1CFM Per Square Foot, Or Follow Heat Load Calculation? - HVAC - DIY Chatroom Home Improvement Forum

 DIY Chatroom Home Improvement Forum > HVAC 1CFM per square foot, or follow heat load calculation?
 Register Blogs Articles Rewards Search Today's Posts Mark Forums Read

02-08-2012, 12:56 PM   #1
Member

Join Date: Oct 2011
Location: Phoenix, AZ
Posts: 115
Rewards Points: 81

So I'm building a bathroom and closet addition on my house and I want to tackle the installation of ducts, supplies and returns on the addition (tying them into my existing 3.5 ton AC/gas pack unit. Total conditioned space will be about 1350 square feet after the addition is complete.

On to my question! I have completed a full calculation for the house according to Manual J using the "HVAC-CALC" program. Here are the specifics:

Bathroom Addition: 180 sq. ft., north and west exposure, R-19 walls, R-30 attic, etc... TOTAL Heat Gain 3,994 BTUH

When I run a duct calculation based on 1400 CFM (the CFM spec'd on my Lennox unit) for the whole house, I get 167 CFM for the bathroom in a 6" round flex duct. This is less than the 1CFM per square foot I keep hearing about. My A/C has 35,000 BTU of cooling capacity. 167 CFM to the bathroom should then provided roughly 4175 BTU of cooling, right?

So do I trust the calculation, or do I increase the size of the duct to provide the 1CFM per square foot? I also have to consider that the duct run is roughly 20 feet (straight run).

02-08-2012, 01:10 PM   #2

Join Date: Mar 2011
Posts: 5,884
Rewards Points: 2,270

This is including that 200 sq ft closet and master bedroom? A system has to be sized accordingly to be able to provide the entire structure proper cfm. What you are doing is taking away from the systems ability to condition the original space when it was originally sized for the home without any addition.

What I would do is have an hvac a manual j performed to verify the proper system size before adding any runs. This new manual j will entail previous and original structure plus the addition. You are more than likely going to have to upsize the entire hvac system with how you are doing this.

I'm actually surprised that no one has mentioned this on your previous threads.

__________________
Thanks.

 02-08-2012, 01:35 PM #3 Member   Join Date: May 2011 Location: NW Burbs of Detroit Mi. Posts: 1,107 Rewards Points: 500 I have never heard of a good out come when sizable additions hvac ducts are just run off the old system with redesign. Plenty of jobs I walked on and let the other guy have. Than I get the call when the new addition isn't heating or cooling properly.

