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Community and Q&A

Longest life with lowest maintenace combi unit for radiant heat + DHW

360beacon | Posted in Green Products and Materials on

Hi, I’m looking to replace and upgrade an existing open-loop radiant floor heat + DHW gas water heater in a three-story ~1200sq/ft townhouse. What design direction would you recommend – Combi Tank, Combi Boiler or Combi Tank + Boiler? I would like to decide based on:
– longest life
– lowest maintenance

Here are units I am considering – let me know if there is a superior alternative:

—Combi Tank—
Bradford White

—Combi Boiler—
Bosch Greenstar
Bosch Buderus SSB
Viessmann Vitodens 100-W
Navien NCB-E Series

—Combi Boiler + Tank—
Viessmann VITODENS 222-F

Also, is it even possible to avoid fossil fuels and install a combi air-to-water heat pump? Or is the technology not mainstream yet?
—Combi Air-to-Water Heat Pump—

Thank you for your insight!

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  1. Keith H | | #1

    I'm going to guess here but I think you are going to get asked for your climate zone and your fuel costs before you'll get opinions.

  2. User avater GBA Editor
    Martin Holladay | | #2

    First of all, can you tell us your name?

    Daikin is no longer selling its Altherma air-to-water heat pump in the U.S. For more information on equipment available in the U.S., see these two articles:

    Air-to-Water Heat Pumps

    Split-System Heat-Pump Water Heaters

    For more information on using tankless water heaters for hydronic space heat, see this article:

    Using a Tankless Water Heater for Space Heat

  3. User avater
    Dana Dorsett | | #3

    Getting to the optimal solution requires starting with a careful heat load calculation using aggressive rather than conservative assumptions (per the Manual-J instructions.)

    The most you can reasonably expect out of a radiant floor without frying your feet is about 25 BTU/hr per square foot, and you'd probably only have 300' of free floor area per floor (best case), or 7500 BTU/hr on any single floor zone. Unless the zone radiation can deliver at least most of min-fire output of the boiler, it's going to short cycle- I'm not aware of any wall-hung combi that modulates low enough. A tank type combi is inherently self buffered against short cycling due to the thermal mass of the water in the tank.

    With most 1200' condos even the whole-house design heat load lower than the minimum--fire output of any of the wall-hung low-mass combi heaters. It might short cycle a bit even on the whole house load, and with separate zones on three floors they would short-cycle themselves into low efficiency and an early grave. Wall hung combis tend to be a good fit for homes with modest hot water needs and higher than average space heating loads. A 1200' condo is the opposite of that scenario. For more on why these are probably not going to work for your situation, run the napkin math on your radiation:

    An HTP Versa combi, or Phoenix Light Duty modulating condensing tank water heater with an exterior heat exchanger would be superior to the Polaris or B-W. The Versa is specifically designed as a combi-heater, whereas the Polaris is not and has it's own short-cycling issues due to limitations in it's controls, that can be modified to work, but not without violating the warranty.

    The Phoenix Light Duty is on only "light duty" in comparison to other commercial water heaters' first-hour gallons numbers, due to its smaller (but still more than enough for your application), 76,000 BTU/hr modulating burner with a 3:1 turn down ratio. At minimum fire it will still have more heat than your likely whole-house load, but it's not difficult to set them up for micro-zoned houses in a way that won't short-cycle.

  4. 360beacon | | #4

    Thank you for your clarifying questions and help! Dana and Martin, your articles are all great resources – full of dedicated research and experience! After reading these and their comments I am leaning towards a Combi Tank:
    HTP Phoenix
    Bradford White

    Dana, thank you for recommending the HTP brand. Does the Phoenix Light Duty and Versa allow for closed-loop radiant heat + DHW? Any other top quality (longest lasting with least maintenance) tanks to compare?

    I’m in marine zone 5 or 4 (Seattle, WA). Energy cost: Gas(~$1 per Therm), Electricity($0.0697 per KWH).
    The existing open-loop water heater is a BW (48 gallons - 65,000 BTU/hr) and it seems to handle the load.

