I have a forty year old natural gas forced air furnace and it’s acting funny.
I set the thermostat on 70 degrees and I see it’s about 20 degrees outside this morning.
Anyway, the furnace has been doing the following for a couple of weeks now:
When room temp drops to 69 degrees the furnace kicks on.
The burners light and a few minutes later the blower fan turns on.
The furnace then runs for about 12 minutes and the inside temp raises to 71 degrees and the furnace shuts off.
Then, after about three minutes, the blower fan kicks back on and it runs for about 30 seconds and shuts off. Why is it doing this?
I will call a HVAC tech tomorrow but I’m just curious whats going on.
Thanks for any suggestions.
Replies
One that old it most likely uses a simple bonet thermostat to control the fan.
Burner heat the heat excahnger and when it gets hot the bonnet thermostat turns of the fan.
After reaching setpoint the gas cuts off and the blower runs until the heat exchanger cools off.
Sounds like the bonnet thermostat is worn and doesn't have enough hysteries or it is set too low.
.
A-holes. Hey every group has to have one. And I have been elected to be the one. I should make that my tagline.
Thanks Bill, I appreciate it.
I just thought of a funny story about my furnace.I bought this house four and 1/2 years ago and demanded the Seller provide me a one year home warranty which he did.During the first winter, during the warranty period, on a cold December day, my furnace quit working.I noticed the house got cold and soon it was 65 then 60 then even colder and I found my home warranty paperwork and called them and asked them to come fix my furnace under the warranty.She said "I'll take care of it" and you will have a licensed contractor contact you to schedule an appointment.That night I sat here with icycles hanging down from my nose and I went to my car and drove to home depot and bought a $19 thermostat and came home and put it in and the furnace started working.To this day the home warranty company never called me back to follow up and see if I was okay. No heating contractor ever called me.I thought to myself "aren't these home warranties really wonderful"LoL
I suspect your furnace has a fan control mounted on the hot air plenum which looks like this (if you remove its cover). http://www.high-performance-hvac.com/hvac-photos/HVACPhotos/fan-limit-control.html There is a long cylindrical bi-metal probe affixed to the back of this device which "senses" the temperature and then turns the blower on and off accordingly.
Note that there is a tab on the disc related to turning the blower on when the temperature rises to X inside the plenum and one for turning it off when the temperature falls to Y. If the "off" setting is a bit too high, the blower will kick on again after the normal cycle completes .......just from the dissipation of the residual heat in the exchanger. If your "off" setting is currently 90F, try changing it to 85F or 80F. Do this when the furnace isn't running, of course. What you don't want to do is set it so low that it's at or below your target indoor temp or it'll never shut off by itself. Hold the center disc firmly and dead-still while you move the temp setting tab.
Or.....try raising the "on"setting just a bit instead. Then that same residual heat won't be as apt to turn the fan back on again. 5 degrees F maybe. Depends where it's set now.
Or it may be that the switch is malfunctioning and needs to be replaced.
Sometimes the appearance of this fan behavior is related to some manner of partial obstruction in the heat exchanger or the flue which isn't allowing as much of the residual heat to escape via that route as it did before. The latter could be a dangerous situation as regards CO and should be checked out if it hasn't already this season.
There's also the possibility that your barometric damper is malfunctioning in a manner which slows down the flue draft and so the residual heat which didn't trip the fan switch last season, does now.
Edited 11/25/2007 11:43 am ET by HootOwl
Thanks - that's a lot of info to digest - I'll spend some time with this this afternoon.Thanks again.
Forgot to mention.........if you have this type of control, there will be three setting tabs involved. The one set to the highest temp on the dial of the three tabs... is the high-limit safety setting and should not be moved. It shuts the furnace down if the blower fails to start or some other type of serious problem occurs. The other two tabs will be fan control settings. One for blower "on" and the other for blower "off".
If you don't have this type of blower control, then you probably have a dip-switch control board. I won't go into that stuff. Your HVAC guy will know how to check those out and alter the settings if needs be......or replace it.
On a cold day my furnace turns on, runs for 12-13 minutes, turns off for about 30 minutes, and then repeat. I'm thinking about leaving my fan on 24-7 Any thoughts on that?
