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Old 12-22-2011, 12:05 PM   #1
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Carrier furnace short cycling between Low and High heat


Hello HVAC specialists. Sorry for the long story.

I am having a couple of issues with my 5 year old High efficiency Carrier furnace. The model number is 58MVB 080-F-1. It is a zoned system with 4 zones. We also have central air as well as the Infinity Air purifier system.

One issue has been present since install and one just surfaced a couple of months ago. Ever since the install we have had a large number of Airflow limited shutdowns. This isn't the problem I need help with, yet. But perhaps it may be the cause of my main issue.

The problem I need some input on is the one that started a month into this heating season. It doesn't occur all the time, but when it does, it drives us crazy.

While the furnace is running on the Low heat setting it will kick into High heat for a few seconds and then drop back down to Low heat. Most of the time this is accompanied by an increase in blower fan speed, sometimes not. When at it's worst, this can occur 3 or 4 times per minute. However, it seems when the furnace really needs HIGH heat, it has no problem kicking in and running as expected.

This issue does not trigger any error events as identified through the thermostat. This has even occurred while doing a system test where the furnace will spend a few minutes on low heat and then a few minutes on high heat. It goes from Low to High and back down to Low.

I have had a HVAC professional (the only Carrier NATE certified dealer in my area) out to look at this "short cycling" issue as well as the airflow limit issue. He thought a newer thermostat would permit greater control of fan speed to avoid the airflow limits and it would also solve the short cycling issue. While we were awaiting the arrival of the new thermostat, we set the old thermostat to LOW heat only as opposed to letting the system determine Low/High heat. The HVAC guy felt it would reduce some wear and tear on the fan blower and it would prevent the Low/High heat cycling.

Well the short cycling continued. The new thermostat arrives and is installed and the short cycling continues, even when the new thermostat is set on LOW heat only. So from my perspective, the problem isn't the thermostat.


We had major renovations done last year and there was tons of dust. The ducts have been cleaned since, air cartridge replace at that time too.

But after searching some posts here, perhaps airlfow problems could be a cause.

- The current air purifier cartridge doesn't look too bad, but I did get a message last week on the thermostat to change it out. So I have one on order. None of the returns are blocked.
- We had the air conditioning guy come out and check on the system this summer. I was worried about dust/dog hair on the coils. They are NOT easily accessible to check. This guy said it wouldn't be an issue as the condensation will wash them clean as it runs off the coils. ???



The airflow limits are still present with the new thermostat but there seem to be fewer of them. We have a couple of ideas for that we can get into later if necessary.

The HVAXC Pro seems very knowledgeable but is so busy with emergencies, he hasn't had the time to research this any further. Soooo I am putting it out there to the DIY community. Note: this isn't something I can/will do myself. Any feedback I get from here will be shared with the HVAC pro.

ANY THOUGHTS??

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Old 12-22-2011, 01:15 PM   #2
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Carrier furnace short cycling between Low and High heat


dog hair DOES NOT ever wash off an ac coil and neither does dust especially drywall dust. sounds like your ac coil is dirty. any experienced tech should know that. I had hair from one dog that was stuck on like glue and had to be pulled off by hand. some type of wire haired terrier I believe. Hair was very fine and next to impossible to get off. the dirt gets between the fins also so the coil probably needs washing/cleaning with evaporator foam cleaner. not familiar with a zoned version of that furnace but I have seen it in regular use. the fan may be hunting to try settle at a speed the board thinks is normal. with an ECM motor once they reach .8"WC they can start to hunt and have a problem setting their speed. some will cavitate/overspeed and the board gets all confused. you need to deal with the obvious mechanical problems like a dirty coil B4 blaming the board or changing more parts IMO. Carrier will also have a minimum size of ductwork requirements for that fan to work properly and if they are not met it will NEVER work properly. Apparently with the housing boom in Edmonton we were having that problem with high end Lennox ECM units being installed in undersized ductwork and hunting/cavitating. May be nothing wrong with the furnace but more of a ductwork and airfow issue is my guess. Is the zoning system an OEM Carrier system or a off market one?

