[engineer_k]

Hello, newbie here and hoping to get some guidance here.

Background:
Have a Single Family home with a 4 ton furnace for heating - Payne PG8JAA048090AEJA. We recently changed all our original 50 year old metal ducts (also did asbestos abatement) to newer flexible ducts (for supply only). Ever since then, we have been having furnace short cycling issues - furnace turns on, stays on for about 4 to 5 minutes, shuts off and starts back up in about 4 to 5 minutes - when the furnace shuts off, our Google Nest gives a "Power Out" error. The home eventually heats but is less efficient than it could be. On consulting with a couple of hvac companies, they mentioned the air return is the issue and furnace is overheating tripping the limit switch. The solution proposed is enlarging the air return and/or replacing the entire furnace. One even recommended operating it without a filter (this did not help and same issue persists).

The dilemma:
While the HVAC contractors say air return is the problem, they have not provided any actual data on if this is really the issue based on any measurements. We purchased the house in this condition and never had this issue with the metal ducts. One HVAC person mentioned that newer flex ducts are more efficient and now the supply air is more so the return sizing needs to be adjusted - I find this hard to believe without any actual data. Also, statements like "increasing air return size may help but not 100%" do not instill a whole lot of confidence that this is actually going to solve the issue.

Some additional information:
We have 13 supply ducts in the house and 2 return ducts - supply registers are in various rooms and the return ducts are only in our living room and passage area. Unfortunately no easy option to add more returns without heavy cutting into drywall, patching etc. - this will be quite labor intensive and cost $$$

Is there any way to go about gathering some data on if return airflow is indeed the issue? Something like the system is currently supplying 'x' cfm worth of air and return is only returning 'y' cfm - so we need to compensate by 'z' to make supply = return? This would atleast make logical sense and be more convincing vs the hvac pros just mentioning air return is the issue. No one has been able to give any convincing solution while we continue living with this issue. Not sure if cleaning return ducts and our exhaust vent would be a good first step before additional work.

Any help or guidance on this would be really appreciated. Thank You

 

[Makeitcold]

Flex ducting is not going to carry the same amount of airflow as hard ducting even when installed perfectly, which is rare. Static pressure of both sides of the duct system needs to be measured and compared to the blower performance chart, it's very, very common for systems to have undersized return ducting which gets worse as more efficient equipment is installed because they move more air. I've never met any tech that will give you a 100% guarantee without being on-site though so get someone out to determine why it's shutting down and ask them to measure the static pressure and temperature rise for you.

 

[Snapperhead]

Has anyone laid eyes onto the Evaporator Coil to be sure its not covered in lint ?

Does this furnace have a Secondary Heat Exchanger that can get dirty ?

Have they checked the drain trap to see if thats tripping the system off ?

 

[BNME8EZ]

Flex duct installed properly will move 80% less air than the same size metal duct.

Static pressure measurements can be taken before the filter, between filter and blower, between blower and coil, and after the coil. This will tell where you have restrictions.

Depending on size 13 supplies and 2 returns sounds short for 4 ton of air.

The bigger question is does the house really need equipment that size?

 

[BNME8EZ]

One other thing, their explanation that the new supply duct can move more air now than the return can, causing the limit issue.

If that is possible then how does that explain it not going off on limit before. A fan is equilibrium, it can only blowout what it can pull in and it can only pull in what it can blowout. Point being if it could pull in enough air before the duct was changed why can that same return not pull in enough after the supply duct was changed.

 

[engineer_k]

Thanks for the insights everyone - I have requested another visit from an hvac tech and will update this thread with more information. The only reason I can think a newer supply flex duct is moving more air now than the metal duct before is because it is potentially larger in diameter than the previous 50 yr old duct - I am not sure of how big the ducts were before though, only have pictures. Also, during the last hvac pro visit, they changed a limit switch and now the furnace doesnt even recover after the initial 5 to 6 minutes of heating - only blower motors runs after that blowing cold air. The only way I found to recover from that is shutting off the furnace, resetting that switch and starting it again - obviously this is highly inefficient and not scalable. I wonder if this is a common issue with some limit switches that they get stuck open even when the temperature in the furnace goes down. I believe the previous switch used to trip when furnace overheated (supposedly due to lack of return airflow as diagnosed by the hvac tech) but as the furnace cooled after shutting off the burners would start again and heating would continue - atleast this was better than the manual way of resetting and gets me back to square one (house is heating but not efficiently). Is there a different kind of limit switch that auto-resets after a few mins that needs to be used here?

 

[pecmsg]

Thanks for the insights everyone - I have requested another visit from an hvac tech and will update this thread with more information. The only reason I can think a newer supply flex duct is moving more air now than the metal duct before is because it is potentially larger in diameter than the previous 50 yr old duct - I am not sure of how big the ducts were before though, only have pictures. Also, during the last hvac pro visit, they changed a limit switch and now the furnace doesnt even recover after the initial 5 to 6 minutes of heating - only blower motors runs after that blowing cold air. The only way I found to recover from that is shutting off the furnace, resetting that switch and starting it again - obviously this is highly inefficient and not scalable. I wonder if this is a common issue with some limit switches that they get stuck open even when the temperature in the furnace goes down. I believe the previous switch used to trip when furnace overheated (supposedly due to lack of return airflow as diagnosed by the hvac tech) but as the furnace cooled after shutting off the burners would start again and heating would continue - atleast this was better than the manual way of resetting and gets me back to square one (house is heating but not efficiently). Is there a different kind of limit switch that auto-resets after a few mins that needs to be used here?

