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Thread: Tankless with varying incoming pressure from well tank/pump?

  1. #16
    In the Trades Tom Sawyer's Avatar
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    What I am saying is that we have installed over a hundred tankless units on well systems running 30/50 and have had zero pressure problems with them. They are designed to handle pressure differences. That's the whole deal with them. Open one faucet get hot water, open another and the fire gets bigger and you get hot water. I suppose that if he DOES have a problem, installing a csv might be a solution but I would just up the pressure switch to 40/60

  2. #17
    Moderator valveman's Avatar
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    30/50, 40/60, or 50/70 it doesn’t matter. The same showerhead will still flow less water when you are at the low end of the pressure than when you are at the high end. When the pressure to the showerhead varies, so does the flow rate and temperature. What I hear is when the pressure and flow rate is low, many times the tankless heater won’t fire off. When the pressure and flow is high, you need to add more cold water to the mix. Tankless heater or not, varying pressure and flow in the shower can make maintaining the water temperature a problem. This is unlike city water pressure, which stays fairly constant.

    Of course all the people I hear from have this problem. So I don’t know how many people do not have this problem or just learn to put up with it. If you have this problem, I know constant pressure from your pump will solve it. If you don’t have this problem, then constant pressure from your well pump just solves a lot of other problems.

  3. #18
    In the Trades Tom Sawyer's Avatar
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    http://www.rinnai.us/documentation/d...nual_EN_FR.pdf

    Rinnai and every other tankless that I have installed have a minimum pressure requirement of 20lbs which is a good 10 lbs below pump cut in. Does anybody really think that tankless manufacturers would manufacture a product that could only be used on city water? Above is the manual link for a rinnai V53. Note that the unit ( and others ) are designed to modulate the fire in order to compensate for varying flow and pressure rates. Again, out of a few hundred installed, low or varying pressure has never been a problem.

  4. #19
    Moderator valveman's Avatar
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    Again, I get a few hundred calls every year where varying flow and pressure from well systems are a problem for tankless water heaters. People figure out pretty quickly that the problem happens when the pressure from the well pump system is varying. Since there is actually nothing wrong with the tankless heater, they probably don’t call the person who installed it. They search for ways to stop the pump pressure from varying, which is why I get a lot of these calls.

    What they tell me is that when running one low flow shower head or one small sink faucet, the tankless heater only works during the higher pressure part of the pumps cycle. Which means there is not enough flow at low pressure to fire off the heater. Keeping the pressure constant also keeps the flow and temperature constant. This has made a lot of well owners happy with their tankless heaters, and kept the plumber from getting calls to replace them.

  5. #20
    In the Trades Tom Sawyer's Avatar
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    If that's so then there is an issue with the tankless heater

  6. #21
    DIYer, not in the trades LLigetfa's Avatar
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    Quote Originally Posted by valveman View Post
    If your micronizer stops sucking at 40 PSI, it is already not working during the upper half of your 30/50 cycle. So I guess your hydro tank is large enough that you get the aeration that you need?
    Yes, but probably only because the micronizer aerated water already in the tank from the first half of the cycle mixes with the non-aerated water from the second half of the cycle. Only about 21% of the air is oxygen and I don't know how much of it gets consumed or how fast, but the consumed oxygen in the tank quickly gets replenished on the next pump cycle. A fish tank without an aerator still has some oxygen from surface exposure but the fish sure notice the difference.

  7. #22
    Moderator valveman's Avatar
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    Hot Water Capacity
    0.6 to 5.3 GPM at 45°F Rise

    “When I was using the hot water, the water got cold.
    If you adjusted the flow from the tap to lessen it, you
    may have gone below the minimum flow required. The
    Rinnai water heater requires a minimum flow rate to
    operate. (See the specification page for the flow rate
    of your model.)”

    “Ensure you have at least the minimum flow rate required to fire unit.”
    “18. What is the minimum water flow required to operate a Rinnai tankless water heater?
    Please keep in mind that the Rinnai tankless water heater needs to see over 0.6 gallons per minute of water flow through the hot side to ignite and to stay in operation. If the flow rate falls below this level, the unit will turn off because it senses that hot water is no longer needed. There are several possible solutions to minimum flow rate issues: lowering the temperature on the unit to a more comfortable temperature to avoid cutting in too much cold water, cleaning the inlet water filter on the cold water inlet of the unit (sediment build up in filter can decrease flow into the unit), cleaning or removing the aerators from showerheads or fixtures, or replacing low flow showerheads or fixtures.”


    Anyway you slice it, low flow through tankless water heaters is a problem. The more your pressure varies, the harder it is to maintain the minimum flow required to keep the burner on. The best I can tell it has a turbine type flow sensor. Flow sensors are notoriously unreliable and troublesome. If you gradually bring the flow up to .6 gpm, the turbine probably won’t start spinning. If you abruptly turn on more than .6 gpm, the turbine flow sensor will probably start spinning. That is until there is a little build up or wear on the flow sensor. And I have never seen a flow sensor that is accurate enough to work at exactly .6 gpm.

    Even if everything is working perfectly, and you mix in some cold water to a low flow shower head, you may have .6 gpm flowing at high pressure, but not at the lower pressure. So the burner turns on and off during a shower, which is the complaint I always hear. The manufacturer even says…. “There are several possible solutions to minimum flow rate issues”. So there are ISSUES with low flow.


