Tk-2 large water pressure drop

Discussion in 'Tankless Water Heater Forum' started by sapo, Oct 31, 2012.

  1. sapo

    sapo New Member

    Messages:
    13
    Location:
    Felton, CA
    i have a TK-2 that has worked great for many years. Yesrerday i noticed that the hot water flow in the shower is about half normal.
    It will start with full pressure then drop to half in about 3-5 sec.
    The TK-2 continues to run, just with low water pressure output.
    When i turn the TK-2 off and run the 'hot' water there is no pressure drop.
    I have cleaned the inlet filter and replaced the house filter i have setup before it.
    I run the water through a sofetner before the TK-2.
    Any ideas?
    Does it have a valve in it that can restrict water flow?
    Also, i'll buy a new unit before i pay someone to come out and fix it.
    Thanks
  2. lifespeed

    lifespeed Member

    Messages:
    321
    Location:
    California
    Although I have no experience with that model, it is my understanding tankless water heaters will reduce water flow to maintain the output temperature setpoint as temperature rise is inversely proportional to flow rate. If you turn off the heater and there is no flow/pressure drop it would appear this is what is happening. If this did not happen earlier in the life of your water heater, it would appear there is a problem related either to the control of the flow reduction valve, or the input of heat to the water passing through the heat exchanger. You mention use of a water softener. A softener, correctly implemented and programmed (a bit of an assumption, not everybody gets this right) should guarantee the heat exchanger is not coated with calcium carbonate. However, internal scaling of the heat exchanger would certainly be one way the heat input to water flowing through the exchanger could be reduced. A correctly-working tankless heater would then maintain correct water temperature in this impaired state by reducing the water flow to compensate.
  3. Dana

    Dana In the trades

    Messages:
    2,812
    Location:
    01609
    It could be an issue with the anti-scald valve on the shower mixer. Check the hot flow at other taps in the house- if it's at the water heater it would have the same problem everywhere, but if it's the shower, it points toward the anti-scald valve.
  4. sapo

    sapo New Member

    Messages:
    13
    Location:
    Felton, CA
    Thanks guys, the shower does not have an anti scald feature to it, thanks though.
    I was thinking i would try descaling it first, before diging into flow sensors or control valves.
    The consistancy of the water softener has been a little hit and miss over the 8 or so years i've had this heater. I probably don't need to mention that it has never been descaled, ignorance on my part. Until i developed this problem i was unaware that it could be descaled.
    If descaling it does not work, does anyone have any sources for parts?
    Thanks again
  5. lifespeed

    lifespeed Member

    Messages:
    321
    Location:
    California
    Obviously descaling works much better when done annually. At some point scale accumulation could be thick enough that chemical removal (mild acid) won't be able to do the trick. Having softened water should almost make this a non-issue, except we probably can't assume the water has always been soft. It may be possible to disconnect the water pipes and do a visual inspection to assess if you actually have a scale problem.

    Given the recent innovations and improvements in tankless WH, I think you are right to be hesitant about sinking much money into this unit at 8 years old. It might be a different cost/benefit proposition if the technology were mature when you bought the heater.

    FWIW, the Noritz NRC1111 is an excellent unit to consider. It has an exceptionally large and thick-walled heat exchanger that does not suffer the pressure drop of smaller units. I have one and am very impressed at the volume of hot water it can provide, while still maintaining good pressure.
    Last edited: Nov 1, 2012
  6. sapo

    sapo New Member

    Messages:
    13
    Location:
    Felton, CA
    Thanks, the inside of the pipes are clear and we only have hard water part of the year. I installed the softner just to play it extra safe. I don't anticipate it being a scale problem. Especially since it happened in the span of time between my morning shower and my daughters evening shower.
    I'll probably throw some money into some parts and see if i can revive it before moving on. I have a hard time replacing something i have'nt thouroughly broken yet.
    But i will look into Noritz and see how they are.
  7. Dana

    Dana In the trades

    Messages:
    2,812
    Location:
    01609
    Takagi customer service has been happy to talk homeowners through diagnosing symptoms over the phone in the past:

    (888) 882-5244

    That fact that it starts out with higher flow for the first 3 seconds is a symptom unrelated to a scale-up issue, which would exhibit low flow all the time. I'm still suspecting something in the mixer valve, not the hot water heater. Before hacking on the heater, verify that the symptom is independent of which tap is being used.

