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Takagi on demand hot water design question (milne)

ScottMP
ScottMP Member Posts: 5,884
that someone one else did to try and trouble shoot it. The other company ( who did a nice install ) used a Takagi Jr. for domestic hot water and also install a Flat Plate HX for heating demand. Owner got hot water untill heating season, then got not heat. Company came back and adjusted unit for higher heat output to satisfy room demand. Owner was'nt getting enough domestic. There where personality issues and after two or three others have said everything looks O.K. .... I got the call.

From what I see the the domestic goes through a Taco mxing valve which is not preasure balanced, only thermostatic. The owner is not getting hot enough water, so we need a new valve right, with preasure balancing to get better responce to the temperature changes and consistant hot water. BUT ... if the owner runs two sinks at a time, the Takagi turns in and operates. If one sink is shut-off, the Takagi will ramp down and shut-off .. ??

Tells me that there is not enough flow through the mixer to turnon the Takagi. The temperature is not hot enough .. so if I replace the mixer and get hotter water, I should get enough flow to keep the heater going ... OR

Should I upsize to a 1" Sparco tempering valve to allow for less resistants and get better flow thought the Takagi.

I think I should be allright with a new mixer but after three different people have looked at this I want to get it right ..

Scott

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Comments

  • S R Denny
    S R Denny Member Posts: 26


    The Takagi requires .75 gal before it will fire. What temp is your Takagi set at? Call Takagi tech support and run it by them. If the tech can't help you ask him to put you through to someone who can.
  • ScottMP
    ScottMP Member Posts: 5,884
    I guess what I am wondering

    is ... What is the flow rate through a 3/4 tempering valve at , lets say, 50 psi. ?

    Because that really is the question, since it seems to shut down if only one fixture is running, or is it the flow rate at that preasure with one 2.5 faucest running.

    Scott

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  • Beam me up Scotty!

    A mix vlv on a tankless? There's a new twist.

    TK Jr lists 140K input. With an EF of .82, that grants you a net of 114,800 Btu's to play with.

    120F target temp & inlet in winter of 40F gives you 80F delta-T. 8.34 x 80 = 667.2 Btu's req'd per gallon & 114,800 divided by 667.2 = 172.06 & divided by 60 = 2.87 GPM of hot water during design conditions.

    If the shower head can deliver 3.5 GPM, at 40F inlet temps & 120F hot water temps with an adjusted delivery of 110F at the shower head's outlet, every gallon delivered at the shower head will need .875 gallons of hot water. 3.5 x .875 + 3.0625 gallons required at 120F. Can't be met by the TK Jr!

    No wonder the heat load kills the deal - they were already riding a bit beyond the threshold.

    Be wary of min cap for the burner in shoulder seasons for the hydronic load. Extreme short cycling can occur & cause premature evacuation to the junk pile.
  • Metro Man
    Metro Man Member Posts: 220
    instant recirc crapolo

    We've run into many of these. Try cleaning the intantant heat X loop with whatever potable cleaner you like. Takagi I thing makes a specical cleaner just for this.

    I think the stuff is a red granular and you mix with warm water.

    When they sit for the summer without circ'n crapolo gets in the mix and H.E,

    Good luck

    Metro Man
  • Metro Man
    Metro Man Member Posts: 220
    Also......

    Check to see if someone neutered unit for full flow for heat... if so you'll need to put a flow restrictor on pot. hot out.
  • ScottMP
    ScottMP Member Posts: 5,884
    Dave

    The second company in turned the dip switch up so its putting out about 150/160.

    When two faucets are running the house gets warm water. I did'nt check but the temp. out of the unit "feels" about 140. When one sink is turned off ( only one running ) the unit modulates down and then shuts off. I think its mxing so much that there is not enough flow going though the TK.

    I am going to replace the mixer with a Sparco 90/140.

    Would a 1" Sparco have better CV rating and allow more flow ?

    " Capn, a kunna adjust the Sparco, she's niegh givin enough floow "

    Scott

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  • Jack
    Jack Member Posts: 1,047
    Not totally familiar with the Takagi, but...

