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Indirect vs Tankless Domestic hot water

steve novak
steve novak Member Posts: 33
I wanted to get some opions on weather to go with a On Demand heater "BW Everhot" or an indirect system. I am adding a second bath with a large whirlpool tub, 75 gallons. I currently have a 40 gal gas fired hot water heater. My heating system is cast iron base board on a mono-flow system. I have a gas fired Burnham Boiler. My HW heater is over 14 years old so I think it is time to replace it and with the new bath I need more GPM available as my wife and I will shower at the same time a few days a week. I also have well water so my supply is a little colder than city water.


  • jalcoplumb_7
    jalcoplumb_7 Member Posts: 62

    I would go with the indirect. Life time warranty and high recovery rate is a plus. Make sure the pump, supply and return piping to the boiler are all sized to Mfg. specification in order to get the best recovery.

    I like the Heat Transfer Products Super Stor Ultra. The Weil-McLain Plus line is also very nice. I like the stainless steel tanks. Call a local professional and find out what brands have the best support in your area.

    I have used various tankless units and have had good results. You need to be careful of the flow rates and the incoming water temperature. As you stated your temp is a little colder. Having a boiler already makes the indirect a better option.

    When it comes time to update the boiler if you go with a 90+ AFUE you will have a very efficient system all around

    Good luck either way.
  • realolman
    realolman Member Posts: 513
    tankless costs too much

    I don't know about the indirect, but I can tell you I have a tankless in my burnham, and it costs too much to operate... at least in the summer. I don't know about the winter.

    I measured my oil usage in the summer when I wasn't using any heat except for domestic hot water, and I can do much better with electric.
  • jalcoplumb_7
    jalcoplumb_7 Member Posts: 62
    Ouch! Tankless coil.

    An indirect will do much better than a tankless coil in a boiler. The tankless coils tend to scale up. Anoter down side with a tankless coil is the boiler maintains temperature all day long even when you are not using hot water, summer and winter.

    A lot of folks put in a stand alone water heater or remove the coil and add an indirect.
  • Jack
    Jack Member Posts: 1,047
    On demand

    I represent Rinnai in New England. I also heat my hot water with a Rinnai, who makes the BW Everhot, btw. I can run two showers all day long. 2.5 gpm heads. Your issue is the tub. The Rinnai will deliver 4.5gpm at a 70 rise (50-120). Mixing that to 105 bath temp is 5gpm. I think you will find you rarely put all 90 gal in the tub, so you figure that out.

    Why do you want to run an oversized boiler all year round. With the Rinnai you will have the capacity to fill the tub but when the tub is filled and the water is off, you consume no energy. Sorry, that isn't true. You consume 2watts for standby.

    If you go this way be certain that you locate the on demand unit where it will give the best performance. Don't put it out in the north forty. Work to its strenghts. This unit will last a long time...assuming you feed it good water. Get a water analysis done.

    Personally, now that I've turned off my indirect for my Rinnai, I find that I have a boiler sized to produce hot water and I have loads of excess capacity. That is not efficient. I'm going to sell my really nice Buderus system and get into a properly sized modulating condensing boiler. Tightening up the energy envelop, so to speak. That might not be a bad way for you to go. It is worth a look!
  • steve novak
    steve novak Member Posts: 33

    When I said tankless I meant an on demand heater, not the coil in the boiler. Sorry about that I used the wrong wording.
  • Dave Yates (GrandPAH)_2
    good points

    made by Jack. And an interesting observation re the modcon.

    Given that you're on well-water, you also need to consider water quality for any of the on-demand tankless units as scale build-up can be an issue. A number of other considerations should be dealt with - such as recirc - during the decision-making stage.

    EF (Energy Factor) ratings should be taken into account. .82 (on average) for tankless nat gas & .84 for tankless propane while an indirect is .87 (on average). EF translates into the overall efficiency within a 24-hour time-frame, so that stand-by heat losses can be taken into account. While an indirect's rating is .87, you also need to consider the energy source & its efficiency. If the source has an 82% thermal efficiency (during actual operation), then making hot water mimics the tankless units operational costs, although you'd need to derate for stand-by losses from a cast iron boiler - meaning the tankless unit would be more efficient. However, if you switch to a modcon, that picture is dramatically different. If your modcon operates at 92% thermal eff during domestic HW production, you'll effectively reduce HW production costs by 10%, but you'll lose a bit during storage via stand-by losses.

