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Navien Combi Sizing 240/xxxH

Member Posts: 47
edited February 2023

-10x 300’ 1/2 joist trak under subfloor loops.
*I have used 395x 4’ uponor 1/2” pex joist trak plates*
I have supplemental heat if needed (2x 30,000 BTU NG fireplaces in these spaces)

-2x 300’ 1/2” per poored concrete loops

How many BTU’s would I require for the heating sustems maximum load (-4C to -6 lowest outside temp In winter)?

Navien makes multiple different sized combi boilers:

NCB 240/80H heat 80K btu
NCB 240/110H heat 1100K btu
NCB 240/130H heat 130K btu
NCB 240/150H heat 150K btu

How would I determine which Navien model would be adequate to supply enough heat while not over sizing it.

• Member Posts: 4,395
You need to figure out wall R value, windows, exposed walls ceiling height, etc 1st...
There are far better options than a navien.
• Member Posts: 812
Do a room by room heat loss calculation (or have one done) figure out what your btuh per sq. ft. needs to be for your preferred indoor temp. Also if a combi: your DHW needs. THEN size your boiler. Consider using: Viessmann, Lochinvar, Ideal, Rinnai, HTP, Bosch. Not Navien.
• Member Posts: 5
psb75 said:
Do a room by room heat loss calculation (or have one done) figure out what your btuh per sq. ft. needs to be for your preferred indoor temp. Also if a combi: your DHW needs. THEN size your boiler. Consider using: Viessmann, Lochinvar, Ideal, Rinnai, HTP, Bosch. Not Navien.
Why do you say "not navien?"
• Member Posts: 2,668
MatthewPK said:

Why do you say "not navien?"

how much time do you have ?
• Member Posts: 21,878
All boilers need to size to the largest load, period:)
With combis in residential U.S. applications the DHW load is often the larger, sometimes by quite a bit.

So start with your desired DHW, be sure to use a reasonable incoming water temperature. That can vary from 33 degrees in Minnesota to 80F in Phoenix.

The beauty of the mod cons is most all can turn down 10-1, and most can be locked down to better match the heat load. So 130K can usually turn down to 11,000 btu/hr.

Additionally the lower you fire them, the more efficient they run. Plug in your operating temperature for the heating mode to get an idea of the efficiency difference between 100% and 25% firing.
Bob "hot rod" Rohr
trainer for Caleffi NA
Living the hydronic dream
• Member Posts: 812
See: neilc response above, as well as others' historically on this forum.
I don't believe Navien has a 10:1 turndown ratio.
• Member Posts: 47
psb75 said:
See: neilc response above, as well as others' historically on this forum. I don't believe Navien has a 10:1 turndown ratio.
Navien’s claim 11:1 TDR.
• Member Posts: 47
edited February 2023
neilc said:
Why do you say "not navien?"
how much time do you have ? https://www.youtube.com/@navienleaks/videos
What are more reliable combi brands in your opinion?  I located am in Canada it did not appear that Bosch was available here when I search. Please include their heating BTU output and TDR if known.

Navien is pretty common their maybe reasons for this? Is it cheaper?

• Member Posts: 47
psb75 said:
Do a room by room heat loss calculation (or have one done) figure out what your btuh per sq. ft. needs to be for your preferred indoor temp. Also if a combi: your DHW needs. THEN size your boiler. Consider using: Viessmann, Lochinvar, Ideal, Rinnai, HTP, Bosch. Not Navien.
Where can I find a good online heat loss calculator that I can input my information?
• Member Posts: 1,799
edited February 2023
Where can I find a good online heat loss calculator that I can input my information?
There’s the rub! They all “work” but garbage in garbage out. They’re usually significantly high. It won’t matter for a combi unless you’re heating a large, leaky house so your DHW load should be much higher.
• Member Posts: 21,878
cloudd said:

psb75 said:

Do a room by room heat loss calculation (or have one done) figure out what your btuh per sq. ft. needs to be for your preferred indoor temp. Also if a combi: your DHW needs. THEN size your boiler. Consider using: Viessmann, Lochinvar, Ideal, Rinnai, HTP, Bosch. Not Navien.

Where can I find a good online heat loss calculator that I can input my information?

There are some free online calculators, some pretty basic, some more detailed. I think LoopCAD has a free trial version online. Watch the demo first to learn the inputs.

\PikesPeak building department has a manual J work sheet for hand calcs on their site.

https://www.calculator.net/btu-calculator.html
Bob "hot rod" Rohr
trainer for Caleffi NA
Living the hydronic dream
• Member Posts: 4,395
https://www.ibcboiler.com/consumer/products/sfc-superflow-1/

The only combi I install. Over 100 installed. Great units.
• Member Posts: 47
edited February 2023
Where can I find a good online heat loss calculator that I can input my information?
There’s the rub! They all “work” but garbage in garbage out. They’re usually significantly high. It won’t matter for a combi unless you’re heating a large, leaky house so your DHW load should be much higher.
My DHW GPM is maxed 3 showers and 2 dishwashers. So I will need 199K btu for DHW for sure. Not like the dishwashers will be on at the same time as the showers anyways but I do need adequate DHW for the 3 showers.

I just looked at Bosch Singular Combi’s: 5200(199K btu DHW / 140K btu heat) & 4000 (155K btu DHW /80K btu heat). They also have 10:1 TDR.

The Bosch Greenstar models are all out because the best model the Greenstar 151 maxes out at 150K btu for DHW and can’t provide enough DHW GPM for my requirements.

It seems if I go with Bosch it has to be the Singular 5200(199K btu DHW / 140K btu for heat) .

*My house isn’t leaky. I just renovated it. New insulation in attic, new windows, new doors. Sealed all of the lights. Plugs seem to have no noticeable leaks.*

Did I miss anything?
• Member Posts: 1,799
The other option would be not installing a combi - you could install a tank, either connected to the central heating or separate. If you have the floor space, it’s a good option.
• Member Posts: 3,248
edited February 2023
Hi, To add the what @Hot_water_fan said, think about lower flow showerheads. If you had three 1.5 gpm heads and used 70% of that as hot, you would need 3.15 gpm of hot water. What equipment can do that?

Yours, Larry
• Member Posts: 21,878
It’s all in the numbers. Shower head flow rates are established at the code maximum 80psi, so if you run 50-60 psi, adjust accordingly. I bought nice 1.5 heads and run 60 psi, plenty of water for us

So if you want 3showers at once crunch the numbers accordingly.

Of course the sky is the limit, the homebuilders show was heavy on multiple head body spray shower displays, 5 gpm and up systems. And they were getting
lots of attention
Bob "hot rod" Rohr
trainer for Caleffi NA
Living the hydronic dream
• Member Posts: 812
I think there can be such a thing as a "hot water fetish." No one has YET mentioned a "rain" shower head coupled with the "body/booty blasters". That's about 5 gpm right there. Home spa? Don't forget the "steam shower." What would the Deadmen think?
• Member Posts: 3,248
Some of the Dead Men might have liked it

Yours, Larry