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Forced-Air Furnace Cycling Too Often...Maybe?

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Forced-Air Furnace Cycling Too Often...Maybe?

Postby wessieball » Wed Nov 29, 2006 4:23 am

Since I've never paid attention until now, I'm wondering how often everybody's furnace turns on and off. Right now I have the temp at 67 degrees and it seems to turn on every 10-15 minutes or so with the outside temp being 19 degrees. It stays on for about 10 minutes (just a guestimate) each interval. While my house is mostly adobe mud brick which insulates quite well, there is no insulation in the attic and one of the two rooms in the addition doesn't have insulation.

How often do you guys notice your heat kicking on in the winter?
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Postby Don M » Wed Nov 29, 2006 12:27 pm

My mother's forced hot air furnace in Denver seemed to cycle on & off rather frequently too. Is your furnace maintaining the proper temperature in your house? With uninsulated spaces you may just be losing heat faster than you should so the thermostat keeps calling for heat. Do you feel drafts in your living space; is the wind blowing outside? That seems to suck out heat faster in my experience. Don
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Postby Danno » Wed Nov 29, 2006 12:48 pm

not that I think this is your problem, but it could happen to someone. My house is kinda drafty, but not as bad as some. Anyhoo, the thermostat is in one of the less drafty rooms. This should make for a colder house in the other rooms. Well, it got cold a couple of weeks ago and I noticed that the furnace was cycling a lot. it seemed to be running almost nonstop. This seemed to be good for my drafty room (i guess there is only one room that is considerably colder than the rest) but the rest of the house was getting hot. Thermostat said 66, but I tell you it felt at least 70 when I got home from work. Wife, of course, said she'd been comfortable all day long. I am painting the thermostat room, so I unhooked it from the wall and "Woosh" (ok not a real sound) came from behind the thermostat. Definitely cold air flowing around in that wall. I let the thermostat hang there for a few minutes and the temp raised up at least 4 degrees. Well, I drilled a small hole through the other side of that wall and filled the larger holes in the room I'm painting and things seem to be in order now. I could have just insulated back there, but I wanted a reason to move that thermostat out of that room anyway.

So, bottom line. Check for cold air in the wall behind the thermostat.
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Postby wessieball » Wed Nov 29, 2006 12:50 pm

The temp is fine. It varies by a couple of degrees. So when the furnace kicks on it gets back up to 67 degrees but will fall to 65 degrees and then the furnace kicks on. I can live with that temp shift but I do notice slight drafts here and there...mainly by windows, but those are new.
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Postby blthur » Wed Nov 29, 2006 1:54 pm

Hi- I've been lurking here for sometime, enjoying everyone's posts, thank you. But your post finally compelled me to log in. Your furnace behavior sounds similar to what ours was doing last winter. It would go on for short periods of time,, then off. We finally called our heating guy to come look at it. He asked my husband if we had had any work on it recently- we said no. After a couple hours, he realized that the duct/pipe venting into our chimney was very hot. Then my husband mentioned we had work done on the chimneys over the summer (including cleaning). We it turned out that all the junk from the furnace flue was shoved down over the vent pipe (there is no clean out for that flue and with 3 flues to a chimney, the chimney guy must have not realized that he only cleaned out two ash traps). So, basically the CO (carbon monoxide) could not vent out properly and luckily the furnace CO detector was shutting the furnace off everytime the CO levels got too high.

So, the furnace guy threatened to shut off our gas, we call the chimney guy in a panick and he came over immediately to detach the duct and clean out 100+ years of junk from the flue. All the while our furnace guy was breathing down his neck muttering ' carbon monoxide, the silent killer..."

In case you are wondering, we also have two independent CO detectors in our basement as well.

So anyway, make sure your furnace is venting into a clean flue!
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Postby oldhouse » Wed Nov 29, 2006 2:34 pm

wessieball wrote:The temp is fine. It varies by a couple of degrees. So when the furnace kicks on it gets back up to 67 degrees but will fall to 65 degrees and then the furnace kicks on.


This sounds to me as if your furnace is working just fine.

What would worry me would be if the furnace was cycling on and off but never getting the house up to the temperature called for by the thermostat.

This has happened twice to me over the years. The first time was several years ago during a bitter cold snap here in Maine. The previous owners had installed a too-small hot water heating system ... and it couldn't keep up with the cold. The boiler would get the water as hot as it was capable of heating it and shut off until the circulating water began to cool slightly ... and then turn back on. But, all the while, the house kept getting colder. Not fun.