    The final cost considerations should be the life-cycle costs. My calculations for a 34-year span indicate the indirect is the winner & will save you more money (than an indirect tankless unit) when coupled to a high-efficiency energy source. In both cases, I included a replacement in the 21st year. You can easily do your own calculations by obtaining installed pricing for a tankless and indirect water heaters. Calculate your net costs to generate a given daily quantity of hot water (I used 50-gallons). Add in a yearly maint fee for service contracts. I used a 5% yearly increase for all costs, although I used a lower number for the tankless to calculate the 21st year's installation because that would be a direct replacement - unlike the first installation, which included T&M for the retro-fit from an older chimney-vented tank-style water heater.
  • Mike T., Swampeast MO
    Mike T., Swampeast MO Member Posts: 6,928

    How much flow can your well pump deliver and at what pressure (both static (nothing running) and dynamic (with fairly high flow)? Hopefully you have 3/4" (or larger) main piping for the domestic water and will continue such (both hot and cold) all the way to the new tub.

    I have a whirlpool tub that requires about 90 gallons before it can be turned on. City water with 1" meter; 1 1/4" main feed; 1" in the mains to and from the water heater; 3/4" branch feeds (hot and cold) to just the tub and 3/4" Grohe tub filler. With about 60 - 65 psi static pressure, I get about 7 1/2 gpm through the tub filler, so it takes about 12 minutes to fill the tub. Frankly, I was a bit surprised that I didn't get more flow at the tub as the 3/4" Grohe thermostatic shower valve can flow about 12 gpm when I turn on a lot of heads while still maintaing decent pressure.

    Unless you take the entire system into account--not just the type/size of the water heater--you can easily wind up waiting 25 minutes or more to fill a tub of that size. I've personally seen this in both new homes and in renovations.

    Say though that your system will be capable of filling the tub in 12 minutes:

    90 gallons & 8.33# per gallon = 750# of water to fill the tub.

    With an incoming water temp of 50F and tub outlet temp of 100F you have to raise that water 50 degrees F. Since one BTU will raise one pound of water 1 degree F you need about 750 * 50 = 37,500 btus to fill the tub.

    Boilers and water heaters are rated in BTUs per HOUR. To get 37,500 btus in 12 MINUTES, it would require a net input of (60 * 37,500) / 12 = 187,500 btus per hour. At 84% efficiency the larger Bradford-White Everhot would not be capable.

    Say now that it takes 20 minutes to fill the tub. Using the same conditions it would require a net input of (60 * 37,500) / 20 = 112,500 btus per hour. Still rather high for most residences. At 87% efficiency the smaller Everhot should prove adequate. I say "should" because even though the unit itself can transfer a sufficient number of BTUs, your entire system (well & pump & etc., piping and tub filler) must be capable of delivering sufficient flow into the tub.

    Some people love their "instant" water heaters; others aren't quite so wild about them as they do have some quirks. At least the Everhot isn't particularly expensive.

  • Rich Kontny_4
    Rich Kontny_4 Member Posts: 73


    I am an on demand water heater fan yet I have come to recognize some variables that need to be included before making a recommendation.

    #1 Would be water quality since this owner has a private well?

    #2 Is the water system one that has a pressure differential of 20# (in a 40 on /60 off system)?

    Since presure-balancing tub/shower valves require a minimum 20# inlet pressure the pressure at the furthest contolling fixture would have to be at the minimum 20#. Does the math work out after factoring in the on demand pressure loss?

    #3 Would be gas availability as far as capacity goes and related costs to upsize?

    #4 Is often the hardest to overcome, are there venting restrictions that would cause the unit to be located a long distance from the fixtures served?

    #5 If the homeowner upgrades to a 90 plus efficient modcom
    heat source with smaller system water volume can you claim that a 82-83% Rinnai can outperform and be more efficient that a highly insulated indirect with negligible heat loss?

    While your selling point is no need to fire that old high volume boiler that is in-efficient, the equation changes if the homeowner upgrades or plans to upgrade to a high efficient, low volume boiler.

    This one requires some definite comparisons with the indirect being the better choice if the mod-com enters the picture.

    Money may be better spent all things considered on an indirect tank at this time even if an upgrade is delayed relative to the heat source.

    For the record I have a "Takagi" in my home in tandem with a "Heating Box" which gives me all the domestic hot water I need and space heating capabilities of up to 2000 sq. ft.

    The difference is, I did not have an existing hydronic system. I use the hydronic side of my system to heat my newly remodeled kitchen and bathroom with in-floor.

    Rich K.
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