Your furnace is probably rated at .5" external static pressure. You can check this with a manometer or magnahelic gauge. Take the 1/4" sensing tube on the meter and place it into the return air duct perpendicular to the air flow just outside the body of the furnace. Note the number. Do the same with the supply and note that. Add the numbers as though they are both a positive number. You may have .2-.3" static on the supply but .8 on the return. The supply is good the return is restarted. Go back to the next fitting and try again. You may find a point where the return static drops to the .2-.3 as well. Frequently, I've found that the main return is good but the drop to the unit is undersized.

How big is your furnace (BTUS)?
What type of filter are you using?
What are the dimensions of your filter?
What are the dimensions of your return?

Your filter may be too restrictive and your your return may be too small. As an experiment, try running the furnace with no filter. if it doesn't trip then you need to get a less restrictive filter. Your return is probably too small and may need to be enlarged.

If this has happened recently then I suspect something else may have changed. Did you close off any registers? Perhaps the gas pressure has increased causing the furnace to burn hotter. This should also be checked.

The furnace is 75,000 BTU and my house is about 1100sqft.

I had an expensive filter but I read they can restrict airflow, so I went out and got a $4 filter. It's a Rheem filter from Home Depot.

It's 14x20, and the return is 13x20. I have to bend the filter a little to get it in, but I see no other way because I haven't found a place that sells 13x20.

I tried removing the filter and the burners last about 3 1/2 minutes instead of 2, but the limit switch still popped.

No registers closed or blocked.

I honestly don't know how long the furnace has been short cycling. The house has always been warm so I haven't paid attention. I only became aware of the problem because I was tired of the noise and rattling so I decided to do something about it. I started researching and studying the furnace and I realized it was short cycling and there was an error code blinking.

Your furnace is twice the size it needs to be for an 1100 sq. foot house. Did you put in this furnace? The return is too small.

I going to assume you have a 3 ton blower which would mean about 1200 CFM. You need a filter size of about 600 sq. inches. The filter should be at least 20 X 25 if it is a 1 inch, you may get away with 16 X 25 if it is a 4 inch media filter. The return should be made bigger. I would not put a install an intake right next to the furnace.

Your original limit switch was fine. The HVAC tech should have realized this. May be you should find a more qualified HVAC contractor to help come up with a good solution.

I'm tossing the B.S. flag on this whole thing.
But for the benift of the few who search arcives before making thier 1st post and lurkers looking to hone thier skills,here are some observations for your consideration.

""So I had an HVAC guy look at it and he noticed two things. 1 - he got a "weird" electrical reading from the limit switch. 2 - if he removed the front panel that covers the blower (providing more return flow) the problem disappeared. So he recommended I first replace the limit switch, and if that didn't work I could add a grill onto the return duct directly above the filter.""
DO NOT PAY THIS IDIOT! Weird readings are the clue to what is wrong,not something that causes a professional to shake thier head and report it to the customer.
Adding an intake where suggested will cause more problems than can be counted but the fact it will likly backdraft the burners is more than enough reason not to do it. See those dark shadows circling overhead? All except one are lawyers waiting to sue for wrongful injury/death. The odd looking one? That's a prosecutor wanting to send you to jail for harming your infant son then riding the noterity to being elected to a higher public office.
Bless your heart for trying to help Jackfre,but I seriously doubt the op and his clueles tech understand a word of the highly techinal procedure using equipment few have seen,much less have experience with.
I am confident Mike would easily nail down the problem using answers to his logical questions,but did you notice how that information was concealed despite dancing all around it? As a matter of opinion only,I believe one or more established Superman members(not nessarly this thread) have recently took to posting as Clark Kent newbies with these seldom seen quirky problems then watch while well intending folks struggle with sketchy clues. After the prescribed leinght of time Superman comes in to show what he does best.
Oops,update! I was called away by swmbo mid-way of my reply. Instaed of editing,I will proceed. As predicted,Mike nailed it after getting his answers. Now let's see how those galvanized vents come into play. LOL

BTW,clean the evap coils regardless what else you do. That "bent filter" has by-passed dirty air and clogged the coils.

The furnace is 75,000 BTU and my house is about 1100sqft.

I had an expensive filter but I read they can restrict airflow, so I went out and got a $4 filter. It's a Rheem filter from Home Depot.