Wears out the blower faster, uses more electric, cold air blows around when the furnace is off. I might do if I had a problem with uneven temperatures around the house.
Most furnace fans today are rated for 'continous duty' - meaning running them all the time makes no difference to the life of the motor.
Jeff
Edited 11/25/2007 11:20 pm ET by Jeff_Clarke
Continuous duty means they can run continuously, but that does not mean they will last forever.
True, but even run continuously a good quality motor will last 20 years (if properly maintained). We had one last about 28 years and another 30 (and still going when the furnace was replaced), even though they ran continuously all summer.
If your view never changes you're following the wrong leader
Thats interesting. I do sometimes run mine continuously, but mainly on very hot days.
Dan have you ever ran your furnace continuously in the winter (as opposed to the furnace stopping and restarting every 15 minutes)Wouldn't it be cheaper to just let it run?It takes a lot of energy to start an electric motor. Why not just leave it running on really cold days
It takes maybe 5 seconds worth of energy to start an electric motor.But we often do have ours running even in winter. Helps to equalize air temps.
If your view never changes you're following the wrong leader
More or less... ditto what MarkH said.
Since your OP said that the furnace kicked on around 69 and off at 71, I'll conclude that the anticipator setting isn't terribly too tight for your own good. If the spread/swing was only 1 degree F, I'd suggest you increase it (via the anticipator setting on the thermostat) to avoid the frequent restarts. That because every time the furnace starts up, a significant amount of BTUs get sent up the chimney before the blower ever kicks on. That's all wasted fuel basically......but necessary with an older non-condensing furnace to prevent condensation in the HE and the flue. Accordingly, the most efficient furnace is the one that runs 100% of the time because that situation never wastes any of those "start-up" BTUs or those remaining in a residual state in the HE when the blower shuts off. In reality, such an ideal is unachieveable, of course. 100% efficiency hasn't been achieved yet as regards gas furnaces......and likely won't be.
How often your furnace has to fire to maintain the target interior temp of your abode is gonna be directly related to the thermostat swing/spread setting and how tight your house is.....or isn't. Lost BTUs are lost BTUs. And the greater the differential between target interior temp and the outdoor temps (and wind), the more often the unit will need to fire to replace the lost BTUs. Our furnace runs about 95% of the time when it's -30F and blowing 40 mph. About perfect or as close as I can get anyway. Very little cycling losses under those conditions.
Needless to say, the larger the house, the larger the losses....all things being equal. Better sealing, better windows and more insulation is a plus, of course. No news there. <G>
It might be that your thermostat location isn't ideal to prevent frequent cyclings. IOW, the thermostat gets satisfied and then the heat in that room soon enough leaks off to the rest of the house and that causes refiring. A better location or some balancing of the duct system might result in longer cycles with fewer firings on the whole. For example, if the register in the room where the thermostat resides is putting out more than its fair share of the developed heat.... that would cause "premature" satisfaction of the thermostat ....... until the heat migrates out of that room. Then there's another call for heat.
Not there, can't say for sure. Just some food for thought.
Edited 11/25/2007 11:31 pm ET by HootOwl
My furnace is a 1968 Rheem Day & Night. It's 100,000 BTUModel number 100CD12ASeries 152These must have been a well built furnace to
have lasted this long.
It's cold here 20-30 degrees at night and only 40 or so in the day.If I set my thermostat on 70 I feel cold all the time.If I move it up one degree to 71 the house feels hot.Am I losing my marbles?
set out several thermometers... and see what the real temp is...
the debate of you having lost yur marbles has been over for some time but the one of you having a defective thermostat has just begun...
Life is not a journey to the grave with the intention of arriving safely in a pretty and well preserved body, but rather to skid in broadside, thoroughly used up, totally worn out, and loudly proclaiming<!----><!----><!---->
WOW!!! What a Ride!Forget the primal scream, just ROAR!!!
ya never did mention which kind of therostst ya had..
Life is not a journey to the grave with the intention of arriving safely in a pretty and well preserved body, but rather to skid in broadside, thoroughly used up, totally worn out, and loudly proclaiming<!----><!----><!---->
WOW!!! What a Ride!Forget the primal scream, just ROAR!!!