    Their solutions include cleaning the water filter, removing aerators, or replacing low flow showerheads. All of these solutions are ways to increase the flow. Maintaining constant pressure from the well pump will also increase flow, compared to letting the pressure decrease as it does on the low end of any pressure switch setting.

  8. #23
    In the Trades Tom Sawyer's Avatar
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    well then, that's it. From now on, every tankless heater that gets installed in residence with a well is going to need a CSV installed, so customers can expect to add another 5 bills or so to an installation that is already costing them 25 hundred or so. I'll be sure and order a case so's I have them on hand LOL

  9. #24
    Moderator valveman's Avatar
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    If the well pump had been set up correctly to begin with, it would already have a CSV and small pressure tank. There would be no problem with varying pressure for the water heater. There would be no excessive cycling, so the pump/motor, pressure tank, pressure switch, check valve, and control box would all last much longer. And the homeowners would already be enjoying city like, constant pressure.

    If the well pump was not set up correctly, this would be your chance to make it so. Then the homeowners would be praising you for giving them the best shower pressure they have ever experienced, plus be able to take as long a shower as they want, because of unlimited amounts of hot water from the tankless heater.

    Adding a CSV to an existing system will cost from $90 to $245. If the CSV had been installed with a small tank originally, it would have saved about $300 over using a large pressure tank. If the system was originally installed with too small a tank, the money that wasn’t spent on a large tank, can be put to better use by installing a CSV now.

    There are really only two reasons a CSV is not used on every pump system. Either the pump installer isn’t experienced enough to understand all the problems a CSV can solve, and all the benefits for the customer. Or the installer does understand all the problems a CSV can solve, and doesn’t want his customers enjoying those benefits and cost savings.

  10. #25
    In the Trades Tom Sawyer's Avatar
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    Nice post, thanks

  11. #26
    DIYer, not in the trades LLigetfa's Avatar
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    Now if only we could solve the recirc problem so easily. How to get instant hot water at the end of a long line? Used to be, you installed a recirc pump but now with tankless, that won't work unless the pump exceeds the minimum flow rate.

  12. #27
    DIY Member rick.a's Avatar
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    Hi, I'm back. Well, this thread certainly grew some legs.

    Ok, this is what I have and what I intend to do. Unless you talk me out of it. :-)

    I have a small cottage with just my wife and myself. I modified the crappy orignial well setup that this place came with 4 years ago with info. that I learned from this forum, which I consider quite invaluable. I have a 1/2 HP Goulds J5S pump (came with house) sitting in a pump house over a shallow well, next to a lake. Water is 15 ft down, and only 2 feet deep, but has never run out. The house is back 150 ft from the well and about 60 ft HIGHER. I have a 4 year old 20 gal. Wellmate low profile pressure tank in my non freezing crawlspace with a 30-50 switch. 3/4 inch PE running from pump up to the house. Everything currently works GREAT. Except for the massive 40 gal powervent Rheem WH in the bathroom. We will be renovating the bathroom in the spring and the WH must go (it's getting old anyway). Thus the need for a tankless in the crawlspace. To repeat, the prime motivation for the tankless is SPACE.

    We don't use a lot of hot water (or cold for that matter). I did not install a CSV originally because we really don't draw water for very long. Only the one shower uses water for more than 1 minute. During a shower the pressure drops and the pump kicks in, but the flow of the shower exceeds the capacity of the pump to push up the hill, and so the remainder of the shower is at a slightly lower pressure. My 2.5 GPM shower head probably goes down to 2 GHP for the last few minutes of the shower. So there is no pump cycling during the remainder of the shower. The low capacity pump acts like an "auto CSV". The lower flow is not a concern for us when showering. But there is no way that this flow is less than .6 GPM, so the Rinnai should still fire for the duration of the shower.

    So, what I intend is to just install the Rinnai as soon as the snow in MA is gone (we have to move the propane tank for this install) and we will see how it goes with no CSV. If pressure fluctuations cause problems with the WH, I have no problem installing a CSV to eliminate the fluctuation. But I don't want to have to install a smaller tank, until my tank needs replacing.

    When my pump craps out, I will probably install a bigger one and then go with the smaller tank and CSV and have more pressure all the time.

    I will update this thread after the tankless is installed. Wish me luck.

    Rick.
    Rick A.

  13. #28
    Scotsman mcconnellplumbing's Avatar
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    Specs say the Rinnia and Navien will fire and operate on .75 gpm water flow. Assuming a shower head with 2 gallon flow rate, and 75% hot water to cold water ratio, it still should stay on if the flow rate is 1.5 gpm (.75 of 2 gpm)

  14. #29
    Moderator valveman's Avatar
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    Quote Originally Posted by rick.a View Post
    My 2.5 GPM shower head probably goes down to 2 GHP for the last few minutes of the shower. So there is no pump cycling during the remainder of the shower. The low capacity pump acts like an "auto CSV".
    That is exactly what is happening, and it works good. Althought that won't happen if you get a bigger pump. Which is when a CSV will be helpful.

  15. #30
    General Engineering Contractor ballvalve's Avatar
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    Sell the junk tankless on g-bay and put in a electric, superinsulated, with switch. You will be thousands ahead in the life cycle of btu use and frustration.

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