Return Duct dia vs. Room Supply Ducts total square inches

[Henry1]

Is a 14" dia flexible return duct (153 square inches) correctly matched for heating a 1500 square foot house with 10 six-inch dia supply ducts (280 total square inches) ?

Since our new downflow 55,000 btu oil furnace shortcycles, one contractor told me I was short 200-300 cfm at the blower. But my installer (who also installed the new return air) said the furnace is getting the right amount of return air and that the fact that it is shortcycling on the fan and limit control 250F is NOT BECAUSE OF HIS new return air installation).

The one contractor showed me the 700 cfm (for return air) on his circular slide rule. My installer showed me 1000 cfm on his rectangular slide rule to prove to me that's what his 14" dia flex duct would deliver to our downflow oil furnace. Both of these gentlemen stood in front of our shortcycling furnace with their slide rules. Who's right on this return air???

This is "general question" since I'm not measuring the length, width, diameter of the rectangular sheet metal supply plenum/trunk lines under house that feed the 10 six-inch dia ducts that connect to the room registers. House has heating system only, we have no air conditioning system. Thanks.

[pstu]

ACCA Manual D is the *right* way to design ducts including return size. The current version says you should not exceed 700 fpm (FEET/MIN) for either return or supply if using flex duct. That strongly suggests that supply area and return area might be close to equal. My understanding is some designers try for lower air speeds in returns such as 400 fpm, that would make the return area rather BIGGER than supply area. That is the case in my own home, after asking the pros on this board I learned that there is little or no penalty for having too *much* return capacity.

Your 14-inch return is 1.06 sqft in cross section, therefore adequate size for 744 CFM without violating the Manual D limits. Try to pull much more air through that, and the air handler will experience much increased static pressure and sometimes the air handler cannot supply the volume. Maybe our pros can say from your BTU rating, what CFM is likely to be required.

It would be good to have your ESP (external static pressure) measured, if the number is excessive then that probably proves your airflow is inadequate. In particular the vacuum at your supply plenum may be the important number. The details depend on the model of your furnace. Many techs are unwilling to measure for one reason or another, and perfer to talk for 5 or 10 minutes rather than do this work. Hope that is not the case with your two techs, because words cannot make a problem go away. In other words if your baloney detector is going off, you may need a better tech.

I am a homeowner in S.Texas so any advice from pros in your region may be more on-target. Where I am air conditioning is king, one of my two systems runs about 0.46 inch w.c. for ESP, the other about 0.70 (gotta think about that one). My air handler is the "VS" type which will operate normally under those backpressures, however yours may not be.

Hope I am not being too harsh on your pros.

Best of luck -- Pstu

[Shophound]

Is a 14" dia flexible return duct (153 square inches) correctly matched for heating a 1500 square foot house with 10 six-inch dia supply ducts (280 total square inches) ?

Since our new downflow 55,000 btu oil furnace shortcycles, one contractor told me I was short 200-300 cfm at the blower. But my installer (who also installed the new return air) said the furnace is getting the right amount of return air and that the fact that it is shortcycling on the fan and limit control 250F is NOT BECAUSE OF HIS new return air installation).

The one contractor showed me the 700 cfm (for return air) on his circular slide rule. My installer showed me 1000 cfm on his rectangular slide rule to prove to me that's what his 14" dia flex duct would deliver to our downflow oil furnace. Both of these gentlemen stood in front of our shortcycling furnace with their slide rules. Who's right on this return air???

This is "general question" since I'm not measuring the length, width, diameter of the rectangular sheet metal supply plenum/trunk lines under house that feed the 10 six-inch dia ducts that connect to the room registers. House has heating system only, we have no air conditioning system. Thanks.

Two contractors stood in front of your short cycling furnace with two versions of a ductulator, but NEITHER ONE broke out a manometer and ACTUALLY MEASURED the TOTAL EXTERNAL STATIC PRESSURE (TESP)??

Unless this is done, all talk about duct sizing is pointless. The furnace has a maximum TESP that it is allowed to run at. If you're above that limit you're going to have problems, such as short cycling. If you can find a contractor in your area that has obtained training from the National Comfort Institute, chances are he might have a clue about air distribution and can tell you without any bullcrap why your furnace is short cycling.

[Shophound]

ACCA Manual D is the *right* way to design ducts including return size. The current version says you should not exceed 700 fpm (FEET/MIN) for either return or supply if using flex duct. That strongly suggests that supply area and return area might be close to equal. My understanding is some designers try for lower air speeds in returns such as 400 fpm, that would make the return area rather BIGGER than supply area. That is the case in my own home, after asking the pros on this board I learned that there is little or no penalty for having too *much* return capacity.

