Not to be oversize or under size on new boiler

Bio

Gentlemen, once I know my Net IBR at what percentage average would I consider being over sized or undersized

Rod

Boiler Sizing

Hi -  With the caveat that I’m a homeowner and therefore have rather a very limited experience with sizing different types of boilers, my answer for your question would be “It depends”.  The first consideration is you want to match as closely as possible the total EDR of your installed radiators. (Use the boiler's Net IBR rating as this has the 1.33 "pickup factor" already included)  You then have to factor in any adjustments.  ie: has the insulation or windows been greatly improved which would mean you could probably go with a boiler capacity on the lower side of the total EDR. If you were going to do an addition to the house in the near future, you might want to go with a boiler with the capacity on the higher side of the total EDR.



     You might want to read the following article by Dave Bunnell “Boiler Pro”, a Chicago steam pro   http://www.heatinghelp.com/article/323/Boilers/1551/Taking-Another-Look-at-Steam-Boiler-Sizing-Methods-by-Dave-Boilerpro-Bunnell



The method I used when looking for a replacement boiler was to take a look at the water capacity of the different boilers in a model line.  If the water capacity is the same,  it indicates that number of block sections are the same and the difference is the size of the burner.     I attached the Peerless ECT table below as an illustration.   As you can see the different size Peerless have wide range of output/ fuel usage while using the same number of block sections (water capacity)   If your total EDR figure is in the middle of this range you can then easily “up fire” or “down fire” the burner as necessary. (Peerless has already down the calculations)

 Be sure to make sure the burner model is the same make / type through the whole block section you are choosing.

 Other boiler manufacturers model lines are similar to Peerless. The information you need to make comparisons in usually located in the installation manual.  Look for the water capacity first.The Peerless ETC has an oil burner so therefore making a comparison varies a bit with gas burners.

If you favor a particular model size and you can’t determine the "downfiring" capability from the manual, contact the manufacturer/distributor for more information. Most burners can generally be downfired a bit.  What you want to avoid it the amateur's first instinct which is to oversize the boiler. Keep in mind that  the IBR rating has a 1.33 "pickup" figure already factored in.

- Rod