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Thread: Tankless Recirculation - Redux

  1. #1

    Default Tankless Recirculation - Redux

    Hi,

    Didn't want to post to the other current thread on tankless recirculation because I have a slightly different situation / idea

    I have a instant-on domestic hot water boiler (NTI Trinity T200). The flow switch to trigger the boiler requires at least 1/2 GPM flow.

    I have installed the Grundos Comfort System Recirculation System - which uses the cold lines as the "return" lines through a heat sensitive "comfort valve" valve installed between the hot and cold supply pipes at the furthest sink.

    I knew this pump wasn't designed for an instant on situation - but thought it was worth a try.

    Suffice to say it didn't work. At first I thought it was because the flow wasn't high enough to trigger the boiler. So I installed a relay triggered by the pump which tells the boiler there is a demand for hot water.

    But, unfortunately, it appears the pump isn't powerful enough to move the water in the pipes (perhaps because my domestic water pressure is >60psi (60 is the top of the pressure gauge's scale?).

    So now I've been trying to find advice on a more powerful pump.

    The problem is that once the value closes as hot water reaches it, the pump continues to operate until the timer turns it off - i.e. by design no water can move. I understand that this requires a special pump design to avoid over heating and damage to the pump.

    Can anyone recommend how I can figure out a suitable pump?

    Thanks

    P.S. I'd be interested in *ANY* recommendations for retrofitting recirculation systems (i.e. no dedicated return), where no power is required at the sinks.

  2. #2
    Retired Defense Industry Engineer jadnashua's Avatar
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    I don't think the system pressure is an issue at all...effectively from one side of the pump to the other, it starts out as zero. If the tankless has a flow restrictor, the pump will have more trouble moving the water. Depending on how the pipes are layed out, it could be recirculating through another loop. Some valves can fail and have an internal cross-over. If the water was moving through that valve rather than back to the tankless unit, it would never see the flow.
    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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    Quote Originally Posted by jadnashua View Post
    Depending on how the pipes are layed out, it could be recirculating through another loop. Some valves can fail and have an internal cross-over. If the water was moving through that valve rather than back to the tankless unit, it would never see the flow.
    I'll definitely double check for cross-overs, but I'm pretty certain I don't have any...thanks for the thought.

    ...the pump will have more trouble moving the water.
    Given the pump has to be able to be "on" while the "comfort valves" are closed until the timer turns the pump off, can you recommend how I can find a more powerful pump which won't be damaged given this somewhat unusal requirement.

    Thanks

  4. #4
    Retired Defense Industry Engineer jadnashua's Avatar
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    I don't think the size of the pump is the big issue...it should move water fast enough to turn on the tankless if it is installed properly.
    Jim DeBruycker
    Important note - I'm not a pro
    Retired Defense Industry Engineer; Schluter 2.5-day Workshop Completed 2013, 2014

  5. #5
    DIY Member ChuckS's Avatar
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    What makes you think your pump isn't adequate? What symptoms are you seeing?

    You said in the other thread that you installed a push button to turn on your loop, does this also turn on the heater?

    I would suggest adding an aquastat to your system to turn off the heater when the water get's up to temp. That should also solve your concern regarding the pump running until the timer finishes.

  6. #6

    Default More Ideas on Retrofit Tankless Recirculation

    Quote Originally Posted by ChuckS View Post
    What makes you think your pump isn't adequate? What symptoms are you seeing?
    Symptom is no hotwater at the faucet after pump and tankless has been operating for 10 mins.

    I can hear the pump turning, and I can see the tankless firing... but no hot water ever makes it to the faucet. Not sure I would expect to see the pressure gauge change...but it doesn't move off "max".

    I've verified that the comfort valve under the furthest sink works - sometimes I get warm water migrating to the cold line - without the pump being on - and this can only happen if the comfort valve is open. (I guess this happens due to a slight drop in pressure on the cold side when I turn on the hot facuet full...which pushes some water through the valve)


    You said in the other thread that you installed a push button to turn on your loop, does this also turn on the heater?
    Yes...you got it...it turns on both the pump and triggers the tankless to fire. Actually its a push button with a fairly sophisticated off-timer - so I can control how long the pump stays on and the tankless fires.


