All areas of North America have a given outside design temperature. The reason for this is that 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 cheat 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 will also shorten the life of the mechanicals, such as the motors and controls, and will require more maintenance.

With this known, imagine an oversized heating appliance. 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 oversized by 100% or more. When it is running, the fuel input will be 120,000 instead of 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 becomes more pronounced than if the unit were properly sized. Every hour of running time, the unit will burn 50,000 BTUs 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 one 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 because not all thermostats will call for heat at the same time, and they will not all be satisfied at the 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 why the boiler should not continue to run the entire time the thermostat is demanding heat.

The system efficiency is as important as, or more important than, the appliance efficiency. I do not want to downplay the boiler or furnace efficiency, but I believe the distribution system, near boiler piping, and duct sizing are crucial to the total efficiency package. I have worked on jobs with properly sized 84% efficient boilers, and the fuel bills were still outrageous. We kept the same boiler and re-piped the near boiler piping. The fuel savings, comparing BTUs per degree day, were 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 BTUs 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 the systems to install and set up the appliance. This in no way discounts replacing the appliance with a newer, more efficient one.

A new appliance properly sized and properly near boiler piping should save between 20% and 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 that 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 is used.

With all that said, you should first upgrade the home with new doors, windows, increase insulation, re-caulk, and 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, and installations should be reviewed on a per-job basis. Any work performed on your heating system should be performed by qualified and experienced personnel only. Comfort-Calc or its personnel accepts no responsibility for improper information, application, damage to property, or bodily injury from applied information found on this website.