Two questions regarding forced air HVAC:
1. Unlike most of the trades, reliable information about products, design, and construction of residential HVAC systems is hard to find. Any suggestions for web sites or publications where I could find reliable information?
2. Specifically, I am interested in installing central A/C on a downflow furnace. There is an ample return plenum above the furnace (and downstream of the filter) where the A coil can easily be installed. HVAC contractors I have talked with all insist that the coil must be downstream from the furnace (which entails relocation of the furnace and replacement of much sheet metal) but don’t know why, other than “that’s how we always do it.” Is there a solid technical reason why coil cannot be installed in return plenum?
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I'm guessing that it's a concern with condensation getting pulled off of the A coil by the air movement and rusting out the heat exchanger.
"Is there a solid technical reason why coil cannot be installed in return plenum?"
Yes, there is a solid technical reason. The coils in most AC systems are not deep enough to fully dehumidify the air and there is moisture carry-over. The AC coil in a residential system, that is close to being setup well, is the greatest pressure drop in the system and the the suction of the blower is at the lowest pressure in the system. These two factors combine to create a fog and seriously gunk up the blower, wet the interior of the cabinet, possible rust the HX, depending on its construction.
With coil downstream, the pressure is greater and the air tends to be less "wet" and the only thing that can get wet if carry-over is severe is sheetmetal.
These are worst case scenarios, like when the unit first starts in a very humid space or at low load, high humidity operation.
As far a general info, search the archives, lots of places and publications have been identified.
Tim
Thanks much for your reply, which is the first solid technical explination I have heard in several years of asking the question.
John O.
most air handler come with the a coil already installed so, do not worry about where the mg install it. The one I installed friday had it in the handler itself so the downflow or upflow space is not a problem. Here most units are upflows.
BB,
Regardless of what you installed or how you innstalled it, the location of the coil is important. Air handlers/fan coil units commonly come with coils in them, but furnaces do not. If you were to check the technical info on that air handler that you installed, it will tell you if it is designed for upflow or downflow. Many furnaces are multi-poise, i.e. can be install in various positions, but air flow direction through the unit is not typically changeable. BTW, what make and model did you install? Is this what you do for a living, install HVAC systems?
"Unlike most of the trades, reliable information about products, design, and construction of residential HVAC systems is hard to find. Any suggestions for web sites or publications where I could find reliable information?"
John,
Unfortunately, there is not a lot of "easy to use" (i.e. rules of thumb), good information out there. There is a boatload of information, if you really want to learn about HVAC&R.
If you interested the following are some of the best references I know of:
The American Society of Heating, Refrigeration and Airconditioning Engineers (ASHRAE), their 4 book series (Fundamentals, HVAC Systems and Equipment, HVAC Applications and Refrigeration) is excellent, though very expansive and technical.
The Trane Air Conditioning Manual - the "blue book", been in publication since 1938, I have the latest, 2001 version - about the best single reference available in this country.
Trade publications include HPAC (Heating, Piping & AC) Engineering and Engineered Systems, and to a lesser degree PM Enginnering. The ASHRAE Journal.
There are organizations such as: The Sheet Metal and Airconditioning Contractor's National Association (SMACNA)and The American Refrigeration Institure (ARI) to name a few, I'm sure there is more.
There is also a web forum, called HVAC Talk.
That ought to get you started.
Tim
There is another reason to avoid a draw thru installation and that is the additional cfm required due to motor and fan heat. In the normal blow thru configuration this heat is added to the return air and is removed by the coil. In a draw thru configuration the chilled air is reheated (sometimes desireble for humidity control) by the fan and motor raising the supply air temperature 1-2 degrees. You must provide additional cfm to provide the same cooling effect.
Ah, isn't that fan temp increase going to be there refardless of the the relationship of the fan and coil?
Maybe this is one of those non-obvious things?
Good point, Bob, and absolutely true. In the sizes that are common to residential, i.e. 1-1/2 to 5 tons, the biggest motor you will see is a 3/4 hp, which adds 1920 btuh to the air at most. At 1950 cfm, this will increase the air temp less than 1 degree F. If that were a space cooling load, at 20 degree cooling this would require 88 cfm. Out of 1950 cfm, most usually and acceptably ignored. BTW, a good designer accounts for motor heat in the load calculations. But the bottom line is, the blower motor is in the air stream and the energy is added regardless of the location. If you were to split hairs, the system will be a little more efficient in a blow through design, as long as the air is equally distributed across the coil.
