properly while its running.

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