Calculating Heat Loss


In order to determine the correct size needed for a heating system in any home, calculating a proper heat loss is required. However, I often wonder what type of heat loss method was used when I see a four-section boiler in the basement of a 1,200 square foot home. Don’t laugh, it happens!

I wonder if we have reverted to the five-finger or boiler tag methods. For those of you who may be new to the business and are wondering what the heck these methods are, let me explain.

First, the five finger method is when a sales person or contractor stands in front of a home and holds up a hand in front of the home. If three fingers cover the front of the home, a three-section boiler would work fine. If it takes more than three, then you increase the size accordingly to the number of fingers.

The tag method is much easier, you just take the Btu rating or the number of square feet of steam shown on the existing boiler rating tag and that’s the size that is used. However, in both cases this is an injustice to the homeowner, who could be paying upwards of $3.89 per gallon of oil and thinks that a new heating system is the answer for cutting fuel cost.

I have found that many cities and towns are now requiring that an actual heat loss is presented to the inspector before a permit will be issued for the job. I’m surprised to find out how few contractors or sales people know how to do a heat loss correctly.

When I ask about the Manual J method, I’m often asked where they can buy the manual or can I do the heat loss for them. Of course, I’m always willing to help fellow trades-men, but if I do the work, how does that really help? I have found that I’m better off by pointing them in the right direction of doing their own heat loss.

The Manual J is the first method that I was introduced to and it still works well today. However, it is time consuming and I know many contractors don’t want to take the time to use this method.

The good news is, like many of the tasks that we do today on our computers, electronic heat loss programs are available and are very easy. There are boiler manufactures that offer heat loss CDs at no cost, so take the time to ask for one and you’ll be amazed at how user friendly they are.

However, before you can do a heat loss on a home or building, you must gather some important information from the owner. This includes what upgrades have been done to the property since the original heating system was installed. For instance, have they added insulation, new windows, siding or possibly planning for an addition? All of these questions are necessary in order to properly size a system.

Something we have all noticed when it comes to oil boiler sizing, on many occasions the smallest unit is often too big to match the required smaller heat load.

I often wonder why are we still using factors like 70 degrees for an inside temperature and a 0 degree outside temperature for sizing a boiler? With oil hovering in the $4.00 range, how many homeowners still have their thermostat set at 70, and how many days do we actually experience 0 degree outside temperatures? I would bet that we have more 25-30 degree days and more homeowners are setting their T stat at 68 degrees or less. In my opinion, using the 70/0 factors can often lead to over-sizing heating equipment.  


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