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Thread: Ridiculous gas bills with Naviens!!

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    DIY Junior Member molejn's Avatar
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    Angry Ridiculous gas bills with Naviens!!

    We have 2 navien units we installed side by side in our garage this summer to replace out hot water tank. We have a recirc pump set up and the temp set to 140 since the master bath is quite a distance from the Naviens. We are going thru over 300 gallons of propane in 7 weeks which is costing a fortune. Everything I read says that these things are supposed to be more efficient than hot water tanks. Our bills are twice what they were. Does any one have any idea why this is happening? I am ready to rip the suckers out and put in hot water tanks again!

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    DIY Senior Member zl700's Avatar
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    Turn the circ function off then or at least program to only run when needed.
    Hot water recirc by many could be considered a luxury, and it comes at a cost to keep that loop hot.

    Its not the heaters, but what you created is a constant heating loop all the way to the farthest point and then back. Depending on whether or not the supply and return pipes are insulated and how well only makes it worse.

    Putting a tank back in and still using recirc would be even worse energy costs. The difference in operating efficiency is about 15% so put the tanks in and use recirc and be prepared to buy even more propane.

    (Guys: note I said 10-15% versus using 30-40% which is commonly quoted. Remember the return temps are coming back elevated to be reheated, thus the unit is out of condensing mode and cycling besides, but without standby/flue loss calculated)
    Last edited by zl700; 01-07-2010 at 06:55 AM.

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    In the Trades AAnderson's Avatar
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    It doesn't sound like you've ever programed the recirc function which would explain the high energy cost. Program this function for when you typically use hot water at the furthest point in the house. Insulate your hot water piping too.

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    In the Trades AAnderson's Avatar
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    Quote Originally Posted by molejn View Post
    We have 2 navien units we installed side by side in our garage this summer to replace out hot water tank. We have a recirc pump set up and the temp set to 140 since the master bath is quite a distance from the Naviens. We are going thru over 300 gallons of propane in 7 weeks which is costing a fortune. Everything I read says that these things are supposed to be more efficient than hot water tanks. Our bills are twice what they were. Does any one have any idea why this is happening? I am ready to rip the suckers out and put in hot water tanks again!
    Why two Navien's? What is your total fixture demand?

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    DIY Senior Member Runs with bison's Avatar
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    I would expect ridiculous costs with constant recirc.

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    Retired Defense Industry Engineer jadnashua's Avatar
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    To make that more ecconomical you'd need a buffer tank. My guess is that the way you have this with the water returning, the thing needs to run essentially constantly. Doing this you'll probably also void the warranty. There are ways to make this more efficient. You'd need to read the user manual to see what they recommend when recirculation is installed.

    In some places, having 140-degree water circulating would be illegal in a residence. The potential for a burn incident is too high. Many places require a mixing valve to limit the max to 120-degrees, regardless of the input temperature.
    Jim DeBruycker
    Important note - I'm not a pro
    Retired Defense Industry Engineer; Schluter 2.5-day Workshop Completed 2013, 2014

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    In the trades Dana's Avatar
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    I don't get why you'd want or need 140F in combination with recirculation under any circumstances, but it's pretty much guaranteeing the highest possible parasitic heat losses. And if it's less than 100' each way to the remote tap the recirculation burns are all short-cycles too. It'll only get to 95% efficiency is when

    A: The water entering the heater is under 100F, and

    B: You're pumping more than 5-6 gallons through it per burn at somewhere around 1/4-1/3 of full-fire.

    Short-cycling it with 2 gallon burns @ 130F return water it's probably getting 50-60% net efficiency at the heater, before distribution losses.

    Then, considering you probably didn't insulate the plumbing runs to more than R2 (if at all) on the distribution/recirculation path your heat loss from that path is HUGE. I don't even run the tubing on my radiant heating over 130F- EVER, and it's doing a pretty good job of keeping the place warm. Your recirculation plumbing is just a long skinny heating radiator.

    If you plunked down a tank in the same spot, at the same temperature, with the same recirculation scheme, you won't save more than a dime or two in operational cost, and it may be a dime or two the other direction. With a tank heater you get an ~80% efficient burner but standby losses that take it way down, and if you're dumping all that standby heat to the garage, so you don't even get the heating-season benefit. You'd still have the huge distribution loss of all of that hot pipe hanging out there.

    Crank the temp down to whatever the hottest you'd need (probably 115-120F, to fill a big tub.) Run the recirculation on a demand basis, not continuous, and insulate the plumbing to at least R4 (5/8" wall closed cell foam), but R6 or more is better. (Some plumbing supply houses may have 3/4-1" wall closed cell foam pipe insulation, box stores won't. Grainger carries some selection too.)

    Also, 2 Naviens? Do you need to fill a big spa at 8gpm at the same time that 3 showers are going elsewhere in the house or something? (That's a heluva lot of burner there!)

