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Community and Q&A

Tankless, tanked, or both?

WinnDesign | Posted in Mechanicals on

I’m in the process of trying to figure out what i’m going to do for hot water on a new custom home I just broke ground on. I don’t want to say money’s not an object but it’s definitely not my first consideration.

A colleague of mine recommended that I explore doing a tankless gas water heater with a 10 gallon standby tank. In exploring this option I came across a Rinnai RH180 hybrid tank-tankless water heater.

Does anyone have any experience or thought about this unit…good, bath or otherwise? Thanks!

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  1. Anon3 | | #1

    >tankless gas water heater with a 10gal standby tank

    Yeah, this is called a hot water heater. Just get the standard type with a 40 gallon tank... They even got these amazing ones with 29 gallon tank from Home Depot.

  2. GBA Editor
    Martin Holladay | | #2

    I suggest you read this article: Domestic Hot Water: No Perfect Solution.

    The article includes a decision tree to help people in your position.

  3. Expert Member
    CARL SEVILLE | | #3

    Your hot water pipe system design is at least if not more important than your water heater selection in terms of efficiency and speed of delivery. It is a complicated subject. Some information available here: and

  4. user-5946022 | | #4

    I think this depends upon what you think your water needs are. The Rinnai Hybrid units come with a 40 gallon tank. If a 40 gallon tank alone suits your needs, seems senseless to add the expense and space of a tankless unit...

    The challenge with tankless is enough flow to trigger the tankless to turn on and heat the water. That is the reason to have a separate tank. Until recently, I thought a 2-3 gallon tank would be just about right. If you have a 0.5 gpm faucet that does not trigger the tank, you would get 4 to 6 minutes of hot water. Any fixture that needed more water would have a higher flow rate.

    That is until I realized you are stuck with a water heater for 15-20 years, but over that time, fixtures will probably become more efficient, such as 0.75 gpm showerheads (google the Nebia), or a Niagara with 0.5 soaping function. If the flow rate won't trigger the tankless heater, you lose the opportunity to upgrade the fixtures.

    I does not seem the tankless available today have caught up to the ultra low flow fixtures, so I've concluded I'm better off with a tank. In my area, water savings is more important than the negligible energy savings of a tankless.

    Ultimately, I think tankless manufacturers will figure out how to trigger the units at low flow, and typical houses will have 2-3 water heaters, one near each group of fixtures, to facilitate hot water delivery with minimal wait and waste. We are not quite there yet.

  5. user-2310254 | | #5

    When I was building, I debated whether to use a tank, HPWH, or tankless unit. Ultimately, I decided to go with a Marathon high-efficiency tank. I didn't have ideal conditions for a HPWH, and I worried about the "cold shots" you get with tankless unit. The Marathon is pretty bulletproof and capable of meeting all out hot water needs.

  6. user-5946022 | | #6

    Does anyone know what the lowest flow rate is for a reputable tankless? For example, the Rinnnai site states they assume a minimum 2.5 gpm mixed flow rate for showers. But the Nebia shower head uses only combined 0.75 gmp and Niagara is selling a 0.35 gpm faucet aerator. Is there any reputable gas tankless unit currently on the market that will provide hot water in ultra low flow rate situations?

  7. Expert Member
    Dana Dorsett | | #7

    C L: Short -answer: NOPE!

    Most gas tankless need more than 0.5 gpm flow through the tankless just to fire up, and don't modulate the flame much lower than 0.5 gpm. And that would be just the fractional hot-side flow to the shower, less than the total shower head flow.

  8. user-5946022 | | #8

    Thanks Dana. That is pretty much what I had concluded.

    I see some of the tankless electric specs have much lower flow rates. The Rheems all indicate a minimum flow rate of 0.3 gpm, and the EcoSmart indicate 0.25 gpm self modulating.

    Although perhaps tankless electric could handle ultra low flow rates, any tankless that might serve an entire house looks it would use a massive amount of power - I can't imagine they are efficient.

    Unless the gas tankless mfg can figure out how to modulate very low, I can see a day when people install a gas tankless for tubs, and then a 0.5 gpm tankless at each bath for showers and sinks.

  9. Jon_R | | #9

    As a hot water system quality issue, I'd think carefully about the time aspect (ie how long till you get hot water).

  10. user-5946022 | | #10

    Jon - you are correct - that is certainly a large part of the equation. The design has very short runs from the water heater to the fixtures and we will be using the smallest size allowable pipe. I wish they would allow 1/4" hot water to sinks, but 3/8" is the smallest. Low flow (water saving) fixtures simply exacerbate the problem of hot water wait.

    For future planning (ie even lower flow rates?), each vanity will have a circuit run to it with J box. That will give me the option to add a point of use in the future (and plug in hair dryers now).

    If houses move from gas to electricity, and move from grid to being more self sufficient, I can see a time when each fixture or group of fixtures has a small electric point of use water heater. Hand sinks might include a 1-1.5 gal internal tank (Bosch already makes these), showers and kitchen sink a 5-7 gal internal tank. Only bathtubs and laundry would get water from the whole house heater. Dishwashers already use so little water that manufactures have realized they need to reheat the water within the dishwasher to sanitize. Even a $250 DW includes this function. Maybe washing machines will also include it in the future. Then every fixture group could get by with the point of use electric except the bathtubs. Since many homes now have some bathrooms with just showers, you could have a single tankless electric serving all bathtubs....

