AC replacement done but now questions

4 years ago

Hi. I have two issues of interest-- I just had my AC replaced with a 5 ton 16 SEER unit but did not replace the 15 yr old furnace. The contractor said the airflow is now 1800 cfm, which he said was acceptable. Shouldn't it be 2000 cfm for a 5 ton unit? Does the lower air flow suggest a problem with insufficient ductwork or the blower on the furnace? Will 1800 cfm mean lower efficiency or cause my AC to wear out faster? Should I press for some fix now?
Also, they repurposed some flexduct rather than installing new when they added a third return in my bedroom and moved the supply register. Previously this flexduct piece was a bypass as part of a two zone system that I removed in this job to replace with a single zone system. In the two zone set up I was having problems with a frozen evaporator coil, a slow refrigerant leak, and poor cooling in some rooms impacted by the bypass. I'm wondering if that flexduct when it was used for the bypass could have been exposed to excess moisture from the freezing coil and caused mold inside the duct. Since they repurposed that flexduct to my bedroom, that very first night afterwards running the heater I had onset of my allergies (typically tiggered by dust and mold). Should they have used new flexduct as I thought they were rather than repurpose the older flexduct? Would long term use of flexduct as a bypass (associated with frozen evap coil and excess water in drainage pan) cause excessive wear or moisture inside that duct?
Thank you for any advice.

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Comments (5)

mike_home
4 years ago
last modified: 4 years ago
I just had my AC replaced with a 5 ton 16 SEER unit but did not replace the 15 yr old furnace.

If you don't have a matched furnace then you really don't know what the actually SEER rating. Mostly likely it is less than 16 SEER.

Was the previous condenser also a 5 ton size? If not then then there is a good chance your duct work is undersized and you are not getting 5 tons of cooling. The 2000 CFM or 400 CFM per ton is a guideline. It is possible to have less and not have the coil freeze. What is more important is the actual air flow. I assume the installer set the blower speed to the maximum setting of 1800 CFM and did not actually measure the air flow. If he did I am confident it would be less.

Flexible duct work is inexpensive. It should have been replaced if there was any concern about reusing it. I assume it was reused to save installation costs.
sktn77a
4 years ago
Sounds like 1800CFM is the maximum setting on your furnace, not necessarily what the true CFM is. So you're probably stuck with it if you keep the existing furnace. However, 400CFM/ton is a starting point - plenty of systems use 350CFM/ton, particularly if dehumidification is a priority.

I hope they didn't up-size your AC condenser from 4 ton to 5 ton (?) and they should not have re-used old flex duct.
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Jo
Original Author
4 years ago
I just received the Title 24 report and the air flow rate is actually 1680 cfm, not 1800 (acceptable is 1500 per requirements). The duct leakage is 235 cfm (acceptable is 300 cfm per requirements). These don't seem like great numbers to me. Trying to assess what if any fix is needed given the permitting review is tomorrow.

My old AC was a 5 ton unit too but it was on a 2 zone system so a bypass was installed years ago. This is in Sacramento region for 2487 sq ft, 17 yr old home with good insulation, 10 ft ceilings vaulted in master bedroom, lots of windows and alot of afternoon sun exposure across back of home.

I was advised by more than one bidding contractor to go with 5 ton unit again. But does this air flow rate of 1680 indicate I would have been better going with a 4 ton unit? Could this air flow rate potentially cause freezing of the evaporator coil? Thanks for your feedback.
mike_home
4 years ago
If the actual air flow is 1680 CFM then you are only getting the effect of 4 tons of cooling. It is rare a residential house has the duct work to handle the air flow for a 5 ton condenser.

Sacramento can get very hot so it is difficult to say what the heat load is for your 2487 sq. foot house. The fact that is only 17 years old would suggest it has better insulation than the average house. This is the reason I always suggest doing a load calculation and measuring the duct work when replacing HVAC equipment.

If your coil did not freeze with the old 5 ton condenser then it will probably not freeze with the new one. I believe your climate is very dry so that will help. You will have to monitor the situation when the temperature is in the 90s.
Jo thanked mike_home
sktn77a
4 years ago
last modified: 4 years ago
Being in Sacramento, you're OK with 1680 on a 5 ton AC. My in-laws have a 60 year old ranch in Elk Grove. 2200 sqft and a 4 ton heatpump. Does the job, but so will yours. Check for any obvious leaks and you should be fine.
Jo thanked sktn77a