    I would suggest adding an aquastat to your system to turn off the heater when the water get's up to temp. That should also solve your concern regarding the pump running until the timer finishes.
    Once I got the system working I was going to time how long the pump / tankless took to get hot water to the faucet and set the off-timer on the push button to turn off both the pump and the tankless after that length of time. This would serve the same purpose as the aquastat I believe.



    I've now done a bit more research and come up with some more possibilities...so want some comments from folks if possible...


    I found the Laing ACT-909 (which is designed to go under the furthest sink and is designed to operate with tankless). It has a much higher max head than the grundfos UP15-10 (max 24ft vs. 4ft) and much higher max GPM (max 8 vs. 4). I assume this is what allows the ACT-909 to trigger the tankless.

    Unfortunately this pump is designed to go under the furthest sink...and I have no way (at this stage of the game) of getting power to that location.

    I also found that the Laing Ultracirc line of pumps offer "thermal dry run protection" (i.e. they shut down if they over heat) which is one of the "special" features Grundfos mentions the UP15-10 has in order to protect the pump if it is running for long periods after the confort valve closes when hot water has reached the furthest faucet.

    BTW I realize with my off-timer the liklihood of this happening is fairly low... but I'm a belt and suspenders kind of a guy :-)



    So if I keep the gGrundfos comfort system valve, but replace the grundfos pump with a more powerful pump with "thermal dry run protection" (similar to the specs of the UCT-909 from lang which is designed to work with tankless) - but locate the pump next to the tankless heater - do you think that will work?

    I will continue trigger and subsequently turn-off the tankless and the pump using a push button switch / off-timer.

    What do people think...will this work?

  7. #7

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    Quote Originally Posted by jadnashua View Post
    Depending on how the pipes are layed out, it could be recirculating through another loop. Some valves can fail and have an internal cross-over. If the water was moving through that valve rather than back to the tankless unit, it would never see the flow.
    Thanks...that is a *VERY* good point. I actually have two comfort system valves installed...so I'm going to try and remove one of the two comfort system valves and see if it works.

    I had thought that my two long loops (to the upstairs bath and kitchen) were "separate"...and so I thought it would be ok to put two valuves in place...but you're right it may be the pump moves the hot water towards the valves but somehow it never get's there?

    But hang-on... if the pump was moving the water at all I'd at least end up with hot water in the cold lines...but this isn't the case.

    At next worst, I should end up with hot water at one of the faucets and not the other... i.e. somehow cold water is pushed the wrong way through the comfort system valve (but again I think I remember grundfos saying there was a check valve in the comfort system valve to prevent such back-flow).

    So...I think I just convinced myself that your thought that the is pump moving the hot water, but somehow the hotwater never gets to the comfort valves is pretty unlikely...

    What did I miss?

    I will try taking one of the valves out anyhow...because perhaps the pump can't handle two valves (although grundfos says it can).

    Thanks again for your help...

  8. #8
    DIY Senior Member zl700's Avatar
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    Grundfos Comfort pumps are low flow, med head circs, that are capable of dead heading when thermostatic bypass valve is satisfied and closed.

    These systems were designed to be added to tank type heaters that store hot water. They are not sufficient to activate to burner because they cannot create enough flow with the design and minuscule bypass, combined with the frction and flow requirements of tankless.

    In order to get it to work you would need a second primary pump at the tankless of adequate size circulating through the tankless creating a hot loop for the comfort series pump to draw from.

    On another note, the Navien has a "A" model that does just that with a internal circ and buffer tank that will allow the comfort series to draw from.

  9. #9

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    Quote Originally Posted by zl700 View Post
    Grundfos Comfort pumps are low flow, med head circs, that are capable of dead heading when thermostatic bypass valve is satisfied and closed.
    This is very helpful...confirms what I had suspected... but wasn't certain...

