[handyman923]

I am working on the design of a new duct system as part of a remodel we are doing. We are replacing an electric furnace from the crawl space with a new heat pump in an upstairs utility closets. I have performed a Manual J analysis and found that I need 550 cfm downstairs and 320 cfm upstairs.

My thought is to do the following for the supply ducts:
- Start with a 14" round duct off of the supply plenum
- Then use a 14x12x10 Wye with the 10" feeding upstairs and the 12" down stairs

My concern is that the 12" duct will be 40' long even before I split off to the individual registers. Do you think I should increase this to 14"? My concern is that the 14" would be too big making most of the CFM's go downstairs and not enough upstairs.

Thanks!

 

[user_12345a]

How did you decide on a 14" duct?

You need to get the stuff sized based on friction rate, determined by total equivalent length and design static pressure.

There are manuals online (as well as published for purchase) which explain the theory and show examples.

Consult a designer if unsure.

You have to get the cfm based on the equipment selected and adjust the values provided by the software.

For example if you were getting a 2.5 ton heatpump (assuming the cfm was calced based on cfm per ton), you would need 1000 cfm.

14" for shows 900 cfm (online duct calculator) at 0.1" per 100 ft -> 0.1 may be too high of a friction rate, and that's if you're using metal.

 

[handyman923]

It will be a 2 ton heat pump (confirmed by Manual J and contractor who gave estimate for equipment), so I will have 800 cfm total. The cfm do not add to 800 cfm since the heat and cooling loads are different.

Using an online calculator, I came up with 14" diameter for 800 cfm at 0.06 friction and 750 fpm.

Is there a way to size the longer main duct line without doing a full-blown Manual D? Since neither of the contractors I had out did a Manual J, I highly doubt they will do a Manual D.

Thanks,

 

[user_12345a]

Do it right, you're spending thousands of dollars.

You may need to hire an independent designer; that designer may work with certain contractors.

This stuff must be engineered.

How much heating capacity are you putting in? What source of heat

For example if you're in a cold climate and putting in 15 or 20kw of heat strips, the cfm needs could be higher than 800.

If you're putting in a gas furnace, same thing - 800 cfm is maybe what is 40 to 50k btu furnace moves.

 

[handyman923]

You are probably right... I should higher a professional designer. But in the mean time, this is fun to play with...

I found a spreadsheet at ACCA that does a Manual D design. I looked at the performance data for the equipment the contractor specified.

I have figured out my TEL (747' which seems quite large), but I have a questioned how to determine the appropriate external static pressure to use for the friction calculation. For the highest speed, I can use 0.40" at 835 cfm or 0.50" at 763 cfm. The performance data says that I need 775 cfm for 15 kW of strip heat. So if I want at least 775 cfm, the max esp I can use is about 0.48".

Two questions:
(1) If I design the ducts using 0.48" and 775 cfm, how does the blower know to "use" this combination of esp and cfm? Is it just about proper sizing of the ducts?
(2) Using 0.48" and subtracting component losses of 0.204", I get a friction factor of 0.04" which seems low. Is this a problem if I use this in the duct design?

Thanks!

 

[user_12345a]

If you get a air handler with a variable speed blower it automatically adjusts. You have to set the cfm using dip switches.

*Variable speed blower air handlers also have a higher max static pressure, sometimes up to 0.8" before airflow starts dropping off.

PSC motors have to be adjusted manually (with static pressure measurements) to get the correct cfm, usually 4 speecs to choose for.

Generally air handler with a 2 ton blower, 1/5 hp would have to be on high anyhow for that size condenser

If you match a 2.5 or 3 ton nominal air handler to a 2 ton heatpump, you'll need to cut the blower speed.

You need 800 cfm for a 2 ton system and it wouldn't hurt to design for 900 cfm -> quieter.

The chart for an air handler includes a coil so you would probably be using 0.3" as the design rather than 0.5".

Design for 0.5" external to air handler and it could be like designing for 0.7" or more if u were using a furnace.

I can't comment on the specifics (what typical equivalent length would be), though you should trust the theory if it's done right.

Components would be grills, filters.

I suspect that you would want at least 0.1" to work with on the supply and another 0.1" on the return after components.

May need to adjust the components to get a lower pressure drop -> larger grills, filters.

I would size the machine right, don't hesitate to oversize the ducts.
A lot of guys will do it the other way around, especially the clown contractors who don't do load calculations. Recipe for disaster.

 

[beenthere]

What was/is the pressure drop of the devices that you came up with .204". I'm guessing the air filter is the main pressure drop.

747' TEL is a lot.