    You should be able to get at least a couple of decades normal use out of a TK2 (maybe only 10 if you're using it as a space heating boiler, which is also done). While the temp control of TK1s and TK2s aren't as stable at low flow as a newer Noritz it's not terrible at all, and the TK2 modulates down to under 20KBTU/hr with reasonable stability, and unlike some others the turn-on delay is pretty short. Like almost any modern gas-burner the flame detectors can develop a carbon-varnish (easily buffed out with a fine plastic scouring pad- avoid hard abrasives like sandpaper, emery cloth, or steel since that would make the problem recur sooner), but most will go a very long time with fairly minimal maintenance.

    Check the filter on the unit again, and purge a few gallons water out back-flushing the filter port when you do. It's at least conceivable you have some scummy crud flapping around in there that get's sucked up to the filter impeding flow only after flow has been established. Those filter screens are pretty tiny, and it doesn't take much.
  8. sapo

    sapo New Member

    Messages:
    13
    Location:
    Felton, CA
    Dana, thank you. Even though i don't have an anti scald shower fixture i was wondering if perhaps there was something in the valve, except it doesn't do it if i shut the heater off and run the hot tap. The other fixtures don't show a pressure drop, but they don't put out the same volume as my shower does.Sinks 3.5 gpm shower 8 gpm, single tap open. The heater currently has reduced the shower to the same 3.5 gpm as the sinks.
    Is it possible to backflush the heat exchange without damaging it? Put the cold on hot side, run water through backwards, dicharge out the cold inlet through a hose to outside. Unit off of course.

    I really appreciate the imput, many minds always work better than one (at least if that one is mine)
  9. sapo

    sapo New Member

    Messages:
    13
    Location:
    Felton, CA
    so I spoke with takagi tech support and they were very nice, never made me feel as though i were an idiot or incompetent in anyway. They weren't even bothered with the notion of me, a semi skilled homeowner, doing the work myself. They highly suggest my first step should be to descale and to remove and clean the burner. they even emailed me two PDF documents on how to do it. I would share them with, but they exceed the file size limits. so if anyone wants it I could email it to you. I will undertake these procedures in the next day or so and see how it goes.
  10. lifespeed

    lifespeed Member

    Messages:
    321
    Location:
    California
    As you pointed out, the scale problem seems unlikely but couldn't hurt. A plugged burner could certainly reduce the heat input to the water, causing the control loop to reduce flow so as to maintain temperature.

    One interesting point of information would be to check if the programmed water temperature is still correct.
  11. sapo

    sapo New Member

    Messages:
    13
    Location:
    Felton, CA
    Well, descaling did not correct the problem. I doubt it hurt any, but did not resolve my reduced water pressure issue. Next up i will be pulling the burner and cleaning it as per the instructions sent to me by takagi tech support.
    Lifespeed, the water temp is still set at 120ish degrees. Good thought though.
    Wish me luck with the burner, a lil more involved than descaling.
  12. lifespeed

    lifespeed Member

    Messages:
    321
    Location:
    California
    Consider that the low burner input (clogged) caused the control loop to decrease the flow valve to maintain temp (as measured) at 120 degrees. Could be clogged burner. Nobody thought descale was the likely issue, given you install and visual inspection. Good luck.
  13. sapo

    sapo New Member

    Messages:
    13
    Location:
    Felton, CA
    Ok, the T-K2 has been descaled, the burner and flame sensor have been cleaned and the fan, heat exchange fins cleaned. No change. One the plus side it still works, just without the original water pressure.

    I am open to any further suggestions.