It's a digital thermostat I bought at Home Depot about 2-3 years ago for
approx $19
it be suspect now.....
Life is not a journey to the grave with the intention of arriving safely in a pretty and well preserved body, but rather to skid in broadside, thoroughly used up, totally worn out, and loudly proclaiming<!----><!----><!---->
WOW!!! What a Ride!Forget the primal scream, just ROAR!!!
I like the digital thermostatsBut I don't like the programmable ones - They just don't work that well for me. I switch them over to manual and over ride the settings.I'll look for something with a little higher qualityThanks guys!
put out a thermometer or two 1st..
see if that is the problem...
those Honeywell round ones are still made in the US...
digital is about garunteed to be from China...
Life is not a journey to the grave with the intention of arriving safely in a pretty and well preserved body, but rather to skid in broadside, thoroughly used up, totally worn out, and loudly proclaiming<!----><!----><!---->
WOW!!! What a Ride!Forget the primal scream, just ROAR!!!
I think Imerc's got ya covered. Most likely anyway.
Could be high humidity. Damp cold feels colder, moist heat feels hotter.
When you set it up one degree the furnace comes on, stirs up the air, evens things out. Just running the fan for a few minutes would have the same effect.(70 is the highest we ever set our thermostat. Usually 68 daytime, 66 while sleeping.)
If your view never changes you're following the wrong leader
I removed the cover and looked at my furnace.My fan limit switch has a lever to set the shut off tempRight now it's set on 105.In other words you slide the lever to the left or right
to adjust it.The numbers on the scale go from 70 - 120 degrees.Should I slide it down to 100 ?Thanks again.
First off, if it's a dial style, don't touch the levers unless you HOLD THE DIAL STEADY. Adjusting without holding the dial can damage the unit.For most efficiency you'd set the lower temp just above the max summertime ambient -- 100 or 105 for most areas -- and the middle lever hard against the lower one. Then adjust the middle lever up if the fan cycles on and off in a burner cycle.For more comfort (less cold air blown) you'd set both levers a bit higher -- the lower at 110 or so. Again, set the middle lever up more if the fan cycles during a burner cycle.Don't touch the upper lever.
If your view never changes you're following the wrong leader
I don't have a dial type of switch.I have two separate switches. The off switch has a lever you slide from left to right to raise
the temp.It's set on 105 now and I'll adjust it to 110 and see how
that works and will report back.Thanks to everyone for their expert recommendations
I changed the shut off from 105 to 110 and it didn't change a thingIt's still doing the same thing.So I'm back to square one.
starting at the top, the highest lever which you should not mess with ,shuts the furnace down if it gets to hot,lets say the burner kicks on ,but the fan motor doesn't start. if it didn't do this pretty soon the chamber would be 400-500 degrees and burn the house down.
the second one that is set at around 110,lets the burner kick on,heat up good so that you don't get a blast of cold air while it heats up.
the third one should be set somewhere around 90,once the furnace has brought the house up to the set temp and burner shut off,this one cools the chamber down to get a little more heat out of it.
now whats happening is the third one is cooling the chamber down,then shutting the blower off. but as it sits there it finds that the chamber is stillproducing a little heat and heats back up and kicks the fan back on.runs for aminute ,cools the chamber again ,shuts off.sometimes this may happen 3 times
my fix for this is do nothing,you are wasting a little energy restarting the fan,how much i don't know but if you buy a new fan limit switch it will take a long time to pay for.
if you decide to replace it's very easy to remove ,then take it down to washer specialty and get a new one. be sure to tell them it's for xyz reality co. they will give a discount. larryif a man speaks in the forest,and there's not a woman to hear him,is he still wrong?
