Help with sizing / installing Rinnai RU180in Tankless water heater?

Discussion in 'Tankless Water Heater Forum' started by Bob Cable, Feb 26, 2019.

  1. Bob Cable

    Bob Cable New Member

    Feb 25, 2019
    San Jose, CA
    Hi All,

    I am looking at installing a Rinnai tankless water heater in my house, but I'm not sure about what size of heater I should buy. I would like to get the RU180in model (natural gas), as the bigger RU199in may be larger than what I need. But I'm not sure if my gas line supports the RU180in. Can you help me with some advise?

    Here is what I have now.
    1) The old tank style heater is a 40 gallon water heater that I would like to replace, with the Rinnai. The 40 gallon is rated at 40,000 BTU, and the Rinnai RU180in I am looking at is rated at 180,000 BTU.

    2) The gas meter has a 3/4" gas main feeding into the meter. The meter then feeds to two dedicated 3/4" gas branches that go into the house. One of the 3/4" gas branches only feeds to the central heater that is rated at 120,000 BTU. The second 3/4" gas branch goes to the Old water heater and the gas range.

    3) On the 3/4" branch supplying the old water heater and the gas range, the 3/4" pipes run about 20 feet across the garage, and then it tee's off to about 3 feet of 1/2" before reaching the water heater. The 3/4" pipe then continues another 20' until it reaches the gas range at the end of the pipe. At the range, it is 3/4" direct to the range.

    4) On the 3/4" branch feeding the central heater, it uses 3/4" pipe all the way until the end, before it switches to a short length of 1/2" pipe, and then the heater.

    5) I believe my gas pressure, after the meter, is 7 C.U. in both of the 3/4" branches.​

    So my question is:
    • Can I just move the gas supply line from the Old water heater tank to a new Rinnai Tankless heater that uses 180,000 BTU? Or do I need to remove that short length of 1/2" pipe first, and go from the Tee to the Tankless heater with 3/4" pipe only? There is only the cooking range at the end of the 3/4" branch about 20 feet away.

    • The Old heater tank vents out straight up through an Oval Type-B vent pipe, that is 6" wide x 2" deep. I want to exhaust the New Tankless heater through the existing Oval Type-B vent pipe to the roof. But the 2" deep oval pipe will not fit a 2" PVC pipe for Tankless exhaust. So can I run 2" PVC up to the Oval Type-B pipe, then go through a Wye to TWO 1 1/2" PVC pipes that will fit Inside the Oval Type-B vent pipe? I calculate that TWO pipes of 1 1/2" PVC will carry more volume than just 1 single 2" PVC pipe, so the exhaust capacity is actually large with Dual 1 1/2" PVC.

  2. Dana

    Dana In the trades

    Jan 14, 2009
    That's probably 7" water column (W.C.) or about 0.25 psi, which is a typical low pressure residential distribution pressure inside the house. I have no idea what units of "7 C.U. " would mean.

    Even though the connections to the Rinnai are 3/4", it is going to need a substantially larger dedicated run of something bigger. It is better if the line isn't shared by branches off to other appliances to have trouble-free operation, even if the gas plumbing is sized to handle both loads. Most houses need a dedicated 1-1/4" gas line between the meter and tankless to support a 180KBTU modulating burner reliably. Modulating burners are more easily disrupted by the sudden blips in pressure as other burners come on/off than dumb on/off burner.

    The chart below is in 1000s of BTU, suitable for 5-7" w.c. supply pressures out of the meter. Be sure to add in the "equivalent feet" for every ell, tee and valve on the path. Also see the chart on p46 of the installation manual.


    The other thing you need to check is whether the meter is big enough to handle the upsized burner. A 120K furnace + 180K water heater + ??K cooking appliance is probably north of 400KBTU/hr, and a lot of residential gas meters are under 300K. When going from a 40K tank to a 180K tankless it adds 140K of max load onto the meter.

    The "central heater" with a 120K burner is obscenely oversized for the actual space heating load of most homes in San Jose. It might more sense to replace it with something right-sized for the load- maybe with a hydro-air air handler running off the Rinnai, if it's a hot-air furnace. If the same ducts are used for both heating and cooling it could mean replacing the air conditioner too- there isn't enough info in the post here to make the call as to what the "right" thing to do is here. But a 120KBTU/80% efficiency furnace is enough heater to theoretically keep my 2400' sub-code 1920s bungalow + 1600' of conditioned basement comfortable at an outdoor temperature of about -100F. (A hundred below zero.)

    Ridiculous oversizing factors makes recovery from overnight setbacks quick, but it's the opposite of "comfort". ASHRAE recommends an oversize factor of 1.4x the load at the 99% outside design temperature. In San Jose the 99% design temp is +37F, a temperature at which the load on my house would be about 18-19,000 BTU/hr, so if I were in your neighborhood the biggest heater I'd need per ASHRAE is 19,000 x 1.4= 26,600 BTU/hr (output), which could be a 30,000 BTU/hr condensing gas furnace or a 2-ton heat pump, or a 2 ton hydro air handler on a condensing Rinnai.

    We can go further down this side track if you like, but you have some homework to do before picking a Rinnai on both the gas plumbing and gas meter end before dealing with the venting issues.
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  4. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

    Sep 2, 2004
    Retired Systems engineer for defense industry.
    New England
    The big thing with any tankless system is related to two factors:
    - how cold is your incoming water
    - how much SIMULTANEOUS hot water uses occur (in gpm)

    The ratings on volume are based on moderately warm water coming into the unit. You may or may not meet that spec. Some places have rather large variations season to season (for example, mine can vary nearly 50-degrees summer/winter, and thus, a tankless' performance would change radically).

    Without knowing both of those things, you can't really know how well the thing will work in your situation.

    If you do not already have softened water, with a tankless, you will either need to install one, or be prepared to demineralize (delime) the thing. Most companies also have an upper limit on how hard your water can be when using their tankless. If you can't do that procedure yourself, figure at least a couple of hundred per year to have it done or you will lose efficiency and capacity...the harder the water, the faster that will happen. Soft water, you may be able to extend that interval.
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