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Undersized HVAC Evaluation


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The neighbor of a client of mine passed this along to me concerning uneven heating/AC in her condo. She hired someone to give their opinion. I don't know if this is from an HVAC guy or a home inspector. I cannot find his name in the Illinois HI Registry. There were some other remarks about a dirty fan from construction dust.

This is a two-year-old HVAC system. Does this make any sense? Sounds like alot of hooey.

1. 1. We observed that near the discharge air vents near the machine we got a temp of 57.7 degrees but near the front of the space we had a temp of 67.7 degrees. This was also the case on the second floor of the unit.

4. The unit seems to be sized right at the bare minimum for your space and the loss of efficiency of the unit would lead to stratification of the temperature in the space as we observed. Running the unit for 90 minutes only reduced the temperature from 81 to 80 degrees.

5. It also appears that the unit is sized just to the space that you have. The unit is working hard to try and maintain a constant temperature. As HVAC systems age they loose efficiency and loose their ability to maintain temperature at the level of when they are installed. If the unit had been upsized by 25% it would not be experiencing the issues that it is currently having. After it is cleaned up of course. With the unit constantly working you might experience more issues with it breaking down

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1. We observed that near the discharge air vents near the machine we got a temp of 57.7 degrees but near the front of the space we had a temp of 67.7 degrees. This was also the case on the second floor of the unit.

Translation: The supply air at those registers that were closer to the air handler unit was measured at 57.7 degrees. The rest were 67.7 degrees. This condition was observed on both floors.

4. The unit seems to be sized right at the bare minimum for your space and the loss of efficiency of the unit would lead to stratification of the temperature in the space as we observed. Running the unit for 90 minutes only reduced the temperature from 81 to 80 degrees.

Translation: The cooling capacity seems about right for that size house but I didn't complete a J-calc for each room so the truth is that I haven't the foggiest idea. The system took 90 minutes to cool the house from 81 to 80 degrees.

5. It also appears that the unit is sized just to the space that you have. The unit is working hard to try and maintain a constant temperature. As HVAC systems age they loose efficiency and loose their ability to maintain temperature at the level of when they are installed. If the unit had been upsized by 25% it would not be experiencing the issues that it is currently having. After it is cleaned up of course. With the unit constantly working you might experience more issues with it breaking down.

I agree with you Mike. It's a lot of hooey. This guy hasn't offered anything of value to your client's neighbor. Go back to square one and try again.

Ask someone to do a J calc to see what cooling capacity is needed in each room and for the whole house then a D calc to see just how bad off the duct system is.

These two calcs are not cheap but if the house is only 2 years old, I think it'll probably be worth the findings.

Marc

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Ask someone to do a J calc to see what cooling capacity is needed in each room and for the whole house then a D calc to see just how bad off the duct system is.

These two calcs are not cheap but if the house is only 2 years old, I think it'll probably be worth the findings.

Marc

Any guesses as to what these calculations would cost by a competent HVAC company for say 2 levels, 2000 sq ft?

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Did the guy bother to compare temps at top of plenum coming out of furnace and the temp at return air boot coming into furnace to get the temp drop?

Did the unit look like it had been serviced recently,was the filter clean?

Id suggest paying $50.00-$75.00 first and have the unit serviced,Being only 2 years old doesnt that the equipment is running as it should be.[;)]

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Ask someone to do a J calc to see what cooling capacity is needed in each room and for the whole house then a D calc to see just how bad off the duct system is.

And then what? Replacing the duct system is only an option for Mike Holmes.

I'd do a duct leakage test and then I'd measure the airflow from each register. Then I'd take that info along with a visual examination of the system and give them some recommendations. At the top of the list is probably calling Mike Holmes. Nowhere on the list is to keep working with the same HVAC contractor who provided the original opinion.

I once did an energy audit for a client who told me that her living room was always cold. One supply register was closed and the other was almost totally blocked by the couch. There might be a simple solution to improve things.

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Taking a few supply temps. tells you nothing about the system's ability to handle the load. What are the superheat/ subcooling, TESP, delta T, fan speed when taken, are ducts sealed, duct blaster test, zone dampers, CFM at each supply and overall, etc, etc, etc,??? Waaaay too many questions without answers but I can assure you that oversizing air conditioning is a sure recipe for inadequate dehumidification, poor overall performance and increased energy consumption. If anything, an air conditioner should be sized on the lower end. As for dropping the temps 1F after 90 minutes, did you check the filter and the condenser to see if they were clogged? AC units can run a very long time with no appreciable loss in efficiency as long as they are maintained properly.

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Some hocus pocus in that inspectors answer. He gives us temperature readings, but fails to give us any information about air movement coming out of those registers. Could some registers be closed? Yup. Could a mouse or a squirrel have eaten a hole(s) in the supply line and all the air pressure is going into the attic or crawlspace? If in the inspectors opinion, the HVAC equipment is correctly sized, then I would look elsewhere before I went and spent the money to have J and D calculations done.

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