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Rinnai Primary/Secondary Kit Required?

Hello all, I'm not a professional HVAC guy but a DIYer. I have been in industrial maintenance though but this is my first time doing anything HVAC related.

I am replacing our 37 year old boiler with a new Rinnai i090SN wall mounted unit (non combi). I was doing an inventory of what I could use from my old system with the new system and ran into an issue.

On the old boiler it was a simple In and Out on the water piping with it being daisy chained throughout the baseboards/radiators (so no separate zoning per room) using a single circulating pump. I was reading in the manual for the Rinnai that it REQUIRES the use of a primary secondary heating kit or a low loss header. Do I really need that if its a single zone though? When I've searched online I keep running in to people having them installed with multiple zones but I couldn't find one with someone using it on a simple single zone and I would personally love not to have to spend $350+ on a kit if I don't need to.

Home is well insulated and 1300sq ft in NW Ohio with 2 radiators upstairs and everything else on the main floor.


  • hot_rod
    hot_rod Member Posts: 19,954
    All you need is two tees, maybe 30 bucks worth. Page 5 shows how to
    pipe PS

    The tees need to be close together maybe 2-4””
    12” on either side of them
    You still need a good air separator regardless

    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream
    Mad Dog_2
  • GroundUp
    GroundUp Member Posts: 1,656
    As long as the boiler's minimum flow rate is met, P/S is not needed. However if it is, you would need an additional circulator in addition to two tees as Bob mentioned, and flanges. It all boils down to flow rate, and the manual does show a diagram for a direct piped unit without P/S.
    Mad Dog_2Teemok
  • Mad Dog_2
    Mad Dog_2 Member Posts: 5,937
    You'll be fine.. listen to the pros above...Rinnai is an excellent product. I have over a hundred out there...no issues..Great tech support 👏   mad Dog 🐕 
  • hot_rod
    hot_rod Member Posts: 19,954
    cast iron rads, large piping, probably direct pipe.
    unless you know or can measure flow rates.
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream
  • Teemok
    Teemok Member, Email Confirmation Posts: 79
    This is a do or don't question many "experienced" hydronic folk get wrong. There is a theoretical advantage to a well designed primary only piping application. No dilution and very low return temps with high delta T but it can go wrong quickly if all is not well thought out. If you have to ask, you should be piping it primary secondary. I've seen great single zone primary only designs go south because someone later on decides to chop them up into small zones with out p/s piping. Proper flow through the HX is a must. I think Rinnai has a don't even try it warranty policy. They don't want to have to vet designs of systems at all and I can't blame them. If you design for primary only, you must shoulder the liability for the potential efficiency gain. I have customers who trust my design and have efficient systems running at close to the wildly advertised but rarely achieved 94-95%
  • Slimpickins
    Slimpickins Member Posts: 334
    The manual says you need to have hydraulic separation and there was even a tech bulletin put out when the i series came out. The plumbing kit sure saves a lot of time and gives it a clean look and ensures the piping is correct.