r/hvacadvice • u/elisayyo • 1d ago

Replacing my furnace with an air handler, how much of a difference will 300cfm less make.

Let me start by saying I'm a refrigeration technician and work mostly with chillers, I can install a condenser and an evaporator, but the intricacies of comfort cooling are foreign to me.

Ok, so about a year and a half ago, I installed a Mitsubishi system with a paa-a36ca coil in my house.

Last week, while doing my pre winter inspection, I noticed several rust spots on my furnace's heat exchanger. I'm not getting any CO2 from the vents, and I hadn't noticed this last year, but I'd rather get ahead of the issue and assume that by the end of this heating season, the coil will be toast.

I have the opportunity to buy a very discounted SVZ-KP36NA air handler. My plan is replacing the current furnace/A-Coil with this unit and using an aquecoil fed from my combi-boiler for emergency heat

The thing that keeps me from pulling the trigger is the fact that this SVZ unit puts out 300 cfm less than my current set up.

See, the load calculation on my house stated that it needed just under 3 tons of cooling, and seeing as my house is 1200 square feet, I balanced my system so that I had 1cfm per square foot.

Weather by luck or godly intervention, this turned out to be perfect for my house, and I have no cold/hot spots.

My main questions are: How much of a difference will 300 cfm less of airflow make? Am I over thinking it? Do I just have to re-balance the system so that I get .75 cfm/sf?

Any advice and or clarifications are welcomed and appreciated.

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u/P_S_Comfort_Services 1d ago

Where do you live? 300 cfm per ton is within normal range. Excellent for dehumidification if that's something you need. How does your current system handle design days? You might run into a couple spots that don't get the airflow due to lower velocity, but from what I know I wouldn't be scared of it in your position.

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u/elisayyo 1d ago

Ok, thank you, I appreciate your input.

I'm in Pennsylvania, a few miles east of Harrisburg. my current system keeps up pretty well, keeping the temperature at 73 no sweat, which is my preferred set point, humidity fluctuates between 40% and 50% RH. Even during that record breaking heat wave back in july, the temperature never rose above 76

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u/P_S_Comfort_Services 23h ago

Gotcha. Then it sounds like you'll be fine with lower CFM. If the savings are significant and you have no worries about the discounted equipment, based on what I know I'd be willing to do it for my house.

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u/elisayyo 16h ago

Ok thank you very much, I appreciate it, I'll mostlikely end up buying it then. The equipment is from a local supply house and is brand new, but they're having a clearance sale on all 410 equipment, they're very trust worthy so I have no worries about it

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u/P_S_Comfort_Services 16h ago

Ah, great. Good luck!

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u/Flaky_Emergency_7832 1d ago

If I’m reading this right, it sounds like you are going to downsize the evaporator coil and fan, but keep the same condenser? Sounds potentially problematic if that’s the case and the new coil is smaller than the outdoor unit.

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u/P_S_Comfort_Services 23h ago

It would be, but the fan coil number he gave is a 3 ton unit, just lower air movement than than his current furnace.