This weekend I got an email from my nephew who lives in New Hampshire. He recently moved into a home and they finished the basement and attic. Now, he had a contractor come in and he was telling him that he needed an additional 50,000 BTU larger boiler to heat the house. Since he knew I retired from heating and air conditioning, he wanted my opinion. My first question to him was, ” How did the contractor determine that he needed an additional 50,000 BTU’s to heat the new areas?” Then I asked, ” Did he do a load calculation? Was he basing it on how many feet of new baseboard radiators he was installing? What water temperature is the existing boiler operating at?” The existing boiler may be large enough to handle additional baseboard radiators. I told him that basements usually are not included in load calculations so that could reduce the sizing the contractor was suggesting. Depending on the water temperature the boiler was operating at, he may be able to go from say, 160 degree water to 180 degree water to handle the additional load. My nephew said that the contractor just said he needed a new LARGER boiler.
My nephew’s boiler is gas-fired hot water. I told him that possibly replacing the existing, inefficient boiler with energy smart new one might be the way to go but there are a few things he needed to consider. That brings us to the subject of this post — Heating Efficiency and Proper Sizing.
Have you ever noticed that the old standing pilot and spark ignition furnaces / boilers appeared to be rated at 80% efficiency? You look at the data plate and it reads 100,000 Btu input and 80,000 Btu output or bonnet capacity. This is 80% efficient, RIGHT? Well, yes and no! In the past, furnaces / boilers were rated by STEADY STATE EFFICIENCY which meant that if the furnace ran 24 hours a day the entire heating season, it would be 80% efficient. This rating never took into consideration normal cycling or milder outdoor temperatures. So, based on this type of rating, it WAS 80% efficient.
Today’s furnaces / boilers, however, are rated by A.F.U.E.’s. This is a measure of the annual fuel utilization of the furnace. So to put this into perspective a 60% furnace is 60% efficient because 40% of the heat it creates goes up the flue. An 80% sends 20% up the flue. An 90% sends 10% up the flue and so on. It is a truer measure of the efficiency of the product.
Why am I telling you this? Well, a lot of dealers still use the rule of thumb and size a replacement furnace / boilers based on the input of the existing equipment and then get themselves into trouble. They say, ” the old equipment was 100,000 Btu’s input so I replaced it with a 100,000 Btu furnace.” The problem here is now the furnace is grossly oversized. Then there are dealers that base the new furnace by using the output of the existing furnace. It still would be oversized (just not as bad) because, in reality, the older furnaces, based on A.F.U.E. were really only 55% to 60% efficient. To make matters worse, if a 92 – 98% efficient furnace or boiler is used to replace the old furnace or boiler and it is sized based on the input or out put, now we really have a over-sizing problem.
An over-sized heating unit will heat the space but the end-user will be sacrificing both comfort and efficiency. With a furnace, it will “blast” a lot of warm air into the space, over heating it, and then is off for an extended period of time. Because of this, there are larger temperature swings in the space and the occupants feel warm and then cold and then warm and so on. Boilers are more forgiving because they heat water that is circulated through baseboard/radiators. The problem here is it just wastes energy to give the heat to an area.
Now, what I told my nephew was that the best way to have properly sized heating equipment was by having the contractor use a load calculation program such as Manual J or Right J. I told him if he was going to make a major investment in new equipment, he really needed to have it sized right the first time.
So back to the subject of this post, with homeowners installing newer energy-efficient windows, insulating attics and walls, and weather proofing their homes, the sizing of the old furnace / boiler is even more out of line and to replace it based on the old rating or “rules of thumb” or guesstimates, is even more absurd. It is a disservice to the customer because they are not getting the true fuel efficiency and comfort they think they are paying for.
So keep this in mind when you are quoting or replacing an existing heating unit. By taking the time to do a proper sizing, you are giving the customer what they are paying for, you are eliminating possible warranty calls due to erratic cycling, and you are providing the proper comfort that the furnace was designed to give. It can also give you the edge in getting the job by showing the customer that you are a “professional” and are truly concerned with their needs and comfort.