ductwork design

[nnmmbj]

Hello everyone, I am not sure I am posting this in the proper place, but here it goes.

We are installing duct work in my mom's house. Due to a financial bind we are doing it ourselves. We had installed duct work in our own home that was designed by a heating in cooling company years ago so we used the same basis for the design of my mom's. We are not contractors and we really were short on cash. This is the design:

We have a 12" round metal supply duct that travels from her new basement, through the old basement, up to through a chase to the attic. It then stretches from a T in two directions. One direction is short about 10' and then it L's for another 5 feet. The long side off of the T is about 30' then it L's for another 5 feet. The total length for the supply 12" pipe is about 100 feet.

The supply travels the same path, but it is 14" round metal duct. Total length of about 100 feet.

The supply and the return both have about 4 90 degree elbows.

The return trunk has 7 returns, one in each room, except the living room has a vaulted ceiling so we put in 2 returns. The returns have grates that are 6"X12" tied to 8" flex duct.

The supply trunk supplies (11) 4" X12" grates with 6" duct.

The house square footage is 2100.

The heating and cooling in the past was 4 tons.

We were hoping someone could tell us if we have enough returns is it based upon grate size or the flex duct size?

Will the duct work be large enough for a four ton unit?

Thank you in advance and I pray that someone understands this post.

[mchild]

The design of a duct system is not a willy nilly process. It is engineering science and if you do not understand the science (fluid dynamics) you will have to be very, very lucky to get it right. Is it to be assumed the home does in fact need 4 tons of cooling? What is the heating load? Has a load calc been performed? The first step in the design of any system is a properly completed load calculation (and no, the fact that home had 4 tons just means that someone else probably guessed at what was needed as most homes have over sized systems). That load calc will detail the heat loss of the home and the sensible and latent heat gain. This is all necessary to properly select the equipment that will be installed (not all 4 ton equipment produce the same output). The load calc will also provide you with the air flow requirements of each room within the home so the duct design can assure that each room will get sufficient air flow to maintain the comfort. Can't design a duct system if you do not know the amount of air that it needs to delivery to each room of the home. That would be like designing a highway without knowing where it was to start or end and what it had to go through in between. For discussion purposes, the air flow requirements of a conventional 4 ton heat pump is 1,600 CFM. In order to control air flow noise and the physical resistance of the air moving through the ducts, the velocity (speed of air through the duct) should be in the range of 600-700 FPM. If you could move 1,600 CFM through a 12" duct it would have a velocity of over 2,000 FPM. The blower in the air handler can not move the air against that kind of resistance and if they could you would think a jet was taking off in the home every time the blower cycled on. For the velocity in a 12" duct to not exceed 700 FPM, the CFM would have to be about 500. This does not factor in any of the other design issues that have to be included and impacts the air delivery system (and you have stated many of them in your description). You stated you have taken on this project to save money. It would seems as though you would have been better off to have a professional design and possibly install the system.

[MikeHolm]

Is the system a heat pump or is it a gas/oil/electric furnace. Where in the USA are you? Heat loss and cooling load is different for each area.

[Yuri B.]

Don't use much flex ducts, they give big pressure losses. As was also pointed, watch that speeds in ducts were not too high.