Oil furnace, limit switch triggering, poor air flow?

[FredHVAC]

Hi,

I have a 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.

Last year the heat stopped and the mechanic found the high-limit switch was open. He removed the switch and it reset itself. He then found that the limit switch was frequently tripping and a few minutes later resetting itself. This is apparently unusual as the limit switch should only trip in rare cases as a safety valve. Is this correct?

The mechanic found the total static pressure was 0.84-0.9" WC. He felt that poor air flow at the heat exchanger was causing the limit switch to trip so frequently.

The mechanic told me that I should replace my limit switch as it failed once and is overused by tripping so frequently. If it failed again in the open position, I could have no heat again. If it failed in the closed position, I could have a major problem. Is this correct?

As I understand, the furnace is suffocating and overheating due to insufficient air flow. The mechanic told me that I should modify the return air duct so more air gets to the burner. Lennox agreed and said this is to protect the limit switch, heat exchanger, etc.
The "cut-in" between the main air return trunk and the furnace is 22" x 14" and I use standard (NOT high efficiency) 16" x 25" disposable filters.

This sounds expensive, including a larger hole into the furnace, larger air filter, and maybe a larger main air return "trunk". Could you please clarify the need for this and what should be done?

These repairs make sense, however the furnace has worked fine for the last year, although I don't know if the limit switch is tripping frequently or not. It also has been working for 10 years with no problems other than this and an unrelated circuit board problem. So why should I do anything? Maybe just let it be?

Thank you very much.

[ascj]

These repairs make sense, however the furnace has worked fine for the last year, although I don't know if the limit switch is tripping frequently or not. It also has been working for 10 years with no problems other than this and an unrelated circuit board problem. So why should I do anything? Maybe just let it be?

It hasn't worked fine for years. It might have made heat, but it's not run properly and safely.

The furnace is suppose to have return air pulled from both sides of the furnaces. It was actually shipped with 2 16x25 air filters tracks, for that reason.

With the frequent tripping on limit, has also put alot of strain on the heat exchanger. 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.

Safety should have no price tag.

[Stamas]

The payback of this should answer that for you. Better efficiency, more reliable, less chance of damaging other components, probably better comfort too.

[heaterman]

That is a LARGE oil furnace. Was a proper heat loss calc done when it was installed? It may be over-fired for your needs. Downsizing the firing rate and properly adjusting the temperature rise is the first remedy that comes to mind. Let’s be realistic, the actual btu output of the unit is compromised by this type of cycling anyway. At any rate, bouncing off the limit is not acceptable and will eventually ruin the furnace not to mention the potential for other dangerous issues.

[FredHVAC]

Thanks for your replies. I don't know what calculations were done to determine the correct size furnace for the house. It seems like it would need a decent size furnace, since it heats a 3,500 square foot house in Connecticut. What type of duct modifications does it need? Do I just need a larger hole between the main air return trunk and the furnace (where the air filter sits) or do I need two holes (for a second air filter?) and/or a larger main return trunk? Or should I just nothing and let it be - nothing bad has happened in 10 years except a one-time stuck limit switch. Thanks for all your help - I am a novice homeowner and don't want to spend a lot of money if I don't have to.

[heaterman]

You have to look at the entire duct system to determine the proper solution. More air in won't necessarily equal more air out. 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 but the manufacturer outlines this in their specs and installation manual along with the other system blower parameters that need to be met. This generally falls way outside the normal DIY range so you most likely are going to need the services of a qualified HVAC company.

[jtrammel]

Sounds like you do need more return air but you may also need more supply ductwork as well to get your system working properly. By running this unit with inadequate ducting it will dramatically reduce the life of the furnace. I would get a pro in to tell you what needs to be done to get the furnace working right if not you could be looking at a replacement in the near future. 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

[skippedover]

I know you're not shopping for a replacement oil furnace, at least intentionally but I've attached a couple of documents that are a little more in depth answers to the questions you're posing. You see, it all goes back to the day the new furnace was installed. It the installing company didn't do their homework before the furnace size was chosen, then you're now faced with the problems you've mentioned.