There are good reasons for draw through designs, but not on small systems.
Edited 4/30/2002 7:45:37 AM ET by Tim
A 3/4 HP motor would add 1909 Btuh (2545*.75) IF it and the fan were 100 percent efficient. Actually fractional HP motors have efficiencies of about 70%. In addition the fan is only 50 to 55 percent efficient. The total heat added is closer to 5,000 Btus. For a residential system with 1500 cfm and 1.25 inches of water total fan pressure rise, that equates to 3.1 degrees of rise, which increases the required cfm in a draw thru system by 13 percent.
A blower/motor that is 100% efficient would add no heat to the air. If the blower/motor were 100% INefficient, all of the horsepower energy would be seen as a temperature increase. Since that is not the case, work is actually done on the air increasing pressure and moving it through the system, you will never see the even the 1909 btuh added. It is conservative to assume so. An operational 3/4 hp blower will not add 5 MBH to the air under any circumstance. As I stated to Bob, a competent designer will account for that fan heat in sizing the equipment. No increase in air flow necessary.
I also question/challenge a couple of the other statements you made.
First, typically the air coming off of a cooling coil is not 50 - 52 degrees, especially in a residential system. 55 is the common design number, but again in residential applications, higher numbers are typical. We're talking about low pressure, standard systems here. There are systems that are designed to cool with 45 - 50 degree air, but they are not common, and very impractical for a house.
Second, you said, "For ####residential system with 1500 cfm and 1.25 inches of water total fan pressure rise..." No residential system that I have experience with will develop over an inch of static, 0.50" is standard. You know of residential system that operate in that range?
Correct me if I'm wrong. I'm always open to learning something new.
Edited 5/1/2002 8:34:53 AM ET by Tim
Even tho the 100 percent efficient fan and motor add no heat there is a thermodynamic temperature rise due to compression heating. Although not exact, this is almost the same as the total energy input. Your correct in pointing out that the system designer should account for this but the point of the discussion is the theoretical basis of blow-thru vs draw-thru.
You are correct that average residential systems have an External Static Pressure rise in the neighborhood of 0.5 iwg. However that is external to the unit. The fan must also make up the pressure drop thru the furnace heat exchanger (~0.125-0.375"), cooling coil (~.325 - 0.5), filter (~.125-0.25") and housing losses(~.25-.5") hence Total Fan Pressure rise of 1.125-1.5 iwg.
"Even tho the 100 percent efficient fan and motor add no heat there is a thermodynamic temperature rise due to compression heating. Although not exact, this is almost the same as the total energy input."
I disagree. The blower imparts a finite amount of energy to the air stream. That energy takes the form of pressure, momentum, heat and noise. Heat and noise are due to mechanical and electrical inefficiencies and are not almost the same as the pressure and momentum. You understand Thermodynamics? Do a quick energy balance on a theoretical air stream and tell me what you come up with after you acount for pressure increase. Use our example of a 5 ton, 3/4 hp residential furnace blower moving 1950 cfm and increase the pressure by 1.0". Assume the blower is 60% efficient and the motor is 75% efficient and the manufacturer sized the cabinet at 500 fpm.
"..but the point of the discussion is the theoretical basis of blow-thru vs draw-thru."
So you don't believe that the temp increase upstream of the coil will not be realized downstream of the coil? Go back to the energy balace again. Assume that the evaporator coil (and the rest of the refrigeration cycle) are not oversized and operating at full, design load. A finite temperature difference across the coil is possible.
True ESP is typically 0.5" for a furnace, but that does not include filter or coil losses, only losses through the cabinet, blower and heat exchanger. The coil (wet) and the filter have to be accomodated (as well as ducts, damper and registers) within that 0.5". The losses through the cabinet and heat exchanger are rarely published in the engineering data avialable to me. Some better furnaces will produce up to 0.8" esp, but not many.
I'm still curious about the LAT of 50-52 degF. If someone out there produces equipment that regulary performs that well, I will start specifying their equipment.
The heat is there regardless but the point was the increase in cfm. Typically air comes off the cooling coil at 50-52 F. If your space temperature is 75 this gives you 24 F. of temperature rise to take care of the sensible space load. With the draw thru fan and a 1 degree rise you now have only 23 F, which will require 4 percent more supply air.