You know Dana I think that what bothers me most are those folks that think everyone in the trade is out there to rip people off.
Words to live by:
Do not ascribe malice to that which could be explained by incompetence.
I'm not in the HVAC trade, but have been on both ends of the contracting stick. The average competence level I see out there is a bit wanting- it's at least 5 hacks per semi-competent HVAC contractor (if not worse). I very rarely hack away on heating systems, but when in the market I'll specify the design parameters fairly well, and take responsibility for the design if for some reason it doesn't work out. (Hasn't happened yet, not that it can't.
)
It's not hard for the DIYer or homeowner to do some napkin-math sanity checking. It's amazing how many boiler-swappers skip even the most rudimentary math when picking a boiler. (Take the contractor who did the original system here, for instance!) Avoiding oversizing to the point of guaranteed short-cycling does not take rocket science level math or even 8th grade algebra. A 5th grader with ADHD could get there with some coaching.
Even among those who measure-up the radiation rarely analyze the total heat load to see if the boiler could be even further downsized for higher efficiency & comfort, but time is money, and if the client doesn't understand or care, it's not necessarily time well spent for the contractor bidding the project, especially if the competing bidders pick larger boilers. To the typical client bigger sounds better, right? "I don't want to be COLD!" But explaining the nuances to the client takes even more time. It's easy to see how some contractors will burn out, and won't bother putting in even another 15 minutes if it doesn't increase their odds of getting the contract.
I'm sure there are rip-off artists out there, but it's the exception more than the rule. But it's more than just a plumbing job, and taking a handyman wrench-twister approach to boiler or system upgrades isn't likely to yield optimal results. Like a lot of HVAC hacks it looks like driger just picked a boiler in the same approximate size range but a hair bigger since "...the old boiler barely kept the house warm".
I'm not sure what the math was behind, "according to my calculation it's 20% oversized. if go with one a size smaller it's 20% undersized." If it was a heat load calc indicating a design load of 70,000 BTU/hr or something the calculation has to be off by a mile, since the radiation can only deliver about 2/3 that. The system may be under-radiated for the load for the house in it's present condition, but not by fully a third or it would be downright freezing during cold snaps rather than barely-warm.
I hope he hasn't already un-packed the boiler, and can take the advice to return it for a smaller one. It's cheaper & more effective to bump up the radiation in rooms that run cold and/or fix the low-hanging fruit on the building's thermal performance than replace an old oversized boiler with a newer oversized boiler that will deliver exactly the same amount of heat as the old one, no matter what the burner size or nameplate efficiency. Just about anybody's 3-plate cast iron gas burner would cover the actual load if the 90' of fin tube was
sort-of keeping up.