Thanks Larry thats some good information. Also, I will file away what you said about Washer Specialty and telling them this is for Real Estate Company and ask for a discount.Washer Specialty is where I bought the elect motor for the blower
several months ago.My furnace is similar to having a car with 200,000 miles on it.If it breaks down to where I have to call a service man, it's not feasible to fix it anymore.It would be a serious mistake to spend $400 - $500 on a forty year old furnace. I have an estimate to replace this furnace and A-coil for
$2,000 but I'd like to find someone who could do it cheaper. If I could get a new one for $1,500 I'd probably go ahead and do it although I'd really rather just sell this house and give the Buyer some kind of allowance to get a new furnace. I would offer to pay 1/2 of the new furnace (and then would not have to pay $400 for a home warranty)Any ideas where I could get a new furnace at a reasonable price? It's 100,000 BTU and the furnace is in a utility room off the garage and easy to get to. It's not in the attic or in a crawl space.They replace these in one day. Why do they have to charge so damn much. The brand new furnace unit itself is probably only $500 or so.
first to tell you the truth,if i could buy a dozen of your furnaces new,i'd do it. they are so simple in operartion,basicily ,fan motor,limit switch[that we've been talking about] fan relay and air cond. relay,gas valve. if the heat chamber is good[yours is if the co det is 0] they are good to go. may use more gas but payback on a new one is pretty long.
as far as a new one you could get one on ebay [i bought 8 central airs this summer on ebay] putting one in is really straight forward deal except you will have to do duct work to mate the new one up.i keep trying to find a hvac guy and just have not really come up with one.i rented a building to one company.,got a couple bids from them and they were higher than anyone,so that didn't work out....there has to be some guys around that you can trust when they say something ,i'll keep looking. larryif a man speaks in the forest,and there's not a woman to hear him,is he still wrong?
Yeah, it's simple and reliable, but he's lucky if his furnace is only blowing 50% of his heating money up the flue. I forget where he is, but in any snow belt situation a new HE furnace would pay for itself in about three years.
If your view never changes you're following the wrong leader
well he's in wichita ks,we have what i call pretty moderate winters here,especially compared to some of the guys on here.
i am not a great believer in all this hi eff stuff. my experence with central air is if you have 10 seer unit that is kept clean and sits in the shade by the house ,it will take many many years to get even let alone save money.
now i will go with you on a furnace that i think hi eff is a great improvement of a 1960 unit,,the plastic flue pipe pretty much confirms that, but....
around here average heating gas bill probably runs around 1000.00 a winter. now lets say you put in a 94% and save 44% on your bill thats 440 a year savings. now you will pay about 3k for a 94%furnace install. so thats at least 6 years to pay back. but then we get into computer boards,electronic ignitors, etc. on the old furnace there isn't hardly a part that cost more than a 100.00 on these new one i'm not sure there is a part that can be bought for a 100. ,a computer board for the last one i bought was like 230.00 and i installed.
my thought is if your old furnace goes bad absolutly look at a new hi eff, but if your old one is still cranking save the money till you have to spend it.
but then i'm a tight az larryif a man speaks in the forest,and there's not a woman to hear him,is he still wrong?
Larry, I would like to try changing a furnace myself.Or if nothing else, I would remove the old one and set the new one
in and let a pro do the "skilled" part of the job.I could attach the gas line and I could wire in the 115 V. I could attach the thermostat wire.I could not pump out and capture the freon prior to starting so that would be a problem.Plus I would have difficulty with sheetmetal where the "old meets the new".Plus I would want a professional to turn it on and check it out.I feel like they're charging too much when they charge a fee of $1,000 for the labor on a one day job. Have you ever installed a furnace yourself?
The sheet metal work is worth paying for if you have a complex situation. A skilled tin bender can produce a much nicer, tighter setup than any DIYer can, and can work out complicated ductwork bypasses, etc. But not so necessary if it's a relatively straight-forward situation (though keep in mind that their "labor" charge includes the "incidental" costs for new ductwork pieces).In our situation one duct had to pass behind the other horizontally, then turn downward and enter a sump for the in-floor ductwork. I never would have figured out how to do it, and could never have made up the ductwork if I had.Likewise you gotta pay for the AC suck out and recharge.$1000 or so doesn't look too bad when you look at what you're getting.
If your view never changes you're following the wrong leader
Fan control is flaky (assuming you haven't monkeyed with it). Should take about 15 minutes to diagnose and replace. Dunno what the control costs.
