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Can we split tankless on-demand hot water unit for hydro air & domestic hot water?

Mike_W1Mike_W1 Member Posts: 8
Can anyone suggest a good way to use one whole-house on-demand lp gas tankless unit (Rinnai for example) and split its use so that it provides tank-less on-demand domestic hot water for a small house AND also provides hot water for an existing central air system (conventional with ducts and an outdoor condenser - one zone) which we want to convert to a hydroair heating system? We live in Putnam County, NY and we currently have an electric hot water heater and electric baseboard heat supplemented by a wood-burning stove. Can we install the on-demand tankless in place of the elec. water heater and hook it up to the air handler (install a hot water coil), get the lp gas to it and install controls that make it work properly? We don't want to install a boiler and an indirect storage tank because we don't have the physical space for the equipment. Plus, we think it'll be efficient and have a short-term pay back on the cost of buying and installing the equipment. Crazy? Complicated? Easy & smart? Thanks! Mike


  • TonySTonyS Member Posts: 849
    Use a combi unit

    They are designed for what you want to do. Navien makes one. Triangle tube makes the challenger and there are others.
  • IronmanIronman Member Posts: 3,525

    I concur with Tony about the combi unit.

    To answer your question directly: no, you cannot use a tankless that way. The hydronic must be separated from the potable (legionella). The tankless is not designed to have anything but cold water entering it: as in 50 - 55*. Not hot, not warm, but cold. It is not controlled for hydronic, but domestic heating. Also, the heat exchanger has an extremely high head that an ordinary hydronic circulator cannot overcome. It's also an illegal installation because the tankless is not A.S.M.E. certified for space heating.

    Here's my usual response to this question: "If a tankless could take the place of a boiler, then there would be no need to make boilers."

    Get a combi unit that is certified as a boiler (Navien, T.T., etc.).
    Bob Boan

    You can choose to do what you want, but you cannot choose the consequences.
  • JasonJason Member Posts: 293
    Boiler ratings

    Boilers are ASME certified and carry the "H" stamp. Many states require an "H" stamp to be used as heating boilers.
  • HDEHDE Member Posts: 220

    It is legal and widely done. In fact Rinnai and Rheem both promote these systems. The loop hole is applying a tankless in a heating system requires a fixture draw making it a combi system.

    However I wouldn't do it because of the low efficiencies especially when pump exercising fires the tankless burner, even in summer. Besides the stagnant waters issues in coil leading to bacteria scare, there is always the odors involved with shared circuits.
  • JasonJason Member Posts: 293

    How do you get around the need for the "H" stamp where required? 
  • SWEISWEI Member Posts: 7,356

    you don't, if you value your liability insurance and your ability to sleep well at night.

    There are ways to make things like this work and still meet code, but economic viability can suffer.
  • icesailoricesailor Member Posts: 7,265
    Tankless heater split with air handler:

    You can do it and there is a way. It has issues though.

    I worked and "fixed" one last Spring/Summer (2011). The problem was that there wasn't enough domestic potable hot water. Whomever had sold the heater to the contractor had supplied a drawing that was very official looking as if it came from the manufacturer. The manufacturer denied the existence and approval of such thing.

    A flat plate heat exchanger was used to make a separate system. The install could be shown here or on HVAC-Talk, Wall of Fame. Beautiful install. But it took me a day or two to figure out the problem. You needed to install a zone valve to stop the domestic water from going through the plate heater when you got a domestic call, and you had to run the tank-less at at least 140 degrees to get the water hot enough on the hot side of a pressure balance single lever shower valve, so you could overcome the cold water inlet water temperature.

    Cost aside, I think that the flat plate heater, though more costly, is a better application here. Those "Tea Pot Tanks" in the combi units will never outlast the units. They are difficult and expensive and will probably end up on the curb sooner.


    You can do it.
  • EddieGEddieG Member Posts: 150
    The key is...

