Oil furnace, limit switch triggering, poor air flow?

[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.

Your post #16 is pretty much implying that new blowers can handle the air flow problem.

[Rewind]

Your post #16 is pretty much implying that new blowers can handle the air flow problem.

Good grief, you have no idea what I was talking about in #16

Here it is again.


"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 "

It is general information, Apparently blower wheel information you do not understand.

Any implication at all would be agreeing with the quoted text. in #16

[beenthere]

Good grief, you have no idea what I was talking about in #16

Here it is again.


"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 "

It is general information, Apparently blower wheel information you do not understand.

Any implication at all would be agreeing with the quoted text. in #16

Or you don't know how to explain what you mean. Plus, all of that has nothing to do with what Udarrel was talking about in general.

[akbrian]

..... 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.

General question;
Why aren't oil furnace controls commonly designed to automatically maintain optimal heat exchanger temperatures through altering blower speed (ECM blower) and intentional burner cycling? While, of course, maintaining an independent high limit safety? One reason for this might be that a frugal homeowner might shut the registers in unused rooms of a perfectly designed duct work system. Just curious.

[FredHVAC]

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
3 zones
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!

[beenthere]

General question;
Why aren't oil furnace controls commonly designed to automatically maintain optimal heat exchanger temperatures through altering blower speed (ECM blower) and intentional burner cycling? While, of course, maintaining an independent high limit safety? One reason for this might be that a frugal homeowner might shut the registers in unused rooms of a perfectly designed duct work system. Just curious.

Same reason gas furnaces aren't. $$$$$ Additional controls cost additional money.

[beenthere]

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.

We can only guess from here. Since we don't can't see and test things for ourselves. Your HVAC pro is the one that can tell you what needs done.

[udarrell]

Good grief, you have no idea what I was talking about in #16 Here it is again.

"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 "

It is general information, Apparently blower wheel information you do not understand.

Any implication at all would be agreeing with the quoted text. in #16

Well, didn't see any disagreement with me.

The newer engineering in size & minor modification allow them to handle somewhat higher static before they begin to unload.

The blower design curves should be in every mfg'ers' manual. I emailed a Thermo Pride Engineer to get a copy of the Oil furnace blower curve in question.The Thermo Pride belt drive blower wheel is large enough to handle 1.5 to 4-Ton of cooling, they just use bigger motors, wheel ratios & increased wheel RPM.

Both you & beenthere are correct in that the wheels, & especially the motors, still won't handle too high a static.
The backward curved blades are designed to handle higher statics.

Come-On; You & beenthere need to get together shake hands & have a beer.

[beenthere]

Using a higher static blower only increases noise and turbulence.

[heaterman]

Still no substitute for a properly designed and installed duct system. Treat the problem, not the symptom.

[udarrell]

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
3 zones
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; IN FACT THERE WILL BE A HUGE STATIC LOSS WHEN READ AFTER THE COIL!

The answers above are good...
I know this is an older thread, however, it wasn't completely answered & there will be a lot of Oil heating situations that will have similar problems!
It is October 25, 2012 & the OIL Heating season is beginning...

Due to severe carpal tunnel in both hands; I can't do enough typing to provide all the details regarding what needs to be done to correct both heating & cooling AIRFLOW & SIZING Problems. (Several Extra large RA Filter areas are very important for Mid to larger equipment.)

First, were I in your shoes, I'd have a Home Energy Efficiency Audit performed; then a room by room heat-gain heat-loss calc done.

IMO, the heating & cooling appear to be way oversized which is creating needless problems.

The Thermo-Pride OL6 Low Boy upflow has a LISTED BTUH OUTPUT ranging from +60,000 to +89,000-Btuh depending on which of 3 nozzle flow-rates is selected, either the 0.50, 0.60, or 0.75 GPH rate.

A 0.50 GPH rate, #2 diesel fuel at 140,000-Btuh a gallon * .50 is 70,000 Input *.865 efficiency is 60,550-Btuh Output.

Well, 140,000-Btuh a gallon using the .75 nozzle is 105,000 Input * .865 efficiency is 90,825-Btuh Output. Here in SW WI I heat my farm home with 56,000-Btuh; it has plenty of reserve left-over.

It uses a direct-drive 5-spd blower which at 0.50" static will get the proper heating temp-rise & tonnage airflow increments from 1.5 to 4-ton of cooling.

That OL6 furnace only weighs 250 lbs & the height of the casing is only 34&3/4 inches; considerably lower than most Thermo Pride Low-boys which will provide more room for more than 6-inches of transition between the E-coil & the top of the Oil furnace - reducing back-pressure toward trying to get to an efficient 0.5" of ESP.

If you live in the northern half of the U.S. & into many southern areas of Canada this furnace should cover the majority of those heating & cooling conditions; of course there are heating exceptions.

Tell us more about where you live & how energy efficient you think your home is; that is something you need to pretty accurately know.

[catmanacman]

is the evap coil clean could have some buildup on after 10 years.