Information on furnace data plate

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Old 01-08-08, 05:27 PM
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Information on furnace data plate

This was accidentally posted in the heat pump/electric heat forum, so I'm reposting here.

I have an old Rheem furnace model RGDA-125-GR. I want to check the static pressure of my system. (125,000 BTU, also 4 ton A/C.)

The data plate lists 2 values of maximum s.p. They are 0.2" & 0.5" w.c. This is with the same blower (10-10DU) w/1/2 h.p. motor.

Why the 2 pressures? Is the larger one for use with air conditioning; the smaller for heating?

Delta L
 
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Old 01-09-08, 06:17 AM
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Hi again...

Blowers in furnaces are multi-speed.
Rheem's RGDA-125-GR has a "4-speed" blower [low, med-lo, med-hi, hi].
Out of these four, only two are used; the other two are capped-off and not utilized.

From factory, this furnace comes wired so that in the heating mode the lowest of all 4 speeds is used; in the cooling mode the "Med-Hi" one is used. You may have to change the factory defaults IF the temperature rise [winter] does not fall within the manufacturer's recommended range, or if the level of cooling/dehumidification [summer] is inadequate.

Why the two different S.P. readings? b/c one is for the blower running in low speed [heating], and the other is for the blower running in med-hi [cooling]

Having said that, DO NOT expect to read either 0.2 or 0.5. That may or may not happen. The proper way to interpret these values is as follows: IF you indeed happen to read a 0.5 in-w.g E.S.P with your manometer, you can be sure that the furnace is moving 1695 CFM's [as per Rheem's performance data for this unit].

For any other values of Static Pressure and CFM, fan laws apply. There is one particular fan law that states as follows: SP varies with the SQUARE of the CFM.

What will your manometer read when you hook it up? that depends on a few factors: the speed of the blower, the total equivalent length of your duct system [both - supply & return], the types of registers you have, the type of air filter installed [and how dirty it may be], the evap. coil being wet or dry, the friction rate the duct designer used when sizing your ducts, type of collars/boots, etc.
The point is, with the above formula, once you get the manometer reading, you may be able to calculate the CFM the furnace is moving.

For example, if the manometer reads indeed 0.2, then:
CFM = 1,695 x (0.2 / 0.5)E0.5 = 1,072
 
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Old 01-09-08, 08:50 AM
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Pflor: Thanks for your very detailed response to my question. Below is your post with some follow up information or questions.


Originally Posted by pflor View Post
Hi again...

Blowers in furnaces are multi-speed.
Rheem's RGDA-125-GR has a "4-speed" blower [low, med-lo, med-hi, hi].
Out of these four, only two are used; the other two are capped-off and not utilized.

From factory, this furnace comes wired so that in the heating mode the lowest of all 4 speeds is used; in the cooling mode the "Med-Hi" one is used. You may have to change the factory defaults IF the temperature rise [winter] does not fall within the manufacturer's recommended range, or if the level of cooling/dehumidification [summer] is inadequate.
I am not about to make changes, but am in the process of replacing my system.

Why the two different S.P. readings? b/c one is for the blower running in low speed [heating], and the other is for the blower running in med-hi [cooling]
I had suspected that that was the case

Having said that, DO NOT expect to read either 0.2 or 0.5. That may or may not happen. The proper way to interpret these values is as follows: IF you indeed happen to read a 0.5 in-w.g E.S.P with your manometer, you can be sure that the furnace is moving 1695 CFM's [as per Rheem's performance data for this unit].
If my a/c is 4 ton, I would expect that the blower would be set at 1600 CFM (maybe not). But, I wouldn't be surprised if it is in fact, set up at 1695. I say this because my dehumidification isn't great, but since my a/c is oversized at 4 ton, cycles are quite short.

For any other values of Static Pressure and CFM, fan laws apply. There is one particular fan law that states as follows: SP varies with the SQUARE of the CFM.
See comment below.

What will your manometer read when you hook it up? that depends on a few factors: the speed of the blower, the total equivalent length of your duct system [both - supply & return], the types of registers you have, the type of air filter installed [and how dirty it may be], the evap. coil being wet or dry, the friction rate the duct designer used when sizing your ducts, type of collars/boots, etc.
The point is, with the above formula, once you get the manometer reading, you may be able to calculate the CFM the furnace is moving.

For example, if the manometer reads indeed 0.2, then:
CFM = 1,695 x (0.2 / 0.5)E0.5 = 1,072
O.K., I don't have the manometer yet, but would expect the static to read higher, since the duct system has some long runs with many elbows. About the only change I would make at this point is to replace my return drop with a larger one (2" narrower than supply trunk), and perhaps a media filter.

According to the formula above, assuming a (heating) static of .2 would give me 1072 CFM. I highly doubt that it is that low based on the sound to my ear (seems louder than a/c), plus the fact that my furnace is oversized (125,000 BTU at an estimate of .60 - 65% efficient [20 years old]. My heat loss calc is 60,000 at design temperature of 14 degrees. I am also located in NJ (Cherry Hill area).

The furnace/air conditioner system that I would like to get is a Rheem Mod RGFD090 with 1200 heating CFM (max) and RANL-042-JEZ with 1400 cooling CFM. Since the furnace will rarely exceed about 80% input, except for morning recovery, which should be less than an hour based on my present situation. To my way of thinking, the reduced CFM in both heating and cooling should result in a lesser degree of esp.

I want to maximize comfort and believe that this system will accomplish that (heat output matched to heat loss, and longer a/c cycles). I have conducted my own heat calc, and have one done by an out of state professional, whose results closely mirrored mine.

I would eagerly appreciate your comments to my proposal.

Thanks for your time.

Hank
 
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Old 01-09-08, 09:51 AM
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Originally Posted by delta l View Post
If my a/c is 4 ton, I would expect that the blower would be set at 1600 CFM (maybe not). But, I wouldn't be surprised if it is in fact, set up at 1695. I say this because my dehumidification isn't great, but since my a/c is oversized at 4 ton, cycles are quite short.
Oversizing is the main culprit for poor dehumidification. oversizing brings about short-cycling, which in turn causes poor moisture removal from the conditioned space.


Originally Posted by delta l View Post
I want to maximize comfort and believe that this system will accomplish that (heat output matched to heat loss, and longer a/c cycles).
I couldn't agree more

Originally Posted by delta l View Post
I would eagerly appreciate your comments to my proposal.
What proposal? you mean...your comments?
 
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Old 01-09-08, 12:24 PM
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Originally Posted by pflor View Post
Oversizing is the main culprit for poor dehumidification. oversizing brings about short-cycling, which in turn causes poor moisture removal from the conditioned space.



I couldn't agree more


What proposal? you mean...your comments?
My proposal referred to the equipment which I specified (heat & cool), based on 2 manual J studies and my personal preference, since I couldnn't get any contractors to do it.

What I was hoping was that you would respond positively that you thought that my new system, operating with less air flow (particularly in heating), could conceiveably overcome any shortcomings in duct design and result in a reasonable static pressure. The expected CFMs would not (in my non-professional opinion) pose a stran on the blower motor (2000 CFM max).

Thanks again for your input.

Delta l
 
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