    Any reason why the following wouldn't work:
    1. a more powerful pump (like the Laing [24ft head and 8 GPM]), mounted in the DHW line next to the tankless,
    2. a relay to cause the tankless to fire when the pump was on (even if the flow rate wasn't high enough),
    3. (of course) the Grundfos Comfort Values still in place

    The Laing has approx 6x more head and 2x more GPM than the Grundfos.

    Thoughts?
    Last edited by DHWRecirc; 08-31-2009 at 08:31 AM.

  10. #10
    DIY Senior Member zl700's Avatar
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    No mater what size circ, it would still have to flow through the 1/8" opening in the Grundfos manifold which I don't believe will meet the .5 GPM requirement of the tankless.

    How could a relay start the tankless? It requires flow to activate. If you bypassed it somehow the temp rise could be too great for the unit than flow available, and ruin it.

    One option is the Metlund and taco system that requires it to be at the sink, which may still be a problem for you.

    If you installed say a Grundfos UPS-42 stainless multi speed and did a loop (from hot outlet back into cold inlet of DHW taps) and switched that, you could hook up your comfort pump to that loop and push to your fixtures.

  11. #11

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    Quote Originally Posted by zl700 View Post
    How could a relay start the tankless? It requires flow to activate. If you bypassed it somehow the temp rise could be too great for the unit than flow available, and ruin it.
    Thanks for your detailed response / thinking...much appreciated...

    The flow switch on the tankless closes a normally open input to the tankless controller. I can trigger the controller to see a DHW demand when the pump turns on via timer or a switch. I already have this part working. When the existing grundfos pump turns on, a relay tiggers the DHW demand. When the pump turns off, the DHW demand stops. Of course this does me no good at the moment because we've now established the grundfos pump doesn't have enough head to get water through the 1/8 grundfos comfort valve :-( .

    When the pump tiggers the relay which triggers the tankless, the tankless then fires, and gets up to temperature... and will then cycle on and off as needed for as long as there is a call for DHW, i.e. for as long as the pump is running ... which hopefully will be <5 mins for the pump to get the hot water to get to the grundfos valve.

    This cycling on and off is what the tankless normally does with lower flow rates given the mixer/tempering valve and the small heat capacity of the circulating loop inside the tankless (from primary heat exchanger to DHW heat exchanger).

    I agree the flow rates would be lower than 0.5GPM... but that just means it will cycle less often....how would that damage the tankless? What did I not understand?


    So my question remains: Since the Laing ACT909 is designed to have enough power to trigger the flow switch in a tankless (but has to be located under the sink which I can't do) what I'm trying to understand is if a similarly spec'ed pump with dead head / thermal protection would have enough power to get the water through the 1/8" grundfos valve if the pump were mounted next to the tankless with an additional trigger for the tankless (in case the flow is lower than the 0.5GPM required by the flow switch due to the increased head of the flow switch)

    More Comments?

  12. #12
    DIY Senior Member zl700's Avatar
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    You could select a circ that is capable of 2-3 GPM taking into consideration of the head loss and friction of the unit, piping and bypass valve and install an additional bypass to eliminate deadheading and over heating. This could be a ball valve, circuit setter or a pressure differential bypass made for hydronics

  13. #13
    DIY Member ChuckS's Avatar
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    My ACT909 is in the crawl space less than a foot from the input of the tankless. It's been working fine there for close to a year.

    I had to install a separate check valve since the one in the ACT909 was allowing cold water to flow backwards through the pump when it was off. I took a chisel and knocked the check valve in the ACT909 out so that I have only one in the system. Now that I think of it, that check valve will cause the symptom you're describing.

    I also noticed in the kitchen our cold water can override the hot pressure. If we turn both the cold and hot all the way up, not only will we get cold out of the facet but you will continue to get cold a few seconds after turning the cold side completely off. To counter this we can only turn our cold about half way up when using hot. I heard there are facets specifically made for tankless that are supposed to fix this problem.

