Combi boiler sizing

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Norco

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After failing to purchase a Navien as a consumer, I have began looking at Rinnai combi boiler since I can source them from multiple vendors.

I went ahead and measured my fin elements and contacted Rinnai for further guidance on their models.

3000sf split level home
Northern ohio
Natural gas
148' of finned elements mostly located on exterior walls.

4 sinks
2 bath
1 Dishwasher
1 washer

After giving the stated information to an engineer at Rinnai, he suggested a I090CN.


From looking at the unit's BTU, it appears to be half the size as the calculators I have used online. Since I am replacing my boiler due to being undersized (70,000 BTU input non high efficiency) and the unit running 50% of the day as per Nest and my own observations, I have become somewhat hesitant to purchase anything close to my current boiler.
 

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John Gayewski

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Boilers are not sized based on your finned tube. They are sized based on your heating needs on thr coldest day of the year. Despite other methods the proper way to size a boiler is based on a manual j hest loss calc for your home.

Slant fin has one you can download for free. I'm betting the person from the manufacturer whom sized this boiler is pretty close.
 

Fitter30

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Any condensing boiler/ dhw water heater is 96%efficient when the return water temp is 130° or less above that temp efficiency drops to 86%. What water temp are you running now? Boilers run more efficient the longer the run time than cycling on and off. Combi boilers are more efficient but they also require more yearly service like cleaning the dhw heat exchanger and other filters.
 
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