    Could a 50 gallon Phoenix Light Duty at 76,000 BTU/hr be appropriately sized? If so, does that factor out the HTP Versa combi or other higher BTU/hr water heater tanks? Or are there a multitude of other factors to consider?


  5. User avater
    Dana Dorsett | | #5

    Both the Versa & Phoenix Light Duty have long lasting stainless steel tanks, as does the Polaris. Any of those should go 20 years or more in this application, despite the 10 year warranty. The Bradford-White and Vertex are glass lined, and would normally need to be replaced in 10-12 years.

    The Versa & Bradford- White are fully designed as combi heaters, with an internal heat exchanger isolating the potable & heating water. The Versa & Polaris & Light Duty are not as combi-specific, but are designed with extra plumbing ports for use with heating systems. To support a radiant system they would need an external heat exchanger for isolation, but any of them can support a non-isolated hydro-air handler. The Versa& Polaris do not have modulatnig burners, and are prone to short-cycling problems when used with a radiant system or low-mass multi-zoned radiation. The Phoenix Light Duty will modulate down to about 25,000 BTU/hr at low fire, 75,000 BTU/hr at high fire, and is easier to design a multi-zoned radiant system around.

    The smallest-burner Versa has a 130,000 BTU/hr burner, which is complete overkill for your applicaion, but has a 5:1 turn down, and will modulate down to about the same min-fire output as the Phoenix Light Duty. In total system cost the Versa would usually be more expensive than the Phoenix Light Duty. Both come in a number of tank sizes. Unless you have three baths or a huge soaking tub, the smallest (55 gallon) would probably be the right one.

    Seattle and all of western Washington is solidly zone 4C, not 5C..

  6. Jay Raja | | #6

    Hi Dana,

    We are also in the same climate zone (4c) and just started building our pretty good house. We have an engineer doing all the calculations for our radiant/forced air system. We are considering the Viessmann boilers/indirect tanks. Do you have any thoughts on how the Viessmann units compare to the other ones discussed here.


  7. User avater
    Dana Dorsett | | #7

    The Vitodens 100 has a fairly low maximum firing rate, which can be an issue for wintertime incoming water temperature in a zone 4C climate. It can support a single full-flow shower just fine, but not much more, and you may have to coordinate clothes washing to not conflict with simultaneous showering use.

    It's minimum firing rate of 21,000 BTU/hr is also likely to be quite a bit higher than your whole-house design heating load, and WAY above the load of any single-floor zone. There are some Viessmann boilers with internal buffer tanks to limit short cycling, but at only a fraction of the thermal mass of 50 gallons of water in tank type water heater.

  8. Jay Raja | | #8

    Based on your explanation, would a separate properly sized boiler like the Vitodens 200 combined with a Vitodens 300 hot water tank make more sense if we have more domestic hot water heating load than space heating loads?

    Sorry to the OP for highjacking the thread. My last question here. I will wait for more info from the mech engr for further discussion on this later in a new thread.


  9. User avater
    Dana Dorsett | | #9

    Jay: No version of Vitodens 200 would not be "properly sized" for Olav's (360BEACON 's) heat load. Even the smallest version has a minimum fire output comparable to his whole-house load, which would make it 3x oversized for any single floor's load.

    With properly sized TANK for the domestic hot water loads it does'n't much matter how big the burner is. Domestic hot water loads are fairly brief, but very high BTU/hr heat rates. Filling a tub with 110F water at 10 gallons per minute with 40F incoming water coming in from cold supply would need a burner delivering 300,000 BTU/hr. But typical 50 gallon standalone water heater has a burner output of 30-35,000 BTU/hr, and does the job just fine.

    The optimal solution 95% of the time is to size the boiler for the space heating load, and size the indirect tank to be capable of filling the biggest tub in the house. When the space heat loads are tiny (as often the case with condos & apartments) it's often better to just serve the space heating load with the water heater.

    With a properly sized hydro-air coil and sufficient thermal mass in your radiant zones you might do fine with a Vitodens 100, providing your domestic hot water needs are modest.

