Tankless with varying incoming pressure from well tank/pump?

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

  1. rick.a

    rick.a New Member

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    84
    Location:
    Palmer, Massachusetts
    I have a small house with a well and pressure tank set to produce between 30-50 lbs. pressure. This works great. We now want (actually need to for space considerations) to install a tankless WH but wonder if the varying pressure from the well tank is going to screw things up with the tankless which probably likes constant flow rates. Tankless will be Rinnai 53i. Small house with 2.5 gph shower as the longest HW draw. Do you think it will be OK?
    Rick.
  2. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

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    What is your supply water temperature year-round? Does it change much in the winter? Compare your incoming water temp with the specs to see what temperature rise you can get out of it, it may be okay in the summer if the water temps are higher, but not in the winter. And, it's not the duration of the draw, but the maximum draw you might want. You can't prevent someone from turning on the washing machine, dishwasher, or a faucet to wash their hands while you are in the shower. If you have a boiler, an indirect may be much more economical, and the tanks are smaller (typically) than a stand-alone tank.
  3. rick.a

    rick.a New Member

    Messages:
    84
    Location:
    Palmer, Massachusetts
    Thanks for the reply. Winter incoming water is about 45 deg. Summer is about 55. The unit that we have spec'ed should give us the flow rate that we need with the 60 deg rise in the winter. What I do question is whether the fluctuating pressure will cause problems with the Rinnai unit if the pressure goes down during a HW draw.
    Rick.
  4. Dana

    Dana In the trades

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    As long at the pressure stays within spec you shouldn't have any pressure. At low/very-low pressures (15psi or less) many tankless units have issues, but not 30-50pis.
  5. rick.a

    rick.a New Member

    Messages:
    84
    Location:
    Palmer, Massachusetts
    Thanks Dana for the confirmation.

    By the way, do you know if the plumbing code in MA requires that the T/P relief valve be plumbed to a drain if using a tankless in a cellar (actually a crawlspace with concrete floor)?

    Rick
  6. valveman

    valveman Moderator Staff Member

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    4,588
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    I get a lot of calls from people with this problem. At 30 PSI the system only has about half the pressure than when at 50 PSI. So the flow through the showerhead and tankless heater vary widely. Many times the flow at low pressure is inadequate to keep the heater working. Then at higher pressure it fires off again. A 40 PSI Cycle Stop Valve will maintain 40 PSI constant for as long as you are in the shower. This keeps the pressure and flow steady and the tankless heater never shuts down. When used with a small pressure tank, the pump is running and the CSV holding a steady 40 PSI before you get the water temp adjusted in the shower. When used with a big tank, you just have to wait for the tank to empty, the pump to start, and the pressure to get to 40 before everything steadies out.
  7. Tom Sawyer

    Tom Sawyer In the Trades

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    30/50 won't be a problem. Tankless heaters have a valve in them that adjusts the fire to the flow rater through the unit. We have installed many tankless heaters on well systems with no issues other than water quality.
  8. LLigetfa

    LLigetfa DIYer, not in the trades

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    One should not make a blanket statement like that. There is another thread here where a low flow shower head was a problem. The pressure wasn't mentioned but the difference in flow at 30 PSI versus 50 PSI could make the difference. Mind you, something like a low flow shower head could be mitigated. I like valveman's suggestion to go to a constant pressure system.
  9. Tom Sawyer

    Tom Sawyer In the Trades

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    Let's see now, a couple grand for the tankless and then another grand or so for the CSV, yep that makes sense.
  10. LLigetfa

    LLigetfa DIYer, not in the trades

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    Not sure where you are getting your prices. I'm seeing $200 for the model CSV1W. If I wasn't limited by my micronizer, I'd drop $200 in a heartbeat to get good constant pressure.
    http://www.cyclestopvalves.com/st-viewcat.php?cid=11
  11. Tom Sawyer

    Tom Sawyer In the Trades

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    Someone gonna install that for free? He needs to change the expansion tank also. And why go through all of that when the chances are he doesn't need too? If his system is maintaining 30/50 he should be fine.
  12. valveman

    valveman Moderator Staff Member

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    A shower head that puts out 2.5 GPM at 40 PSI, will do less than 2 GPM at 30 PSI, and more than 3 GPM at 50 PSI. Varying pressure in a shower is annoying and makes it hard to keep the temp adjusted in the shower. Many times the tankless heater won’t stay on because of the varying flow caused by the varying pressure. Constant pressure keeps the flow constant and the temperature from a tankless heater constant.

    Depending on which model you choose, the CSV will cost anywhere from $90 to $245. This size CSV can be used with a 4.4 gallon ($75 tank) instead of an 80 gallon ($500 tank). CSV’s always more than offset their cost by greatly reducing the size of pressure tank needed. A CSV that cost $925 will control a 150 GPM pump with as little as a 44 gallon size tank. A system that size without a CSV would need a minimum of (six) 80 gallon size tanks. So a $925 CSV would save you $3,000 worth of tanks and do a much better job.

    The CSV will also work with a micronizer. You just need to set the CSV close to the off pressure of the switch. Ie; 40/60 switch, set the CSV at 58 PSI.
  13. LLigetfa

    LLigetfa DIYer, not in the trades

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    Sounds strange, coming from a DIYer... As for the expansion tank, we don't even know what the OP has or if it even needs changing.

    Anyway... sounds like you've already formed an opinion on CSVs and I'm not trying to change it. Just don't care for misinformation and FUD. I never suggested the OP had to go with a CSV, only said that I like valveman's suggestion to go to a constant pressure system. If a low flow shower head presents a problem, there is always the option to switch it out. God knows, I pay a lot of money to condition my water and I want a good constant high pressure with good flow in the shower. Not everyone does.
  14. LLigetfa

    LLigetfa DIYer, not in the trades

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    Location:
    NW Ontario, Canada
    I don't want to derail this thread, but I will respond to this statement. My micronizer stops sucking at 40 PSI because the flow rate at that pressure is reduced. A CSV will flow limit at or above the setpoint so for the duration of the shower, there will be no aeration at the micronizer and the only aeration would be in the aerator tank which is considerably less. Any iron that does not oxidize and precipitate will pass through the filter and foul the softener beads. What gets past the softener smell like a bloody nose in the shower.
  15. valveman

    valveman Moderator Staff Member

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    4,588
    Location:
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    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? That is kind of the same thing that happens when the pressure goes above the setting of a CSV. I have been told this still allows plenty of aeration, by many who have this type system.
  16. Tom Sawyer

    Tom Sawyer In the Trades

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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
  17. valveman

    valveman Moderator Staff Member

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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.
  18. Tom Sawyer

    Tom Sawyer In the Trades

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    http://www.rinnai.us/documentation/downloads/U287-182100_V53i_and_V53e_Manual_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.
  19. valveman

    valveman Moderator Staff Member

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    4,588
    Location:
    Lubbock, Texas
    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.
  20. Tom Sawyer

    Tom Sawyer In the Trades

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    If that's so then there is an issue with the tankless heater
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