For you own edification you should know that the manufactuers of warm air furnaces do NOT install high limit controls as operating controls. That is to say, they are not there to normally cycle the burner off while the t-stat is still calling and the blower still operating, trying to deliver Btus to the living space. So the furnace should be properly sized and then when the t-stat calls, the temperature rise on the furnace is appropriately controlled by the blower speed and the duct system is sized to allow the proper airflow. So 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 knowledgable oil technician, who also knows about airflow and duct design, to try an adjust the burner to more closely match the blower and duct system.

But no matter your intended solution, to do it correctly mean undoing a lot of wrong work and that's going to cost handsomely. The results will be stunning if done properly but...getting it done properly may be the first major hurdle.

[beenthere]

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.

[udarrell]

Hi,

I have a 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. (Clipped...)

The mechanic found the total static pressure was 0.84-0.9" WC. He felt that poor air flow at the heat exchanger was causing the limit switch to trip so frequently.

The mechanic told me that I should replace my limit switch as it failed once and is overused by tripping so frequently. Is this correct? Should correct airflow problem!
(Clipped...)
The "cut-in" between the main air return trunk and the furnace is 22" x 14" and I use standard (NOT high efficiency) 16" x 25" disposable filters.

This sounds expensive, including a larger hole into the furnace, larger air filter, and maybe a larger main air return "trunk". Could you please clarify the need for this and what should be done? The need is gigantic!

These repairs make sense, however the furnace has worked fine for the last year, although I don't know if the limit switch is tripping frequently or not.

It also has been working for 10 years with no problems other than this and an unrelated circuit board problem.

So why should I do anything? Maybe just let it be?
Thank you very much.

First, Oil furnace 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!

How did the Tech check the static Pressure? These two videos by Bacharach show how it ought to be done.
http://www.youtube.com/watch?v=YkCXg...x=2&playnext=1

http://www.youtube.com/watch?v=5hq7_...eature=related

You have a 5-Ton A/C that requires at least 1750-CFM of airflow. Even if the Return Air an added filter are accomplished, but the coil is not 6" or more above the furnace, it is unlikely it will deliver 1750 to 2000-CFM of airflow for cooling at, say 0.50" ESP, which is a proper static pressure.

Too low a heat load on the evaporator can be a big threat to the A/C compressor. A TXV metering device would have helped some to protect the compressor.

Ample airflow is necessary, or it won't get anywhere near 5-Ton of cooling. A lot of Oil furnaces have the cooling coil improperly installed!

[skippedover]

First, Oil furnace 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.

Darrel, you usually give very solid information but this statement is just plain incorrect. Not the measurement but the reason. The distance between the top of the furnace and the AC coil has nothing to do with static pressure and everything to do with melting the drip pan on the AC coil. Oil burns much hotter than gas and therefore the temperature at the top of the heat exchanger is easily capable of melting the less sophisticated drip pans. I believe the Trane and maybe some other pans are made of a material that is ablel to stand up to the oil units and therefore the 6-in measurement is not a requirement. So the coils have no more or less restriction/static drop on an oil furnace v. a gas furnace but you might have a very leaky AC system is the coil is not properly placed and the drip pan isn't approved for closer distance from the coil heat exchanger.

[udarrell]

[QUOTE=skippedover;9354292]Darrell, you usually give very solid information but this statement is just plain incorrect. Not the measurement but the reason. The distance between the top of the furnace and the AC coil has nothing to do with static pressure and everything to do with melting the drip pan on the AC coil.

The melting part is true.

Some Oil furnace companies state in the owners manual that the evaporator must be at least 6" above the furnace or airflow will be restricted. There are links on my pages to those companies.

I know a Tri-State (WI,IA & IL) Contractor that specializes in Oil furnace installations, he says never install a coil less than 6" above the Oil furnace if it has a large HT/EX near the top or it will restrict airflow.

The amount of the back pressure varies with each installation situation. My brother's belt-drive is still only delivering half the airflow it needs for a mere 1.5-Ton A/C with a 2-Ton coil.

The problem is that the evaporator coils entry area is not large enough & the airflow is near the outer area, it hits the other areas & bounces back like a stream of water would causing turbulence & increased static. There is not enough air-space room between coil & the huge solid HT/EX to the entry area of the coil. Those factors do make a considerable difference.

If you install a filter grille or just a grille too close to the entry of a RA duct it will compromise airflow; it needs a sufficiently deep box to realign to the duct entry area. Does that register with you?

