Setting Fan Speed

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Jrv

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Got a new variable speed 2 stage furnace with 2 stage ac installed last week . I’m just wondering how you know how high you can go on fan speeds before it freezes the coil. Would that be in the manual somewhere?
 

Jrv

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No it won't be in the manual. Why do want to run just above freezing? To lower RH?
It’s a new install and the company is trying to tweak the speeds a little bit and I just am curious how you know what fan speed is too high that it starts causing problems. My first stage right now it’s just a little bit on the low side and I want to bump it up to the next setting
 

LLigetfa

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I’m just wondering how you know how high you can go on fan speeds before it freezes the coil.
I'm confused. How would a higher speed freeze the coil? I would expect too low a speed to freeze the coil, not too high a speed. AFAIK, it is the speed/volume of the air flowing over the coil, which is secondary to fan speed.
 

Jrv

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I'm confused. How would a higher speed freeze the coil? I would expect too low a speed to freeze the coil, not too high a speed. AFAIK, it is the speed/volume of the air flowing over the coil, which is secondary to fan speed.

Thank you for the reply. Then maybe Im not understanding this correctly. When the furnace was installed they adjusted the first and second stage speeds. AC is having trouble cooing the house so they are coming back to adjust the blower speeds. I asked if they could just set both stages at their highest levels and just be done with it but they said running the fan to fast causes issues, could freeze your coil.

So is it adjusting to high that causes problems or slowing fan speed down that causes issues. My blower is currently on 350cfm , but can also be set for 375 or 400.

Thanks
 

LLigetfa

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If there is not a large enough volume of air flowing over the coil, the surface temperature on the coil could be below freezing which causes ice formation.
 

Fitter30

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It doesn't matter if its a basic system or high efficiency system have to set correctly. Temperature drops across the coil,/ air flow and refrigerant charge. Coils don't freeze from to much air flow. To much air flow lowers the temperature difference across the coil = less RH removel. Brand and model of furnace to see if theres a installation operating manual online.
 

WorthFlorida

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HVAC for both cooling and heat uses a different fan speeds. Cooling the fan speed is usually higher, for heat the fan speed is lower. Depending on your model, same fans are multi speed, others are variable speed. It all depends on the design of the controller and the SEER rating. The condenser fans can also be variable or multi speed, again depending on the model. Variable speed fans will usually get a 1 SEER improvement for its rating.

Coils will only freeze if the airflow is restricted (such as by a dirty filter) or the refrigerant is low (pressure) and the temperature of the coils will get below 32*F. For cooling the fan runs higher than in heat mode. Too fast and the air won't have time to cool and humidity extracted from the air. Too slow and the airflow to the furthest rooms may not be sufficient. From the old R22 units to today's HE units, air volume has been increased by larger duct work and not by fan speed. If your ductwork is undersized, increasing the fan speed will help but the efficiency may drop just a bit. Anyway, you'll see a big drop on your electric bill and the home should be more comfortable. Two stage cooling is usually a high end unit and most likely variable speed fans are used.
 
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