    There seem to be alot of benefits to a setup like that - you would not lose water waiting on the hot water, if a water heater goes out, you would not lose hot water for the entire house, if there is a seldom used guest bath, you could simply turn off the water heater at that area of the house, etc. For those already heating water with electricity, I'm wondering what the drawbacks are. These point of use units cost $100-$200. For the typical 2.5 bath house, you could install a $300 tankless for the bath with the tub and the laundry, a $200 tankless for the bath with the shower, and one $100 tankless for the kitchen sink and powder room. Total cost similar to a whole house tank or tankless. I'm not sure these small point of use units are as energy efficient as, for example, an electric heat pump water heater, but they could sure reduce waste while waiting for hot water.

  11. user-5946022 | | #11

    Jon - good idea. I was wondering about that - is you restrict the flow at the fixture, but whenever a fixture calls for flow, have a recirc pump going, would the flow in the recirc line be based on the pump flow, which you could calibrate to be sufficient to trigger the water heater? In that case you would need the recirc pump to operate the entire time the fixture is used.

    As you say, every fixture would need to trigger the recirc pump. If you triggered the recirc pump based on flow, you might be able to solve the problem of insufficient flow to trigger the water heater. However, to solve the problem of waits for hot water and to eliminate wasted water, the recirc pump needs to trigger before the flow starts.

  12. Jon_R | | #12

    Agreed, post faucet opening recirc would reduce, not eliminate waits and wasted water.

    Looks like the min btu limit on a Rheem gas tankless would work with a .75 gpm (total) shower in most climates, but not with a .35 gpm faucet.

  13. user-5946022 | | #13

    So does a device exist that could trigger a pump to be turned on by turning on a light (to get the hot water recirculating to get to the fixture) and then receive a signal from another device to stay on as long as there is flow?
    if so, are the pumps quiet, and how energy efficient are they? I would seriously consider this to get the higher energy efficiency and space saving of a gas tankless...

  14. Jon_R | | #14

    I can imagine a system where every hot water faucet has a flow activated pressure regulator/valve that diverts excess pressure and flow to a return line. Then a centralized pump (similar to a TacoGenie) that is automatically activated by the first minute of increased hot water flow. The result would be much faster warmup without the need to first press a switch (or cause unneeded preemptive recirc as a motion detector would).

  15. Expert Member
    MALCOLM TAYLOR | | #15

    I'm trying to imagine a plumber coming in to trouble-shoot this system when it doesn't work. Does he bring an electrician with him? An engineer?

    Like many of the systems we contemplate adding to houses in the name of efficiency this would require maintenance and specialized trades to keep functioning. The reality of housing is that it isn't inhabited by people with the interests or aptitudes of the people who post here. Surely all new technologies should be evaluated in part by how the building functions when they don't.

  16. user-5946022 | | #16

    Agree somewhat. But this isn't really all that complicated. And the same could be said 5-10 years ago on building a tight house and then adding an HRV/ERV.

    Taco already sells pumps with on/off intervals down to 1 minute. You can wire the power to the pump into the light switch for the bath, or a nearby switch. A friend did this with standard household wiring, where somehow the pump portion of the circuit is on a 4 way switch - any time anyone turns on the bath light in any of three bathrooms, the pump runs for 1 or 2 minutes...

    So the only thing you would add is another trigger - keep the pump running when water is flowing.

    This seems simple compared to some of the home automation systems on the market today...and massively simple compared to commercial controls systems. Electronics can help immensely in curtailing waste of resources. It is all a matter of degree. If a 1970's HVAC tech looked at a standard current system, they would wonder why it is so complicated.

    If the controls described above are possible, there is no reason not to implement.

  17. Expert Member
    MALCOLM TAYLOR | | #17

    I agree in part. The HRV example is an interesting one. I wonder how many of the homes that rely on them, especially in the rental market, have well functioning ventilation systems after several years? Perhaps the systems themselves need to be designed to indicate more clearly when they aren't working.

    There is a tendency with engineered solutions to keep adding technological fixes to each problem in the system that comes up until you end up with something both complex and inelegant. Perhaps what is needed is a fundamental re-think of how tankless heaters themselves work, rather than requiring the whole plumbing system to rely on valves, controls and pumps?

  18. KeithH | | #18


    Whats your climate? Is natural gas cheap or are you on propane? Are you putting solar on or not? (Why not?)

    Maybe skip the complicated questions regarding tankless and get a tank but not a crappie 29g from Home Depot (what the heck Anon3? Small tank and small burner doesnt equal tank plus tankless).

    If it is hot in your climate, what about a split system water heater like the Sanden? Platinum price and bleeding edge refrigerant (CO2)? Please do report back ....

    Or if it is cold, maybe a stiebel Elton hot water heat pump?

    Disclaimer: i dont have and havent installed one of these though I'd like to (not convinced the sanden is viable in CO)

  19. WinnDesign | | #19

    Hi Keith,

    I'm in Northern Virginia, climate zone 4 and we have natural gas. I assumed it'd be gas fed whether it was tanked, tankless or both. Unfortunately, as much as I'd like to add PV panels to the roof, I have both heavy tree coverage and the orientation of the rear roof slope is NW.

  20. KeithH | | #20

    Bummer about your aspect and lack of solar. I think Ng will be cheaper to install and cheaper to operate; my electric option suggestions were oriented towards using a presumed pv array. Still, the sanden is awfully interesting. CO2 refrigerant, coefficient of power 5.0(!), plumbing only between compressor and 80g tank (no refrigerant lines), but the equipment alone is ~$4k+. In my climate, we very rarely see temps below -20F. I'm guessing in your climate you never do.

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