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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?