Heat Loss calculation

Benefits of Manual "J" Heat Loss & What Not to Do

A Manual-J heat loss or Manual-J heat gain is going to save you more money on your heating bill then just a rule of thumb furnace replacement. Are you planning to replace your home heating appliance to save fuel? Are you choosing the most efficient warm air furnace or hot water boiler to maximize your fuel savings? Are you aware that just replacing the appliance will not maximize your fuel savings? There is more to it than just sizing off the old unit or measuring the radiation or duct size. Do you realize that most equipment installed today is 100% over sized? This practice is a waste of fuel and increases maintenance.

Below is the top three ways of improperly sizing the appliance. When the heating or cooling unit is over sized it costs more to install, short cycles which lowers the efficiency and increases the maintenance costs. All the below sizing methods will size the heating or cooling appliance incorrectly. The proper way is a heat loss or heat gain calculation.

  • Measure the existing radiation
  • Measure the square footage of floor space and multiply or divide by a magic number
  • Put in the same size as the one being removed

The above does not work to your benefit!!!! The rules of thumb listed above will always over sizes the equipment. If your equipment is not sized by performing a Manual "J" heat loss your contractor is doing you a dis-service. The need for a heat loss today is critical with the fuel costs as high as they are and using higher efficiency equipment.

Most consumers believe by replacing the heating appliance they will automatically save the largest amount of fuel. This could be no further from the truth. In most cases, just replacing the heating appliance without a heat loss, a fuel savings will be met, but you will never maximize your fuel savings unless the unit is properly sized and properly installed. Choosing the right size unit is more than an educated guess or a rule of thumb calculation!

All areas of North America have a given outside design temperature. The reason for this is the greater the temperature differences between the inside and outside of your home the greater the heat loss. As the outside temperature warms up the heat loss is reduced. When the equipment is sized with a heat loss, it will heat the home with the lowest fuel input. As the outside temperature warms up the unit will start to short cycle. Longer run cycles are more efficient than shorter run cycles. To sum it up, the unit will short cycle when outside temperatures are warmer than design temperatures. These shorter cycles also will shorten the life of the mechanicals such as the motors and controls and will require more maintenance.

With this known, imagine a heating appliance that is oversized. Let's assume we need 70,000 Btu's to heat your home at a design outdoor temperature of 10f, and the actual unit was improperly sized and is 120,000 Btu's. This unit is twice the required heat loss, and this is not common. Statistics say that 85% - 90% of all units being installed will be over sized by 100% or more. When it is running the fuel input will be 120,000 instead of the 70,000, which is the actual heat loss requirement. This is the start of your problems. At the outside design temperature of 10f, the unit will short cycle. As it continues to warm up outside the short cycling gets worse than if the unit were properly sized. Every hour of running time the unit will burn 50,000 btu's more than needed on the coldest days. The unit becomes more oversized as the day gets warmer; the short cycling gets worse. There goes your efficiency and wear on the motors and controls increases driving maintenance costs up. The fuel bill is higher than needed. Fuel is wasted every time the unit is running. Happy heating!

The proper sizing of the heating or air conditioning appliances will maximize the efficiency of the appliance. Let's assume we properly sized the hot water boiler and the old system had 1 thermostat (1 zone). We would get good boiler efficiency. We then decide to break the system into multiple zones (thermostats). The properly sized boiler now becomes oversized as all the thermostats will not all call for heat the exact same time and they will not all satisfy the exact same time. The cycling of the different zones will overlap causing the boiler to short cycle reducing the boiler efficiency. The system efficiency will improve but the boiler efficiency will go down. There are several ways to control the boiler cycle rate. Here is a short list of things that can be done to extend the cycle time and increase efficiency. 1) Buffer Tanks, 2) Outdoor Reset Control, 3) set point circulators, 4)modulating condensing boilers. After your new boiler is installed I would suggest run times of at least 7 - 10 minutes to peak your efficiency on cast iron boilers and 10 - 20 minutes on a high efficiency boiler. If the high efficiency boiler is sized and installed properly there should be no reason the boiler should not continue to opperate the entire time the thermostat is demanding heat.

The system efficiency is as, or more important than the appliance efficiency. I do not want to downplay the boiler or furnace efficiency but the distribution system and near boiler piping, duct sizing, I feel makes or breaks the total efficiency package. I have done jobs that had a properly sized 84% efficient boiler, and the fuel bills were still outrageous. We kept the same boiler and re-piped the near boiler piping. The fuel savings comparing btu's per degree day was 62% savings for the season. I had another job where the homeowner had an 88% efficient properly sized unit installed and saved a decent amount on his annual fuel bill. We increased the load by 22,000 btu's of baseboard in a newly installed basement bathroom to the system, installed a boiler bypass pipe and still reduced the fuel bill by 9%. Purchase the proper sized appliance and get a good, qualified contractor with a good understanding of systems to install and setup the appliance. This in no way discounts replacing the appliance with a newer more efficient one.

A new appliance properly sized and proper near boiler piping should save between 20% - 50%. As in warm air furnaces, boilers come in a multitude of efficiencies. Boilers and furnaces that run at efficiencies above 90% are called condensing, modulating/condensing, or mod/con units. Most units running in this range modulate the gas input, as needed which can create added fuel savings. Units running below 90% are noncondensing units and do not modulate the fuel input. Mod/cons will try to match the heat loss as the outside temperature changes. What this means is the gas input will change. As the day warms up the unit will input less gas and if the temperature gets colder the unit will increase the gas input. The efficiencies on today's equipment will range from about 80% to 97%+. The higher the efficiency the less fuel used.

With all that said you should first upgrade the home with new doors, windows, increase insulation, re-caulk, replace weather stripping if these areas have not yet been addressed. This will reduce the heating appliance size required which will lower your installation cost plus use less fuel and require less maintenance.

Disclaimer: The information found on this website is for informational purposes only. All preventive maintenance, service, installations should be reviewed on a per job situation. Any work performed on your heating system should be performed by qualified and experienced personnel only. Comfort-Calc or it's personnel accepts no responsibility for improper information, application, damage to property or bodily injury from applied information found on this website.