Its cutting out on a SAFETY. That SAFETY is doing it's job. That job is NOT being an operating control! Get it serviced or shut it down before you damage it more.

 

[engineer_k]

Thanks - yes I tried the manual workaround a couple of times but have shut it down since then pending further investigation by an hvac tech - I did not want to tinker too much with something as complex as an HVAC system. Surviving with a space heater till then..

 

[BNME8EZ]

Since the new limit is worse than the old one it makes me wonder if it is the correct one.

 

[kdean1]

Too many contractors are nothing more than mechanics or salesmen.
Try the links in my sig to find someone who can measure static pressure and air flow volume. Ask if they have a Digital True Flow Grid and a flow hood.

 

[Snapperhead]

during the last hvac pro visit, they changed a limit switch and now the furnace doesnt even recover after the initial 5 to 6 minutes of heating - only blower motors runs after that blowing cold air. The only way I found to recover from that is shutting off the furnace, resetting that switch and starting it again - obviously this is highly inefficient and not scalable. I wonder if this is a common issue with some limit switches that they get stuck open even when the temperature in the furnace goes down. I believe the previous switch used to trip when furnace overheated (supposedly due to lack of return airflow as diagnosed by the hvac tech) but as the furnace cooled after shutting off the burners would start again and heating would continue - atleast this was better than the manual way of resetting and gets me back to square one (house is heating but not efficiently). Is there a different kind of limit switch that auto-resets after a few mins that needs to be used here?

They evidently installed a universal switch they carry (has manual reset)

Yes, there are automatic resets

 

[kdean1]

Flex duct installed properly will move 80% less air than the same size metal duct.

Static pressure measurements can be taken before the filter, between filter and blower, between blower and coil, and after the coil. This will tell where you have restrictions.

Depending on size 13 supplies and 2 returns sounds short for 4 ton of air.

The bigger question is does the house really need equipment that size?

I think you meant to say “80% of the air flow of metal duct” rather than “80% less.”

 

[BNME8EZ]

I think you meant to say “80% of the air flow of metal duct” rather than “80% less.”

Yes, thank you for catching that. I was trying to say that say a metal duct moves 100 CFM a flex duct the same size moves 80 CFM.

I guess that's what happens when my fingers are moving faster than my brain.

 

[engineer_k]

Quick update here, I had an HVAC tech visit to come take another look, here is a summary:
* They proved air return is the issue - we did a test with the furnace and blower motor door open so there is unrestricted airflow (this was for testing only) - system runs constantly without any power/safety cut off etc. My HVAC unit is in the garage.
* They refused to change the limit switch they put insisting the newer one with the manual reset is there for safety and older one which was auto-reseting every few mins was not a good thing and dangerous for the home - I did not argue with this, safety first

Eventually, I was back to square one with at least return air confirmed as the definite issue - still could not use the furnace. The only recommendation was enlarging the return - I have reached out to an HVAC company for an estimate.

Brainstorming through options in the meanwhile, I ran a test with the filters for both of my return vents out - this resulted in the system running continuously without cutting off (ran it for 30min+) and home was heating just fine. This makes me wonder if I was using too restrictive of an air filter causing issues with return air - I have 2 return vents - one with an FPR 9 filter and one with an FPR 10 filter - the best available I found for air quality. I presume with the improved filtering efficiency, airflow was just being too restricted - my next step would be to choose some sort of middle ground and try an FPR4 or FPR7 filter to see if that provides sufficient airflow while also doing the filtering function for dust, pollens etc. Perhaps the system I have with the 2 return vents is just not sized enough to handle a highly restrictive air filter - would love to hear expert opinions on if they have seen this sort of issue - thanks!

 

[pecmsg]

Did they take any static pressure readings?

 

[ss120396]

Can we see some pictures? I bet the new ductwork is a hack job. Should have kept the 50 year old ducts lol, they would have worked better, even with the likely oversized 90K modern furnace.

How big is this house and where is it located?

 

[engineer_k]

no static pressure readings taken, keep in mind this was a contractor assigned by my home warranty - so I had very limited expectations of them. They did take measurements on temperature, CO etc. at multiple vents
House is a 4 bedroom ~ 1850 sqft 2-story home, SF Bay area. I will post some pictures in a bit

 

[kdean1]

no static pressure readings taken, keep in mind this was a contractor assigned by my home warranty - so I had very limited expectations of them. They did take measurements on temperature, CO etc. at multiple vents
House is a 4 bedroom ~ 1850 sqft 2-story home, SF Bay area. I will post some pictures in a bit

Home Warranty?! :D:

Run, don't walk, to your phone and call a real contractor. HW techs work for the lowest pay and have the least training. MEASUREMENTS are essential.

 

[pecmsg]

no static pressure readings taken, keep in mind this was a contractor assigned by my home warranty - so I had very limited expectations of them. They did take measurements on temperature, CO etc. at multiple vents
House is a 4 bedroom ~ 1850 sqft 2-story home, SF Bay area. I will post some pictures in a bit

Run like your life depends on it because it DOES!

BTW

You got SCREWED!

 

[kdean1]

no static pressure readings taken, keep in mind this was a contractor assigned by my home warranty - so I had very limited expectations of them. They did take measurements on temperature, CO etc. at multiple vents
House is a 4 bedroom ~ 1850 sqft 2-story home, SF Bay area. I will post some pictures in a bit

4 tons for 1800 ft2 is likely to be terribly oversized equipment on undersized ducts. You need, really need, to have the entire HVAC system checked and have a Manual J load calculation performed. Chances are you need equipment 1/2 that size. Then the duct system will be a better match. Oversized equipment dies an early death and has multiple problems before it finally gives up.