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Is this a good pump for my tankless water heater/system?

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toeknee
toeknee Member Posts: 20
Hey guys,

Building a really small basic living quarters at my personal shop and trying to size a water pump that would work for my system. Supplied water is hauled in and stored in a tank. Here is the layout of the system:



There is also another toilet and sink on the second floor directly above the ones in this image, so a total of 2 toilets, 3 sinks, and 1 shower (and a few teaspoons of water once in a while to the closed loop hydronic system I suppose). And as you can see, they're all close together so not much head loss in the piping. But I do know the tankless water heaters cause quite a bit of head loss. How much? I have no idea, I've looked all over for that info to calculate what my head loss would be, can't find it.

Indoor Tankless Rinnai RU130IP- Min. Flow Rate .4gpm, max 6.6gpm. But I figure ill see 5 to 5.5gpm after Delta T is factored in. Recommended 60-80psi This is higher than a lot of the pumps I've seen are set up for and are even capable of.




I chose this because I am at 7,000ft elevation and it doesn't look like many I've seen are readily capable of elevations above 5500ft. This one is capable of 10,200.




The pump I'm interested in is the Grundfos JP PS 13 03 137. And please let me know if you think this would even be suitable and if you know of one that would work better.













Says max system pressure is 87 psi. Is that the pressure it is capable of putting out? It comes with a 30/50 pressure switch adjustable I believe, but probably not up to 60/80 so I'd need to replace that. Also, is it possible for a pump to have too high of a flow rate for a tankless? It's my understanding that the tankless chokes down flow to get correct heat transfer to the water to match the set point. But do you think this pump would cause any issues at the tankless, like water hammer or anything? My check valves will all be the spring type by the way.

Brings me to my next question. Water filters have a max flow rate as well. Im not sure what filters I'm going to get yet (haven't researched enough to know what I even need) but just a quick google search for various filters and It seems somewhere in the 16gpm range max for some carbon filters:




Am I correct in saying I should keep my pump max flow rate below that? Or am I understanding that rating wrong? If someone were to hook the next size up pump, the JP 18, to those filters would that cause problems?

Thanks for your time, any input is appreciated!
Arizona

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  • Jamie Hall
    Jamie Hall Member Posts: 23,454
    edited March 2021
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    You need to look at the whole pump curve, not just the maximum. That pump will put out about 5 gpm at a total head of about 40 psi. Increase the head loos -- such as by partly closing a faucet -- and the flow will, not surprisingly, drop. Decrease the head loss and the flow will increase. It is true that the maximum flow it is capable of is about 16 gpm -- but that is with essentially no pressure at the outlet: just a short length of straight open pipe. You aren't going to do that (at least not intentionally!). Your actual flow will be determined by what faucets or other fixtures you have open and, on the hot water side, the additional (considerable) resistance of the tankless unit.

    The 30/50 pressure switch setting is pretty close to standard for any water system for a one or two story building. There is no need to go higher than that. That 30 psi cutin pressure is ample for all the fixtures you plan to have.

    I might point out that the shutoff head -- the pressure at which it will pump nothing at all -- is 56 psi. If you were to set your pressure switch to 60/80, the pump would never shut off at all. Not quite what you want -- even if the pump could run continuously without flow, which it can't.

    What will happen in practice is that you will open a tap somewhere -- say 1.5 gpm or so. The water will flow out of the pressure tank until the pressure drops to 30 psi. The pump will cut in and start to pump -- a little over 10 gpm at that pressure going into the pressure tank. As the pressure rises, the flow will drop to about 3 gpm at 50 psi and the pump will stop. And the cycle will repeat. If you open enough fixtures to get over 3 gpm, the pressure will never get up to 50, and the flow and pressure will stabilize at some intermediate level.
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
  • rick in Alaska
    rick in Alaska Member Posts: 1,457
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    In my limited experience with that particular Rinnai, it is a condensing unit and does have a high head loss to it, and if I remember correctly, does need at least 50 psi minimum to make it work. Therefore, a 30/50 switch rating would not work. I believe you should be able to raise the pressure switch on that pump to get there, but you will also need to raise the pressure in the expansion tank, assuming that pump isn't one of the Grundfos pumps with it built in.
    Rick
  • Jamie Hall
    Jamie Hall Member Posts: 23,454
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    In my limited experience with that particular Rinnai, it is a condensing unit and does have a high head loss to it, and if I remember correctly, does need at least 50 psi minimum to make it work. Therefore, a 30/50 switch rating would not work. I believe you should be able to raise the pressure switch on that pump to get there, but you will also need to raise the pressure in the expansion tank, assuming that pump isn't one of the Grundfos pumps with it built in.
    Rick