Remember on some Oil furnaces the coil is very close to the HT/EX.

On a belt-drive blower motor that will work on heating without the coil, - few of them will move enough air due to the added back-pressure static, at their rated 0.50".

If the rest of the duct system is perfect & you have a multi-speed direct drive blower it may get by,but at a higher static than it would if the coil provided more room for the air to get to the entry area of the coil.

Oil burns much hotter than gas and therefore the temperature at the top of the heat exchanger is easily capable of melting the less sophisticated drip pans. I believe the Trane and maybe some other pans are made of a material that is able to stand up to the oil units and therefore the 6-in measurement is not a requirement. (The 6" is to elevate the restriction that the HT/EX causes, not the melting factor.)

So the coils (that is true, drop through the coil is the same, but the coil characteristics is not where or why the airflow loss occurs) have no more or less restriction/static drop on an oil furnace v. a gas furnace but you might have a very leaky AC system is the coil is not properly placed and the drip pan isn't approved for closer distance from the coil heat exchanger.

[QUOTE]The gas furnace heat exchanger does not force the air to come up the sides of the furnace & its a different airflow equation. I'm not saying the required CFM couldn't be obtained, but it would be at a higher static & blower work than if the coil were installed higher to relieve an air-path restriction.

However, in many belt-drive cases it will take a jump in HP & blower RPM, or airflow won't meet requirements. It is strange that far too many companies don't mention these realities. Many switch to direct drive blowers with adequate HP & RPM.

Older Thermo Pride belt-drives use the same big blower wheel to go from 1.5-Ton to 4-Ton. They use different size wheels, HP up to three-quarter, & increased RPM. However, you can't violate airflow laws without paying a price.
-----
An "Oil Furnace" Airflow Problem Fix
"Boyer- Installation manual"

"Download" the pfd file below & scroll to page 8, or type in 8 & hit Enter, scroll down to it.
http://www.boyertownfurnace.com/Prod...nual042909.pdf

Below goes for Thermo Pride & many other Oil furnaces:
"If the oil furnace is used in connection with summer air conditioning the evaporator coil must be installed at least 6” above the oil furnace for proper airflow. Distances less than 6” will result in decreased airflow.

Make sure outlet supply takeoffs are NOT blocked by the coil. In all cases, refer to the manufacturers’ data for static pressure losses to ensure the total system static pressure does not exceed 0.5” WC." - mfg'er

[beenthere]

Udarrel is correct about the static pressure. Many oil furnace heat exchangers are so close to the top. That the coil does impede air flow.

[udarrell]

Udarrell is correct about the static pressure. Many oil furnace heat exchangers are so close to the top. That the coil does impede air flow.

Thanks for the support 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...

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

[Rewind]

Originally Posted by udarrell
First, Oil furnace 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.
---------------------------

Can do............due to the physical configeration of the exchanger and the configeration of the coil base.

[Rewind]

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.

-------------------------

Actually every blower has a design curve, giving various CFMs at different SPs and RPMs.
Hard to find any curves of blowers in the residential literature or at least it was. You have to go to the commercial or industrial catalogs to see what thay look like. If you do venture there, be sure to look at the design curves of forward curved blades verses backward curved blades. (lots of curves there) g

[beenthere]

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.

-------------------------

Actually every blower has a design curve, giving various CFMs at different SPs and RPMs.
Hard to find any curves of blowers in the residential literature or at least it was. You have to go to the commercial or industrial catalogs to see what thay look like. If you do venture there, be sure to look at the design curves of forward curved blades verses backward curved blades. (lots of curves there) g

Still don't get enough air flow. Udarrel is right. Thermopride is one of the ones that gives the most trouble with poor air flow. Unless you raise the coil.

[Rewind]

Still don't get enough air flow. Udarrel is right. Thermopride is one of the ones that gives the most trouble with poor air flow. Unless you raise the coil.

Not sure if you understood either of my two post on this thread.

[beenthere]

Not sure if you understood either of my two post on this thread.

Maybe you didn't understand Udarrel post.

The newer blowers don't do any better then older blowers did.

[Rewind]

Maybe you didn't understand Udarrel post.

The newer blowers don't do any better then older blowers did.

Now I'm sure you have no clue !

WFIW I agreed with him in my first post.
The second post was general information.

Maybe you need to slow down and get a grip.