I installed a high end Lennox Harmony 3 zoning system with a G61V which is comparable to yours. They use a discharge temp sensor which must be PROPERLY installed in the plenum or it will hunt between low/high fire and cause fan problems. Not sure what Carrier uses but that may be a problem also.

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Last edited by yuri; 12-22-2011 at 01:29 PM.
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Old 12-22-2011, 02:44 PM   #3
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Carrier furnace short cycling between Low and High heat


Hi Yuri,

Thanks for that feedback. What do you mean when you say .8"WC?

Yes, we had a fair amount of drywall dust and an excessive amount of dust from MDF trim/crown. So I wouldn't be surprised to see some dust on the coils. We have a sheltie who sheds more hair than a herd of bison. Wouldn't our furnace filter (air purifier) catch all that hair? I ask that because I have never really seen much hair stuck in that filter.


To answer your question, the zoning is OEM Carrier. I asked for 4 zones and the salesman eagerly sold it to me. Little to no effort was made to ensure the new (smaller) zone trunk lines and associated vent runs were adequately sized. The HVAC Pro, along with Carrier, are supposed to be reviewing the duct sizes.

I will share the airflow issues (ie. dirty coil) and ask if carrier has a discharge temp sensor with the HVAC pro and see what we find.

Keep the suggestions coming guys.

Thanks.
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Old 12-22-2011, 03:25 PM   #4
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Carrier furnace short cycling between Low and High heat


In laymans terms static pressure drop is the literal amount of "resistance" the fan has to work against. Static pressure is the force of the air generated by the fan (or pressure) against the sides of the duct. measured in inches water column.

http://www.engineeringtoolbox.com/eq...od-d_1028.html

http://extension.usu.edu/files/publi...ry_Vent_03.pdf

If the ducts are too small or the ac coil dirty it has a harder time to move air. Kinda like a straw and you sucking on it. 1/2" diameter straw and your face turns red 1" straw and you can move a fair amount of air thru it.

I hate to say it but some tech support guys from the manufacturers are not always the sharpest knives in the drawer. You actually need a mechanical engineer to look over the duct layout and compare it to Carriers specs. That link has some good info. In the trade they use a "ductulator" type of slide rule device or nowadays a computer program.

If the filter is working it should stop the dirt but drywall dust is nasty, devious and seems to get thru the smallest cracks and is fine like flour and gets into everything. If you have a company that uses the robotic duct cleaning method you may want to get them and some have a video camera on the machine to show you the innerds. LOTS of the other clowns just do a cheap dog and pony show, use undersized vacuums in small vans etc etc and are a waste of time. You get what you pay for with them.
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Old 01-11-2012, 01:04 PM   #5
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Carrier furnace short cycling between Low and High heat


Hey Yuri,

I am learning you are correct about tech support, and I am beginning to wonder if my HVAC pro is in over his head or is just really really busy.

I have talked to another HVAC guy who thought it could be the furnaces main circuit board could be going too. Any thoughts?

In the meantime, I thought I would check out the evaporator coils myself to see if they were dirty, but the plenum doesn't have an access panel. Is there a trick to accessing the coils without cutting in an access in the plenum?

Would a couple of photos of my setup help?
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Old 01-11-2012, 01:13 PM   #6
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Carrier furnace short cycling between Low and High heat


Photos would help. We have other guys here who may be more familiar with that Carrier setup (Houston204?) or a Carrier zoned setup. I have not seen or know the Carrier zoning system to be real helpful to you.
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Old 01-11-2012, 03:31 PM   #7
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Carrier furnace short cycling between Low and High heat


I'll post a few photos.

Thanks Yuri.
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Old 01-11-2012, 04:05 PM   #8
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Carrier furnace short cycling between Low and High heat


a very easy test would be a temperature rise, it would tell you if the plenum or in this case the A coil is dirty. you would need to have all 4 zones calling for heat for about 15 or 20 minutes to get accurate results. This is a measurement of the supply temp. and the return temp........the difference between the two is your temperature rise.

