Further Clarification Concerning Airflow Variables...
Thanks for the support beenthere.
Originally Posted by beenthere
I need to explain further differences between most Oil furnaces & gas furnace applications.
First, the Oil HT/EX goes all the way up from the burner chamber therefore the blower is off to the side blowing directly at the side of the HUGE HT/EX.
The 80% gas furnace's blower blows up through the HT/EX'ers & there is airspace room between them.
If you were to put a 5-Ton coil within 3" of the two setups, which one would cause the most restriction of a 2000-cfm of airflow?
If you were to place the coil directly on top of each one, the Oil HT/EX would block off perhaps all the flow, the gas EX would allow air to flow between the HT/EX'ers.
You can get easily fooled when checking static pressures to determine airflow on an oil or gas furnace that has the blower wheel blades full & the E-Coil partially blocked with lint; because checking above the coil, on an upflow system, will show a low static. Looking at the chart you'll think it's delivering plenty of airflow; an airflow check will reveal that error.
Here in the cold north country I had a contractor tell me that a1.5-Ton system should have a 1.5-Ton evaporator.
The reality is that the system we were discussing had a 140,000-BTUH input & 112,000-output which called for around 1200-cfm of airflow in heating mode.
The 2-Ton coil could go to 900-cfm, but it really needed a 3-Ton coil rated at 1200-cfm at 0.50" ESP.
In the cold climates it is important to FIRST Weatherize to reduce furnace sizing & airflow requirements. Then make the airflow system as efficient as possible so you can work with a proper condenser & coil sizing setup.
Why the oversized A/C system with extremely low airflow doesn't cool many contractors will simply say you need a larger A/C... :playing:
Well, there is more, that I've never found in the best text books, but I need to quit typing. These airflow realities are extremely important, but I never see or hear anything on them, in textbooks, only in rare mfg'ers' manuals & no videos - illustrate those critical airflow design variation situations.
The old blower wheels would simply unload if the ESP static went much above 0.50", newer blowers are engineered to handle a higher ESP without unloading.
If you pulled those old blowers out of the furnace, as I did, & used a ply-board to change the resistance - using an amp-probe; when you moved the board up away enough, the blower would move too much air & the motor would overload & bog-down.
Moving the board closer would unload the blower wheel & amp-draw would move lower.
Got to quit, but never underestimate the airflow variables...
Skippedover was just having some fun with me, which I appreciate. - Darrell
Oil furnace, limit switch triggering, poor air flow? (Continued)
I appreciate all the replies so far, however I'm still confused and would appreciate anyone who can clarify the situation. I am using a professional HVAC contractor and was hoping to get a second opinion from this expert group. Thanks for any clarification or additional advice that you could provide.
I summarized the situation below, my 2 questions, and the answers so far. The answers to question #1 (do I need to fix this) are fairly consistent, however there are many different professional opinions regarding question #2 (what specifically needs to be done).
Thanks again for all of your expert advice with this!
Situation (summary of description in post 1 and 5)Lennox 023Q5-140 oil furnace, with forced air, which is approx. 10 years old.
Lennox told me that it is a 5 Ton 2,000 CFM unit.
3,500 square foot home in Connecticut
High limit switch was stuck in open position 1 year ago.
The mechanic found the total static pressure was 0.84-0.9" WC.
The "cut-in" between the main air return trunk and the furnace is 14" x 22"
I use standard (NOT high efficiency) 16" x 25" disposable filters
The main return air trunk is 12" x 24"
Question #1. Do I need to do anything? It has been working for 10 years with no problems other than a one-time stuck high limit switch and an unrelated circuit board problem. So why should I do anything? Maybe just let it be? Will it work for another 10 or 20 years if I do nothing?
Summary of answers so far:It hasn't worked fine for years. It might have made heat, but it's not run properly and safely.
Better efficiency, more reliable, less chance of damaging other components, probably better comfort too.
Bouncing off the limit is not acceptable and will eventually ruin the furnace not to mention the potential for other dangerous issues.
By running this unit with inadequate ducting it will dramatically reduce the life of the furnace
Its been working for 10 years, but has had the same problem for those 10 years, just its catching up to you now. If the heat exchanger fails. You'll spend more getting a new furnace, since warranty doesn't cover failure from low air flow.
The need is gigantic!
Question #2. What specifically needs to be done?I understand the concept of "more air" or get the static pressure to 0.5" WC, but I want to know specifically what the HVAC professional needs to do to achieve this, e.g. enlarge cut if for a 20" x 25" filter, add another cut in for two filters (perhaps by wrapping the return duct around the furnace so I can have a filter on the side AND the back, enlarge the main return trunk, replace the high-limit switch, etc.
Summary of answers so far:
The furnace is supposed to have return air pulled from both sides of the furnaces. It was actually shipped with 2 16x25 air filters tracks, for that reason.
I would first find out if you really need a 140kbtu furnace....and if so, have the heat exchanger inspected for any cracks or nasty stress points. And if that all checks, then proceed with the proper duct modifications to get the air proper.
Downsizing the firing rate and properly adjusting the temperature rise is the first remedy that comes to mind.
Typically, when you get in the 1600 cfm range and above, a single 16x25 side return won't cut it. Either double intake from both sides or the bottom
You do need more return air but you may also need more supply ductwork as well to get your system working properly. Also the chimney and flue compartment in furnace is probably severely clogged and needs to be cleaned by a pro that could be why the limit tripped
You could have a new duct system designed and installed to accommodate the furnace. Or you could replace the furnace to match the duct system. Or you could have a knowledgeable oil technician, who also knows about airflow and duct design, to adjust the burner to more closely match the blower and duct system.
Oil furnaces usually have a very large heat-exchanger very near the top of the furnace; if the evaporator coil is not installed at least 6 inches above the top of that kind of oil furnace it will cause a bad airflow restriction with a lot of back-pressure.
Is the cooling coil installed at least 6" above the furnace? Also, if cooling coil is directly on top of furnace, he probably didn't get a static reading between the blower outlet & the bottom of the cooling coil. Therefore, that high restriction static area - would NOT have been read!