    The problem is typically the minimum flow/temp rise issue. Whenever anyone has a problem with a "water heater" of any type the conventional answer is to "turn the temp up". If you are looking at a low flow faucet and some other restriction, in this case the tempering valve, you should turn the temp down with on-demand. The higher water temp will require less flow to mix to the proper temp, dropping below the minimum flow to actuate the unit. See how your domestic circuit runs iwth the temp valve set at 120 and the unit set at 120. In fact, if you by-pass the temp valve and put 100% capacity to the dhw loop does the unit provide enough hot water to satisfy the customer? That answer will help define the problem and solution.

    Avoid tempering valves with on-demand water heaters. It is about impossible to get a high temp heating circuit and low temp dhw to peacefully coexist.

  • why? (Plus - your question now aswered)

    It won't matter where the temp is set if the flow rate and/or heat load exceeds input capacity. Sounds like one screwed up mess that needs to be revised. I'd suggest you start over & ignore what's installed. How would you approach the installation? What would you do to ensure operation & performance? I'd quit trying to herd those wild ponies and offer them a system I knew would work flawlessly. Once you touch it and alter anything - it's your baby.

    I'd have to know the total demand for potable hot water during design conditions. Then I'd determine the design and shoulder-season hydronic demands to ensure two things: adequate cap at design conditions & no risk of beating the little beastie to death via short-cycling; and a bit more than adequate cap to accomodate all connected loads at design conditions.

    My experience in situations like this is that I've got but one opening to squeeze resolutions through said opening. After that, any continuing issues are percieved to be my fault.

    And, I forgot to answer one of your questions. Sorry about that! The mix vlv is seeing 160F and you're using 40F to temper it to 120F during design conditions. As a result, few lav fcts will give enough flow to trigger the burner because only half the flow (at best) will be drawn from the tankless. Instead of .75-GPM to trigger a burn, you'll need 1.5-GPM (split flow) and new lav fct aerators are set for 1-GPM. The warmer your incoming water temp gets, the worse the flow rate through the Takagi will be & the higher the delivery GPM will need to be to obtain that .75 burner-triggering flow rate.
  • hot_rod
    hot_rod Member Posts: 22,103
    The \"Hot Box\"

    is about the only way I have seen to mix DHW and closed loop hydronics together properly. IF you really want to walk that path :)

    Sell them a boiler with the DHW built in Mascot, Vitoden, MZ, and a few others do DHW from the "boiler" approch.

    hot rod
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream
  • kal_2
    kal_2 Member Posts: 60
    hear hear!!! finnaly someone's making sense!!!

    there is so much water flow trouble with tankless - and the Takagi’s have the extra problem of needing air from the boiler room, if you have a clothes drier near it fighting it for air, foh-ged-abow-eet!!! – don’t get me wrong I use lots of tankless- but they are water flow guaranteed, gas pressure guaranteed, truly sealed combustion, and perfectly matched to the load at hand – this is why I call them “thank-less” water heaters ;)
  • tim smith
    tim smith Member Posts: 2,752
    Regarding Takagi

    Scott, look in the install manual for the Takagi, they show their recommended layout for heat/hot water system that they say works, maybe this will help. If it's not in there I can send you their drawing or you can call Takagi and they will fax it to you. PS they are quite helpful. Tim
  • Mike T., Swampeast MO
    Mike T., Swampeast MO Member Posts: 6,928


    Sounds to me as if the general scheme of the system is at fault. The use of the mixing valve on the DHW side of the flat plate HX is a serious flaw. You're seeing the consequences when a single lav is flowing--there's simply not enough flow through the water heater itself to trigger it to fire because of that mixer.

    I believe you need to remove the mixer and come up with some sort of "DHW priority" control. Easy with a true boiler, not so easy with a "water heater". Can you use a flow switch in the cold supply to the DHW to detect a "call" for DHW and disable space heating until the call ends?
  • Take a minute to read Siggy's P&M article

    on page 46, right side of top "No instantanious water heaters used as boilers"

    If I could add one thing to John's article on what's not seen at ISH in Germany, it would be that you won't find anyone advocating open combined potable hydronic systems.
  • Home Depot Employee
    Home Depot Employee Member Posts: 329


    I think a few people missunderstand how the mixing valve is being used. The mix/temp valve is not on the flat plate. The flat plate is used to exchange heat via a domestic circ to a closed hydronic side. The Therm mix valve functions as a safety and to cut the water temp down on the domestic side thereby allowing the heater temp to be set higher to satisfy the heat loads.

