Lochinvar Noble: How big is too big?

RC_MI

We are replacing a 100 MBH (100,000 Btuh) cast iron boiler with a Boiler-Only Lochinvar Noble. It will be connected to the indirect water heater ("boilermate") that is already in place, providing both baseboard heating and DHW.

My question is: How big is too big?

My HVAC guy wants to install the 150 MBH Lochinvar Noble. When I asked him why he wanted to bump up to the 150 instead of the 110 MBH Lochinvar Noble, his response was, "Well, with the 10:1 turndown ratio it really doesn't matter, and I once installed one that was too small and I regretted it."

Well, I wish I could just accept that. This weekend, I went through the rigors of a heat loss calculation for our northern Michigan home and came up with 59,810 Btuh. Then I calculated how much heat our total footage of baseboard could deliver and came up with 73,200 Btuh. I realize that the DHW demand would be on top of that, but in the Boiler-Only Lochinvar Noble, when DHW is called for, it shuts down home heating: they wouldn't both be demanding heat at the same time. Moreover, you can only cram 48,000 Btuh through the 3/4" copper pipe that feeds the indirect tank and our DHW.

So, 150 MBH seems like overkill to me. I mostly don't want the boiler cycling on and off all day in the winter. Also, in the reading I've done, it seems that a frequent comment is that boilers are often way oversized. Additional cost as well.

Am I overthinking this? Should I push back more on this and demand the 110 or go with my HVAC guy's recommended 150 and quit worrying about it? Thanks!

GGross

Have you sought out a couple of quotes? maybe another contractor would take the time to properly size the equipment? For me personally the oversized new boiler isn't quite as big of an issue as the contractor not taking the time to properly size the equipment.

Jamie Hall

Quite agree with @GGross — but that doesn't excuse the error of trying to sell you a boiler half again as big as you could possibly use — which is just being lazy.

GroundUp

Lose that guy's number. While he is somewhat correct in that an oversized modulating unit is of lesser consequence than an oversized cast boiler, he's either lazy or uneducated- very possibly both. If your old boiler was 100k input, its output was 85k at most. If that <85k kept everything adequately heated, there is absolutely no reason to go larger. If your heat loss calc is correct, an 80k Noble is more than sufficient for both space heating and DHW. Even if they won't run simultaneously, they still have a given load on a design day so this is a situation where I might upsize to the 110k given your baseboard's potential output with added buffer for the indirect, but the 150k has absolutely no place here.

EBEBRATT-Ed

Agree with @GroundUp

Firs you don't add anything for the DHW load. If your baseboard can only output 73,200 then that is all the boiler you need especially with a heat loss of 60K the 80K boiler will be plenty

JUGHNE

I put a 100,000 CI 87% in my house. My wife looked at the physical size of the Weil Mclean and said that will never heat our house……Eventually went to an 80,000 Loch floor mount. Wish I would had went to the next size down but there were no floor mount in the smaller size. No easy wall for mounting the small guy.

And I have a 120 gallon indirect tank with tube in shell exchanger. The 80 still has short run times even in Northern Nebr.

hot_rod

I would go with a 110,000 also. Covers the fin tube output potential, and plenty for the DHW.

Read up on how the outdoor reset works. With that dialed in, maybe 80% of the heating season you will run 90% efficiency in condensing mode.

JohnNY

110,000 BTUs is the more-right boiler if your numbers are correct, but the contractor is right about load matching which is pretty much the point of modulating boilers. I'd back off on the name calling and assuming the guy is incompetent.

NB: I'm not the contractor :)

RC_MI

https://forum.heatinghelp.com/discussion/comment/1807536#Comment_1807536

Thanks. Good advice.