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    Unwanted reverse flow between tees (38 Posts)

  • sgorrill sgorrill @ 5:50 PM
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    Unwanted reverse flow between tees

    Hello All,

    I recently replaced the on demand electric water heater for my in-floor heating system with a Stiebel Eltron "Hydroshark". I also re-piped it to be a Primary/Secondary loop system. I added temperature gauges on the supply and return lines of both P & S loops so I could see what was happening on each. Based on the temperatures there is reverse flow between the closely spaced tees. I'd like to have 130 to 140 going out on the supply but with 3 or more zones open it only reaches 110 to 120 (I have 6 zones). I used Grundfos 26-99 3 speed pumps on the P/S loops and have tried a few different settings but still cant stop the reverse flow. What would you say are my options to resolve this? Is the Secondary pump moving too much water? Can I increase the flow on the primary? If so how is this done?

    Any suggestions would be great! Thanks...
    This post was edited by an admin on November 29, 2012 6:02 PM.
  • Gordan Gordan @ 7:01 PM
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    Post a sketch or a picture of piping and gauges

    Should help with figuring out what's going on.
  • sgorrill sgorrill @ 7:18 PM
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    A Few Pictures

    Here are a few pictures to help. Be kind, I was working with space restrictions ;) I'm moving the cold water supply to where the relief valve is now in the near future. I was trying to find a fix for the reverse flow before taking it apart.

    I'm no plumber/heating specialist and after doing this I have a whole new respect for them..

    The first picture is what I started with.

    This post was edited by an admin on November 29, 2012 8:51 PM.
  • Gordy Gordy @ 7:51 PM
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    Piping pics

    Please sounds as though something may be piped wrong.

    Those are big circulators. Could be the boiler circ is over pumping, and the boiler can't heat the water fast enough to get the temp you are looking for in the secondary.

    The reverse flow is puzzling. Are you sure the circulatory are pumping in the right direction
  • sgorrill sgorrill @ 8:07 PM
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    Unwanted reverse flow between tees

    Before the P/S it was piped direct and was able to heat up to 140 without any problems. The temperature gauge on the supply confirmed this. The boiler has a light that blinks when it's heating on full power and is on steady when up to temperature so I know its reaching 140 after the P/S was added. I can also feel the pipe is hotter on the primary supply just before the tee when compared to the the secondary supply above the tee.

    I was considering replacing the secondary pump with a smaller 3 speed but wasn't sure if that was the fix or not.. I'd hate to spend the time to tear it down only to have the same issue afterwards.

    Yes they are pumping in the right direction.

  • RJ RJ @ 8:24 PM
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    looks like your primary loop is piped into your boiler, can you draw a schematic showing pump flow direction,  also recommend balancing valve on secondary loop as well as petes plugs on supplys and returns.
  • sgorrill sgorrill @ 8:31 PM
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    Unwanted reverse flow between tees

    The supply for the primary comes from the left side of the boiler and the secondary supply from the left hand side of the tees. The primary pump flows away from the expansion tank and the secondary flows away from the tees.

    Thanks for the other suggestions RJ.
  • Paul48 Paul48 @ 8:20 PM
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    have 130-140 on the primary supply when it is running?
    This post was edited by an admin on November 29, 2012 8:24 PM.
  • sgorrill sgorrill @ 8:48 PM
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    Unwanted reverse flow between tees

    Based on the temp difference I can feel when touching the pipe on the primary supply just before the tees, when the secondary supply reads 130, it is much hotter. So I'm assuming its at 140.. The light indicator on the boiler is also on steady so its reached temperature which is set to 140.

