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Rinnai Advice DHW Combi

I'm looking into the Rinnai E75C (or 110C) Combi boiler or the Q85S to handle both DHW and hydronic heating. Two issues that I'm stumped on is the GPM of DHW and the rather large oversize of the boiler for heating.

I have a Mitsubishi ducted split system heat pump (non-Hyper Heat) and added a hot water coil in the ductwork for supplementary heating for really cold days and when the system is in defrost mode. The coil measures about 42" wide x 8" in height. For DHW, I have a 50 gallon natural gas fired  A.O. Smith water heater. It's okay if one person takes a shower, but if two people use the shower without allowing the heater to recover, it's an endless supply of cold water. My DHW needs include shower, bath sink, kitchen sink, dishwasher, and washing machine.

The heating coil will probably not require more than 30-40k BTUs and that's on the really conservative side. I think with the modulating burner, it should be alright without constant short cycling, although I'm not really sure. I wanted to use the E75C as the heating BTUs were good, but the GPM on the DHW aren't. It flows 2.1 GPM @ 75 Delta and would limit me to using one sink or appliance at a time. In addition, I didn't feel like there was much cushion if the shower or kitchen sink flowed more than 2.1 GPM.

I think I've narrowed the choices down to the E110C & Q85S since they both use the same venting, have DHW capability, and provide hot water for heating. The primary difference is that the Q85S will need to use an indirect fired tank and three way valve to have DHW which will need to be sourced. The E110 flows 3.1 GPM @ 70 degrees Delta, but I'm still on the fence if this will be adequate. I don't mind using the shower and starting the dishwasher afterwards but didn't want to suddenly have a rush of cold water if someone opened the sink to wash their hands. This is one of the reasons I'm considering going with the Q85S as the indirect tank will provide some storage and a buffer, but I'm not sure if this will provide more hot water than the E110.

Anyone have any advice on how to proceed or something I missed?


  • Jack
    Jack Member Posts: 1,045
    You understand the combi question perfectly

    And your outputs are correct for the 75/110. I will say that I think you need to run a flow check on your kitchen sink, labs, shower, etc. With my Rinnai tankless set at 120f with a 2.5gpm shower head I am running 2.1 gal of hot to .4 cold. The Rinnai boilers come with the 3"low loss header and the unit, in heat mode is doing nothing all day every day but maintaining a 36* temp rise across the unit, and short cycling "shouldn't" be a problem, but that said, what is your heat load?

    If you are uncomfortable with the DHW output and want more your best bet would be the Q85s with an indirect. Rinnai has an optional 3-way valve that mounts to the unit and allow you to use the internal boilers variable speed pump to drive the indirect. You will like that system a whole bunch. If you do not want to use the optional valve you can run the indirect conventionally off the secondary side of the LLH, but you will need more pumps/controls for that.

    I have to acknowledge my bias as I represented Rinnai for a long time and still consult with them.
  • RJmacreedy
    RJmacreedy Member Posts: 4
    Combi Special

    I did a rough heat load calc and came up with about 25,000 btus. For getting a general idea of flow rate needed, I did the ole plastic gallon container with stopwatch. The shower is about 2.5 GPM, bath sink is 1.5, kitchen sink is 2.2, and slop sink about 3 gpm.  It's difficult to measure the dishwasher and washing machine, but I don't think they'll make a huge impact except when the fill solenoid is open for 30-60 seconds.

    I'm kinda partial to Rinnai equipment. I have one at work that feeds a toilet sink and hand washing station. We've had it about 5 years and never had an issue with it. It'll pump out hot water all day long.

    I think I could get away with the 110, but would have to manage hot water use when using the shower or doing dishes. I like it as the unit is pretty much self contained.

    I believe you're right about the Q85 though and wouldn't have to worry about DHW as much if at all. The modulating circulator is also a nice addition.  Yeah, I would go with the internal three way valve. Besides the additional piping, which isn't a big deal, the indirect tank isn't cheap and isn't a deal breaker.

    With the Q85, I was looking at the Super-stor Ultra stainless tank model SSU-20. Do you feel this would be adequate or should I step up to the 30 gallon? According the their chart, the 20 gallon would flow 168 GPH or 2.8 GPM @ 115*. The 30 gallon would flow 212 GPH or 3.5 GPM @ 115*.  I'm not really sure what their chart means with 180* and  200* boiler water first hour rating.

    I was planning on using the Johnson Controls 450 temperature controller with the sensor seated inside the immersion well. This controller will allow me to set the temperature differential with accuracy and plan on using the internal relay to fire the boiler for DHW. For hot water heating, I was going to use a simple 24 volt valve with an integral normally open endswitch.
  • Jack
    Jack Member Posts: 1,045
    25kbtu at design condition?

    Is this a baseboard system? I have to say that with that small a heat load I would look at a couple Rinnai Energysavers DV's. One unit could easily handle the heat load but depending upon floor plan two may be best. A Rinnai tankless with that and, well, Bob's your Uncle! Much less dough. This is worth a look for sure. It's the way I heat my house.

    The first hour rating of the indirect at 180 &200f is the hot water capacity with those output temps from the boiler. You will never see 200 out of the Rinnai. I'd use the 30 minimum if you go indirect. Is this ng or lp
  • RJmacreedy
    RJmacreedy Member Posts: 4
    Hot Water Coil

    Yes, at design. The hot water coil is located in the ductwork and will be activated when the heatpump cannot achieve setpoint or the system goes into defrost. It's basically going to be used for supplemental heat only. The heat pump output is about 25,000 BTUs. This number decreases as ambient outdoor temperature decreases. At about 45* +, it can handle the heat load fairly well. Anything less, it struggles and goes into a ten minute defrost more often.

    I was looking at the RU series to handle hot water needs and the R50Lsi (I Think) to handle DHW. It almost half the price as compared with the Q85s, with indirect tank, valves, etc. One of the problems was space and running dual vents, PVC or Concentric polypro. I gutted the mechanical room out, sealed the old window with concrete block, and added a natural draft air makeup louver for the tenants hot water heaters. While I did all of this, I installed the termination kit and cemented the pipe going through the wall which is about 18" thick of solid concrete. To be honest, I don't wanna rip it all out even though I would probably save 1k going with your suggestion.

    Thanks for clarifying that. I didn't think I would see 200* out of the Rinnai either. According ot their chart 180* entering water would give me 212 GPH or 4.5 GPM if I'm doing this correctly. The system would be natural gas. I don't know, with this setup, would I ever run out of hot water? lol Don't get me wrong, I really love the E110, I just wish the DHW output had better numbers. I feel it's a pretty amazing machine in a nice package.

    I guess there are lots of different ways to achieve the same results and also different budgets.
This discussion has been closed.