Difference between cooling and heating static 1

[bawhb]

I had a customer call me up and ask if he could use a different furnace for a design i did for him. When checking out the blower specs for the furnace he was asking about, it gave me fan speeds for up to 0.8" w.c. for cooling, but limits me to 0.5" w.c. for heating.
Why would the pressure ranges be different between heating and cooling?

Thanks in advance.

 

[willard3]

Difference in air density.

 

[chicopee]

Check your blower specs again as there should be temperatures listed or under which standard these specs were determined.

 

[EnergyProfessional]

willard3 is right. but I wouldn't have expected that much difference from 55°F and 120°F air.

 

[urgross]

I woudn't have expected that much difference, either, as relation ship to density is linear. Assuming linear, that would be closer to 0.71 IWC.

Is the cooling coil after the heating section? You might be adding drop from the cooling coil.

 

[317069]

What about the furnace you have specified at the beginning, does it give fan speed for heating and cooling too?

 

[bawhb]

Thanks for the replies.

I know i must be missing something really simple here. I understand that matter changes density with temperature, I just don't know how a blower is able to produce an external static pressure of 0.8 in.w.c. with cool air, and only able to produce 0.5 in.w.c. for warm air. Is this because the warmer air would be more "spongy"?

The blower data is just your basic ecm chart, with a column each for heating and cooling of cfm amounts and the tap settings for these, along with a header telling you at what ESP at which the blower can operate. What confuses me, is that all the other furnaces with ecm and psc motors i design to, do not limit the heating design to 0.5 in.w.c. ESP. Could a "delta" or "wye" wiring pattern of the blower motor have something to do with this?

Don't get me wrong, i still like to keep my ESP to the 0.5 in.w.c. and don't want to go against the manufacturer's specs. It is just that i like to understand the workings as much as my poor brain will allow. Also, it will take me a bit of a re-write to my calculation sheet in order to take advantage of the two different static pressures this furnace offers, and i would like to know the reasons i am doing something.

Thank you all again for very quick replies.

 

[BronYrAur]

Are they factoring in a wet coil for cooling that you won't have for heating?

 

[bawhb]

BronYrAur,

Nope they are not. The give external static pressure, which does not include anything outside the furnace itself (maybe a 1" disposable filter is included). They have no way of knowing what coil is being put on the furnace, and besides the biggest difference on a residential "A" coil is 0.04 to 0.05 in.w.c.