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Rinnai tankless for domestic and radiant

Has anyone out there installed a Rinnai tankless unit for D.H.W. needs and a radiant floor.

Thinking about using this method for a small guest house.

If anyone has done this, do you have any install pics or sketches i can peek at?


  • Jack
    Jack Member Posts: 1,047

    no longer approves their tankless waterheaters for closed loop heating systems and has not done so for the last 18 mo or so. They will be happy to sell you one of their new wall hung boilers for these applications.

    While I know of many out there working successfully I have always felt that a unit that is happiest with variable flow and large delta t's (water heater) will not be as happy long term with pretty much fixed flows and small delta t's.
  • Jack
    Jack Member Posts: 1,047
    I forgot...

    to add my usual disclaimer. I represent Rinnai.

    If this is a small space, you might want to consider one of Rinnai's direct vent wall furnaces. The first modulating gas appliance.
  • Kal Row
    Kal Row Member Posts: 1,520
    btus are btus but within reason..

    a rinnai is a water heater not a boiler - besides the legality of using it as a boiler there are practical differences - namely the need for serious head though it - like a taco 009 for low gpm single unit applications or a taco 1400-50 for 5 at once,

    also the unit is looking for a delta-T of 40f , if it starts closing in, the unit will rapidly modulate down, and not give you the BTUs ,or worse lock out if it delta-t is closing really fast 

    it's best to use the commercial unit with the mc91 controller so that you can set it for 180F and operate in the 140 range 

    u need a low boy 30-40 gall electric water heater to use as a buffer tank and low flow decoupler - which give you that added advantage to electric backup heat – as the rinnai is not meant for the continuous duty use – u would do this for domestic hot water recirculation also

    you pipe the heating load, with the thermostat driven pump, through the hot/cold inlet/outlet of the tank - and the rinnai with a 009 from the drain outlet through the rinnai and back into the upper relief port (put in a Tee so that the relief can be put back)

    so, the thermostat runs the load pump,

    the lower aquastat on the tank, runs the 009 to the rinnai,

    and the rinnai, runs itself when it senses flow from the 009

    this is simple, neat, and in my experience quite reliable

    rinnais are really really reliable – and they do what they can with the available resources, eg: not enough gas pressure, or water press, – no problem – just runs on low fire and give you what it can instead of bitching and moaning like other devices 

    the new rinnai modcon boiler – is a work or art – if you can afford it…the last time i checked it was a lot more than the competition
  • Kal Row
    Kal Row Member Posts: 1,520
    being that you represent rinnai...

    please critique this diag
  • Jack
    Jack Member Posts: 1,047
    Nice drawing Kal

    I'm entirely to dim to draw that without using a pencil.

    A couple things. I would get rid of the strainer and put a cartridge filter on the cws to the system. If the cws is clean you don't need the wye strainer ahead of the Rinnai's. To much pressure drop. When you are finishing any tankless install and everything is tight and right, as you are picking up, run the heck out of the hot water...making sure you pick fixtures which will not flood the rim...go ahead, ask me how I know that! The last thing you do is shut off the water at the service valves and clean the filters. Then you know the water supply debris is out of the system. If you get a call that ends up being the filter related you have dirty water coming in.

    I do not use the EZ Connect when pumping to tanks. When I am using a tank I want max recovery, so I want max flow. I will let the unit modulate on delta t. Set your a-stat for a 20 delta T. Turn off your elec elements. You do not need them.

    I like an Aquabooster or a tank with 4 penetrations. CWS to the bottom tap. Pump off the lower portion but not the bottom (debris). Return to the relief valve or top (dhw outlet) of the tank. You have shown it taht way pretty much. What I like about this system is that you satisy the tank from the top down to the a-stat. That tank is full of HOT water. If you look at some of Rinnai's drawings they show the hot water from the tankless coming to the upper tank penetration but still in the area of the a-stat. The hot water influences the A-stat and you can get a perfect sine-wave type operation. Putting the hot water to the top of the tank makes sense as every gallon of water exiting the Rinnai's is at set-point temp and therfore ready to go to the tap...or tempering valve. I have used this system on apts, restaurants and hotels with great success. As well I've done some large homes with it.

    Have you tried the new condensing Rinnai water heaters. RC80 or98HPi ? 
  • Kal Row
    Kal Row Member Posts: 1,520
    tnx-a-bnch for the input...

    am waiting for supply house to get the new condensing units - i use the electric tank as it is less expensive - i know it seems insane that a whole water heater would be cheaper than just a tank - but they are making way more heaters than tanks, so it's the econmy of scale - it important to come into the side of the tank not the top as the cold inlet with it's dip tube and ball check is restrictive
  • Jack
    Jack Member Posts: 1,047
    On the condensing units...

    Be aware that Rinnai does not allow PVC for the vent. They have made a new pipe in the same 3/5" concentric pattern with a Polyprop inner pipe.

    We, the sales force, pushed pretty hard for PVC for competitive purposes but engineering came back and said "no joy" with pvc. Their feeling is that on a furnace or boiler pvc is ok as it has either no water or low water content. In a boiler system, you fill and heat a few times and pretty much create an inert fluid. With a water heater however, every gallon of water is fresh water and the long term liability due to scaling and resulting rising stack temps is not unacceptable. PVC can take I think it is 142F where the PP pipe will take 245F. It is the same pipe the new boilers use. The PP system is a bit more expensive but not a deal breaker. Make sure you have the right pipe for the install. You cannot mix and match PP and metal. Metal for R75/94. PP for RC80/98HPi
  • Kal Row
    Kal Row Member Posts: 1,520
    there is fantasy and then there is reality...

    attached is what i installed in at the wedding hall in the end - as the old way wasn't giving enough flow - this way we have two 3/4" in and out - i also got rid of the ez-connect cascade - don't need low flow mode as that is handled by the storage and when the storage gets cool or the flow ramps up i want all three - it was hunting a lot on the ez-connect cascade - the 140f is needed by the kitchen but to high for public use - so those sink groups have asse point of use 120f mixers

    the system is live and preforming well even when the water off the street is 40f - this is really really inexpensive for a commercial setup
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