It's 14x20, and the return is 13x20. I have to bend the filter a little to get it in, but I see no other way because I haven't found a place that sells 13x20.

I tried removing the filter and the burners last about 3 1/2 minutes instead of 2, but the limit switch still popped.

No registers closed or blocked.

I honestly don't know how long the furnace has been short cycling. The house has always been warm so I haven't paid attention. I only became aware of the problem because I was tired of the noise and rattling so I decided to do something about it. I started researching and studying the furnace and I realized it was short cycling and there was an error code blinking.

I've lived in this house for 3 years and the furnace was here well before me. So how does a return that is too small get built and exist this long without being noticed?

Someone that "knew" what there were doing must have installed the furnace, and an inspector must have inspected it. I had a home inspection before I moved in, and I've since hired two HVAC companies to clean/diagnose the furnace. How is it possible that with all of these professionals nobody called out that the return is inadequate?

Should I call a new (3rd) HVAC company and suggest the return is too small?

I am going to assume the previous owner replaced the furnace prior to selling the house to you. Perhaps the original furnace was smaller and someone decided a bigger furnace would be better. A good contractor would not have installed a 75K BTU furnace in your size house with an undersized return.

Home inspectors don't find these kinds of problems. They check to see if the furnace operates and it is in good condition. You have to hire a HVAC tech if you want a proper furnace inspection.

You should hire a company that knows what they are doing and capable of increasing the size of your return.

Something else to point out - it is a downdraft furnace and the return comes down from the ceiling into the top of the furnace. The exhaust vent also exits the top of the furnace. There is literally no more room, the return duct takes up 100% of the top of the furnace besides the exhaust vent. So the return on top cannot be made bigger. Would a furnace really be designed this way?

Here is a link which I has the service manual for your Heil furnace. Take a look at the down flow installation. It shows on page 14 the exhaust vent should be installed on the side and not the top. The exhaust may be incorrect and causing the pressure limit switch to be tripping.

I assume your furnace is 19 inches wide, so I think it think it can accommodate a bigger return bigger than 13 x 19. If it were smaller, the return could be made wider above the furnace.

You need someone to evaluate the complete installation. You are wasting gas and if you continue this long term the heat exchanger is going to have a short life.

Here is a link that might be useful: Heil 8DNL Service Manual

Here is a link which I has the service manual for your Heil furnace. Take a look at the down flow installation. It shows on page 14 the exhaust vent should be installed on the side and not the top. The exhaust may be incorrect and causing the pressure limit switch to be tripping.

I assume your furnace is 19 inches wide, so I think it think it can accommodate a bigger return bigger than 13 x 19. If it were smaller, the return could be made wider above the furnace.

You need someone to evaluate the complete installation. You are wasting gas and if you continue this long term the heat exchanger is going to have a short life.

Here is a link that might be useful: Heil 8DNL Service Manual

Thanks for the manual. It's interesting because I have the installation and maintenance manual, and both clearly show the exhaust out the top. But I will read this manual over and probably call the HVAC service guy back tomorrow.

Thanks!

Thanks for the manual. It's interesting because I have the installation and maintenance manual, and both clearly show the exhaust out the top. But I will read this manual over and probably call the HVAC service guy back tomorrow.

Thanks!

So another HVAC guy is telling me I need all new duct work. Supply and return. Several thousand dollars. Can this be correct?

I am not surprised given the size of your furnace and house.

If you plan to live in this house another 10 years, it may make more sense to replace the furnace with a smaller size rather than making the ducts bigger. It is something to consider.

I think the limit switch is operating correctly and the return vent size if not optimal probable worked fine at one time. But you you do have an airflow problem. normaly corrected by a filter change and yet not helping much now..Adding greater air supply by removing filter and/or blowier housing door helps some ..."(Next step would be to install the grill but I have some hesitations. I took the blower panel off and it does prevent the limit switch from popping, but I noticed that the air coming out of the registers was significantly hotter than I'm used to.)" ...This tells me the xchanger is getting hot but there is not enough air flow to remove the heat and put it into the house..ie less air at higher temp. So if it runs fine with the blower door off I think the return air grill from the living space may be blocked (pile off clothes or stack of newspapers , or maybe replaced a door without inset grill to allow air to return grill) the other possibility (klem1 was right) is the AC A-coil is blocked by a dirt hair moisture mat on the supply side,pull the blower and look there for your eureka moment.....