    Make sure that you have a load calculation done for your heating needs and figure out what your domestic hot water needs are. The combi's work great if installed properly in the right application. Just make sure it fits your needs. The combi's have limitations. I have found jobs they wouldn't do the job. Others they were fine. One thing nice about a boiler, you have room for expansion!
  • icesailoricesailor Member Posts: 7,265
    Combi Unit Sizing:

    Very good advice.

    But you should have pointed out that the domestic potable hot water heating load will almost always If  not always) be greater than the heating load. That a combi boiler should be sized for the DHW load, which is higher. Unless you store it.
  • EddieGEddieG Member Posts: 150
    I agree...

    I agree Ice, but I was trying to point out that a combi isn't always the answer. If a combi doesn't meet the requirements, you have to look at another option. A boiler or a tanked type combi unit (like HTP stuff).
  • Mike_W1Mike_W1 Member Posts: 8
    a two year later reply to those who helped

    I didn't have the notification set properly so I'm just seeing all these replies today. Thank-you for taking the time to help. We haven't done the work yet as other remodeling has taken precedence. The wood stove is gone. Two of the cheapo electric baseboard units have been replaced with better quality units. Better insulation and draft reduction in some areas have made a nice impact on our heating dilemma. But, I'm still thinking about upgrading our heat and hot water systems and it's two years later so maybe technology has advanced in this niche area? I'm a carpenter, not a heating guy and much of the subject matter in the replies is way beyond my work experience. I re-posted the question on this today, 4-17-2014 and have set the notification to, hopefully, alert me to any replies either here or on the new post. Again, thank-you for taking the time to help! Mike
  • SnowmeltSnowmelt Member Posts: 751

    New technology but same concept, know everything can be vented outside with 2 inch material, what's your heating load? Do you have traditional ac and want to install ac with a hot coil ?
  • Mike_W1Mike_W1 Member Posts: 8
    reply to snowmelt

    Hey, I'm just seeing this from you too. No heat load calcs yet. A very basic 1 zone retro-fit a/c system is in place. Flex-duct runs, 1 main return, barely engineered you know what this probably looks like, lower budget equipment, 1400 s.f. house, the air handler has a coil in it for adding hydro but was never done. There's really close to zero room for equipment in this house. 2nd flr Knee-wall attic space for the ex. air handler. 2 @ 3/4" pex runs were futured in for supply and return for when/if we change the ex. elec. ht wtr htr (9' away from the air handler) to an on-demand system. Still would need to run a line for the lp gas but we're still renovating. Always renovating. ;)
  • SnowmeltSnowmelt Member Posts: 751
    3/4 supply

    & return, that doesn't sound like much, I think you want to run a 1 inch pex to give it a little bit more flow.
  • JackJack Member Posts: 940
    Actually you can do that

    It is the only type system Rinnai approves their tankless for. As well they have a DHW priority switch available. You need an AH that will operate the pump on a regular basis. Rinnai will not allow a "closed" loop system application with their tankless.

    A Combi boiler is the better choice, if it has the hot water capacity.
  • Mike_W1Mike_W1 Member Posts: 8
    yes to the 1"

    Thanks again Snowmelt. Sounds like a smart idea and we can run new 1" pex when we reno that area. A little snaking but not too bad. I really like using pex and sharkbites for all the water line work I've been doing. Saves carpenters like me that just can't/won't sweat a nice joint, lol. We'll be replacing the refrigerant line set soon (in the way of future work, the equipment is around 8 years old and has never been serviced- girlfriend who owns the home hasn't a clue, lol) but a real mechanic that can braze will be doing that for us. :)
  • Mike_W1Mike_W1 Member Posts: 8

    Thanks for the reply Jack. I'll definitely be studying this possible solution to our dom. ht water and heating needs. I really like collecting a ton of info from a wide range of people. I kinda think of mechanical equipment as more than just devices that either work or do not. The system as a whole just seems to be a little alive and with a personality that gets along with an owner that understands it AND cares. :)
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