    Recap, happened overnight, full pressure when heater is off, starts at full pressure then drops after 3-5 seconds. Aside from this all is normal operation.
    Filters are all clean and set temp is same.
  14. lifespeed

    lifespeed Member

    Messages:
    321
    Location:
    California
    Try measuring the water temperature at the nearest faucet (hot only) and see if it is exactly at the programmed setpoint. Starting at full pressure and then dropping implies the temperature control loop is closing the valve, rather than the valve is inoperable, although a faulty flow valve may still be a possibility. Also the control loop could be receiving erroneous input (temp sensor).

    I think your temp sensor, control circuitry, or flow control valve are to blame. Unfortunately that is a few different possibilities that must be narrowed by troubleshooting techniques. Does the manufacturer offer troubleshooting procedures, manual diagnostics with the aid of the electronic control? For example, allow you to manually open/close the control valve in diagnostic mode? Display the temp sensor reading to compare against a thermometer? Depending on how thorough they were in including self-test and diagnostic capability and documentation this could be relatively straightforward to diagnose . . . or not.

    Good luck.
  15. sapo

    sapo New Member

    Messages:
    13
    Location:
    Felton, CA
    I called tech support again, less helpfull than the first time. I guess it depends on the tech you get. Their suggestion, which makes sense, is to buy the remote control so i can read any error codes.
    So i guess this will be the next step.
    It makes sense that the control valve is functioning and the problem lies in a sensor.
    Thanks again, when i get the remote i'll update.
  16. sapo

    sapo New Member

    Messages:
    13
    Location:
    Felton, CA
    Hi, so i finally got the remote for my T-K2. It does not show any trouble codes :(
    Any one have any other suggestions?
    I was thinking it must be a non sensor issue since it gets no codes. Perhaps the control valve is acting on it's own.
    Any help would be great.
  17. lifespeed

    lifespeed Member

    Messages:
    321
    Location:
    California
    Did you ever measure the temperature of the hot water and compare it to the setpoint? You already know the heater is shutting down the flow rate, now you have to figure out why. If the temp is correct then perhaps it is incapable of putting out enough BTU, so it throttles the water flow to maintain temp.

    If you can't do the troubleshooting you're going to have to hire somebody who can, or buy a new heater.
  18. sapo

    sapo New Member

    Messages:
    13
    Location:
    Felton, CA
    I checked the output temp with a thermometer at the faucet. It is the correct temp. The new remote also verified the set point and output temp as being accurate.
    When the problem started back in october the incoming water temp was warmer than it is now, yet the output temp and gpm has remained the same. This would suggest to me that the burner capacity is not taxed.
    I emailed the company, i guess i'll see what they have to say.
    At this point my wallet prevents me from hireing someone or buying a new unit.
    The control valve is only $90 so i might throw a new one at it and see if it sticks.
  19. lifespeed

    lifespeed Member

    Messages:
    321
    Location:
    California
    Most tankless heaters will reduce the output flow, using their flow control valve, in order to maintain water temperature. Given your problem (reduced flow rate when the heater is plugged in and controlling output flow), it would appear that your burner is not being asked to produce maximum rated BTU. There is a possibility it is not capable of max BTU output, so the water heater throttles the flow to maintain temperature. So maybe a problem on the burner side, or it's gas valve and control.

    There is also the possibility your flow control valve won't open up all the way, so the brain throttles back the gas to avoid overtemp. But you said when unplugged the flow control valve opens up, right? So not a mechanical blockage, but possibly electrical control and or the servo within the valve itself?

    Talk to Takagi and see if there is a diagnostic mode available, where you can manually command the flow valve. You need to identify if the problem lies in the water control side, or the flame control side. Either one could potentially cut down your water flow.

    Unfortunately these water heaters are somewhat more complicated. Hopefully they have provided some helpful built-in diagnostics and manual controls to allow you to determine the problem without guessing and parts-swapping too much.

    Edit: Here's a test - turn the temperature up all the way, hopefully it will allow you to go up to 140F or so. See if the water flow rate is reduced. Then turn the temperature down to 110F. See if the water flow increases. This should tell you if the heater is limiting water flow based on limited burning capacity.
    Last edited: Feb 2, 2013
  20. sapo

    sapo New Member

    Messages:
    13
    Location:
    Felton, CA
    Thank you, excellent idea. I will try it and see what we get.

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