Jad is right, but, though calculating heat load from fuel history sounds good, it only takes into account one season for a house that was. There is no accounting for improvements in the envelope that are often contracted with the addition of a new boiler. A proper Manual 'J' heat load analysis uses the accepted mean taken over a longer time period and building improvements can be easily and accurately accounted for. We specialize in condensing boilers and use Wrightsoft HVAC software to calculate heating and cooling loads.
I am not a fan of the online load calculators as they are highly simplified (inaccurate) but it is quite obvious from our experience that many "professional" boiler companies are still using "rules of thumb" for sizing residential boilers. We wish they would use ANY heat load calculator. In fact, it has gotten so bad that we write are contracts for boilers "sized to the load" as we find that we are the only heating company that performs a heat load at all (some have their suppliers "knock out" a load to satisfy the Minneapolis/St. Paul building officials). This is most disappointing as the results are evident from double-size boiler described here and the double-size Buderus GB142 that I looked at last week. The practice of over-sizing boilers goes on through mass-ignorance.
Rules of thumb are a lazy man's tool. There are useful benchmarks, to which one might compare the results of a proper heat load, even if you use the marginal HDD method. But having performed hundreds of heat loads and often guessing within 5% the load and design water temperature for a given radiant or radiator system, I still do the math.
In the case at hand, the owner has changed windows and should improve insulation in the attic (I would have chosen the latter first) which, would naturally change the load (throw out the fuel/HDD method) and perhaps the size of the new boiler.
A few facts about modulating condensing boilers:
Some make more noise than others, but all are quieter than any power-vented water heater. If hung on an outside foundation wall, few will be heard by anyone, anywhere in the house. It is more likely that the over-sized pumps will be heard through the distribution piping. We install many brands of ModCon boilers, both wall-hung and floor mounted and choose them for size, low-fire capabilities, control, ease of installation and service. You should too.
The novice should not over-size a boiler for any reason he might make up. This goes for the professional (most only install a few retrofit boilers a year, many only do a few in a lifetime). When sizing a condensing boiler an experienced designer pays careful attention to the rated maximum AND minimum output of the boiler he wants to install. If the boiler is twice too big it may very well bump off the bottom (cycle on the lowest fired setting) giving the owner poor efficiency and service headaches over time. Think of it as highway vs city miles. Never buy a used Taxi.
All ModCons now have weather sensitive controls which, are properly set by an experienced designer, whom has measured the existing or new radiation, selecting the design water temperature according the the Manual 'J' mean for the area in which he is working.
Rule of Thumbers need not apply.
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