Rinnai Tankless Water Heater

We need to replace our water heater, and are considering a Rinnai Tankless water heater. The household is two adults living in Minnesota. So, the temperature rise would typically need to be 70 degrees. The biggest draw in the house is a soaking tub. This would be the only thing drawing hot water when in use.
I see that Rinnai has two models, 2532FFU and 2520FFU. Looking at their charts, both units perform the same after you have to heat water 60-65 degrees or more. Are there any real world advantages to the 2532FFU? Does anyone have experience with these units that lives in my neck of the woods? What kind of maintenance is needed on these systems?
I notice that Rinnai recommends a 3/4 NG supply. The plumber that came out to take a look at our setup, said that my existing lines will work fine. There is currently a 'main' line in our utility room. He said they have installed units in the area without having to change the NG lines. What are the possible ramifications of using the smaller NG line?
Anything else I should know before making a decision?
Thanks for any feedback,
Steve
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lefty wrote:

You don't guess at the supply size. It's determined from the appliance capacity and pipe size/length.
http://dana.ucc.nau.edu/~fc26/asst1/natural_gas_pipe_sizing_and_asso.htm http://www.firelogs.com/gasline.htm http://www.tanklesswaterheaters.com/sizingtable.html some help.
The appliance needs a minimum pressure at the regulator inlet (usually 7 in H2O) when its own useage plus all other appliances is taken into account.
Jim
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What is the difference of the 2 models. Here is my experiance with a Bosch 117000 btu unit and my take on what you need to know. You can`t guess at Ng supply it needs to be measured with a manometer with competing apliances on, if you dont get the flow you wont get 100% output when you need it when in winter water is at its coldest. This means knowing main input also. A Ng 3/4" line, im guessing you are thinking 190000 btu Rinnai, My 117000 btu Bosch needs 3/4 at 10 ft or more run, as I mentioned competion affects flow, I measure 2f drop in water temp with my furnace competing.
Inlet water temp in winter for me can be 33f at -15f, you need to knows your lows and take off 5f for saftey in sizing, but you probably don`t know your low incomming temp as now is mid March.
Now measure output temp at your HWH and at a furthest shower, mine drops 6-10f with insulated pipe in 30ft. I have no problem with a 112 shower temp with 33f incomming with unit not even on high so the big Rinnai can handle most everything unless your gpm is to high or Ng flow to low, I only have 4gpm. For a tub it will be fine, it will fill a swimming pool. What is your Gpm incomming, tankless are rated at a temp rise of a gpm. So sizing everything is critical. Get a unit that keeps output temp even, my bosch monitors rise, not output temp. A remote thermostat is nice to, the better Takagi and Rinnai keep an even output temp. Takagi makes Bosches better unit.
Ive only heard you have no Rinnai Warranty unless a Rinnai certified installer does the job, and a "plumber" that just says, without checking, that your Ng supply is great is a, well, maybe, might be, a hack. Poor testing, poor planning, you might be unhappy.
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I've done quite a bit of looking into for the Tankless solution.
Stay away from Bosh, especially the low end units Only go rinnai if you have a Licensed person install it as if you do it, the warranty is void. The Takagi is what we chose. the T-K2. Its for a 840sqft Granny unit. 1.5 Bath, Kitchen and garage Sinks and Dish Washer. LP though.
Let me know if you want more info.
Scott<-

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depending on your water pressure, some places like ours need cold and hot turned on for sufficient flow at the upstairs showerhead. at our home we only have 42 psi from the city water coming thru a new one inch main. you may want that hot water at 140 degrees for you and the dishwasher. this means a rise from cold city water 105 degrees at whatever gallons per minute you have. whether any tankless unit can achieve that depends on the entire home system for gas, water pressure psi, water flow gpm, personal preferences, and future owner needs. i can't imagine we'd enjoy waiting for the tub to fill with barely hot water more slowly without the cold flow that speeds up the process. we use a regular gas 75 gallon water heater.
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tankless have poor reputations and in other countries are low cost option. you might look into.
http://www.americanwaterheater.com/WHBrowser/gas/Polaris50.cfm
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Even in the most Northern of the States, inlet water temperature below 50 degrees Fahrenheit is very unusual (I'm in MI, btw). Expected faucet temperature is generally in the 120 degree range (hotter can scald), giving a max expected rise of 70 degrees. I've never heard of anywhere where the inlet water is essentially ice at 35 degrees. I suppose if you were taking water out of a shallow pond this could be the case, but water pumped from underground will never get this cold.
For Lefty, I'd suggest you are on the right track with your questions, and the only advice I would give you is to do a complete assessment of your simultaneous volumetric needs. For example, while you mention a soaking tub (which either of the heaters you mention will handle just fine on its own!), you should assess whether a simultaneous dishwasher, clothes washer and shower situation might ever occur. Maximum draw is not based on a single application. If you are convinced that your maximum need will be 5gpm or less then the Rinnai will suit you fine. If you need more, then you can hook up 2 or more in parallel (NOT series) giving higher heating capacity (we're starting w/ 1 and planning to add a 2nd as our children get older).
As for the 2532 vs. the 2520, I'd ask the rep some questions - the volume flow rates in their online brochures simply don't make sense (for example, GPH should be GPM * 60, but isn't; 2 tanks should be 2x volume flow from 1 tank but isn't.
d-
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I guess you have never heard of house incomming supplys freezing needing a big welder to thaw them out at -20f. Well im south of you and many "summer" homes here on the lake or anywhere can have substandard hookups. My case is my hill is eroding where my main is embedded and Ive measured 33f with an acurate thermometer on a -18f morning. 50f is only way way below frost line, sure the City pumps 50f water but unless you test it yourself on your coldest day you might be out of showering next winter on a major freeze spell. Now thats kinda dumb to not figure in a 15f more drop, isnt it.
A big 190000 btu Rinnai or Takagi will handle a house, my 117000btu little Bosch gives me a hot shower at 34f incomming without it being even set to its highest.
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Yup, good point - designed poorly enough any water inlet can freeze.
Given, however, that the original poster indicates he believes only a 70 degree rise will be necessary, I think we can safely assume that his house was built correctly and the city water pipe is below the frost line. Otherwise the city would be out there with a giant blowtorch as well....
d-
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The true point is people that are unhappy with tankless just sized or installed them wrong.
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lefty wrote:

they are maintenance free. The main consideration is line sizing for the gas lines. They fire at about 3 times the rate of a tank type unit. Hence the need for proper line sizing.
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There are cheap options, many used overseas, 100$ units. These are not the 600-2000 dollar Bosch- Rinnai- Takagi , units. Its like comparing cars, I nor would you buy a yugo.
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