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sizing tankless system

Jack_21 Member Posts: 99
Frequently people only look at the high end btu's to determine "correct" unit. Todays on-demand water heaters are mostly modulating. The only on demand unit you want is one that does modulate. The low end btu range is very important to you also. Pardon the bias but I represent Rinnai in NE. A Rinnai 2520FFU in Propane has an energy factor of .87. Low end flow is .6gpm min. to fire the burner and min low fire is 15,000btu. Max output at 55F temp rise (tr)is 5.3gpm. 60F tr is 5.2, 70f tr is 4.5. The FFU unit is a sealed combustion direct vent. Check for certified dealers in your area.


  • KMD
    KMD Member Posts: 10
    sizing tankless system

    I posted a question yesterday about tankless -vs conventional system and you all seemed to agree that a tankless is more efficient if sized correctly. I want to make sure the one I have installed is the correct one so I want to do a little homework first. I live in a 1800 sqft rancher with 2 bathrooms, myself wife and 3 year old girl. we never use hot water while washing clothes, I dont think we have ever used both showers at the same time, I take very quick showers my wife isnt too far behind me. Should I size the system to handle both showers at the same time? I know you should size a system for worst case but I dont want a system that will cost me twice as much to run so that once a year we can take to showers at the same time, what do you guys think? I do want it sized so the dishwasher or sink can be running and a shower that is more practicle and will happen more often. I did some calculations and here are my flowrates:

    Kitchen sink=1.36gal/min
    hall shower=2.5gal/min
    hall sink=1.875gal/min
    master shower=2.3gal/min
    master sink=1.875 gal/min

    I measured the cold water temp out of the sink and it was 60 deg F, I didnt measure it from the well pum and my crawl space is about 68deg F so I am sure there is a little temperature rise from the water running through the pipes under the house.

    What size tankless system should I put in my house? I want to make sure we can take a very hot shower if wanted. I dont have natural gas so it will be a electric system.
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