    With one or two zones on it takes awhile to get to 130 on the secondary supply because of the reverse flow from the return but it does get there. Given enough time to heat up, the secondary supply temp gauge reads 130 or so. If 3 zones or more come on it goes down to 110/120 even when left for awhile. In my mind anyway this tells me the GPM in the secondary is higher than what can come from the primary so it pulls more water from the secondary return to compensate and reduces the temp. This will eventually heat the loops but I'm not sure how it will respond when the weather turns colder. I don't think it'll push the BTU's that the boiler is rated for when I need it.
    This post was edited by an admin on November 29, 2012 8:53 PM.
  • Paul48 Paul48 @ 9:15 PM
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    You're not running a circulator like an Alpha that adjusts to delta-p. When you add a zone, you're adding head , which is slowing the flow through the zones.There is something flawed in your diagnosis, because things don't seem to add up. I can't put my finger on it. I feel like Abbott& Costello...."Who's on first....What's on second.....I don't know.....Third base".
  • sgorrill sgorrill @ 9:30 PM
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    Unwanted reverse flow between tees

    I won't disagree with you there Paul. What you wrote makes sense. I'm not sure why I'm getting reverse flow across the tees.
  • Gordy Gordy @ 9:05 PM
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    Start from the beginning

    Did the original electric on demand water heater perform as expected?

    Did you use the hydro shark plumbed direct as in pic 1? If so did it supply the water temps you are looking for?

    If the old unit worked as expected, is the new hydro shark the same output?

    I think your flow is to high through the boiler. Have you tried speed one on the primary loop (boiler loop), and speed one one the secondary.

    Is your radiant staple up? Wondering about the high temp you think you need.
    This post was edited by an admin on November 29, 2012 9:11 PM.
  • sgorrill sgorrill @ 9:20 PM
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    Unwanted reverse flow between tees

    No, the original was undersized based on the heat loss assessment we had done recently. The new Hydroshark is 67,500 BTU where the old unit was 47,000. (house heat loss is 56,000). It also over heated/burnt out more than once during its lifetime. It wasn't the best brand and was made more for domestic hot water from what I've read. It came with the house.

    Yes I used the Hydroshark direct for a few weeks but didn't want to leave it that way for the winter. It did deliver 135/140 with that piping. P/S was the recommended piping for it.

    I have tried speed one on the primary and secondary but the heat on the secondary supply went down at that setting. It seems to work best a 2 on the primary and 1 on the secondary.

    Yes, radiant staple-up. I was told 130 to 140 works best. Do you agree with that?

    This post was edited by an admin on November 29, 2012 9:23 PM.
  • Gordy Gordy @ 9:30 PM
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    Depends on the

    Staple as to what floor coverings lay above the floor. Higher the r value the higher the temps needed to heat the space. And is the staple up insulated.

    Sorry for all the questions....trouble shooting can be more than the immediate problem in front of you. The more details about the whole system helps.

    The piping is correct. The whole concept of primary secondary is that both are decoupled hydraulically. So if the boiler pumps is pumping, and the secondary is not there should be no flow in the secondary, and vice versa.

    What is the boiler temp set at?
  • sgorrill sgorrill @ 9:45 PM
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    Unwanted reverse flow between tees

    The floor coverings are wood snap flooring and ceramic tile. Yes they are insulated well between the floor joists but I don't have any heat diffuser plates.

    No worries with the questions that's why I've posted this here. I've had more than a few drinks sitting in front of it trying to figure this out myself ;)

    The boiler temp is set at 140.
  • Gordy Gordy @ 9:59 PM
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    One thing

    Did you have the temp gauges before? If not and you do now maybe the old setup was never doing what you thought it was. Now that you know it's driving you crazy.
  • sgorrill sgorrill @ 10:04 PM
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    Unwanted reverse flow between tees

    Good question but no the temperature gauges were there when it was piped direct. I actually added one to the return so I could baseline it for reference before adding the P/S.
  • Gordy Gordy @ 10:06 PM
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    Delta t

    Between the secondary supply, and return lines?
  • sgorrill sgorrill @ 10:19 PM
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    Unwanted reverse flow between tees

    There was about a 20 degree difference between the supply and return once it ran for a bit. So 135+/- supply and 115+/- return. When it first started to call for heat the boiler was on full to get the 135 but started modulating after the return temp went up. There's a light that blinks telling me when its running on full verses modulating.
  • JaredM JaredM @ 10:15 PM
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    Reverse Flow and BTU's

    What goes in to a tee has to come out of a tee.

