Need assistance on combi boiler size

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Norco

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I have been researching combi boilers for what seems like forever trying to come to an answer on what size combi boiler is right.

3000sf 2 level house with lots of large windows located in Northern Ohio. House has new windows and doors with added insulation. 1 kitchen and 2 bath.

I currently have an 80,000btu 80% unit that struggles to keep up in the winter and the propane bill proves it. My Nest report shows it's running 50% of the time in the winter $$$.

Using simple square footage calculations, I need a 150,000btu unit.

The quote I received listed a Navian 160kbtu unit.

On the Slant/Fin app, it recommends 150,000btu unit, but my requirement is ~112,000. See pic.

Everything I keep reading about ~150,000btu combi units and similar sized houses has replies stating that it's "WAAAY too big". I'm trying to do my due diligence by researching on the forums but what I read only leads to more confusion and make this decision that much harder.

I'm currently looking at the Navian NCB240/130H

Thanks in advance.
Screenshot_20220714-173402_SlantFin.jpg
Screenshot_20220714-175347_SlantFin.jpg
 

John Gayewski

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Fitter30

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What water temp in the coldest days? Any combi has to run 130° return water or less for 95%+ efficiency above 87%. DHW need to know max gpm flow, coldest incoming water temp and what temp for the hot water. 80k boiler for 3k sq ft seems small. Running 50% then boiler should have no problem heating the house what the water temp and type heat emitters. Counting the basement sq ft?
 
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Norco

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You took the time to correctly fill out All of the info for each room in the slant fin app? That can be a very tedious task are you sure?

I do heat loss calcs by hand and then check random rooms with that app and it's pretty accurate.

There is also a way to size it based on your usage that a member here likes to promote https://www.greenbuildingadvisor.com/article/sizing-a-modulating-condensing-boiler
I feel like I filled it out correctly. What terms I didn't know I googled to find out what it was asking for.
 

Norco

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What water temp in the coldest days? Any combi has to run 130° return water or less for 95%+ efficiency above 87%. DHW need to know max gpm flow, coldest incoming water temp and what temp for the hot water. 80k boiler for 3k sq ft seems small. Running 50% then boiler should have no problem heating the house what the water temp and type heat emitters. Counting the basement sq ft?
Are you asking what the incoming temp of the well water is on the coldest day? That's one thing I have never took measurements of.

I forget the exact hot water temp, but it's on "B" which is still too hot for us. I have it in "B" because of possibility of bacteria growing in lower settings...or so I read.

Our shower times are offset and there's only three of us. 2 showers, 3 sinks, 1 HE dishwasher (dont use-still handwash), 1 HE clothes washer =11.5gph. However our lifestyle would be a max of 6.5 gph.

I have the current boiler set to 190*.

The current boiler does a great job of heating the house, but it's the constant run time which leads to excessive fuel usage $$$.

3000sqft includes the entire house. 1500sf upstairs and 1500sf basement.
 

Fitter30

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If your running 190° for heating and why what type of heat emitters do you have fin tube? If you have fin tube pipe size and how many feet of fin upstairs and basement? According to your nest stat said boiler is only running at 50% time.
 

Norco

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If your running 190° for heating and why what type of heat emitters do you have fin tube? If you have fin tube pipe size and how many feet of fin upstairs and basement? According to your nest stat said boiler is only running at 50% time.
3/4in 2.5 finned elements.

148' of total elements.
 
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