where do i put my hvac dump? (posted by john)

[ASKNACHI]This question was posted on AskNACHI.org by john (from hope valley ri). [/ASKNACHI]cape style 2 story house
1800 sq ft
2 bedrooms, kitchen and living room 1st floor
2 bedrooms on second floor
i have a 3 ton central air, it is a a frame coil
and a blower directly above the a frame coil.
directly on top of the blower is
the supply plenum,
about 6 " above the plenum there
is an even split of 2 ducts ( 10w x 10 h)
one going left and 1 going right
the one going right has a 8 x 8 hard duct cut into it about 24 " from the split.
This 8 x 8 is all that feeds the 2nd floor.
Afer the 8 x 8 cut in the duct continues and has 3 flex hoses to feed 3 rooms on the first floor.

The 10 x 10 duct going left supplies cold air to the kitchen and living room on the first floor.

There are 3 zone dampers, 1 on the 10 x 10 duct going left, 1 on the 10 by 10 duct going right (this damper is AFTER the 8 x8 cut in and 1 zone damper on the 8 x 8 ductfeeding 100% to the 2nd floor.

The 8 x 8 hard duct is the only feed to the 2nd floor w/ 2 bedrooms.

It is always 10 degrees hotter upstairs
and the 2nd floor is always calling for cold air and the 1st floor cools off and the 2 out of the 3 zone dampers are shutting off the cold air to the first floor and all the 3 tons (1200cfm’s) are trying to get thru the 8 x8 (300cfm capacity) duct that feeds the 2nd floor
My coil is freezing us within 24 hours and there is plenty of gas
and it is a brand new unit.

When the 1st floor zone dampers shut, I want to put a dump to get rid of the xtra 900 cfm’s
and my hvac guy says the dump should be put on the
8x8 duct AFTER that duct’s zone damper.

It seems to me that there is 1200 cfm’s trying to get into the 8x8
from the 10 x 10 supply and the extra cold air is going to get pushed back towards the supply plenum and freeze up the coil.

I think we should put the dump after the plenum splits into the 10 x10" but Before the 8 x 8 cuts into the duct .
what do u think?
and should the dump be put on the 10 x10 on the right side which is the side the 8 x 8 is cut into it?
or should the dump be put on the left side
but of course before the left side zone damper.

With that kind of detail you should be a mechanical engineer …and thats what you need.
Maybe someone will help on that , but duct calculations are beyond SOP for most Home Inspectors

John go here…http://hvacmechanic.com/forums/resservice/thread_full450.htm

I do not want to sound negative. But quite honestly, I have never seen a zoned sytem work properly yet. I wish you good luck. That system will diffently be a hair puller.
You will need to give at least the square footage of each floor, supply and return ducts size and length for each level, how many return and supply ducts on each level, and the tonage of the blower.
It would be best to give this information to somebody who does that every day. It has been awhile for me. I am not a big fan of dumps. Your duct work was not engineered right if you need a dump. But that is just my opinion.

We have talked about your supply air. Let’s talk about your return air.

There is a reason for your upper floors to be 10° warmer. What is it?
The second floor of a Cape Cod style house has a greater heating/cooling load due to attic design and walls and ceilings with unconditioned spaces on the other side. Roof and attic temperatures are higher than the outdoor air creating a greater cooling load.

How are you getting this heat to the unit?
Your return duct must be balanced the same as your supply air duct needs to be.

There is also a problem in system design that must be overcome when attempting to condition two levels of one building. Air conditioned air is denser and wants to follow the path of least resistance (down). A balanced system must overcome his forces.

The reason your system is freezing is because the thermostat will not reach set point. You must answer this question first.

The zone actuators must be adjusted to a minimum closed setting (correct system balancing) if you are running one zone which is too small for the HVAC system size. You should never be forcing 3 ton capacity through 33% of the duct system. 100% shut down should not be occurring on any of the circuits in this application.

3 tons air conditioning appears to be too big for 1800 ft.² with a zoning system. The purpose of a zoning system is to be able to use a smaller system and rotate its use throughout the house using a smaller sized equipment.

From your description you have a 10 x 10 inch duct and three smaller takeoffs supplying the first floor (an area with more square feet but lower cooling load) and one, 8 x 8 inch branch to the second-floor. This layout does not appear to be designed for a zoned system.

Where did you come up with 300 CFM capacity for an 8 x 8 inch duct?
If you have access to this information then you should be able to calculate the settings of your actuators and the location of your barometric damper. You should realize that 1200 CFM produces about 2700 feet per minute velocity in that size duct.

You need to talk with a testing adjusting and balancing technician before you start hacking on anything.

No matter how much information you try to provide over the Internet, you will never supply enough information to get an accurate answer.

Zoned systems require a bypass duct to work properly, weather it is actuated or of a barometric design depends on the quality of the main controller. Take the first advise and enlist the help of a professional HVAC engineer.

Only about 2/3 zoned systems are dumped. Dumping a zoned system is an option, the cheaper option in the short term. You will be paying more in the heating and cooling bills if you reheat or recool the air in your duct work. Dumping is not required if the system is engineered correctly.

I think the cheaper option is not to do it at all if it’s not going to be done right in the first place.

I agree, “engineer it correctly”.