    If you are getting reverse flow, and your description seems to describe it, then you have more flow in the secondary loop than the primary loop.   This sort of makes sense because you said you get reverse flow with more zones open.  Think about it and draw it out on paper or your mind's eye.....if the secondary has a higher flow than the primary some of the water from the secondary return HAS to mix with the water delivered to the first tee of the primary loop.   This may be explained by what I imagine is a high head loss (resistance to flow) inside that electric boiler.  I have never worked on an electric boiler so I am not familiar with them.

    Now, does it matter to your application?  When you operate all or most of the zones does the boiler cycle off or is it heating continuous?  If it's delivering the heat to the radiant floor (and ultimately the house) you have achieved your goal.  Right?
  • sgorrill sgorrill @ 10:26 PM
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    Unwanted reverse flow between tees

    Yes I agree with that 100%.

    Once it heats up the return with more zones open it modulates but with the reverse flow due to the higher GPM on the secondary I'm getting a cooler supply temp. What can I do to increase the GPM on the primary? I assume nothing because of the head on the primary so that leaves me with decreasing the flow on the secondary. Would changing the secondary pump to a smaller one do this or am I at a point of having to accept this as the way it works given the piping setup?

  • JaredM JaredM @ 10:30 PM
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    How about this

    Can you raise the setpoint on the boiler to compensate?  Let's say you raise the setpoint to 150F, with your mixing tees you will then raise the delivered supply temp closer to your goal of 140F.   
  • sgorrill sgorrill @ 7:12 AM
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    Unwanted reverse flow between tees

    No I can't raise it any higher. It's at the max temp it will deliver.
  • Gordy Gordy @ 10:40 PM
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    Did you try

    Speed 3 on the primary circ. and speed 1 or 2 on the secondary?
  • sgorrill sgorrill @ 7:18 AM
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    Unwanted reverse flow between tees

    Yes I tried that but I still get reverse flow. My guess is that the primary is at its max flow for the piping it has so I cant raise the GPM on it by setting the pump to a higher speed. I don't think its a matter of the boiler not being able to heat the water fast enough when the pump is on 3 because if it were the light should start blinking telling me its on max but its not.
  • sgorrill sgorrill @ 9:28 AM
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    Unwanted reverse flow between tees

    After more research I believe I have an over-pumping problem on the secondary loop because the circ is too big. Because this was an existing system I have no way of determining the "right" circ to have on the secondary. I can only go by what was in place before which was a B&G NRF22. The new Grundfos 26 should perform the same as the NRF22 on setting 1 based on the comparison chart on their website. This clipping from another site explains why I'm thinking this way:


    “That’s all fine and dandy as long as the flow in the primary loop is higher than the flow in the secondary loop. But what happens if this is not the case?” Well, let’s take a look. (See picture below)
    Suppose a circulator capable of generating a flow rate of 15 gpm was installed in the secondary circuit, and the piping was scaled up accordingly.

    Notice that flow between the closely spaced tees has indeed reversed. This reversal is the only possibility given the flow rates in the surrounding pipes.
    Also notice that the flow rate in the primary loop for all practical purposes does not change. Why? Because the insignificant pressure drop between the closely spaced tees effectively prevents the primary loop from “feeling” any change in its hydraulic resistance. Remember, the primary loop doesn’t even “know” the secondary circuit exits.

    An even more interesting result is what happens to temperatures and heat transfer in the secondary circuit. Again, we have to follow the energy into and out of the tees to predict what will happen.

    The flow reversal between the tees now creates a mixing point at the upstream tee. This mixing point lowers the water temperature supplied to the secondary circuit.

    Thoughts on this guys? Should I be swapping out the secondary Grundfos for a smaller model? Possibly a Grundfos Alpha 15?

