View Full Version : Cycling of furnace
We have a comfortmaker furnace about 4 or 5 years old. We just bought house in april 2004. I noticed that the furnace cycles on and off three times before the thermostat is satisfied. The fan still is running the entire time the fire cycles off and on. The furnace is a 100,000btu for a 1500sq ft house ranch. Had a teck come out and check things. Everything checks out. He thinks furnace is to large for house and cycle switch for the fire box is telling it to turn off to cool and then bacl on again until thermostat is satisfied. He set the blower motor for high speed for heat and high for summer ac. So I was looking for another opinion. Any more info needed let me know.
02-15-2005, 06:11 AM
Makes sense to me. That is a lot of furnace for a small house. But the only person who could give a 2nd opinion is a competent tech who sees the installation and takes temp readings.
02-15-2005, 12:21 PM
That's a mighty large furnace for a 1500 sq ft house, but it will work just fine. Maybe the furnace was oversized to handle the A/C, if you have that (standard procedure). Any way to reduce firing rate? If it's an oil furnace, drop nozzle size alittle, that will reduce cycling and possibly increase efficiency too.
No it"s a natural gas furnace.
02-15-2005, 01:36 PM
Tell you what, turn the stat up and take the supply air temperature at the plenum and post it. Odds are you are either cycling on the limit and reducing the life of that furnace or it could be as simple as a slow drain if its a 90+ unit. Really, you should have it looked at. Ask the tech if the temperature rise is within the nameplate range.
It"s not the 90%. It is just a 80. He didnt check anything except the limit switch in the fire box, and the filter to be sure they were clean.
02-15-2005, 04:29 PM
You might want to get another contractor, these are simple symptoms.
02-15-2005, 08:38 PM
100000 btu furnace is a bit oversized but in my opinion not grossly. This is rather rudimentary but is everyone sure the furnace is not going out on high limit cooling down closing limit and firing back up? It sounds as though restricted air flow (filter, blower losing RPM, overfired burner etc.) is causing high limit to open, if this is the case the furnace could operate like this all day long. Say goodbye to HX if you dont take care of it.
02-15-2005, 08:55 PM
I don't know what you are in but in northern Ohio that would be an oversized furnace unless you have a window missing or somethig. If the furnace was sized to match the cooling then it was misapplied they should have just matched the blower on the furnace to match the cooling but I wouldn't suspect that your cooling unit is anymore than a 1.5 to 2 ton unit. As for the problem you are seeing, how do you know it is not the stat turning it off ? The guy was on the right track although he should of went a little further and came up with a definite answer for you.
02-15-2005, 09:57 PM
1500 sq ft - 1.5 or 2 ton ??? Sounds closer to 3 from here 100000 btu is not so oversized to cause short cycles like that sorry.
I"m not sure of the ac size, will check it today. After setting the blower to high for heat, and then it only cycles off and on two times instead of three times during the heating of the house. The teck did checa a sensor, that ran fron the fire box, but I cant remember what he said it was, I do remember he said it was to sense something to do with the fire in the box but it was not designed to be used all the time , maybe for safety purposes. I might need to get another opion, might cost for service call, might be worth it. I also noticed the vent pipe to outside is hot.
[Edited by j1cs on 02-16-2005 at 04:26 AM]
02-16-2005, 08:40 AM
Has anyone thought that the cooling evaporator coil may be dirty, causing a reduction in cfm's. Hard to see the cased coil. but should be checked.
Is the house on a slab or off the ground, 100,000 for 1500 sounds like a lot to me, especially if the cfm output is not right.
Doc gave good advice. It is likely cycling on the limit switch but having a supply temp would verify it. This will damage the furnace and double your utility bills. Sounds like you still need more air flow - may even need more duct. Depending on how oversized (if it is) your furnace is, you may stop the cycling on the limit problem but may still have short cycling issues.
Went and talked to another teck today. Explained things to him. He thinks it might be the trunk only 14in wide with 10 runs. He says the trunk needed to be at least 20 I think. Anyway am going to get someone to come out and go over the whole thing. Thanks for replys.
02-16-2005, 08:12 PM
Have a proper load calculation done before any duct modifications are done as that may just be a bandaid to a oversized furnace... I would recommend a room by room load calculation and have the ductwork checked.. As the furnace may not be sized right,, the ductwork is likely to be incorrectly sized also..
Things your tech should check:
1. Temp rise..
2. A-coil(is it dirty)
3. Check gas pressure..
4. Check for closed dampers or crushed flex duct..
And many other checks...
As stated,, this furnace seems way oversized for the size home you stated,, however only a proper load calculation can tell the truth behind our guesses..
Find a quality contractor and have this properly checked out..
You can run your own hvac load calculation with the home owner version of hvac calc 4.0.....Just click the bullseye and Don Sleeth will help you...
02-16-2005, 08:19 PM
1,500 q ft = 3 ton !! Where is this glass house located ?
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