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Here are three different rating tables from three different boilers

You can clearly see before there was a Department of Energy in the 1950s boiler rating were dome by the American Gas Association (AGA) and the Institute for Boiler Ratings (I=B=R)  and all the boiler manufacturers used these ratings to rate their boilers.  The Weil McLain model D boiler that has an I=B=R Gross output of 90,000 was about 80% lower than the Input of the boiler at 112,000  

When the 1970s oil boiler was rated they used GPH for the I=B=R input and Gross output was changed from I=B=R to DOE output. That would make that oil burner about 84% efficient since one gallon of fuel oil is equal to about 140,000 BTU of energy.  At this point there are no yellow AFUE stickers yet but the Department of Energy has been formed to keep the cost of oil from getting out of hand.

In the current gas boiler you can see that the numbers are still the same but they all are under the direction of the DOE.  

• Input
• DOE heating capacity or output, and
• DOE NET

Still the same numbers as in the 1950s but the names have been changed to protect the innocent. So look at the NET number when the boiler says 70,000 it is really going to do a good job all the way up to 56,000 BTU heat loss on a design temperature day.  I believe you will be just fine with the GV90+3

EK is considered to be top of the line as long as you have a good installer. EKs technical support is among the best.

EK is on the forum and willprobably chime in.

There are 3 separate BTU numbers on each boiler. Prior to 1968 there were only 2 but when the government got involved with the department of energy they needed to have an equal footing to measure furnaces and boilers for those yellow Energy Guide stickers on each heating appliance to make it easy to compare the different systems. But I digress

Back to the three different BTU numbers.

1. Gross Input aka Input aka GPH on oil fired equipment = How much energy goes in
2. Output or DOE output is how much energy gets put into the home (maybe). Or better described as what is left over after what is lost out the vent pipe.
3. NET output aka I=B=R NET and now known as DOE NET. This is the number that you are supposed to use when you know the actual heat loss of the building. It is actually a 15% lower number that takes into piping and pickup consideration on a water boiler. (it is 30% on steam boilers)

So when you know that you need 46,000 BTU per hour on a design day (that is what we consider the average coldest day of the year for an area) Then you use the NET number which is much smaller that the Input that the boiler usually is sized by.