Air Conditioning - Cased coil installed directly on furnace?

Am installing a Goodman CAPF3636B6 cased coil with a Goodman GMV950453BXa 95% gas furnace positioned to work as an upflow unit. My specific question is in regards to the installation of the cased coil. Should it be installed directly on top of the furnace or should a small duct transition piece be fabricated? The coil plenum opening is designed to fit right over the furnace plenum opening (and does fit) but there is no way to attach the furnace plenum "tabs" to those of the coil itself. I am thinking it is just supposed to sit on the furnace, be secured in place with a few pieces of trunk duct straps and then mastic sealed where the finished surface of the furnace meets the finished surface of the cased coil unit.

I have done about 4 weeks worth of reading/calculating to make sure the system is installed correctly and this is about the only factor of the installation I cannot find any information on. Any opinions will be welcomed.

The coil is usually installed directly over the furnace.
A transition between them would improve your ability to clean and inspect it though.

A transition between them would improve your ability to clean and inspect it though.

All the cased coil I've seen are easy to get to, just pop the front cover off, and there you are. Unless I have not come across one that you are talking about?

The cased coil does have a front inspection plate so no issue with accessing it. It also has a double drip collection system so one of which is a high temperature rated plastic (rated up to 300 degrees) so I don't foresee ever having to access the drip pans.

The main reason I asked about this was because I assumed if they were meant to fit together from the factory there would be some obvious connections points to secure the coil and a method of sealing the connection to ensure no air leaks.

I guess I will just set the coil on the furnace, secure it however is easiest and seal the seam between the 2 pieces with regular duct mastic.

Thanks for the input guys

That can work okay on a gas furnace, however, oil furnaces with large heat exchangers near the top of the furnace should be installed on a transition at least 6" above the furnace.

Otherwise, there would be an airflow restriction with a lot of back-pressure & turbulence.

A large portion of the velocity & static pressure is lost before it enters the evaporator & the duct system.

Do a Google search on the above oil furnace problem.

I thought the issue with coils and oil furnaces was the heat. I never would have figured coil restriction would be an issue with oil furnaces since they do produce such hot air and thus require fewer CFM to heat.

Why would a 6" transition make a difference for flow? If the size of the opening on the furnace doesn't change and the size of the inlet on the coil doesn't change why would a 6" gap between the two have any bearing on that?

I did read that a transition piece between and UNCASED coil and furnace is necessary but didn't find any info on the matter regarding cased coils.

Thank you for the input fellas.

I usually dread cleaning these coils. It always feel like I'm going to see some white smoke before I can access the dirty side of it.

http://i151.photobucket.com/albums/s160/Houston204/evapcased.jpg

oh .. duh! Yeah, you are right about that!

I thought the issue with coils and oil furnaces was the heat. I never would have figured coil restriction would be an issue with oil furnaces since they do produce such hot air and thus require fewer CFM to heat.

Why would a 6" transition make a difference for flow? If the size of the opening on the furnace doesn't change and the size of the inlet on the coil doesn't change why would a 6" gap between the two have any bearing on that?

It has a huge bearing because it provides room for the air to flow between the huge oil heat exchanger & the coil.

I did read that a transition piece between and UNCASED coil and furnace is necessary but didn't find any info on the matter regarding cased coils.

Thank you for the input fellas.
It is because the huge heat exchanger comes close to the top of the furnace & forces the airflow to the outer portion of the furnace.

Therefore, the airflow velocity hits the bottom of the evaporator coil drain pans & causes excessive back-pressure.

Below the evaporator coil is the worst place to lose velocity and generate back-pressure.

It is a real blower & airflow killer.

Where the duct system needs the velocity and static pressure is at the diffusers.

It has a huge bearing because it provides room for the air to flow between the huge oil heat exchanger & the coil.


So even though there is no where for the air to go with a 6" tall duct support (the exact same size as the furnace supply plenum and cased coil "intake side") it still helps reduce back pressure? I guess I don't understand how that helps since the same amount of air still has to travel through the same size openings. I understand how change of direction - like elbows - add flow restriction and back pressure but the furnace-to-coil connection involves no change of direction.

If you don't want to explain it to me can you give me a site or two that further explains this?

So even though there is no where for the air to go with a 6" tall duct support (the exact same size as the furnace supply plenum and cased coil "intake side") it still helps reduce back pressure?

I guess I don't understand how that helps since the same amount of air still has to travel through the same size openings. I understand how change of direction - like elbows - add flow restriction and back pressure but the furnace-to-coil connection involves no change of direction.

Sorry, but in this scenario it does involve changes of direction & a lot of back-pressure.

My brother's little 1.5-Ton system split-system is NOT delivering even half the required airflow due to the E-Coil being installed directly on top of the furnace. Yes, I measured the airflow.

If you don't want to explain it to me can you give me a site or two that further explains this?

Sorry, I don't check my gmail very often, so maybe kept U waiting?

The huge heat exchanger very near top of furnace is either octagonal or round & takes up the entire center area of the furnace.

This causes a restricted area that creates back-pressure & a lot of turbulence before the air can even enter the evaporator coil intake area.

Maybe someone can make a drawing illustrated what that does to airflow. That causes the loss of velocity & there is nl transition area long enough to regain static so it's all trapped prior to entry i to the coil.

For one thing, that extreme back-pressure will also unload the blower wheel further reducing airflow.

U don't understand how that can cause a restriction? Wow.

Do several Google & Yahoo searches with different key words for this specific problem, include "oil furnace" in it!

On this DIY Forum, I'm not allowed to post links to my non-commercial web pages!

All U have to do is test an oil furnace scenario, "directly on top of the furnace & then 6" above the furnace on a transition," that I have described & U will know what it does to airflow!