Below is the top three ways of improperly sizing the appliance. When the heating or cooling unit is over sized it cost's more to install, short cycles which lowers the efficiency and increases the maintenance costs. They will all size the heating or cooling appliance incorrectly. The proper way is a heat loss or heat gain calculation.
1. Measure the existing radiation
2. Measure the square footage of floor space and multiply or divide by a magic number
3. Put in the same size as the one being removed
The above does not work!!!! The rules of thumb listed above always oversizes the equipment. The need for a heatloss today is critical with the fuel costs as high as they are.
Most consumers believe by replacing the heating appliance they will automatically save the maximum amount of fuel. This could be no further from the truth. In most cases, just replacing the heating appliance without a heatloss, a fuel savings will be encountered, but you will never maximize your fuel savings unless the unit is properly sized and properly installed. Choosing the right unit is more than an educated guess!
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. To sum it up, the unit will short cycle when outside temperatures are warmer than design temperatures. The shorter the cycles the less efficient the operation becomes. These shorter cycles also will shorten the life of the mechanicals such as the motors and controls.
With this known, imagine a heating appliance that is over-sized. Let's assume we need 70,000 Btu's to heat your home at a design outdoor temperature of 10º, and the actual unit was improperly sized and is 120,000 Btu's. This unit is almost twice the required heat loss and this is not uncommon. 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 10º, the unit will short cycle. As it continues to warm up outside the short cycling gets considerably worse then if the unit were properly sized. Every hour of running time the unit will burn 50,000 btu's more than required. 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 appliance 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 fairly 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. There are a number of 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, buffer tanks, Beckett Heat Manager, set point circulators, modulating condensing boilers and outdoor reset to mention a few. I would suggest run times of at least 7 minutes to peak your efficiency.
The system efficiency is as, or more important then the appliance efficiency. I don't want to down play the boiler or furnace efficiency but the distribution system and near boiler piping, 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 per btu's per degree-day was 62% 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 added 22,000 btu's of baseboard to the system, installed a boiler bypass pipe and 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 set-up 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% - 40%
As in warm air furnaces, boilers come in a multitude of efficiencies. Boilers and furnaces that operate at efficiencies above 90% are called condensing or modulating/condensing (called mod/con) units. Most units operating in this range modulate the gas input, as needed which can create added fuel savings. Units operating below 90% are non-condensing units and do not modulate the fuel input. Mod/cons will try to match the heatloss as the outside temperature changes. What this means is the gas input will actually 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. With all that said you should first upgrade the home with new doors and windows, add insulation, re-caulk and replace weather-stripping if these areas have not been addressed. This will reduce the boiler size required which will lower your installation cost plus use less fuel.
The efficiencies on today’s equipment will range from about 80% to 95%+. The higher the efficiency the less fuel used.
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