    Then there's a real problem. As I noted, that pump has a shutoff head of 56 psi. Does that Rinnai need 50 psi to activate it, or is it that the pressure drop through it at full flow is 50 psi? Somewhere there's another thread here which has the flow/head loss curve for those things. See if I can find it...
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
  • EBEBRATT-Ed
    EBEBRATT-Ed Member Posts: 15,634
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    You need to find the pressure drop of the Rinnai
  • rick in Alaska
    rick in Alaska Member Posts: 1,457
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    I can't remember, but it seems like it needs at least 50psi to operate properly. Maybe I can check it out tonight when I get back home.
    Rick
  • Jamie Hall
    Jamie Hall Member Posts: 23,454
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    Ha. Found it. The pressure drop curve for a similar Rinnai is attached. My interpretation of the pressure drop characteristics and the minimum pressure requirement for the units in that graph is that to get the full rated flow out of the thing you do need 30 psi, but that it isn't a fixed drop -- so that at less flow you will get less drop. Not sure what the reference to 50 psi minimum for maximum performance is.

    In any event, the referenced pump won't do, if it really requires 50 psi. I'm not sure where Grundfos is getting that 87 psi maximum pressure -- cutoff. That doesn't correspond to the pump curves shown at all.
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
  • toeknee
    toeknee Member Posts: 20
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    @Jamie Hall I see what you're saying now. If Im not mistaken 87psi would be about 203ft of head. And thats at no flow, like you said. None of the pumps on the graph have that capability. Back to the drawing board. Ill keep searching for a pump

    @rick in Alaska point taken on the expansion tank, that hadn't occurred to me yet. Ill put that in my notes.

    thanks guys
    Arizona
  • Larry Weingarten
    Larry Weingarten Member Posts: 3,338
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    Hi, Might a tank type heater be worth considering?

    Yours, Larry
  • toeknee
    toeknee Member Posts: 20
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    @Jamie Hall I did a little more digging in the Grundfos documents and it is now apparent to me that "max working pressure" is not the max pressure that the pump can produce. See below that all the pumps on this list have the same max working pressure but produce different pump head. So I assume max working pressure is the pressure that the weakest mechanical component in the pump can handle?



    After looking at the available pumps and the enlightenment I received from @Jamie Hall about pump curves, I've discovered that to achieve the pressure I need at the flow rate I need for this tankless, I would need a 1.5hp pump at minimum.

    The pump also can't be cast iron according to Rinnai documentation. So far I haven't seen any shallow well jet pumps with those specs. I've seen the cast iron up to 3hp, but the stainless steel up to 1hp. The search continues. Maybe I could use 2 pumps? Or a different type of pump?

    @Larry Weingarten switching to tank type might be my only feasible option. I was trying to avoid having to heat a tank of water when the place sits empty often and keep the place as efficient as possible, but if I can't come up with a simple solution for the tankless I may have to
    Arizona
  • toeknee
    toeknee Member Posts: 20
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    I wonder if there exists a filter I can put on after a cast iron pump that would satisfy Rinnai's water quality requirements in terms of cast iron pumps not being allowed
    Arizona
  • Jamie Hall
    Jamie Hall Member Posts: 23,454
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    You don't really need a shallow well jet pump, so long as whatever pump you use can be mounted below or at worst beside your storage tanks. Such as this one: https://www.northerntool.com/shop/tools/product_200329827_200329827 (I don't know whether that particular one is stainless steel or plastic.). Or this one, which is plastic with a stainless steel shaft https://www.northerntool.com/shop/tools/product_200622983_200622983. -- there are stainless steel models available; the one in the link does have cast iron inlet and discharge fittings, but the rest is plastic or SS. You would need to add a suitable pressure tank (which you'll need anyway, because of the hot water heater) and a pressure switch. This link: https://www.northerntool.com/images/downloads/manuals/43571.pdf covers all the pumps in the Star range.
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
  • rick in Alaska
    rick in Alaska Member Posts: 1,457
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    Not sure how your storage tanks are set up, but there are a lot of set ups like that around here, and quite a few of them are using a deep well pump as the main pump, stuck in the bottom of the tank. Most of our tanks are 1500 to 2000 gallon tanks though, and it can be a little more challenging to use it in a multiple tank setup. If you have a good crossover pipe to each tank, you could put the pump in one tank and just draw off from it. I am just winging it because of lack of time to research it, but I think you could get better pressures and flow out of it then with a jet pump.
    Rick