5 ton system pressure drop questions

red_oakOctober 28, 2011

Once again, I have read lots of information and what we are being told and what exists does not fit when it comes to calculating ESP. As, I have mentioned before I have a 5 ton unit and the coil itself is .498 pressure drop and most other 5 ton coils are @ 0.5 and that is what all these furnaces are rated to handle @ 2000 CFM. That leaves no room for ductwork, grilles and even filters to be installed and keep the system balanced within this guideline or maintain the SEER rating of the system. What am I not understanding as I see everything below 5 ton units the coils pressure drops are much lower allowing room for all other ESP to be factored in. Can someone please explain the secret here if there is one?

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Just curious: is the coil the same width as the furnace? If smaller, what type (if any) transition?


    Bookmark   October 29, 2011 at 9:39AM
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First, thank you for responding back as I think I have been sold and installed equipment that as another HVAC guy said, is possibly just junk related to efficiency being impossible. So, on my own I am trying my best to correct this a best as possible without spending enormous amount of money. I am so happy you have asked me this question, but on the other hand I see it very bad news. Enough venting, the opening of the furnace is 19.6 inches. The opening of the coil case is 20" but the coil itself is 21". I see they sold me a discontinued furnace and the newer furnaces are in a couple of sizes going up to 24.5" inches and they sell a coil that is also that dimension with a pressure drop rated of 0.395. I am so disappointed in this and I hope someone can help me. Here are the model #'s of the furnace and the coil if you can please verify what I said and suggest what I can do to improve this problem. GOODMAN FURNACE GME81155CXCB and COIL CAPF4860C6DB.

    Bookmark   October 29, 2011 at 6:48PM
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First, let me say that I am NOT a PRO, but have become educated in matters HVAC, by plodding this and other HVAC forums for years.

I could not find information on your furnace and coil. I am however, curious as to your location, size of home, and your need for that large of a heating/cooling system.

Would you also detail exactly how your ESP was determined? Also, can you give information on what was replaced, and the outdoor unit?


    Bookmark   October 30, 2011 at 11:01AM
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I understand you are not a pro and neither am I, but I have been rapidly learning about this process also, and I see that I might could have made it with the 4 ton unit that I had. I did the Manual J and Manual D methods and if you use the standard of 95 degrees and 18 degrees for Memphis, TN area then it is a 4 ton, but that is incorrect as the last several years that is not the case here. So, I used, and it may be high, 100 and 10 degrees. I know now a lot of things I could have and need to improve on related to the duct work in general, insulation, etc. Go to www.alpineair.com and you can get these values for these units. It was a Trane 4 ton unit XE1000. The builder's spec. said 2222 sq. ft. home. Measurements of the floor space was 2,000 sq. ft. I am being told it is going to be impossible to make this unit operate at the 14 SEER it is rated at, which I completely understand after the fact. Since the coil is rated at .498 pressure drop and the furnace is designed to operate at .50 @ 2,000 CFM there is no room for duct design. I am disappointed in myself for letting this happen and the HVAC industry for manufacturing equipment particularly 5 ton units that has these kind of specs. I tell you anything you want to know in what is all wrong, I just can't straightened it all out on my own, and I can make it a lot worse, but I know if I continue to try and improve this I can do a better job than most HVAC service companies around here can or would do. Most of them don't even own an anenometer to check airflow. I bought one and have used it and like I said I could go on and on about what is wrong, but getting it ultimately right can only occur with the proper equipment installed to begin with/

    Bookmark   October 30, 2011 at 4:16PM
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Regarding your SP, the CFM delivered is dependent on the SP, not the other way around. Merely setting the CFM by DIP switches, or speed taps, depending on your equipment, does NOT 'give' you x amount of CFM.

Since I can't find specs on your furnace, I don't know if you have an variable speed blower (DIP switches), or a multi-speed blower (taps). The former, will attempt to deliver the set CFM by ramping up, causing higher amp draw and static pressure. Were temperature rise and delta-T values within specs?