  10. Jay Raja | | #10

    Got it. Thanks for your valuable and informative posts. It really helps educating owners like me to have an educated conversation with the contractors/engineers while designing these systems.

    Much appreciated.

  11. 360beacon | | #11

    With the HTP Pheonix using an external heat exchanger for heating, would any tank essentially work? But I assume the Pheonix has better programming for heating (along with a stainleas steel tank)?

    Jay thanks for asking the question about the Vitodens boiler units. I have also been considering one, especially because there may be a $800 rebate from my gas company for a tankless unit used for heating and DHW.

    But even with a $800 rebate - could a tankless be more maintenance and not handle usage spikes unless it's oversized? Is there a tankless unit still worth considering?


  12. User avater
    Dana Dorsett | | #12

    Olav: I'm not sure what "With the HTP Pheonix using an external heat exchanger for heating, would any tank essentially work?" means. Do you mean any tank type water heater? (answer: Maybe- it depends.) Or do you mean any size Phoenix? (answer: Yes- size the tank for the domestic hot water load.)

    With any of these it's all conjecture until the actual heat loads (per zone) and radiation (per zone), become better defined. I'm guesstimating that the design heat load of a 1200' condo located in zone 4C will come under 10,000 BTU/hr, and that no individual floor would have a load more than 4,000 BTU/hr, maybe even less than 2000 BTU/hr but you have to do the math to verify that. Heat load isn't a function of square footage of conditioned space- it's all about exterior surfaces, which condos have less of per amount of floor area than single family homes. A town-house in the middle of two other townhouses has barely more than half the exterior surface area of a comparably sized house. At town house on the end of a building can have have more than 3/4 that of the house though- there are no simple rules of thumb that will get anywhere near reality. It must be calculated.

    Also, to be clear, I am not a hydronic heating designer by profession, but I know how to do the math better than the average hydronic contractor seems to, having been forced into figuring it out for myself due to the wealth of ignorance/complacency out there. An oversized burner can still heat the place, but it's not the right thing do do, since it comes at the cost of efficiency & maintenance, and too often takes hit in comfort as well.

    Tankless combis can be sized for the domestic hot water load, but the minimum fire output would be way oversized for the zone radiation, possibly even for the whole-house load. Even the smallest tankless combis have minimum firing rates over 10,000 BTU/hr though, so it's never going to modulate with heat load, and it's always going to cycle with zone calls. A tank based system eases most the design constraints on the zone radiation allowing micro-zoning to the Nth degree if desired, since it's utilizing the thermal mass of the water in the tank rather to limit the numbers of burn cycles rather than insisting on sufficient radiation on each zone to emit the full min fire output.

  13. Jamie B | | #13

    Hi Dana,

    Are you aware of the htp crossovers?

    They have both a wall unit with a 3.5gal tank and a floor unit with a 30gal tank. What are your thoughts on them. I

    I was planning on getting the floor unit mostly based on your discussions on short cycling of radiant systems. And should my domestic use drain the tank, I'm going to assume 100,000 BTUs would be enough to act as a tankless. (Please Tell me if I'm wrong)

    What do you think about these units?


  14. Richard McGrath | | #14

    Jamie ,

    The wall unit is not something you wanna use for space heating . although you can , the heat exchanger has a high resistance that needs to be overcome by the circ , This because it's made for street pressure to be the force behind the water flowing through it , the mechanical energy needed from a circulator makes it hard to justify or make sense of .

    The floor unit however would suffice , however it does not have a programmable differential to really fine tune the use of the mass . For the extra 400 - 600.00 the Light Duty is a much better unit for combi use .

  15. Richard McGrath | | #15

    This home was designed from the ground up for comfort and efficiency . The source for heat and hot water is an HTP Versa Hydro . Embedded slab radiant , 4 active adults who cook , shower and wash clothes often , Ann Arbor , Mi . , 4000 sf .

    You mentioned open loop earlier , could you elaborate on whether this system is truly closed or open ?

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