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    Journeyman & Gas Fitter Doherty Plumbing's Avatar
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    Quote Originally Posted by molejn View Post
    We have 2 navien units we installed side by side in our garage this summer to replace out hot water tank. We have a recirc pump set up and the temp set to 140 since the master bath is quite a distance from the Naviens. We are going thru over 300 gallons of propane in 7 weeks which is costing a fortune. Everything I read says that these things are supposed to be more efficient than hot water tanks. Our bills are twice what they were. Does any one have any idea why this is happening? I am ready to rip the suckers out and put in hot water tanks again!
    You should pipe the recirc through a little electric hwt that takes care of the minute heating. Not the tankless units. A nice little 120v corded 2 gal tank works perfect.



    http://www.bradfordwhite.com/product...&product_id=18

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    Quote Originally Posted by Doherty Plumbing View Post
    You should pipe the recirc through a little electric hwt that takes care of the minute heating. Not the tankless units. A nice little 120v corded 2 gal tank works perfect.



    http://www.bradfordwhite.com/product...&product_id=18
    Why would heating water with a 120v tank be cheaper than heating it with a tankless if the tankless units are so efficient? Nice pvc relief line

  10. #10
    Journeyman & Gas Fitter Doherty Plumbing's Avatar
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    Quote Originally Posted by Hackney plumbing View Post
    Why would heating water with a 120v tank be cheaper than heating it with a tankless if the tankless units are so efficient? Nice pvc relief line
    Because the tankless units aren't super efficient when they're heating water up 10 or 20 degrees. Any gas heating appliance with a low run fraction are terribly inefficient. You don't know this? A condensing appliance actually needs to be condensing to get it's super efficient ratings.

    The electric tank is very efficient all the time...

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    Quote Originally Posted by Doherty Plumbing View Post
    Because the tankless units aren't super efficient when they're heating water up 10 or 20 degrees. Any gas heating appliance with a low run fraction are terribly inefficient. You don't know this? A condensing appliance actually needs to be condensing to get it's super efficient ratings.

    The electric tank is very efficient all the time...
    Ah the navien units i've seen have their own integral buffer tank and the Factory set it up to be used without an outside tank. I suppose thats a design flaw that should be avoided by the consumer. In the pic of that rinnai what if you happened to be standing at the rinnai looking down when the relief valve for the small electric heater's relief decided to blow? Maybe a face full of 212 h2o? Not that uncommon either with a stuck thermostat. Becareful!
    Last edited by Hackney plumbing; 01-08-2010 at 05:36 PM.

  12. #12

    Default Metlund!

    Quote Originally Posted by Runs with bison View Post
    I would expect ridiculous costs with constant recirc.
    I agree with this an other posts. Constant recirc in a house is begging for high bills with any hot water source, not just Naviens.

    Look at this pump set up and switch your system out to it!

    www.gothotwater.com

    From what you say, it will be paid for in the first year!

    you want a circ system to help you not wait for hot water and to save water, but NOT EAT YOU OUT OF THE HOUSE WITH ENERGY COST!

    This is how you do it.

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    Master Plumber master plumber mark's Avatar
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    Talking Nice job....

    Quote Originally Posted by Doherty Plumbing View Post
    You should pipe the recirc through a little electric hwt that takes care of the minute heating. Not the tankless units. A nice little 120v corded 2 gal tank works perfect.



    http://www.bradfordwhite.com/product...&product_id=18
    THAT IS REALLY A NICE LOOKING SYSTEM...except for the pressure relief
    drain pipes.....


    but I would hate to be the fellow that has to fool with and service it someday

    It amazes me what folks will go through just to get a tankless system
    and then realize they dont have instant and hot water and have to add that storage tank underneath it,,,,
    and then they find out they have to de-ime it every 6 months
    and it goes on and on from there.....


    but on the bright side,
    look at all the extra space you have now
    over a common tank type heater!!! LOL..


    ...



    ..


  14. #14
    Moderator & Master Plumber hj's Avatar
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    Default relief valves

    Someone must not have read where you should not connect multiple relief valve discharges together, especially if the size is not increased. And what is the deal with the "vent" which is where the water will overflow if either valve discharges.

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    Quote Originally Posted by hj View Post
    Someone must not have read where you should not connect multiple relief valve discharges together, especially if the size is not increased. And what is the deal with the "vent" which is where the water will overflow if either valve discharges.
    If your talking about the rinnai pic then its not a vent ,its for the condensate line from the tankless vent. 1 temp/pressure relief,1 pressure relief and 1 condensate drain all ito the same 3/4 PVC pipe. Wrong type pipe too. Here the pressure and temp valve would have had to go out seperate.
    The recir loops not insulated also,since we are talking about saving energy.

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