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Heil furnace with small return duct


msteger
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A client called me a few days ago after I did an inspection for their new home and the end of Sept. Home was occupied and seller moved out a few days before new buyers moved in.

The reason for the call has to do with a gas fired Heil furnace not running properly. From what he describes, it appears to be short cycling. He called a HVAC tech who says the issue is related to the return duct not supplying enough air to the furnace and this is causing things to overheat and shut down prematurely. From the serial number, the unit was manufactured in 1994 and if I recall, it was a mid-efficient model (approx. 80%). Generally, these types of furnace's have air grills either at the top or in the front grill, or both.

At the time of the inspection (Sept.), the unit was running fine. I typically run furnaces for at least 10 minutes (normally more) to ensure proper operation, etc. and found no issues in Sept. Also, we had several cold October nights and seller made no observations that the furnace was working improperly b/w the inspection and moving out. I would think they'd notice something irregular on those cold nights, but who knows.

The only service records visible at the inspection showed November 2007. I recommend C/S on all HVAC systems not showing recent (12 months or more recent) service records.

I would imagine that, assuming the tech is correct, that the duct issue would probably have existed from day 1 of the unit's installation (either in 1994 or 1995). It's not something that was worn out over time, but rather something that was there at the original install. Has anyone out there ever heard of short cycling due to a small return duct? First time I've heard of it.

The client purchased a home warranty, so they are taking care of it, but I am still curious as to the cause.

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Every time I have found a unit short cycling it has had to do with the burner compartment. Either a screwed up sensor, a burner out of alignment, or other issues with the heat exchanger or burners.

I have a hard time seeing how the return air would be an issue, unless it was almost completely blocked off or restricted to the point that the unit was overheating. If this was the case I would think that you would have seen it.

A 1994 gas furnace is reaching the end of its life anyway, so it could be several things.

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Originally posted by msteger

. . . Has anyone out there ever heard of short cycling due to a small return duct? First time I've heard of it. . . .

Yes, I have. But, as you say, it's not a problem that comes & goes. Did anyone check for other obstructions? A clogged air filter, a second air filter that no one realized was there? A dirty evaporator coil, etc?

Also, you can restrict air flow by damping down the supply registers. Did anyone try opening all of them to see if it made a difference?

- Jim Katen, Oregon

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Sounds fishy to me, too. Probably just a bad limit switch, or something similar, like Scott said. I've had furnaces with clogged coils produce 180 degrees on the supply side and the burners continued to fire.

Did you check the supply temp. while you were in the house, Matt?

Don't forget that most young-fart HVAC techs are comfortable with installations, but fall short when it comes to service calls that require independent thought and exploration.

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Slipping blower belt, clogged filter, Barbie and Ken in the return ducting...anything that reduces air flow will cause the symptom. A likely cause is someone dinking with the thermal limit settings as John suggested.

In my house it'd be several large house plants placed strategically to hide the floor mounted return air registers.

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I believe the supply vent temp was around 130~140 degs or so. I take the cover off, take the filter out, check to see if anything is on the other side of the filter, etc. Yep, I have found Barbie dolls, 2 or more filters, socks and other clothing, etc. stuck in there. I typically watch the unit fire for a few minutes and wait for the blower to start, then continue the basement inspection. Then, I go upstairs and check for heat in most if not all of the rooms.

The furnace was not at the bottom of the basement stairs, but rather approx. 15' from the bottom of the basement stairs along the outside foundation wall. It was a duplex from around 1950.

According to my report, the air filter was clean (serviceable), so it wasn't cruddy like some of the filters we all see from time to time.

I, too, thought the return duct idea was a little odd based upon what I didn't see stuck in there and I don't recall seeing an extra small return duct plenum or anything like it. It appeared normal at the time. That's why I posted it here since I thought it was rather odd. The first thing I thought of when the client called either was the limit switch, the thermostat, or burner.

According to the client, the return duct was replaced Tuesday and all is well. Oh well. Go figure.

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Originally posted by Bain

Don't forget that most young-fart HVAC techs are comfortable with installations, but fall short when it comes to service calls that require independent thought and exploration.

Many are trained to turn service calls into $ales calls. A neighbor's heat pump quit and was told the indoor portion, outdoor portion, supply ducts and returns all need replacement.

It worked fine after we replaced the start capacitor.

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Originally posted by inspecthistoric

Originally posted by Bain

Don't forget that most young-fart HVAC techs are comfortable with installations, but fall short when it comes to service calls that require independent thought and exploration.

Many are trained to turn service calls into $ales calls. A neighbor's heat pump quit and was told the indoor portion, outdoor portion, supply ducts and returns all need replacement.

It worked fine after we replaced the start capacitor.

Those capacitors go bad all the time. They're one of the first things the old fart--read competent--HVAC folks check on a system that isn't working.

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  • 1 month later...

The sad truth of the matter is there are many improperly sized ductwork. The other sad truth is that many service technicians have no clue on how to use manometers to measure Total External Static pressure of furnaces or air handlers and comparing those readings against the appliances' blower static pressure chart to determine air flow. Many systems have worked with marginal ductwork for years. When a good technician comes along that knows their salt and takes measurements finds that the ductwork is indeed marginal. Especially on homes that have had additions and supply and return runs are tied into the existing trunk ducts. Their findings are called into question. Another company is called that does no airflow testing only temperature testing and declares the system good.

Other possible cause of short cycling may be a system with marginal ductwork that may work fine with disposable fiberglass filters have reduced air flow when allergenic filters, with high restriction, are used.

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Supply grills covered?

Fan speed to low?

Damper's on supply drops? Did they vibrate closed?

Is the thermostat an old T-87. Check the heat anticipator.

You can see if the return is to small simply by taking off the blower door, taping the blower switch down in the closed position and see if the unit runs longer.

Did you measure the supply air temp in the plenum?

Is the flue under sized? Is it too long, maybe to long of a horizontal run?

So many simple things may be causing the problem. Me, I love to figure this type of stuff out. Piece of cake. Let me know if I can help you out via the phone or email.

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Undersized ducts should be reported. They will only exacerbate other conditions such as dirty filters and blowers, limit switch settings etc as well as restrict the furnace and a/c intended function. Remember that it always goes back to what the appliance manufacturer requires and not what an inspector or HVAC tech thinks will be ok.

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Originally posted by Eric B

Undersized ducts should be reported. They will only exacerbate other conditions such as dirty filters and blowers, limit switch settings etc as well as restrict the furnace and a/c intended function. Remember that it always goes back to what the appliance manufacturer requires and not what an inspector or HVAC tech thinks will be ok.

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tn_20081223141629_Undersized.jpg

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So, you're saying that if the return plenum changes in overall size between the basement ceiling and where it connects to the HVAC system, you'd report it as possible undersized return plenum?

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