Looks like 1600 CFM.

Very small return would probably be a large part of the problem.

 

lots of things affect cfm ratings in a furnace. a few are:

blower speed
duct sizes
duct quantity
duct quality
duct type
filter quality
filter cleanliness
coil cleanliness
blower wheel cleanliness
blower wheel rotation


if that model number reads like most others it looks like you have a 4 ton/1600 cfm blower. where i come from, that's a huge amount of air.

a tech onsite would be able to measure sizes and help you better determine what is best for your situation.

 

400 cfm per tom
500 btu per ton

 

really ........

 

Thank you! Seems like the single-speed blower is definitely not the culprit.

I wonder though from where the air is coming from registers. The only return is 30x2 inches and for an experiment I tried closing it completely with a cardboard for 30 seconds to see what happens. With closed return the air was still continuing coming from the registers at the same low speed. The house has a fresh air intake, might the blower be sucking air from there to compensate ?

 

Just my opinion

 

Yes, its probably getting a lot of its air from the fresh air intake.

 

You could run a little test with your furnace operating to get a rough idea of the CFM that the air handler is delivering. Of course, you have to shut off the ventilation air from outdoors, additionally there could be some return air leaks from a cold outside source which could throw the temperature off.

You could use the closest supply air register temperature or you can drill a hole in a safe place on the Plenum and check the temperature there; you subtract the room temperature from the temperature that you get with your probe. Then you look at the furnace and see what the output BTU per hour is; take the temperature-rise-split over the indoor air temperature and multiply it by 1.08 then divide that into the BTU output, that will give you just a ballpark CFM number.

Let's say your furnace output is 100,000 BTU and the temperature-rise-split is at 64°F, that's after multiplying 1.08 times temperature-split-rise; that would produce a ballpark figure of 1562 CFM.

I doubt very much that it's getting 1562 CFM, with that small return and probably other ductwork problems therefore, CFM could be much less than that. Additionally, the furnace output may not be what the tag says it is, it could have lost a lot of BTU output; there are variables in this method that are a little bit difficult to compensate for.

It works best on fairly new furnaces with right sized and properly sealed duct systems, and of course, with new clean filters installed.

 

Could have a clogged Blower/A-coil.

 

What 54regcab posted.
Also, could have fair amount of leaky supply ducts.

1600-CFM / 18 runs, that's only 88.8-average CFM, say they're all 6" ducts, that's .19635-sf area (depends on each diffuser's square foot (sf) (Ak) open-air-area); 88.8 / .19635 is around 452-fpm velocity; 452-fpm /88 is about 5-mph velocity...

Doubt it's delivering that CFM & velocity to the registers in the rooms...

 

I have put the probe into the closest register, and calculated CFM=1216=(115000*0.8)/((138-68)*1.08), but the furnace is doing something very strange with the thermostat set for fan=ON and temperature=80 (so I can run these experiments without thermostat interfering)

The temperature in the room is 68F. The probe shows 93F with very little air coming of out register. Then the probe temperature rises within 1 minute to 138 degrees and quite a lot of air coming out of register. Right after that the probe temperature starts going down and airflow decreases quite a bit (when I go to check the furnace, the burner is off but the blower is still blowing! But why did the airflow at the register decrease??) This takes about 5 minutes for probe to reach 93F and then cycle repeats.

As far as I can tell this is a "dumb" single stage furnace with constant speed blower and it is supposed to work continuously till desired temperature is reached. Could it be that the furnace overheats (as not enough airflow) and does some kind of emergency shut-down till it cools down?

 

Well, it's possible the furnace or plenum limit switch is shutting the burner off.

Are you sure the blower is actually ON all the time; it may not have a fan relay so fan-ON wouldn't cycle the blower ON. Many older furnaces' with a single speed blower motor only cycled using a furnace or plenum temperature activated fan switch.

If you recall how an old gravity airflow furnace worked, there could be some airflow when the temp was approaching 93°F; before the blower cycled on, due to the plenum air-temp-rise.

 

That model has an IFCB. The blower is brought on by a timer on a heat call.

 

Tripping high limit due to low airflow. 115,000BTU is a BEAST of a furnace and requires LOTS of airflow to stay off the high limit.

 

So if cfm is low and blower is on high what would good techs recommend

 

Thanks everyone for helping to diagnose the furnace tripping high limit every 5 minutes due to low airflow. It is amazing that the poor furnace was operating in this mode for the last 19 years.

@udarell – after observing the furnace more I am not that certain now about my claim that the blower keeps going all the time. There’s definitely quite a bit of rumbling coming from it (maybe motor on, but fan disengaged?) before the burner re-ignites, but I cannot actually see the fan, so I will take that claim back. It seems like the furnace is not that "dumb" after all.

I have done some more experiments with trying to see if I can have the furnace not tripping by having fresh air intake on all the time (the fresh air dumper and blower fan are controlled by timer). I can certainly see the somewhat increased airflow (I put a windsock-like object next to the register to measure changes in airflow objectively), but the furnace is still tripping when air at the register reaches 138F. It seems like there should have been some codes in place prohibiting builders from installing equipment in such manner.

 

Thanks everyone for helping to diagnose the furnace tripping high limit every 5 minutes due to low airflow. It is amazing that the poor furnace was operating in this mode for the last 19 years.

@udarell – after observing the furnace more I am not that certain now about my claim that the blower keeps going all the time. There’s definitely quite a bit of rumbling coming from it (maybe motor on, but fan disengaged?) before the burner re-ignites, but I cannot actually see the fan, so I will take that claim back. It seems like the furnace is not that "dumb" after all.

I have done some more experiments with trying to see if I can have the furnace not tripping by having fresh air intake on all the time (the fresh air dumper and blower fan are controlled by timer). I can certainly see the somewhat increased airflow (I put a windsock-like object next to the register to measure changes in airflow objectively), but the furnace is still tripping when air at the register reaches 138F. It seems like there should have been some codes in place prohibiting builders from installing equipment in such manner.

There are codes in place, but that doesn't mean they are followed. The "bigger is better" mentality lives on, except when it comes to ductwork. Install giant furnace that can't breathe, customer never knows it's hitting high limit because it's so oversized that the house stays warm even when it's running 1/4 of the time.

 

Need to call a service tech.dirty blower motor,dad cap.,etc

 

Thanks hewitt, the furnace very well could have some other issues as you suggest. But do you really think the return below would be able to supply the furnace with needed airflow? The joist return in picture measures 30x3, goes through at least 3 turns in the house with the total length of around 15ft (I cannot see them, they are in-walls and in-floors), then connects to 25 feet of 16 inch flex duct which goes back into the furnace.

I was in the process of hiring someone to improve the return path by installing properly sized return box / grille much closer to the furnace, and decided to check on furnace CFM which led to discovery of furnace tripping high limit.

 

How does it blow out of register in ac mode

 

The limit will shut the system down or eventually burn out the heat strip or crack the heat exscanger a failed capiciter will make blower fail also.

 

@hewitt, there is no ac in this system, only furnace. The only way I can make a bit more airflow is by causing fresh air intake to be open. As weird as it sounds, I think the house might warm up faster that way (but I'd have to run a few experiments to be sure of it) -- fresh air (around 30F-40F) causes better airflow which causes furnace to run a bit longer before tripping. After the furnace trips, the colder air / better airflow cool the furnace faster so the burner can re-ignite sooner to repeat the cycle ; - )

I do have a CO monitor on the floor where the furnace is, so hopefully if the crack in heat exchanger were to happen, the monitor will let me know.

 

You have a serious problem.Problems with gas furnaces are for qualified techs.U should seek help immediately