    This post was edited by an admin on November 30, 2012 9:43 AM.
  • JaredM JaredM @ 9:00 PM
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    Old Circulator

    You are thinking along the right lines.  A grundfos alpha would probably do well as someone else suggested.  If you still have the NRF22 you could try that and see what happens.  I just re-read everything and noted the 26-99 pump....that's alot of pump for a 55,000 btu system.  Have you calculated the head loss of your secondary circuit?
  • sgorrill sgorrill @ 12:11 PM
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    Unwanted reverse flow between tees

    This post was edited by an admin on December 1, 2012 12:19 PM.
  • sgorrill sgorrill @ 12:12 PM
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    Unwanted reverse flow between tees

    I'm not sure how I can get that number because I don't have any way of knowing what the loop lengths are through the house. All I can do is assume the original installer did the math and used the right pump for the job. From what I seen online the Grundfos on 1 would perform the same as the NRF22. I think I'm going to try a small Alpha to see if that helps rather than replacing the pump twice.

    This post was edited by an admin on December 1, 2012 12:17 PM.
  • Jason Jason @ 9:43 AM
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    Reverse flow

    This is elementary. When you have primary secondary with hydraulic separation the flow in the tees will either flow left to right, right to left or stop flowing between the tees. This is in relationship to the flow in both zones. The boiler flow is set by the heat exchanger and manufacturer.
    For example:
    If you have 10 gpm in the primary (boiler loop) and 15 gpm in the secondary (system) you will have 5 gpm backward flow. turn that around, if you have 15 gpm in the primary and 10 in the secondary you will have 5 gpm in the proper flow. The last example is 10 gpm in primary and 10 gpm in the secondary no flow between the tees.
    The problem with zoning with pumps is the system usually ends up with too much flow through the tees as more zones pumps start.
  • RJ RJ @ 11:01 AM
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    I agree with all,  remember the law of the tee, what goes in tee must come out, still would like to see a schematic, you need some circut setters in your system to get it balanced.
  • Gordy Gordy @ 5:45 PM
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    I agree with what you are saying, but he is zoning with valves, and a fixed speed either 1,2,or 3 now every time a zone opens flow rates change especially if all loops are not equal in length. along with return temps dropping. If all six are calling the OP may get the proper flow if only 3 are calling or 1 then the flow rate is higher in the secondary than the primary. Sounds like an application for a circ with delta t or delta p capability on the secondary may help this situation. Now with the boiler temp limited to 140* there may be an issue getting the 140 supply temps the OP is looking for the more zones calling the worse it may get.
    This post was edited by an admin on November 30, 2012 5:51 PM.
  • RJ RJ @ 4:46 PM
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    send a detailed piping drawing and we can get to the bottom of this  Taco has a website that used to have some good radiant heat info.  Usually the boiler is on the secondary loop.
  • Gordy Gordy @ 5:03 PM
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    Primary secondary

    Usually the primary loop is where the expansion tank , and air scoop is located which is usually on the boiler side of things to enhance air removal. But the expansion tank could be located in the secondary loop thus making it primary.

    The bottom line here is that the primary flow, and secondary flow must be equal so there is no mixing in the boiler circuit. That way the hottest boiler water gets to the circuits which is limited to 140* by the highest possible setting on the hydro shark 3, and this is what the OP needs to his staple up radiant.
    This post was edited by an admin on December 1, 2012 5:10 PM.
  • RJ RJ @ 6:37 PM
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    primary secondary

    In most commercial systems Tel. Larrs, bell&gosset the primary loop (system loop) travels around the bldg to the individual terminals, the primary (system loop) includes the expansion tank, main pump,and air seperator. The secondary loop is usually where the tees take off and where the boiler is piped into. the secondary loop adds heat to the secondary as needed.  The terminology is often flip flopped in residential and light commercial radiant systems calling the primary-secondary and vise versa. the individual radiant zones can be there own secondary loop off the primary loop with there own pumps,  once again drawing would help  also have pumps been sized forTEL and or press drop of system.
  • SWEI SWEI @ 9:18 PM
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    commercial primary loop

    In a typical residential primary/secondary arrangement, the closely-spaced tees and zone pumps are "collapsed" back into the mechanical room instead of being spaced around the building, close to the loads they serve.  The smaller ECM pumps typically used in Europe for this are not easily available here.
  • Mark Eatherton Mark Eatherton @ 10:35 AM
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    Primary Schmimary...

