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Rinnai RC80HPI

I am using a tankless as a boiler for the first time and have a few questions. I have already ordered the unit as well as materials to install. It will be piped primary secondary with a 15-58 pushing thru the unit. I'm in IL outside of Chicago.

Primary loop is 2 Zones in a 4200 sq ft moderately insulated clear span building used 1/3 as showroom and the remainder as warehouse. heat load is 85,754 btu at design temp.

My question is how to control loop temp and boiler, i'm considering the following

1 .. Aquastat to cycle boiler pump, flow will trigger TWH

2 .. Outdoor Reset control to cycle boiler pump.....

3 ...Tekmar 356 reset control... run boiler as vs injection pump.

Has anyone used a 356 in this application?

I'm open to suggestions on how best to install.


  • Ironman
    Ironman Member Posts: 7,376
    edited October 2013
    You're Headed for Problems

    This subject comes up almost weekly on here. There are numerous responses by the pros and they are unanimous: don't do it.

    A tankless is NOT a boiler. It is not designed, controlled or approved to operate in place of a boiler. In almost all cases, the manufacturer will not warranty it if used this way.

    It is designed and controlled for domestic heating with and average temperature rise of 77* (50* in and 127* out) and high pressure drop through the heat exchanger.

    Existing hydronics systems, on the other hand, are designed for 20* delta T (160* in and 180* out) with very little resistance to flow. A 15-58 circ is not gonna begin to move enough through the heat exchanger and the temp range will be so much higher than what it's designed for that it probably won't last 3 years. Not to mention the performance problems that the system will have.

    If a tankless could take the place of a boiler, there would be no need to make boilers.

    Do yourself and your customer a favor and use the right tool for the job. Sure it will cost more; it always does to do the job right - initially. But it will be much less expensive in the long run.
    Bob Boan
    You can choose to do what you want, but you cannot choose the consequences.
  • Steamfitter66
    Steamfitter66 Member Posts: 117
    It is rated and warranted for space heating.

    The RC80i is rated for and warranted if used as a space heating appliance.

    you can set the supply temp from 90 to 140 and it will modulate to maintain that temp.

    a 15-58 will handle 15' at 4 gpm and the RC80i has 10' at 4 gpm.

    At design my return temp is 73* and the temp rise thru the RC80i at 4 gpm would be 42* so a setting of 115 supply temp on the RC80i would work fine.

    The customer is a friend and he understands its not a boiler as do I.
  • Jack
    Jack Member Posts: 1,047
    Rinnai is approved for

    "Open loop Heating" only. Close loop apps are not approved. Don't do it! You are cruisin' for a bruisin'
  • Gordy
    Gordy Member Posts: 9,546
    Read the warranty

    There is no coverage when used in a closed loop heating system.

    That should be a red flag in itself.

    Also don't be fooled by the rated maximum output. It's a whole different game when used for space heating. Their outputs are based on high deltas not found in hot water heating systems.
  • Steamfitter66
    Steamfitter66 Member Posts: 117
    You guys are correct

    There is no warranty if used on a closed system.

    I don't need the full capacity and it will work at the numbers I posted above.

    This system is for a good friend and I tried to explain all the short comings.

    New company short on cash. We will install a TT in a few years or when it craps out.

    I believe properly installed it will last at least 5 years.

    What I was getting at with my original post is how do you post purge purge on the HX
  • Gordy
    Gordy Member Posts: 9,546
    What type

    Of emitters besides RFH would operate at a 115 supply, and 73 return?

    Possibly fan coils?
  • Steamfitter66
    Steamfitter66 Member Posts: 117
    Its a heating appliance

    I believe that this model is approved to be used in hydronic applications.


    I did find the exclusion for space heating only. Jack thanks for pointing that out.
  • Steamfitter66
    Steamfitter66 Member Posts: 117
    Its RF

    It is radiant floor and the primary loop temps are 82 supply 73 return. Secondary is 115 supply and 73 return
  • Steamfitter66
    Steamfitter66 Member Posts: 117
    RF design temp

    Actually my temps on the floor would be lower as those are the show room zone. The primary loop would be 70 return temp. Supply of course would be the same.
This discussion has been closed.