    Having done this application numerous times successfully, I am sure that the domestic temp and flow issues are related to the domestic water backflowing through the domestic side of the heat exchanger and bypassing partially or fully the demand water heater. This will happen due to the water taking the path of least resistance and avoiding the heater itself.

    Properly placed circ and check valves corrects this.

    And in case you are thinking it I will answer the next question.
    During a call for heat the domestic side circ on the heat exchanger is functioning in a closed system. Once a faucet is opened and the circ between the heater and the exchanger is still running, the system becomes open thus, the performance of the circ changes and you end up with domestic priority naturally.

    See an install picture included which has a therm mix valve, plate heat exchanger and heats 25' rec room baseboard along with 300' radiant. No matter what I set the temp of the heater at, be it 110 or 140, I always have no more that 120 going to fixtures.
  • Home Depot Employee
    Home Depot Employee Member Posts: 329


    Before anyone says anything

    1. I properly vented it the soon after
    2. I added pipes to the relief valves to the floor
    3. I tied up all the loose ends

    I did this one Saturday in my own house and just wanted to get cleaned and go to bed. I finished the install the following weekend
  • ScottMP
    ScottMP Member Posts: 5,884
    O.K. here's what I found

    Went to the job today with a Takagi rep and just by accident he sut off the ball valve coming from the HX. We got Hot Water.

    Look at the photo .. There is a check valve that was to stop cold "feed" water from back feeding through the HX BUT it was letting it by. The cold water was mixing at the tee below the Mixing valve and we'rnt getting enough flow thought the Takagi to fire it.

    I will recommend a Sparco mixing valve, a sring check to replace the swing check and a flow switch to shut off the bronze pump to prioritize the domestic out put.

    As I said the originall installer did a great job but this may have been a solder gob stuck in the swing check.

    Scott

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  • I must be one of the unelightened ones!

    OK, I'll bite - give me the flow dynamics through the mix vlv when the Takagi is targeting 160F, the inlet cold water is, let's say, 60F (the yearly average for that area according to the DOE), and the lav's aerator is a 1-GPM model.

    Cold water flow rate in GPM =

    Hot water flow rate in GPM =

    Then again, you could use inlet cold service line temps from August, which will hit close to 80F. I'd be curious to know what your calculations reveal about flow rates at that lav fct through the Takagi when cold inlet is at 80F.

    Help me see the picture.
  • Rich Kontny_4
    Rich Kontny_4 Member Posts: 73
    Heating Box

    Hot Rod,

    I think you are thinking of the "Heating Box" by Navien America.An add on that was specifically designed to allow up to 2000sq ft of space heating in combination with an on demand water heater.

    I have installed two systems and they both work fine. One is in my own home. Water quality is important with these systems.

    Priority is given to the domestic water.

    Rich
  • S R Denny
    S R Denny Member Posts: 26
    Dave, why don't you just say it?

    This Takagi is too small for peek DHW demand during high ∆T winter conditions.
  • Rich Kontny_4
    Rich Kontny_4 Member Posts: 73
    Some Pics

    Would like to share some photos with you.
  • Rich Kontny_4
    Rich Kontny_4 Member Posts: 73
    Heating Box

    Some photos showing on demand heaters complimented by the "Heating Box" to use for space heating with domestic water heating as priority
  • maybe

    Don't know until the heat load and domestic peak demand are calculated/measured and then compared to what any given heat source can produce in net energy. Toss in a split-flow and low-flow fixture(s) & you might sputter when hot water is wanted. I've surveyed more than one tankless installation where homeowners are not at all pleased by having to jam their hot water wide open at lav fcts in order to get the units to fire. And, that was without a mix vlv robbing part of the flow. Add a wee bit of debris to a 1-GPM aerator (like we ever see that!) and all bets are off.
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