 

[handyman923]

My losses include:

a) Air delivery performance correction component pressure drop at Indicated Airflow (Dry to Wet Coil) = 0.076"
b) Electric resistance heating coil = 0.02" (1 additional to what's included)
c) Filter = 0.048"
d) Supply outlet = 0.03"
e) Return grille = 0.03"

Do I not need to include (a)?

I took a look at my TEL. I can reduce it by 60' by changing the register boot. Do you know what the R/D ratio is for an adjustable elbow? I was assuming 0.75, but if it is larger, it would help reduce my TEL.

Thanks!

 

[user_12345a]

You don't need to include the coil, it's already in the air handler.

You need to include everything outside the air handler and use a fairly low friction rate on the chart.

0.5" is high for an air handler -> the coil produces one of the largest pressure drops. (0.1 to 0.2" or higher)

0.5" is for a furnace with a coil.

what's r/d ratio?

 

[handyman923]

Thanks for all of the help!

What do you recommend I target for my Available Static Pressure? My component losses are now 0.13". I have reduced my TEL to about 600' by changing a few fittings. If I want to get my friction up to 0.06, then I need an available static pressure of 0.36" and a External Static Pressure of 0.49".

For the 90 deg elbows, the R/D is the ratio of Radius to Diameter. In Manual D, I get an EL = 30' if R/D = 0.75 and EL = 20' if R/D = 1.0.

Seems like my TEL may still be too high...t's tough trying to work new ducts in an old structure without too many soffits and low ceilings...

 

[user_12345a]

When reading these posts keep in mind that i'm not a experienced pro, so someone like beenthere would have better ideas.

Air handler model number? that may help to decide what kind of static to design the system around.

Main issues to me are noise and getting proper airflow. Doubt u would have noise issues at 0.6 so i may have been wrong.

Another thing to look at is balancing.

To me it would make sense to use a lower friction rate for nearby branches.

May also make sense to use a lower friction rate if there's a shorter trunk.

If you design the system all using 0.6", branch lines near the air handler will get lots of airflow while branches near the end of the 40' would get little.

it's far from an exact science.

 

[beenthere]

Most els will be .75.

 

[handyman923]

The contractor is specifying a Carrier FB4C024.

The existing electric furnace is in the crawl space and has about 5 - 6" ducts attached to the plenum. There is currently no ducts going upstairs. One idea I had to run the ductwork (and hopefully eliminate some of the long TEL wyes) is to do the following:

a) Run a main duct off of the supply plenum
b) Use a wye to split off a 10" duct to feed the upstairs
c) Run the main duct downstairs into the crawl space and connect to the existing supply plenum
d) Then I could re-use the 5 existing 6" ducts that radiate from the old supply plenum. I may need to add one or two to make sure I have adequate airflow downstairs.

Would this work?

Thanks!

 

[user_12345a]

I can't recommend that and don't see how it would reduce the tel.

I know that in typical two story homes the trunk lines are all in the basement or crawlspace and the branches run up to the second floor.

I don't know what kind of space you're working with.

Are you using flex or metal?

If it's like a mini-basement (not a dirty, ventilated crawlspace), maybe 4 to 5 ft' ceiling height it can be done with rectangular metal ducts.

 

[handyman923]

One more question on Manual D...

When I calculate the TEL for the fittings, do I need to adjust these for a duct velocity different than the velocities in Manual D? For example, I have a duct velocity of about 500 fpm, but Manual D lists an EL for an elbow as 30' at 900 fpm. I read in a paper online to adjust the EL by (Vx/V)^2. Is this correct? This would really help reduce my TEL.

Thanks!

 

[beenthere]

Yes, you can do that and get a lower TEL.

 

[handyman923]

I have been working on sizing my ducts using my heating and cooling loads. As you can expect, my heating loads are higher downstairs and cooling loads are higher upstairs.

As an example, for heating, I need about 600 cfm downstairs and 200 cfm upstairs. For cooling, I need 450 cfm down stairs and 350 upstairs.

I am wondering if it is possible to install seasonal automatic dampers? I would size the downstairs for the max heating cfm and the upstairs for the max cooling cfm. When the heat is on, the upstairs damper would close to a minimum set point (say 75%), and the downstairs damper would be fully open. The opposite would happen when the ac is on. Is this possible?

I would rather not mess with a fully zoned system since the upstairs and downstairs loads are so different.

 

[beenthere]

You could automatic dampers. You would probably need to use relays, as your thermostat may not take the load of the damper motors.

 

[user_12345a]

Easier to put manual dampers and adjust them seasonally.

75% open doesn't correspond with a 25% reduction in airflow.

Have to measure airflow at each vent and no one does that.