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Choosing the most efficient natural gas water heater

user-270695 | Posted in Mechanicals on

This project involves abandoning a chimney and replacing a natural gas water heater.

We were going to go solar (with condensing direct vent back up), but after realizing how efficient the new generation water heaters are, the savings of going solar are quite small, the payback after incentives AND self install still around 20 years.

Our decision is to just get the most efficient, durable water heater out there and forgo the added expense of solar thermal. We currently have a conventional 50-gallon 40,000-BTU model with an 89 FHR. Our family of six has been well served by the capacity of this water heater. We currently have no space-heating requirements for the water heater.

I’m having a hard time finding a 95% or better efficient water heater in this size and capacity. The best units (listed below) seem to be much larger and often designed for space heating as well. We are not adverse to spending $2,000 to $4,000, as long as there is real value.

We will be adding a 200 sq. ft. room addition in the next couple of years. Its long side faces due south and passive solar heating opportunities abound. Maybe one of these larger water heaters with side-mounted taps would be wise for a radiant slab in the future addition? Is it wise to put radiant in a solar thermal mass? I have seen it done before, locally, but never experienced it.

I have narrowed it down to:
A.O. Smith Next Hybrid (
A.O. Smith Vertex (
Phoenix (
Bradford-Whites Residential Ultra High Efficiency Energy Saver Gas Water Heater (

Does anyone have experience with these units, recommendations of others, or a different strategy to offer?

Many Thanks!

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  1. Riversong | | #1

    Be aware that some high-output units (such as the Bradford White) will require a 3/4" gas line, so you may have to re-pipe your house.

    If you can spend a little more for a higher quality unit that can also provide space heating, you might consider the Polaris with stainless steel tank (no anodes required). Even their smallest 34 gallon tank (100,000 BTUs) will provide 129 gph recovery rate (at 90° rise).

  2. user-869687 | | #2

    What's the best way to set up a water heater like the Polaris for both domestic hot water and hydronic space heating? I understand the need to avoid running water directly from the tank to the floors, to allow lower temperature water for space heating without risk of Legionnaire's disease. If there were an indirect tank for space heating, that could draw water from the main tank (at say 130°), but there would have to be a circulator for that short loop (between the Polaris and the indirect tank) and then another circulator for the radiant system. The radiant system would be a closed loop.

  3. wjrobinson | | #3

    Phoenix and Polaris have stainless steel tanks etc... I like that aspect if going gas. Marathon has plastic tank if going electric. Phoenix also does integrated solar models for those that desire such. Extra exchangers can be used for radiant floors, air handler coils, or with any system an external exchanger can be plumbed in. Thomas, typically the Polaris used for both water and space heating would use a wall mounted heat exchanger plumbed with a circulators.

    The sky is the limit from simple to space shuttle style set ups.

    Robert Post-
    Water source and quality matters along with amount of water you may need such as just a shower or the whole house pulling at once... all baths, laundry, dishwasher.

  4. user-270695 | | #4

    Thanks for all the input. I am choosing between the Polaris and the Phoenix. Both are made in the USA and have stainless tanks, but the Phoenix is edging ahead due to reliability and refined technology as described by a few expert plumbers I know.

    One question. With a very high FHR (2-3x the existing conventional), it would seem to me that one could do with a smaller tank size. Are there any online sizing tools that would account for this?

  5. user-869687 | | #5

    An indirect tank would have an internal heat exchanger coil. If using a wall mounted heat exchanger would there not be a tank for the radiant system? I could see the advantage of a wall mounted unit in being able to have a couple of smaller units for different zones.

  6. Riversong | | #6


    The sizing rule for DHW is to match the first hour rating to your peak demand. Tank size is irrelevant except in so far as the smaller tank will have less standby loss (and take less space).

    I'm not familiar with the differences between the Pheonix and the Polaris, but all the Pheonix models seem to require 3/4" gas piping and the smallest unit has a larger tank than the smallest Polaris with almost identical outputs. I suspect that the Polaris heats more effectively because of the in-tank symmetrical burner and larger condensing tubes. But I can't speak to reliability.

  7. Robert H | | #7

    what are the installed cost for the water heaters.

  8. wjrobinson | | #8

    Robert, I agree with your plumbers. Way too many people have loved and hated their Polaris water heaters. The ignitors needing replacement so often... changing those for no charge at 2 AM gets old.

    There ignitor issue is solved by changing the Polaris to less efficient operation parameters. That all said, the Phoenix is just set up easier to work on when one does have to.

    Go Phoenix the smallest unit if not going solar. I like the idea of solar though. You can still go solar later and hook into what you do purchase, not that big a deal.

  9. user-270695 | | #9

    Well, once again this has been a very valuable exercise. Thanks to all!

    Robert R.: Correct- only Polaris has .5" gas piping options. This alone swings my decision back to them. Thanks for pointing out what might have been a costly oversight. Otherwise, I would have chosen the Phoenix for the previously mentioned advantages plus the 316L stainless tank as apposed to the 444 Stainless- a minor difference, but noteworthy.

    AJ: Many Thanks! Please expound on "changing the Polaris to less efficient operation parameters": Not sure I follow. Also, as I mentioned above, even if I self-install solar AND take advantage of all incentives and rebates, the ROI is longer than the life of the products when measured against the efficiency of a Mod-Con HWH. It would be a fruitless exercise. I don't need the bragging rights.


  10. wjrobinson | | #10

    In days of yore... to stop the drip of condensate water taking out the ignitor on a Polaris, they had us turn up the temperature on the unit to 140 I think which is above the condensation temp of the flue gas making the unit the same efficiency as a modern power vented unit. There have been tank failures too though not too often. Keep your warranty papers in order, make sure you will be under warranty depending on who installs and properly maintains said water heater.

    As I said, up this way we used lots of them in the early days and now stay away from them.

    Buy an ignitor at time of purchase and practice replacing it when you install the unit.

    Now if this issue has finally been taken care of by a design change, then let me know and we will try them again here.

  11. wjrobinson | | #11

    Robert Post, Just rereading your post about payback. I think a modern power vented water heater would be the least expensive option if I were to run the payback calcs you are.

    Most payback calcs lead to not spending any money as the winner. Buffet is a prime example.... still driving his old station wagon that I think he bought before I was born.

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