Shopping for new mod/con, set me straight!

ochy38

Hi All,

I'm in upstate NY, 1960's vintage house, fin tube baseboards, original Crouse 150k btu boiler.

A tornado came thru and trees fell on my house this summer. Initially thought chimney was OK, but unfortunately learned the top of my flue was damaged. Had a mason come look and tell me it was clear, and OK to use temporarily until he could fix it next year (he just looked from the top) but I decided just before firing up the furnace to pull the exhaust in the basement. Turns out the flue tiles are collapsed, even at the basement level.

Instead of fixing the chimney, Ive decided to pursue a new mod/con boiler since mine is so old. Done tons of research, both here and elsewhere, and have learned a TON. Zeroing in on a heat only mod/con and pairing it with an indirect hot water tank.

I've done my own rough manual J and landed somewhere in the 80-85k BTU load for the house on the design day as is. Im currently tightening up insulation, windows, etc so that should drop a bit in time. I have a mix of high output and "regular" slantfin, a kickspace heater, radiation load w/ approx pipe losses:

Zone	110°f	180°f
Downstairs	13116 btu	45520 btu
Upstairs	7980 btu	26820 btu
Total	21096 btu	45520 btu

Total of all zones @ 180 = 72340 w/ pipe losses.

The manual J tracks as being slightly higher than radiation, we had a period of a week or so of sustained -5°F -ish temps a couple years ago and the house couldn't get above 63 degrees for a few days.

Unfortunately, I'm scrambling here to get a system designed and in. I've called about 9 companies to come and i've received callbacks/quotes from only 3. Only one of them cared to measure anything, he asked for my radiator lengths. Interestingly, he is the one with the smallest boiler proposal but it's still too big, from my research.

2 of the quotes want to install 150k systems, an NHI and a Navien, "thats the size we install in every house"

The guy that measured my baseboards sized a 120k Rinnai w/a 32 gallon Buderus tankless. All 3 are well established local companies.

Unfortunately, all three only turndown to 15k btus. Of the 3, I'd chose the Rinnai based on cost alone, its cheaper than the navien by HALF and the NHI by about 40%.

I called a couple more companies to come in the next week or so. I know one of them installs viesmanns, and the 85k btu viesmann will turn down to 8.5k, which looks good compared to my zones above, but with a 70k AHRI rating, is that teetering on too small with an indirect attached? Should I push for that? He'll probably also say "We only install 150's"

Am I making a grave mistake picking the Rinnai with its 15k min? I'm sort of in a tough situation here, being the beginning of heating season and needing a solution like yesterday.

I so appreciate any advice

mattmia2

How are your baseboards zoned? Is it one zone upstairs and one downstairs or are there multiple zone on each floor?

It wouldn't hurt to throw in a couple panel radiators to make up the difference between the heat loss and the output of the fintube if your calculations are correct.

Keep in mind that the mod con will be more like 80%-85% efficient at 180 swt so about 100k btu input is probably about the right size.

ochy38

https://forum.heatinghelp.com/discussion/comment/1868240#Comment_1868240

One zone upstairs and one downstairs.

Yea additional radiators isn't a bad idea. I could probably achieve by swapping a few more sections with high output. And it wouldn't be too urgent.

I hadn't considered the efficiency drop and it's affect on output. Thank you very much for bringing that to my attention!

mattmia2

With only 2 zones you should have enough load even with a 5:1 turndown ratio that a 100k btu/hr boiler won't short cycle.

ochy38

Even if I'm only calc'ing 8k on a zone @ 110° water?

I was under the impression that ideally you want the turndown lower than your lowest zone draw to prevent short cycling. Maybe that's getting too nitpicky though?

mattmia2

it would cycle but it wouldn't short cycle if you chose something with the right controls and it is unlikely you would want to run fin tube at 110f, the output would be almost nothing.

EBEBRATT-Ed

I would look at Lochinvar.

hot_rod

Lochinvar and probably others have adjustments to lessen cycling

Anti Cycling and Step Fire for example. Lock the firing rate to the design load, then use other functions if needed

ochy38

https://forum.heatinghelp.com/discussion/comment/1868314#Comment_1868314

Ahh. That makes sense. At first glance of the manual, looks like the Rinnai has some anti cycling measures, like min off time etc. Thanks for pointing it out.

Mad Dog_2

As far as quality, The Rinnai unitsa verg good..mad Dog