  14. #14

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    Quote Originally Posted by ChuckS View Post
    My ACT909 is in the crawl space less than a foot from the input of the tankless. It's been working fine there for close to a year.
    Thanks for your response...very helpful...

    Its good to hear the ACT909 does work if you install it close to the tankless rather than under the furthest sink.. a few follow-up questions...

    1. Did you use the Grundfos Comfort Valves under the furthest sink and use the cold line as the "return", or do you have a dedicated return line?

    2. Any reason you put the pump before the tankless rather than afterwards...is it on the dedicated return line (if you have one) or on the main cold water line into the tankless? (I have a space problem...plus was worried about the pump restricting the flow when it was off if I put it on the main cold water line into the tankless?

    3. Did you use the temperature sensor that comes with the ACT909 - if so how? Or did you just put the pump on a timer?


    I had to install a separate check valve since the one in the ACT909 was allowing cold water to flow backwards through the pump when it was off. I took a chisel and knocked the check valve in the ACT909 out so that I have only one in the system. Now that I think of it, that check valve will cause the symptom you're describing.
    Any idea why the pump's check valve didn't work?


    I also noticed in the kitchen our cold water can override the hot pressure. If we turn both the cold and hot all the way up, not only will we get cold out of the facet but you will continue to get cold a few seconds after turning the cold side completely off. To counter this we can only turn our cold about half way up when using hot. I heard there are facets specifically made for tankless that are supposed to fix this problem.
    Not sure I follow this... you're saying the cold pressure is high enough when you have both hot and cold on that the cold water flows back through the return (either dedicated or via the Grundfos comfort valve) to the hot side preventing any hot water coming out if you have the cold facuet open more than half way?

    That must be some pressure drop on the hot side for that to happen...

  15. #15
    DIY Member ChuckS's Avatar
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    Quote Originally Posted by DHWRecirc View Post
    1. Did you use the Grundfos Comfort Valves under the furthest sink and use the cold line as the "return", or do you have a dedicated return line?
    Dedicated return line.

    Quote Originally Posted by DHWRecirc View Post
    2. Any reason you put the pump before the tankless rather than afterwards...is it on the dedicated return line (if you have one) or on the main cold water line into the tankless? (I have a space problem...plus was worried about the pump restricting the flow when it was off if I put it on the main cold water line into the tankless?
    It is on the dedicated return line.

    Quote Originally Posted by DHWRecirc View Post
    3. Did you use the temperature sensor that comes with the ACT909 - if so how? Or did you just put the pump on a timer?
    I tried but it was set too low for me. I have one that is 120' and my heater is set to 125'. Also, the one on the pump caused the loop to run too long after the water was hot, I have my aquastat on a small piece of copper pipe that is not far from the output of the heater. The copper pipe heats up fairly quickly which turns off the pump a lot sooner. This made a big difference on the energy bill since washing hands etc... kicks on my loop.

    Quote Originally Posted by DHWRecirc View Post
    Any idea why the pump's check valve didn't work?
    Mines went bad relatively fast. I think it worked for about a month then one day no one could get hot water. I traced the problem down to the flow valve so I drove a chisel through the one on the pump then got one off **** and things have been great since.

    Quote Originally Posted by DHWRecirc View Post
    Not sure I follow this... you're saying the cold pressure is high enough when you have both hot and cold on that the cold water flows back through the return (either dedicated or via the Grundfos comfort valve) to the hot side preventing any hot water coming out if you have the cold facuet open more than half way?

    That must be some pressure drop on the hot side for that to happen...
    Sorta, I am saying if I turn the cold up completely then the cold will dominate the hot and you will feel cold water going back down the hot water pipe under the sink. They make facets that are supposed to counter this but my fix is don't turn the cold all the way up.

    I also noticed it has a lot to do with the outside temp since as you know, your tankless will have a higher output when the input water is warmer.
    Last edited by ChuckS; 09-03-2009 at 09:52 PM.

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