Airflow in a home is as important or even more important than the Size of the AC unit for cooling a home

Summer arrived in Utah last week during a cool spell.  We have had some fairly hot days 85 deg + but Friday and the weekend was not.  Now Monday is back to 85 deg and tomorrow will be 90 or so and gaining from there.  We are back to cooling weather for the next 2 to 3 months here.

Here in Utah, we often run into customers who are adding an AC to there home to replace a evaporator cooler or a older, much less efficient AC unit.  Most of the time we can not get the best performance from our AC no matter how good it is.  Without good or very good airflow in the house the AC can not cool the house well.  Since about 2017 manufactures have gone to constant torque, variable speed fan systems.  This provides much more air in the home.  If we also do the measurements and help the duct systems both outgoing and return, we get very good results.

Often a customer will ask what we recommend to solve there problem or get the best performance.   If we are dealing with furnaces older than 3 years and many older than 2 years that are not the top units, we have a major airflow problem.  the best solution is to replace the furnace with a 96%-97% multiple stage unit with constant torque, variable fan system.    We can often replace what they have for less than $4000 and they get the best heating, cooling and air flow.  The value of their home just went up about $8000+ doing this.  In the winters this year and next they have little or no cost to heat their home.

On the AC side with good air flow we can design a system that will run about 20% of the time and the cost to cool the home perfectly is 1/3 of what it was before.  We put in many 3 ton units, 3.5 Ton units, 4 Ton units as well.  We pick very efficient Evaporator coils and line sets to get very close to 20 SEER on the performance.   We can do this for just under $4000 and the value of the house just went up another $8000++

If the customer can not replace the current furnace, we can replace the fan motor for $500 or less, but it is not a variable speed unit (they cost more than $900 each and need a controller to get good performance).  Total cost is often well over $1500.   You still have a old furnace when done.   In contrast, you can get the best performance from the fan system and best performance from the furnace for less than $4000.   A replacement fan will get more air and better performance,  but not close to what we get with a new high efficient furnace system.

Some people here do not believe what we say, even though we have years of experience and many years of formal training.  If they just do not like the truth, or how we measure the results, there is little that we can do except go to the next customer.  We get a lot of referrals here and all over where we work.  Tells me we are generally on the right track on what we say and do.

We make measurements on  system airflow, on cooling capacity, on heating capacity, on BTU gain, and on how a gas furnace is tuned.  We not only know how to do all this, we have the best equipment, understand how it works, and how to apply it.  Some people ask me if replacing their old windows will help with heating and cooling their homes.  The obvious answer is yes, it will help more than most things that can be done.  We can measure the difference before and after on this as well.

Many other things matter here.  Return air is a major issue here in Utah.  It takes about 1.8 times more return air than feed air to not cut the airflow going out.   We use 2X because it is easier and not exact thing.

Two good examples here from 2019:

#1    A couple here in Utah wanted a good AC unit that would keep there home cold.  I went and accessed the furnace, the size of the home, windows, doors, air leaks, etc and gave them a bid.  A 3 ton unit was perfect for this home.  They had an old furnace that had an old fan system; and I urged them to replace the 80% builder grade unit with a 96% two stage furnace with a constant torque,  variable speed fan system.  This included a computer driven smart thermostat that operated the furnace properly and could be done from there phone.   They insisted they did not want to do this.  I then explained that I could replace the motor in the old furnace but it would not be near as good as a new furnace and fan system would perform.  They decided to not go with a new furnace and go with a fan motor on this and a high efficient AC.   It turned out the old 80% furnace would not support any AC at all.   Before I replaced the board and relay at my cost, I  urged them to go with a new furnace again in place of putting now $700 in that old unit.  They said they did not want a new furnace.  I replace the board in the furnace and a relay to drive the ac and fan system.   The new AC worked very well and the fan system was much better than I started with.  That system worked OK but not as good as with a new furnace and thermostat.   They were unhappy later and wanted $500 back for all this.   They were told in advance about the risks and performance.   They have cold air.   Total cost just under $4000 The value of there home went up the $7000 from the AC but is down by $5000 due to the old furnace that needs replacing.   So all taken into account, about $2000 more is all.

#2  And older couple needed a AC to replace there evaporator system that no longer worked well here.   I went over to their home and told them they need both an AC and furnace along with updates in the main plenum duct system to support the extra ducts they wanted.    They listened and we put in a good 3 ton AC and 84,000 BTU furnace with variable speed fan system.   We doubled the flow in the plenum to support the air flow in much of the house.    The system works great and provides very good flow and cooling all over the house.  Their home is now worth about $18,000 more due to this.  The cost to run the AC will not be much more than they were paying for the evaporator system.  They are very happy  Total cost just under $8000

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Terry

Professional Engineer with 30+ years experience

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