New condensing boiler sizing

dough041

Please bear with me as its been suggested all the ways to size a new boiler through heat loss etc. but I have a hard time following and am hoping some people on this excellent forum can provide me with an approximation for a new sized boiler for Northeastern Pa winters.
An old school hvac guy recommended a rinnai i120sn heat only. My current and dated boiler is a Utica DV 125A 3 zoned via taco relay/3 circulation pumps w/ tekmar outdoor reset that has served me very well but is 24 yrs old and Im preparing to replace it during the off season this summer.
My home is 24 yrs old and a ranch 52x26 ft. With 3 zones of slant fin type baseboard. The main living area is 2 zones with approx 33 ft of baseboard in each zone. The remaining zone is my finished basement with 24 ft .
Its 2x6 insulated outside walls . 2 entry doors approx 36x72”. 7 double pane double hung windows about 40x56” and 2 smaller 28x32 windows.
I tried several times to figure out on my own and I seem to come up with the rinnai i90sn as sufficient where I was told the 120 was approximately same output as my utica and that would work better?
Just wanted some opinions of persons on this forum as I value what is exchanged and discussed here. Thanks for any help and or advice.

Hot_water_fan

Both are huge actually. Your baseboard can only emit 90 linear feet x 540 Btu/lnft (that’s at 170 average water temperature) = 48.6kbtu/hr IF your baseboard is perfectly sized. If it’s oversized, your heat loss is even less. The 60 is probably the best size. 

That said: for the Rinnai product line, it doesn’t matter. They can turn down to 15,000btu/h for the 60,90,and 120 sizes, so over sizing doesn’t hurt you with this brand. For other brands, you can get better turndown ratios: for example, you could get a 80kbtu boiler that turns down all the way to 8kbtu. 

I don’t think it’s worth doing any more calculations, especially if your contractor isn’t on board going smaller. However, if you feel inclined- this is another, quick way https://www.greenbuildingadvisor.com/article/replacing-a-furnace-or-boiler. 

GroundUp

90 feet of baseboard can only put out approximately 54,000 BTU with 180 degree water. What you have is roughly double the size it needs to be, unless for some reason you're using a water temp over 180 degrees or the thermostat doesn't meet setpoint on the coldest day of the year. A 120 won't heat any better than a 90, or possibly even a 60 depending on the actual water temp. The closer the boiler is sized to the actual design heat loss, the more efficient it's going to be IMO.

dough041

I really appreciate the feedback. Always worked on most things on my home, and my thoughts were where you guys were leading. Now Im closer to leaning to the 90 or maybe 60 as you guys suggest. My friend is definitely old school and I believe he mentioned to replace what was there with something similar but am so glad you guys felt similar to what my researching has lead me to think. ( of course its good to hear from people that know heating so well - I dont ). I can do the install of the piping etc but not the fine tuning.

Rich_49

Your home if terribly built quite probably has no more than a 40,560 BTUh ( 30 BTU sf )load @ design . It is really very possible that the load is more like 27,040 BTUh ( 20 BTU sf ) . I would suggest contacting several qualified contractors and getting their opinion and pricing .

What type gas is available where you are ? What town in NE Pa ? Really seems to me that this house could use a 50K mod con utilizing really low temps and eliminate some of the complexities on the existing system .

What do others think ?

dough041

Appreciate your time and advice Rich..Thanks.

Nom_Deplume

Any contractor who proposes boiler size based on existing boiler size, and even more, is incompetent and should not be hired. He/she is just ****-covering while trying to do as little as possible.

Apart from the max baseboard output criterion discussed above, another one is as follows. (You have to know what degree-days are, and also the design temperature for your area.)

Look at your gas bill for a winter month, and look up on the web the degree-days for the month. Calculate the BTU consumed per degree day. Multiply it by the degree-days in a design day (coldest likely day). This tells you the BTU/day needed, divide by 24 for BTU/hr needed. Add 10% as a cushion, not more.

You'll probably get the same answer as above, that your boiler is way oversized. Sure some boilers have a large turndown ratio, but why waste that capability by oversizing? A right-sized boiler with a large turndown ratio will run like a champ if you set it up with variable water temperature (outdoor reset).

Hot_water_fan

@Nom_Deplume  agreed, but mod cons come in limited sizes, most of which are oversized with similar minimum fires. There’s only so much optimizing to be done: 8kbtu is very similar to 10kbtu or even 15kbtu. 

dough041

Appreciate all the input. My take of all the replies is with my limited footage of slantfin baseboard is the sizing if I'm considering a Rinnai I series heat only would fall with either an i90sn or an i60sn?

I have a rinnai tankless water heater which is good ( like the rinnai brand?)

My local supplyhouse insists the i120sn can modulate down so that's their recommendation?

Hot_water_fan

@dough041 they're trying to upcharge you or maybe that's all they have in stock. Regardless, the Rinnai's all have the same minimum output so it doesn't matter.