Heat loss calc..

DIY_Pro

Sizing my heat load for a new boiler.. looking at a Rinnai 90k unit

My heat loss using Fuel bill/design days is 36,000btu/hr for my 2000sqft house in NJ..

Manual J using loadcalc.net was about 42,000…

Really? These numbers seems low to me…is this correct…my old failing boiler is like 120k btu.!

Cyclist77

I am currently in the process of going from a coal stoker boiler to NG boiler on our radiant heating system. I did one heat loss calculation that came out showing I need a much smaller boiler. The local supply house is doing another one . Looking forward to seeing the comparison!

pecmsg

https://forum.heatinghelp.com/discussion/200551/heat-loss-calc

Not supprised at all.

jesmed1

Trust your fuel bill. It is correct. That's a design heat loss of 18 BTU/hr/sq ft, which is easily achievable in your zone with average amount of insulation and reasonably tight envelope.

Hot_water_fan

unsurprising !

Kaos

Typical residential boiler is around 3x oversized, residential furnace 2x.

Fuel use is usually the most accurate method, the only time it will be off is if you have a lot of glazing and solar gain, like an MCM house with a wall of glass.

EdTheHeaterMan

@DIY_Pro, Do you still have the old 120,000 BTU boiler operating? If yes, how long is a burner cycle when the temperature is near zero? (0°F). The reason I ask is that if there is a call for heat for say about an hour and the circulator operates the full hour but the boiler cycles on for 3 minutes and off for 6 minutes, then the entire hour, then your 120K boiler is only using 40,000 BTU in any given hour. That is how you know you are way oversized.

Next thing is that the input of a boiler is how you size the fuel line The NET output is how you size the heat loss. A boiler with a 65,000 BTU input has a 56,000 Output and a 48,000 NET is the correct boiler for you. if you were to put a 42,000 BTU Input boiler, then expect to have problems heating your home when it is below 30°F. That 42,000 input boiler might only have a NET of 31,000 BTU, and that is not enough for your home that requires 42,000.

hot_rod

a 50K mod con should give you 43,500 btu per hour at 87%. Higher efficiency if it can run in condensing mode some or all of the time.

The fixed output boiler could drop into the 70% range if it shorts cycles as @EdTheHeaterMan described.

Do you have fin tube baseboard? How many feet of finned tube?

If the home heats comfortably now, then the boiler doesn’t need to be sized any larger than the heaters attached to it.

DIY_Pro

Ok, the team has spoken! Thank you everyone for your input!

I'm a long time reader of the forum..

@EdTheHeaterMan , yes the old 1970's Burnham America boiler is cycling on/off in short 3 minute-ish cycles as you describe, while the 20yr old Bell&Gossett pump runs a bit longer at maybe 30 min? Your numbers make sense, thanks.

@hot_rod, I currently have 2000sqft + 95' of finned tube over 2 zones (65' 1st floor, 30' 2nd floor). House is comfortable when the heat is running.

I'm planning on combining both zones into a single 95' split zone to get me into condensing temps with a new mod con combi, and avoid a small second floor zone that may cycle.

The Rinnai mod con Combi I'm considering only comes in 60k or 90k for CH, both with199k DHW, both turn down to 15k for CH.

Favoring the 90k unit as I will be adding 1000sqft (+45' finned tube) +1 bath addition next year (a final total of 3000sqft + 135' finned + 2.5bath).

LRCCBJ

95 feet of fin tube ELEMENTS (not the shell) can deliver no more than 47,500 BTUH. So any boiler that can deliver MORE than 47,500 BTUH is too large for your house.

If you actually measure the elements (inside the shell) the length will probably be about 85 feet………………resulting in a maximum delivery of 42,500 BTUH. Figure 75% efficiency and the boiler you require is 56,000 BTUH. NOT MORE.

Buy more………pay for more for the entire life of the boiler!!!😫

Ironman

I’d highly recommend that you go with a boiler + an indirect water heater. It’s a much more reliable setup with less maintenance and a longer life expectancy.

7” fin tube BB has a heat output of 500 btus per linear foot at 170* AVERAGE water temperature. So, your BBs can produce 42,500 btus - right where the load calc put you. Load calcs typically have about a 10-15% fudge factor built in to cover extreme temperature and high wind. Realistically, you’re not gonna need more than 60k net output even with the proposed addition.

A mod/con Boiler will give priority to an indirect water heater which means that there’s usually no need to upsize the boiler beyond the space heating load to include the domestic. So, 60k btus is your realistic load.

hot_rod

so 135’ of fin tube X 500= 67,5000,

so an 80 or 90,000 btu/ hr mod con covers you nicely. It would modulate down to 8,000 btu/ hr for a zoned system to prevent cycling.

Paired with an indirect you have plenty of capacity for DHW. Go with an 80 gallon if you have a high DHW load, or a large tub to fill.

Consider a mod con that has all the control features to maximize the system efficiency.