Ah, what sequence is really happening? Does the the following sequence occur:
-- burner on
-- fan on (about 30 seconds later)
-- reaches thermostat setting
-- burner off
-- fan off (about 30 seconds later)
-- fan on (burner still off)
-- fan off (burner still off)
If this is the sequence it's "normal". What's happening is that there's a lot of heat in the mass of the heat exchanger, and when the fan stops the plenum temp rises until the fan switches back on.
You can "fix" this by lowering the bottom (fan off) setting or raising the top (fan on) setting, increasing the "dead band" between the two. But there's no harm in it the way it is.
(If this isn't the sequence, then please describe it in more detail.)
The sequence you described is what's happening.
I've got good news.I set the fan limit off switch down to 95 degrees and that took
care of the problem!I just wanted to express my appreciation to
the kind souls on this website as it's saved me from
paying for a heating and air service call !Tip O' The Hat and Thank You So Much
Glad you found the cure.
Actually, it wouldn't hurt a thing if you were to set the fan "off" settting to 90 or 85.
At 95, you're pulling more of the residual heat away from the exchanger and that's what was previously causing the fan to trip again. While a setting of 95 has resolved the secondary fan trip, set at 90 or 85, you'll get even more of those residual BTUs delivered around the house instead of having them leave up the chimney.
Thanks hoot owl.
You're more than welcome.......if my rambling helped a bit.
Hope you have at least one quality carbon monoxide detector (like a Nighthawk) in the house. It's a "must have" for everyone .....and the older the furnace, the more likely that a leak develops somewhere. Maybe take the money you saved on the service call and get one, if you don't have one now.
Be warm, but be safe.
Yes I do have a Night Hawk carbon monoxide alarm that plugs into the wall. I bought it a couple of years ago at Home Depot for about $45.It has a digital screen which reads 0It has two buttons, one is a reset button.The other is a button that tells you "peak level"When I press peak level it reads "33"What does that mean?
"When I press peak level it reads "33"'that means that since the last time you reset it that measurement of 33 (ppm??).Reset it by pressing both buttons. Then check it from time to time. That is too low to cause an alarm, but if it is continous at that level you have have some problems..
.
A-holes. Hey every group has to have one. And I have been elected to be the one. I should make that my tagline.
The problem is that at 85, with the standing pilot and the thermostat set high, the fan may never go off.
If your view never changes you're following the wrong leader
Possible I suppose. Have never personally seen that happen yet at 85 though.
Edit: Forgot the smiley. :-)
Edited 11/27/2007 12:40 pm ET by HootOwl
I think some people get attached to their old furnace.It's kind of like a member of the family.I call my furnace "Ole Bessie":)
Couldn't wait to get rid of mine. Too many evenings spent sprawled on the floor replacing bearings, etc.
If your view never changes you're following the wrong leader
i see your just luring me into the trap for everybody to flame me....lol yes i have installed my own several times.
first since you are thinking of selling your house you gotta do a permit. here you can go downtown and take a small open book test and be able to pull the permit your self.test is easy and my experence is the inspectors will work with you,there just happy to see you pulled a permit.
what you first notice about new furnaces is they are about 12-18" shorter than old ones,thats what kinda complicates there install. do you have a updraft or down.that can make alot of difference.
one thing you will get into is fresh air into the area.m,ost installs i have had done ,theyput a 6" pipe in the attic into the closet and stop about a foot down from ceiling.and a second one right beside it that goes down within about 6" of floor
now as far as 80% of the work goes it's pretty simple.hook up the gas line.110 line thermastat wires and flue.flue should be nothing unless you go with a hi eff 94% unit that pipes in plastic,that can be easy to a hole big bunch of trouble as you have to get the flue out and fresh air lntake in. tell me a little more on where it's located,center of house next to a outside wall?????
you know if your a coil is good you don't have to replace it or the case around it. it is a perfect time to do it but if that really complicates things it can stay. let me know on some of these things and we'll go from there.larryif a man speaks in the forest,and there's not a woman to hear him,is he still wrong?