    Secondary schmecondary... They are just words. In the 35 years that I've been doing hydronics, I have YET to see any of my associates in the industry really try and pin down which is which, and why.

    Some say that its a "heating" system, therefore, the loop that services the boiler is considered the "primary". I can blow that theory away by having numerous "heat" sources, all on their own pumps and piping systems.

    THE one most critical component, in my humble opinion, is the expansion tank, and its connection to the system. THIS is what I consider the primary loop, and it can be on the heat source loop, or not. And as we all (most) know and acknowledge, ALL circulators SHOULD pump away from this critical point, in order to guarantee trouble free, quiet operation. So, you can call it whatever you want, so long as ALL circulators are pumping away from the Point Of No Pressure Change (PONPC), which is the expansion tanks connection (open, closed captive, non captive whatever) to the system.

    To the original posters problem, welcome to the world of hydronics. It is a wonderfully amazing world, and just when you think you understand whats going on, it jumps up and smacks you in the head and shows you something you didn't think would/could happen.

    What you are describing is perfectly acceptable and normal. You must size the circulator for the heat source based on maximum potential flow demand from the "system". The system is actually only going to be at maximum demand for less than 10% of the time, so for the other 90 percent of the time, the fluid will flow backwards through the closely spaced "tweener" tees, causing a mixing action that results in a a lower fluid temperature.

    This is actually used quite commonly on larger district heating systems. They will run 200 degree F fluid temperatures through the mains, and on the secondary branch serving the building, the flow is such that reverse flow occurs, cooling the supply water temperature down to a lower (180F) temperature.

    If only one zone is calling, reverse flow can be expected between the heat source connections, and mixing will result. As more zones open up, the flows can flip flop, whereby the main is flowing more fluid than the boiler circuit, and then the cooler fluid doesn't regurgitate and cause mixing, but at that point, demand can exceed heat source capacity, and you may not be able to achieve the 140 F you are looking for.

    If your heating system is extremely dependent on 140 F regardless of the outside temperatures, then your "system" of delivery has other issues. It sounds like a "staple up plateless" system which IS dependent on higher operating temperatures in order to overcome all of the inequities (convection, radiation and minor conduction in that order) that are built into the heat emitter design. Conduction os the king of heat transfer, and anything you can do to enhance that mode of heat transfer will be to your advantage, and allow you to operate at a significantly lower fluid temperature,which has monetary savings and increased comfort benefits.

    If you do slow the boiler flow down to accommodate the low demand flow condition, as it get colder outside, and more zones begin calling create more flow, the heat source temperatures will fall initially until the whole BTU train gets rolling, and if the heat source is properly sized, should back come up to a higher temperature.

    All of this is predicated on the assumption that a proper heat calculation loss has been performed, and the heat source is properly sized. And at this time, it sounds as though this is a large ASSumption... Without knowing for sure, everything else (pump sizing, etc) is just a guess. If you want to do a quick SWAG (scientific wild ass guess), use 35 btu's per square foot of heated space per hour as your load factor, and see if your heat source is anywhere near where its supposed to be. If its shy, that's a part of your problem, and it will become even more pronounced as it gets colder outside.

    Hope this helps.

    It's not so much a case of "You got what you paid for", as it is a matter of "You DIDN'T get what you DIDN'T pay for, and you're NOT going to get what you thought you were in the way of comfort". Borrowed from Heatboy.
  • sgorrill sgorrill @ 4:11 PM
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    Unwanted reverse flow between tees

    Thanks for everyone's feedback with my issue. This is a great forum... Because I don't know what the exact specs are past the boiler room, a lot of this has to be guess work which makes it hard to get it right. I appreciate all your comments and help...


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