Author Topic: carrier ECM blowers: understanding continuous fan cfm (and wattage)  (Read 1014 times)

Offline mdhare

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Hello, for newbie but enthusiast for all things mechanical.    I'm a homeowner, not an HVAC tech.

In short I need to replace my furnace shortly and I'm trying to better understand continuous fan cfm options.

The preferred contractor uses carrier so I've been looking at a 60k btu 59MN7A (multistage) and 40k btu two stage (59TP6A). 

When it comes to product selection, I know it's odd to focus on continuous fan, but I have a tight home (getting tighter) and realistically cannot run separate ductwork for fresh air ventilation so I'm looking to use the existing ductwork for that purpose.  Ultimately I'd like to leave the blower ON nearly 24x7 which means finding a low cfm and efficient (wattage) blower setup.

I would like to understand which unit would allow the lowest cfm/watt continuous fan option.  I (think I) understand cont fan speed is ultimately tied to AC tonnage and selected cfm per ton. 

With that intro, the questions;

1) I see the 59MN7A blower claims to go down to 300 rpm whereas the 59TP6A blower claims to bottom out at 600rpm.  Usually lower rpm is better cfm/watt however I don't understand if that low rpm is selectable for continuous fan purposes.

2) Do the ECM blowers in these two units have a significantly different cfm/watt profile?  It seems hard to find this info.  Within a fan family (ECM vs PSC) usually lower rpm = better cfm/watt.

3) If a blower is rated @ 350cfm/ton at ESP 0.1 = 575cfm for AC, is 575cfm the min, med or max cfm for continuous fan (I presume you can toggle the fan speed option with either a communicating or non communicating thermostat)?

Regarding product selection, looking at spec sheets if I choose a two stage 59TP6A 59TP6A040E141110 [40k btu] I'm going to pair it with a single stage 1.5ton (such as the 24ANB6).  If you derate to 325/cfm it looks like you can achieve ~540 cfm.

If I choose a multistage 59MN7A 060-14 my understanding is that I can pair it with the 24ANB6 if I choose.  However if I choose to pair it with a two stage or modulating 2 ton AC unit (my understanding is 2 ton is the smallest non single stage I can choose), for parity sake assume 325cfm/ton, does this force me to choose a 325cfm * 2 ton = 650cfm continuous fan as a basis?  Or can I still reach the 488 cfm theoretical minimum in the spec sheet (325cfm * 1.5 ton = 488) for continuous fan?

I know that's a lot.   Thanks for any words of wisdom.

-Michael

Offline Sergroum

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Re: carrier ECM blowers: understanding continuous fan cfm (and wattage)
« Reply #1 on: December 17, 2018, 01:03:19 AM »
continuous fan and air conditioning speeds are two entirely different speeds.

You can have your air conditioning speed going full 1200 cfm, while your continuous one will be in 300s.

The reason why a modulating AC has a minimum of 2 ton, is because it's actual tonnage will vary up and down with a maximum of 2 ton. So if you pair the modulating ac with mn7 and a communicating thermostat, your ac will not be running at 2 ton, unless the heat load settings require it. Your furnace blower speeds for heating/cooling will also vary by the increments of ... 2-4 percent, depending on the modulation level.  Should you install a non-communicating thermostat, then mn7 will just turn into a 3 stage furnace. Which I suppose, is still more electrically efficient then tp6, but not anywhere near as much as it would be with a communicating t-stat.

Again, the continuous blower speeds and cooling blower speeds have nothing to do with each other.

Offline Hvacpimp

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Re: carrier ECM blowers: understanding continuous fan cfm (and wattage)
« Reply #2 on: December 17, 2018, 07:27:22 PM »
Sergroum is right. These are two different things that you are talking about. I think what your looking for is based on continuous fan. Minimal power usage power is a motor that is using DC current running on a constant low. Don't worry about ac fan speeds. If you contractor sized the equipment properly the furnace will bump up and down as required (higher speed for air conditioning etc etc)