Sizing A Furnace And AC
Sizing a Furnace and Air Conditioning System
Having your new heating and cooling system properly size is very critical to getting the best energy efficiency, comfort levels in your home, and the lowest maintenance over the life of your new heating and cooling system. Surveys conducted by the heating and air conditioning industry have determined that at least half of all HVAC contractors do not do any type of sizing for your new heating and cooling system. They simply take a guess or they look at the equipment that you have now which may be over or under sized and put the same equipment in. Doing you a big dis service.
If the equipment is not sized properly your utility bills will be much higher than normal and you will not know the difference and you will accept this as normal when in fact it is not. A very common mistake is over sizing the equipment many contractors will do this to cover themselves. By doing this the equipment cost more to be installed because it is larger, it cost more to operate, and it will break down more often because the equipment will overheat and cause short cycling and premature failure.
Wrong Sizing Methods
It is the HVAC contractors job to properly size the equipment for your home. Many HVAC installers only check the name plate which is the label on the unit that has the current size of the equipment in your house now, and replace it with the same size or they’ll put one in larger which makes the condition worse. This is not the right way to size the equipment for your home and it’s not in your best interest. Other installers use rules of thumb, based on the type of home you have and then making an educated guess.
Manual J and Manual D is the HVAC Industry Standard
Properly sizing your new heating and cooling system requires much more than reading your current name plate or guessing, important factors will include the following:
- Your local climate
- Mean temperature for your area
- Air leakage rates
- Insulation thickness
- Type and shape of house
- Quality of windows and type
- Number of people living in house
- Quality of doors
- Direction the house is facing
You should insist that your HVAC contractor uses a correct sizing program before you sign any agreement with them. There is no cost to you to have a manual J completed on your home.
Sizing a Steam Boiler System
When sizing a steam boiler system the boiler has to be sized to match the radiators, the boiler that’s in there now maybe over sized for the radiators that you currently have so the contractor cannot just put in the same capacity boiler he has to do a Manual J proper sizing calculation for your home. The new boiler may be more energy efficient and you may need one or two less radiators or smaller radiators installed in your home.
When having a new steam boiler installed all the steam air vents on all the radiators should be replaced at the same time, and all the shut-off valves at radiators should be checked for leakage. A properly installed steam boiler should activate the water feeder very little, if your water feeder is coming on often that means that you are losing steam through the system.
A Good Contractor Will Perform the Following Steps for Sizing
There are many factors involved that effect your home’s Heating and Cooling requirement which is also called a “load”. A good HVAC installer will measure the floor space, walls, windows, and doors to determine the room loads and will determine the R value of the homes insulation, windows and other building materials. He will also have to determine as best possible the air leakage in your home.
A good HVAC estimate will also include the condition of the duct work seals and joints, if they are located in an attic or unconditioned space, to see if they are insulated if they’re not, they will need to be insulated and to be sure that the supply air and return air registers in your home are of the right size and location.
Be sure that the contractor uses the correct outdoor temperature and humidity for your area, if he uses a higher temperature and humidity level your equipment will be over sized and this is not what you want.
When the contractor is finished with your estimate be sure to get a copy of all the calculations the Manual J and the finished worksheet so that you have proof that the job was done right.
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