Your 14-inch return is 1.06 sqft in cross section, therefore adequate size for 744 CFM without violating the Manual D limits. Try to pull much more air through that, and the air handler will experience much increased static pressure and sometimes the air handler cannot supply the volume. Maybe our pros can say from your BTU rating, what CFM is likely to be required.



For a trunk return, recommended feet per minute is 600 fpm, flex or metal. For branch returns, 400 fpm is recommended. Careful not to confuse cfm for fpm. The feet per minute limit of 600 is a design aspect primarily for noise concerns.

[Henry1]

Total ESP measured .34 W.C. which is mid-range between the furnace spec of .15 to .50 W.C.

[mchild]

Henry1,

Are you sure that is the total and not just the supply side static? Do you have the individual readings for the supply and return?

[pstu]

You should count your blessings even though you have a problem as yet unsolved. Seeing as your tech has already taken an ESP measurement, you are hooked up with someone actually using instruments and training to diagnose a problem. One thing is parent, one or both are not hacks.

If you can find separate readings for supply and return components of ESP, that would help everybody feel they are working with valid numbers. FWIW when my return vacuum was first measured it was 0.45, so my system was worse off than yours for puny return capacity. That improved to 0.15 when return capacity was more than doubled.

I think it's prudent to re-visit, perhaps confirm some facts in your original message. Your one tech said 1000 cfm... why again? Ditto for the tech who said 700 cfm... can you confirm he was speaking CFM and not FPM (although with a duct size of 1.06 sqft the numbers are close). I am interested in asking what principles they were using, whether they were calculating a measurement of actual airflow or not.

The fact remains that air speed limits are similar or lower in return ducts, therefore there is a mismatch when your return is smaller than supplies. I had been hopeful to find a solution based on so little return that airflow was too small for the furnace, it is not clear so far that is the case.

While I believe my questions are useful, I am not a pro here. If there is only time to address some of the questions asked of you, feel free to pay more attention to those coming from actual pros. But I *do* want to see your problem solved.

Best wishes -- Pstu

[Shophound]

Total ESP measured .34 W.C. which is mid-range between the furnace spec of .15 to .50 W.C.

If I recall, in another post we had this discussion:


As an aside, why do I get the feeling the tech had two static pressure probes going simultaneously, one in the supply, one in the return? Maybe I misread the post..."

You are correct. The Mag had a hose going to the supply port in the trunk line and another hose going to the return port in the return air plenum right above the furnace at the same time.


This was also posted pertaining to the correct way to use a Magnehelic:

http://www.achrnews.com/NEWS/2003/15/Files/Images/81043.jpg

Was there ever another set of readings done correctly?

[ch4man]

a furnace overheating many times is low airflow,but can be overgassed(i dont do oil but is it firing corroectly?) also a loose belt or dirty blower will do two things....decrease air flow and..lower static readings!!..get another contractor out to properly diagnos your furnace...

[Kevin O'Neill]

I have installed 14" flex on 2.5 ton systems and measured full required air flow with a flow hood. However noise is usually not a problem with flex duct.

That being said, a long flex or a duct system with lots of fittings or length WILL reduce air flow. I would want to measure static then check it against fan curves. You would also need to know what blower speed it is on if it is a multi tap motor.

The other option is to break out the flow hood and measure actual air flow. An under performing blower will give low static readings and low air flow as well.

If you have poor air flow across the heat exchanger where the limit sensor sits, you will have short cycling even with adequate OVERALL air flow. I saw one like that once. The return duct connection did not provide good air flow to the limit switch but the furnace had a proper temperature rise. Drove me nuts finding it.

Size is not everything. :D

[deejoe]

Henry1..... what size is the burner nozzle? a .50 or a .65 or (yikes) larger?
Too large a nozzle can cause your problem.

[Henry1]

Shophound and other members:

I have contacted a National Comfort Institute specialist to arrange a static test and I will get back to you with the reading for the return and the reading for the supply. At that time I will answer the questions you have above.

Thank you.

[dmctechtrainer]

If you have a cooling coil on this job the supply static will need to be measured upstream of the coil...you want to be directly across the furnace at return and supply for the ESP reading when you compare it to the furnace data...as the other dudes said - overfiring can also cause it (nozzle size too large or supply pressure too high...your tech can check air temperature rise and net stack temp / if both are too high its overfired...) A venting problem can also cause overheating...don't overlook the basics...has the tech pulled the limit and checked for insulation or other foreign debris from the install?...has he verified that the actual temperature at the limit location is reaching the listed cut-out point?