Tankless with varying incoming pressure from well tank/pump?

Discussion in 'Tankless Water Heater Forum' started by rick.a, Feb 15, 2011.

  1. LLigetfa

    LLigetfa DIYer, not in the trades

    Messages:
    4,001
    Location:
    NW Ontario, Canada
    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.
  2. valveman

    valveman Moderator Staff Member

    Messages:
    4,472
    Location:
    Lubbock, Texas
    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.)â€

    “Ensure you have at least the minimum flow rate required to fire unit.â€
    “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.â€


    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 issuesâ€. 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.
  3. Tom Sawyer

    Tom Sawyer In the Trades

    Messages:
    3,184
    Location:
    Maine
    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
  4. valveman

    valveman Moderator Staff Member

    Messages:
    4,472
    Location:
    Lubbock, Texas
    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.
  5. Tom Sawyer

    Tom Sawyer In the Trades

    Messages:
    3,184
    Location:
    Maine
    Nice post, thanks
  6. LLigetfa

    LLigetfa DIYer, not in the trades

    Messages:
    4,001
    Location:
    NW Ontario, Canada
    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.
  7. rick.a

    rick.a New Member

    Messages:
    84
    Location:
    Palmer, Massachusetts
    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.
  8. mcconnellplumbing

    mcconnellplumbing Scotsman

    Messages:
    43
    Location:
    Wellston, OK
    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)
  9. valveman

    valveman Moderator Staff Member

    Messages:
    4,472
    Location:
    Lubbock, Texas
    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.
  10. ballvalve

    ballvalve General Engineering Contractor

    Messages:
    3,261
    Location:
    northfork, california
    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.
  11. rick.a

    rick.a New Member

    Messages:
    84
    Location:
    Palmer, Massachusetts
    OK, it's done. I have installed the tankless (several weeks ago) and have used it long enough to report back to everyone.

    I have a Rinnai 53i and this unit just fit into the tight vertical clearance in my crawlspace. I got this specific model because of it's low vertical requirement. Space consideration was the #1 reason for switching to the tankless.

    Propane company did the gas part of the install. I did the plumbing. Very straight forward, especially since I redid all the plumbing in the house as part of a bath remodel.

    The 30-50 water pressure fluctuation from my well pump/tank system does not seem to make any difference. It works fine. We experience about a 10-15 second additional wait for hot water, and this is acceptable. Flow rate is fine (but the incoming water temp is warmer this time of year). We don't like the cold water sandwich and don't like it when the hot cuts off when the flow is too low. We hand wash dishes. I may install a small electric "buffer" tank downstream of the Rinnai which should solve those issues.

    By the way, does anyone know what the hit may be in flow rate/pressure if I install a sediment filter (just after the well tank)? I fear sediment buildup in the heater since it may be prone to plugging up. I installed a 10" cartridge housing but have not yet put the filter into it yet. If I use a 30 micron filter will it block much flow? By the way our water is crystal clear and no sediment is showing in the clear empty housing. Might this mean that I don't even need a filter, or is this false logic and small particles are flowing through?

    thanks,
    Rick
  12. ballvalve

    ballvalve General Engineering Contractor

    Messages:
    3,261
    Location:
    northfork, california
    My well water could be bottled, but the electric elements build up huge deposits of calcium that hinder their operation unless they break off and drop to the bottom of the standard tank unit. Which they do if using a HIGH watt density element.

    If I had a tankless, I would spend hours a year filling it with lime away and trying to get a brush throught it to remove the lining that makes the efficiency about 1/2 of advertised.

    tankless are foolish when the best is a $200 tank that has only 2 or three parts and sit on every shelf in America. Many life cycle cost tests prove it.

    Sediment filters dont help with dissolved solids which tankless units like to extract.
  13. LLigetfa

    LLigetfa DIYer, not in the trades

    Messages:
    4,001
    Location:
    NW Ontario, Canada
    Ja, mine too, as mineral water! Despite having both an iron filter and water softener, I still see a lot of mineral buildup on my fixtures.
  14. rick.a

    rick.a New Member

    Messages:
    84
    Location:
    Palmer, Massachusetts
    Well in that case, I should be fine. The tested hardness level in our water is very low. We also have never seen any buildup at any fixture.

    But I still would like to know how much pressure we would loose if I install a 30µ cartridge filter. Or for that matter is it just a waste of time to use such a filter.
  15. DonL

    DonL Jack of all trades Master of one

    Messages:
    4,201
    Location:
    Houston, TX
    Like BallValve said; "Sediment filters dont help with dissolved solids."

    You would just have put a filter in it and see what you get.

    The flow rate changes as it gets plugged up.


    DonL
  16. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

    Messages:
    22,051
    Location:
    New England
    Any filter acts as a restrictor, and it gets worse, as mentioned, when it starts to actually collect debris in the filter media. Now, how restrictive depends on the size and design of the filter and the filter housing, which should be stated in the specifications. It will have some nominal pressure drop across it that will increase as it fills up. It isn't a bad idea to plumb pressure gauges on the inlet and outlet, as they will give a good indication (while there is a flow) of how plugged the filter is without having to take it apart to look (which may not tell you much). Note, with no flow, the gauges (should) read the same on the inlet and outlets...it only changes while there is flow.
  17. ballvalve

    ballvalve General Engineering Contractor

    Messages:
    3,261
    Location:
    northfork, california
    I DO use filters in order to save getting junk in fixtures. Usually 5 or 10 Micron.
  18. hardness of the water...

    i think I would be more worried about the incomming pressure
    and when the well kicks on ....

    how hard is the water anyway... they claim you need a water softener if you have more than 11 parts hard water...

    good luck with this one
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