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Old 07-01-2018, 01:32 PM
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Cement Bench Cement Bench is offline
 
Join Date: Apr 2008
Location: alberta
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Quote:
Originally Posted by MK2750 View Post
I see lots of people throwing around the square footage of their homes in hopes of an estimate on AC sizing and cost.

AC units are not sized on the square footage of a home or even the heat loss of a home. It is a factor, but just a factor in the equation.

The number one concern of a qualified HVAC estimator should be the duct work. Furnaces and air handlers are not pumps. The amount of air they are able to move is completely dependent on the size of the duct work, number of supply outlets and return air. Any contractor that does not count your vents, measure the exposed mains and calculate the air flow using a calculator, should be shown the door. The only exception is an experience contractor that actually designed and installed the duct work in your home or knows the exact duct design of the house you are living in. Always make certain he checks for proper return air in the lower levels or your nicely finished basement will be colder than a meat locker. This is especially true when your house was designed around in floor heating where inexperienced or lazy contractors feel they can get away with inadequate air flow. He/she should also ask specific questions about the comfort of your home and if you have had any airflow concerns or hot/cold spots. These are often easily dealt with by better balancing a system, adding a run or two, or adjusting the furnace or air handler.

The second step is determining the air flow capacity of the furnace/air handler. The furnace/air handler CFM out put is generally clearly marked on the manufacturers plate. It is also included in the model # of the unit. An experienced contractor should be able to point this out to you however you can look your furnace up on line if you have doubt.

If you have not got it yet, air flow is everything in HVAC, especially the AC part. An AC coil needs 350 to 400 CFM of air flow per ton or it will ice up and cost you a fortune to run. Some may still cool your home but when you get your electrical bill you will still be over heating.

Next up is determining the electrical requirements of the unit and install of both high and low voltage wiring. Plumbers, sheet metal techs and refrigerant techs are NOT electricians. You require an electrical permit drawn by a master electrician to legally install your AC. Do not assume, ASK. Low voltage requirements can be rectified with a new modern thermostat that runs off a couple or three wires or by adding a fast stat. Don't allow your contractor to move around low voltage wiring to make the AC work. Many an inexperienced contractor will use wires designed to stage the furnace or induce the fan during central exhaust cycles, to run the AC. You generally don't notice until heating season when your circulating fan, staging or accessories are no longer functioning. By this time the contractor is gone and you are left paying the money you saved on install fixing these issues.

Now that we have determined what the system is capable of doing, we can finally look at the square footage, heat loss and general exposure of the home. Have the windows been upgraded? Are we dealing with 2x4 or 2x6 construction? What is the R value of the insulation? Is the home exposed to southern sun all day? Is there an unusual amount of electronics like computers running? Is the home a walk out, bungalow or bi-level?

A well insulated 1400 square foot home on the north side of hill or well shaded with a bungalow type basement may be easily and efficiently cooled with a 2 ton unit. The same house moderately exposed on the hill top may require a 2.5 ton unit. If we build this same house as a walk out on the south side of the hill without any shading, 3 tons is not out of the question.

OR you can get a price/sizing over the phone, internet or big box store, wait for slick to come by and measure the top of the furnace for coil size and roll the dice that you luck into a good deal. Do you feel lucky?
Correct our quote was based on redoing the furnace with a complete new install and he wanted the house plans to get the cubic meter air volume and window sizes before gives us an answer. We are happy.
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