 02-08-2012, 01:47 PM #4 I'm Your Huckleberry   Join Date: Mar 2011 Posts: 5,884 Rewards Points: 2,270 Small, single runs such as one 5" to a restroom I've added without any problems but that's a single run to a tiny space and not much cfm. OP, you may get lucky and your existing system was over sized for the original structure to begin with. Just have the man j performed to know where you stand. __________________ Thanks.
 02-08-2012, 01:54 PM #5 Member   Join Date: Oct 2011 Location: Phoenix, AZ Posts: 115 Rewards Points: 81 The unit was replaced two years ago to be a bit larger than needed since we knew we would be expanding. The bedroom is existing and is not being altered - it has the same supply its always had. The closet is separate from the figures I listed in my post above and has a heat gain of 1,418 BTUH. The Calculator calls for 63CFM for that 97 sq. ft. area. The manual J for the entire house comes to 33,699 BTUH total, which assumes an outside temp of 110 degrees, inside temp of 72 (pretty low). These numbers take include the bathroom and closet addition. If I change the manual J calculation to the specs of the house PRIOR to the addition being built, I end up with a 33,610 BTUH heat gain. The reason it is so similar is because the addition covers up/eliminates two average sized single pane windows with western full sun exposure. It also covers up 18 linear feet of the existing house which is 8 inch concrete block with no insulation whatsoever. Additionally, we tore down a 67 square foot bathroom that was factored into the original Manual J (when the A/C was replaced) but had no air supply in it. That bathroom was poorly built and poorly insulated with only R-11 insulation, a flat roof and had full south and west exposure. I had a HVAC contractor out who felt that all things considered, the A/C unit could handle everything with no problem. The problem is, he wanted \$2K to do the duct work which is far more than I want to pay. I feel I can do the duct work myself, but I'm struggling with the calculations I have from the HVAC-Calc program.
 02-08-2012, 02:00 PM #6 Member   Join Date: Oct 2011 Location: Phoenix, AZ Posts: 115 Rewards Points: 81 Here are the Manual J's I did for the house both *prior* to the addition and separately, including the addition. Thanks for all the continued help and input! Belmont__Prior_to_Addition__ByRoomEachComponentSelectionsSqFt.pdf Belmont__Includes_Addition__ByRoomEachComponentSelectionsSqFt.pdf
 02-08-2012, 02:02 PM #7 I'm Your Huckleberry   Join Date: Mar 2011 Posts: 5,884 Rewards Points: 2,270 http://contractingbusiness.com/enews.../cb_imp_46087/ http://www.servicor.com/airflow.html What you need to know is how much cfm the room requires and then what size duct will accomodate the required cfm. __________________ Thanks.
 02-08-2012, 02:03 PM #8 I'm Your Huckleberry   Join Date: Mar 2011 Posts: 5,884 Rewards Points: 2,270 If you've done a heat load and had a bigger system installed in light of the addition than no manual j is required, just the cfm of the room(s) and again, the correct sized duct to deliver the air. That's it, you're done. __________________ Thanks.
 02-08-2012, 02:32 PM #9 Member   Join Date: Oct 2011 Location: Phoenix, AZ Posts: 115 Rewards Points: 81 I'm still not clear on how to calculate how many CFM I need to cool a specific room. The calculator program spit out the following, but it falls short of 1CFM per square foot for the bathroom addition which is why I'm questioning it. Belmont__Includes_Addition__CFMReport.pdf From what I can find, it seems that bathrooms typically require 3 air changes per hour. Using the bathroom's volume of 1,530 (15'x12'x8.5'), this comes out to 4,590 CFH for three air changes per hour. 4,590 CFH divided by 60 minutes gives me 76.5CFM, which is way below the calculator's number, and below 1/2CFM per square foot. I'm thoroughly confused. Edit: I don't know if it matters, but I also have a 150CFM bathroom fan that will vent to outside as needed to remove moist air from the room after showers, etc. This is already factored into the Manual J.
 02-08-2012, 03:26 PM #10 Member   Join Date: Oct 2007 Location: Manitoba, Canada Posts: 400 Rewards Points: 380 When you get into calculating your duct sizing there are a lot of factors to take into consideration. What you have for static pressure in your ducting system, what your unit is actually delivering for CFM (not necessarily the same as rated CFM) due to pressure drops, leaking fittings, filter restrictions, etc. Of course size of the duct you are using will directly effect how much heat/cooling you are delivering to the space, it will be hard to determine how much CFM will actually come out of the register without knowing the pressure in the ducting system (the size of the main trunk line/plenum and where the other runs tie into it will all contribute to what the velocity in your new duct will be (which will determine what your delivered CFM will be. All that being said ductwork can be very forgiving, provided it isn't undersized too much. I think that your original size of 6" to give you 169 CFM seems a bit optomistic, you could install a 7" line and then install a hand damper to reduce airflow down as required. Just some thoughts on your project from a general view.
02-08-2012, 04:10 PM   #11

Join Date: Oct 2008
Location: Somewhere
Posts: 34,413
Rewards Points: 7,816

There is no actual 1 CFM to the sq ft rule.

167 CFM could be 4452BTUs, 5010BTUs, or 5724BTUs, all depending on what your blower is set to move per ton.

 Thread Tools Display Modes Linear Mode

 Posting Rules You may not post new threads You may not post replies You may not post attachments You may not edit your posts BB code is On Smilies are On [IMG] code is On HTML code is OffTrackbacks are Off Pingbacks are Off Refbacks are Off Forum Rules

 Similar Threads Thread Thread Starter Forum Replies Last Post JeepNick HVAC 13 02-07-2012 10:21 PM GuitarPlayer Electrical 16 12-17-2008 11:48 AM Mechanically Challenged Roofing/Siding 15 10-23-2008 09:05 PM darren Roofing/Siding 8 04-13-2008 09:29 PM RM575 HVAC 9 07-04-2007 12:37 PM

Top of Page | View New Posts