The second time was this week, in a newly purchased house in New Hampshire. Even though it has been mild, the boiler was having a hard time heating the house. And worse, it kept cycling on and off. Source of the problem? When we finally got to the bottom of stuff, the previous owners (or their heating repair people) had messed up an internal setting on the thermostat. We changed a setting and the boiler now works like a charm.
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Postby moneymgt » Wed Nov 29, 2006 3:13 pm

Last year my house did this and I got fed up with paying double in heating costs what my neighbors were paying. After a little inspection I realized that the attic insulation had flattened to almost nothing over time. $300 worth of rolled insulation and a staple gun fixed most of the problem. Insulating the walls is an issue I'm not ready to tackle just yet.....
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Postby S » Wed Nov 29, 2006 3:57 pm

When I first moved into my house my furnace (1920's gravity furnace) was cycling on and off quickly. I had a furnace guy come out.....and he pointed me to a little lever on the thermostat that you can adjust to cycle longer. Not sure if the new fangled thermostats have them, but there should be one on the old round style.
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Re: Forced-Air Furnace Cycling Too Often...Maybe?

Postby Patchules » Mon Dec 04, 2006 11:58 pm

Some things about forced air heating.

Since the system makes you feel warm by heating the air which then warms the objects including you, losing the warm air will make you feel cooler. The faster the warm air leaks out and cold air leaks in, the more the furnace will cycle. Since furnace capacity is typically oversized, it starts and then stops after a short run, especially during periods of not so cold weather. As the outside temp falls, the house looses more heat per hour and the furnace cycles will get longer. But as soon as it shuts off, the high heat loss rate in very cold weather will cause it to turn back on in a short time.

If the furnace is sized to the house heat loss for worst conditions, when you are in those conditions (say -10 and very windy and leaky house), the furnace will run continuously because you are losing heat as fast as you can make it. Ductwork is rarely properly sized for heat distribution based on needs of each room. In a leaky house, if the wind is from the west and then from the east, are the ducts auto-resizing to take care of the changing heat losses? Nah.

Are the ducts passing thru unheated space? Do the ducts leak? Is the furnace shutting down on overheat from poor air flow versus thermostat control? Is the anticipator on the thermostat set correctly? This adjustment turns off the burner before the setpoint is reached while the fan continues to run, the residual heat in the furnace brings the air temp to the setpoint. Set wrong and the room temp tends to overshoot or not reach the setpoint. When it overshoots, you get too warm but the heat cycle will not start again untill the room temp drops below the setpoing. Install a thermostat with a tight temp control band and you can increase cycles but reduce the temp swings.

The end result - you have many factors that contribute to the cycle times of furnaces. Tighten the house envelope up and you will cut heat loss and energy requirements. The furnace will cycle but probably will be on for shorter cycles with more time between cycles but you will still feel the hot and cold spots because of all the other factors.

It is not easy to fix poorly designed air systems in leaky homes. If it were a hot water system there are some options. It might be easier to find another house or add a hot water heating system and use the air system for AC. Depends on how much it bothers you and how much you want to spend. Can you provide more details on the house and heating system?

My city row home is scorched air and it is the pits. My home in the north north is hot water and one can go barefoot on the warm floor. If I ever remodel the city home I have 5 radiators in storage waiting to be installed!!
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Postby wessieball » Wed Dec 06, 2006 4:29 am

Thanks for the info Patchules.

The house is about 900 sq. ft. but one of the rooms is unfinished and unheated with only plastic sheeting up in both doorways leading to the room to keep the dust out of the livable areas. I've calculated my heating bills and it looks like I'll probably average 4.5 CCF per day during the cold winter months (nov-feb) which is high given the square footage.

I plan on insulating the unfinished room off from the livable areas since I can feel a draft coming from that room. Maybe just put up some styrofoam insluation in each doorway.

The attic isn't insulated and I doubt I'll be able to insulate the entire thing since some areas aren't accessible. I can easily insluate the two biggest rooms of the house as well as the back bedroom which has no insulation in the walls (just drywall over clapboard right now).

The ducts do run through an unheated crawlspace. I've taped up all retuns and any joints in the sytem to ensure that it is as airtight as I can get it. There were a couple of areas where hot air was pouring out of one of the ducts into the crawlspace :shock:

I'm hoping that insulating the attic and the back bedroom will help somewhat. If I find time and the motivation I'll put up insulation under my floor within the crawlspace too.

But I think I'll have to grin and bear it this winter. As long as gas prices don't spike I can handle a $150 heating bill if I have to but I should be paying maybe $100 at most for this small of a house.

If it's one thing I DON'T like about old houses it's this very issue.
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