Were there any static pressure measurements taken *and how)? Using altered design temps to increase heating or cooling needs leading you to increase blower size may be mostly responsible for your problems.

What was there before replacement?


    Bookmark   October 31, 2011 at 11:03AM
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Go to www.alpinehomeair.com and you'll find it all. But, it is a multi-speed furnace and they have it set on T5 which is @ 2,000 CFM @ 0.5"IWC. Someone else mentioned about changing condensing unit, coil, motor, etc. to be like a 4 ton system. But you have got to be kidding me, and even if it did, why? Would the manufacturer warranty this arrangement if you had problems? I just don't understand why this manufacturer and looks like most others that are designed to operate @ 2,000 CFM have no room there period. I am going to take static pressure readings and let you know later. I forced these guys to add at least one 12" return because we were short with the 4 ton unit and they were going to just cut a hole in the wall where one return already existed and they thought it would be good enough. But, now this individual return is mounted in the ceiling and is just a humming as I know this motor is ramping up like you say as the CFM is @ 750 and so is the FPM of @ 750 FPM. So, like some others have said and your saying that apparently you cannot order a standard 120K btu furnace and 5 ton condensing unit and matching 5 ton coil and operate within the specs of 0.5" IWC and (in this case 14 SEER rating) without manipulating various components in the system to try and come close or meet the efficiency that the manufacturer originally said it is capable of performing, right? If this is true, then manufacturers are not telling the truth of what a true full sized 5 ton residential system will operate in the real world and HVAC companies are covering this problem up and seems to me that there is something wrong with this picture. I had a 4 ton system that I later found out was undersized in return of at least one 10" or 12" return and I now know it would have helped considerably, but I still feel it was undersized, but again what I feel and what the professionals know and might could have done to make it work is another matter. The problem is my wife and I want 72 degrees year around inside whether it hits 115 degrees outside or -15 degrees and if it causes the system to be oversized for those conditions and makes it less efficient, that is our choice there also. Then it becomes a matter of how much oversized is too much or not enough yet, right? Just like in the opposite direction of when you perform Manual J it goes down to a 4 ton unit if your conditions are such it says 4.5 tons needed. But, what is that based on 2 days out of the year, a week, a month, you know?

    Bookmark   November 2, 2011 at 11:55PM
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"Go to www.alpinehomeair.com and you'll find it all."

A direct link would be nice.

"...it is a multi-speed furnace and they have it set on T5 which is @ 2,000 CFM @ 0.5"IWC."
"....as I know this motor is ramping up like you say as the CFM is @ 750.."

Mult-speed motors do NOT ramp up. The speed is set by the tap. The air flow delivered will depend on the total ESP SEEN by the blower. If you got a manual for your furnace, it should have 'fan' data, which lists air flow at different values of SP.

"I had a 4 ton system...."

So, you increased tonnage. Your duct system was likely undersized ( I needed a 10 or 12"...), so total ESP is likely higher. How's the noise? Try blowing through a straw. Not much breath getting through, but the pressure on the walls of the straw will be great.

"But, what is that based on 2 days out of the year, a week, a month, you know?"

The OUTDOOR design temperatures are such that about 98% of the time, the temperature will be at or BELOW that temperature (cooling). This is calculated from accumulated data.

Short cycling due to oversizing will result in poor humidity removal.

    Bookmark   November 3, 2011 at 11:58AM
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I apologize,I have not done this link thing before, so here is one for the furnace:

I read 2,014 CFM at 0.5".


For the coil: http://www.alpinehomeair.com/viewproduct.cfm?productID=453057261.

For the condensing unit: http://www.alpinehomeair.com/viewproduct.cfm?productID=453057626.

For the filters: http://www.alpinehomeair.com/viewproduct.cfm?productID=453059851.

All this information is also on Goodman's website and if you can't find it I will get you links there also.

I am sorry, but when I said ramping up I should have said loading up (current increasing) as I know the return and supply is undersized, so I would expect the motor is under constant strain being the velocity IS running very high and so, the RPM is probably lower, right?

Thanks, RC

    Bookmark   November 5, 2011 at 4:05PM
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