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Old 01-11-2012, 08:05 PM   #9
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Carrier furnace short cycling between Low and High heat


How's he going to get an accurate reading with it vacillating between high fire and low fire. The blower will speed up and slow down too, adding even more confusion to the reading.



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Old 01-12-2012, 10:41 AM   #10
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Carrier furnace short cycling between Low and High heat


Thanks Harley, I'll mention that to the HVAC pro. It would be lot easier than cutting into the plenum.

Hvac5646, when the furnace needs to bring the temperature up a couple of degrees, it has noooo problem kicking into, and staying, in high heat mode. So we won't have any problems performing that test by setting the thermostats high enough.

The vacillating (good word!) only occurs when the furnace is trying to maintain a temperature. And it only happens occassionally.
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Old 01-12-2012, 10:52 AM   #11
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Carrier furnace short cycling between Low and High heat


I was just chatting to an retired HVAC guy and I was describing my problem.

He mentioned that these furnaces will automatically switch to high heat if they have been running on low heat for an extended period of time, like 10 to 15 mintues. I cannot be 100% certain of how long the furnace has on low heat when this cycling occurs, but I think he may be correct on determining when this occurs.

We still have to answer why it cycles only on these occassions and works fine when the furnace gets a major call for heat: Air flow? Main circuit board? Inducer?
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Old 01-12-2012, 12:47 PM   #12
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Carrier furnace short cycling between Low and High heat


Aren't you using a two stage stat? And did you mention that it would go into high fire in your post? I got the impression that it was stuck going from hi fire to lo fire.
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Old 01-12-2012, 01:30 PM   #13
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Carrier furnace short cycling between Low and High heat


Quote:
Originally Posted by hvac5646 View Post
Aren't you using a two stage stat? And did you mention that it would go into high fire in your post? I got the impression that it was stuck going from hi fire to lo fire.


Hi hvac5646, I do appreciate your input. Keep it coming.

Yes we are using the Carrier Infinity thermostats that accomodate the dual stage. The HVAC replaced our 4 year old thermostat with a new model in hopes it would solve the cycling. It hasn't.

And yes, the furnace does go into high fire without issue when the furnace needs to bring the temperature up a couple of degrees (like in the morning when we increase the temperatures). No cycling at that point in time. It seems that the cycling only occurs when the furnace is trying to maintain an even temperature. It will be running on low heat, then click to high for a few seconds and then drop back to low. At its worst, it will do this 2 or 3 times a minute. Most of the time the blower speed increases in conjuntion with the increase in heat, but sometimes not. And we may go days without this occurring and then have a day where I actually shut the furnace off to stop it.

Does that clarify the issue a bit better?
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Old 01-12-2012, 04:09 PM   #14
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Carrier furnace short cycling between Low and High heat


Thank you.
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Old 01-12-2012, 05:12 PM   #15
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Carrier furnace short cycling between Low and High heat


Quote:
Originally Posted by drtbk4ever View Post
I was just chatting to an retired HVAC guy and I was describing my problem.

He mentioned that these furnaces will automatically switch to high heat if they have been running on low heat for an extended period of time, like 10 to 15 mintues. I cannot be 100% certain of how long the furnace has on low heat when this cycling occurs, but I think he may be correct on determining when this occurs.

We still have to answer why it cycles only on these occassions and works fine when the furnace gets a major call for heat: Air flow? Main circuit board? Inducer?
I'd start with the programming set point differential of the stat. Two ways it will bring on second stage, either by time (how long first stage runs without hitting set point, 10-15 minutes) or by temperature meaning if the temp above set point is a few degrees higher than stat is set at both stages kick in to bring temp up quickly, I do believe. It can vary from half a degree to 4 or 5 degrees and again, I think. I am not very familiar with your stat but it's not unusual for a majority of them to act as such.

(Re?)-read the manual of the stat very carefully and see what it says.

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