Sorry I put you on the spot Larry. But you do seem like the type
of guy who can figure things out on your own and find a way to do it.My furnace is in an unheated utility room beside the garage. You walk through the garage to get to the utility room.I think it's in an easy location to work with. No stairs, no crawl space, no attic installation.But I'm going to run my old furnace the rest of the winter and then
when I sell this house someone will probably want a new furnace and I'll give them some money and just let them deal with it.I just remembered when I was in college I helped a guy put a new furnace in. He was a do it yourselfer. He did just fine. This was back years ago when I was going to college at Hays.Thanks again for your info.
I hear ya.
Nothing more comforting than hearing the old girl fire up in the middle of a -20F night.
Put a new one in here about 7 years ago. Took a while to get used to the new sounds.
Now if Ole Bessie was aptly named Ole Smokie instead.....you might be more anxious to part with it. <G>
Reminds me of the father in A Christmas Story fighting it out with his old heat plant. Funny stuff.
I've never had a furnace named ole smokey but I've had a few cars
with that name.:)I had a 1959 pickup that smoked so bad I got pulled over by
the police:)
LOL
Tell 'em it isn't actually smoke. You're spraying for mosquitoes. Voluntary public service. :-)
you may have a restricted return air duct (furniture, drapes) . I had the same thing in my home after removing some walls (open floorplan) and installing new return airs. Was told by my HVAC man that not enough cross section will cause the fan switch to short cycle, blower will keep going on
Thanks I really appreciate it.Just the vent grills or covers restrict the air a lot.It I take the grill off it increases the air flow 2-3 timesThanks again
supply or return???
Life is not a journey to the grave with the intention of arriving safely in a pretty and well preserved body, but rather to skid in broadside, thoroughly used up, totally worn out, and loudly proclaiming<!----><!----><!---->
WOW!!! What a Ride!Forget the primal scream, just ROAR!!!
The grills for the air supply.They're on the floor in every room of the house.
you are under sized...
do the current ones make air flow noise when the fan comes on???
Life is not a journey to the grave with the intention of arriving safely in a pretty and well preserved body, but rather to skid in broadside, thoroughly used up, totally worn out, and loudly proclaiming<!----><!----><!---->
WOW!!! What a Ride!Forget the primal scream, just ROAR!!!
no they don'tBut I do live in an old (1950's) house 1250 sq feet which only has ONE return air duct in the ceiling of the hallway.This is probably not a good system.I hear the air making a sound as blows through the grill and into the return air duct system in the attic of the home
if ya just hear a "sound" that's pretty much normal....
if ya hear noise or whistling.... generally that means the ducting is under sized...
is this an original furnace to the house???
Life is not a journey to the grave with the intention of arriving safely in a pretty and well preserved body, but rather to skid in broadside, thoroughly used up, totally worn out, and loudly proclaiming<!----><!----><!---->
WOW!!! What a Ride!Forget the primal scream, just ROAR!!!
Well, the house was built in 1954 or 1955 and the furnace is a 1968
model.So it's not the original furnace but boy it's an old timer.I replaced the blower motor in July
that old...
consider replacing it a 90+%'er...
Life is not a journey to the grave with the intention of arriving safely in a pretty and well preserved body, but rather to skid in broadside, thoroughly used up, totally worn out, and loudly proclaiming<!----><!----><!---->
WOW!!! What a Ride!Forget the primal scream, just ROAR!!!
I've got a bid for $2,000 for a new 80 % furnaceThat would include a new A coilBut this house is for sale and I would like to sell it and move on.I'll probably offer to pay 1/2 of a new furnace (if I receive a full price offer of course)
2K for an 80...WOW!!!!
1.4 to 1.6 tops....
Life is not a journey to the grave with the intention of arriving safely in a pretty and well preserved body, but rather to skid in broadside, thoroughly used up, totally worn out, and loudly proclaiming<!----><!----><!---->
WOW!!! What a Ride!Forget the primal scream, just ROAR!!!
I need to get a couple more estimates.I think this would be an easy installation.The furnace is in a utility room on the main floor off the garage.It's not in an attic or crawl space. Not in a basement.I'm guessing the furnace and A coil retail for $1,000 and so I'm being quoted a price which would include $1,000 for labor to install the furnace and I feel like I'm dealing with Jesse James