Well, not mine, but my daughters’ viola teachers’
He asked me to have a look at his system because “the register in the back bathroom isn’t working like it used to”.
The register is fed by 6″ hard pipe, off an 80% gas furnace, about10′ from the plenum to where the duct passes into an addition crawl space (and transitions to insulated flex).
There is a Tjerlund booster in the hard pipe, about 6′ from the plenum (yeah, wrong place, should be nearer the terminal end), and it seems to be running properly.
I stuck my head in the crawl, didn’t see any obvious drops in the flex.
So, I don’t really know what might have changed. But, I did notice while I was there that the furnace never stopped running (the fan, anyway….not sure about the burner). Fan was set on Auto, T-stat @ 69 degrees, and reading 69 on the display. I turned the heat to off, and the system shut down normally. FWIW, the T-stat is an older Honeywell programmable.
Anyone got any idea why the fan was running constantly?
Replies
Maybe check the air filter on the furnace? If it's clogged it will have a hard time pulling enough air through the system.
The fan is controlled by a either a fan limit thermostat in the top of the heat exchanger or a time delay relay.
Continuous running can be cause by a stuck contact or a bad relay. Or in the case of the limit thermostat it might be set way to low.
William the Geezer, the sequel to Billy the Kid - Shoe
Thanks, nice to hear from you.
How hard to investigate these items (typically, I'm sure it varies from OEM to OEM....)?
You said 80%, so that implies a modern pilotless unit. More of the those are going to be on control board then identifiable components.The fan limit switch you can get with a voltmeter if you can find it.Other than that I don't know of anything that you can easily check without some electronics and/or HVAC experience. Some units might use plug in relays, but you still need to identify which one it is..
William the Geezer, the sequel to Billy the Kid - Shoe
Should note here that an 80% unit is not a condensing unit, but a unit with a conventional flue, and likely ambient combustion air intake.
The mark of the immature man is that he wants to die nobly for a cause, while the mark of a mature man is that he wants to live humbly for one. --Wilhelm Stekel
Mine seems to do this when I have a clogged filter and the furnance set to gas. (Combo Heat Pump with Gas backup) My deduction is that the furnance has a heat sensor on the exchager to keep it from overheating. The clogged filter limits airflow to cool the exchanger so the gas shuts down but the fan still runs. The thermostat never gets to the temp it really wants to because the gas keeps shutting off. As soon as the exchanger cools the gas comes back on and the whole thing starts over again.
Anyway, thats what I told my wife to sound smart and changing the filter did fix it.
On a related note of glorious endings to furnances, I had one that sucked in one of those cheap metal mesh covered filters into the blower. It locked up the fan and the motor burned itself out. There really should be a piece of hardware cloth over the cold air return to stop this from happening to those of us too lazy to change the filter.
Edited 12/29/2008 1:53 pm ET by cussnu2
Here is a question for some of you who are apprently very up on calculating this stuff.
Someone claimed 40 degrees is the cutoff point on the heat pumps which is just plain crazy. But what I would like to know is what is the ideal time to cut over? I have a heat pump with electric resistence aux and back-up gas. When I run the heat pump, the resistence heat comes on by itself but I have to manually switch to gas.
I have a 12 seer unit which runs on .06/kwh rate when the heat pump is running but the resistence heat would run me .11/kwh and the blower runs on .11/kwh. If I stick with the heat pump, the blower runs a lot more with the heat pump than it does witht eh gas. Any easy way to calculate what the optimum point would be for me to switch to gas?
Right now I just switch it when it drops into the teens.
There are 3412 BTU per KWH. 100,000 BTU per therm. So you get 31018 BTUs per dollar with pure resistance heat. The problem is we don't know at what point the HP cuts off, and what it's thermal efficiency is just above the cut-off point. But if you get more than 31018 BTUs per dollar of gas then certainly you should switch to gas when the HP cuts off. Determining whether you should switch earlier requires more info.
The mark of the immature man is that he wants to die nobly for a cause, while the mark of a mature man is that he wants to live humbly for one. --Wilhelm Stekel
"There are 3412 BTU per KWH. 100,000 BTU per therm. So you get 31018 BTUs per dollar with pure resistance heat. The problem is we don't know at what point the HP cuts off, and what it's thermal efficiency is just above the cut-off point. But if you get more than 31018 BTUs per dollar of gas then certainly you should switch to gas when the HP cuts off. Determining whether you should switch earlier requires more info."
Well, I get a therm of Natural Gas for about $1.11 if you ignore fixed charges and all the taxes. So 100,000 BTUs of electric resistance would cost me $3.22. Since I'm not very knowledgeable about the system settings and have no intention of trying to find the owners manual, I think I just stick with switching it over in the teens.
Thanks!
Never run the electic backup. Gas heat will always be cheaper.Ozlander
"Never run the electic backup. Gas heat will always be cheaper."
Don't have any choice, the aux heat automatically kicks in if the furnance senses the heat pump isn't producing enough heat. Since my AUX is electric resistence, I have no choice. Not sure why they even put in the resistence coils since they installed all of it brand new. They new they were installing gas backup. Seems like they just would have defaulted the system to gas for aux heat then I would never have to worry about it. as it is, I'll just keep manualy switching between heat pump and gas depending on the temp. Thanks.
It should not cost too much to get that setup correctly.I could be a simple system with an external thermostat that switches the modes.Or a new thermostat that senses both indoor and out door temperature and the logic to select the operating mode..
William the Geezer, the sequel to Billy the Kid - Shoe
"I have a 12 seer unit which runs on .06/kwh rate when the heat pump is running but the resistence heat would run me .11/kwh and the blower runs on .11/kwh."You have a different meter on the heat pump than for the heater?.
William the Geezer, the sequel to Billy the Kid - Shoe
Yes I have two meters. My CO OP offers a controlled Heating and Cooling Rate. In exchange for my permission to shut down the heat pump in the event of peak usage, the co-op sells me electricity for a little more than 1/2 the regular rate. If they get into a peak situation, typically in the summer, they will shut down the heat pump for either 15 minute intervals or 1/2 hour (don't really remember which). The blower will still come on to circulate air but you won't get any AC during the shutdown periods.
Its all radio controlled
We have a similar deal for our AC in the summer, except it's just a flat % off for a period during the summer. Considering that we only ran the AC for 3 days this year, it's a pretty good deal.
The mark of the immature man is that he wants to die nobly for a cause, while the mark of a mature man is that he wants to live humbly for one. --Wilhelm Stekel
If the rollout switch is tripped, the fan will run continuously on a call for heat. It must be manually reset.
Thanks...
Is the roll-out switch another name for the limit switch BH mentions up-thread.....?
There are several switches in a modern gas furnace. A high limit temperature switch is internal to the heat exchanger and resets automatically. A rollout switch is located in between burner nozzles on the inlet to the heat exchnager, to sense when the flame "rolls out", usually an indication of a draft issue. The other switches in a high efficiency furnace do not apply, but include high and low pressure switches on the induces, a gas pressure switch and a pilot flame proving switch.
TimWhy would a rollout cause the air circulation fan (which I assume that it what he is talking about) to run continuously?Generaly any kind of air circulation fan are turned off in any fire or smoke problems.Now maybe I could see a draft inducer fan to continue running to try and clear the unit..
William the Geezer, the sequel to Billy the Kid - Shoe
I will state two things before I give my 'maybe' solution:1. I am NOT a heating/cooling expert.
2. Every problem is unique - possibly.I had the same problem after I replaced an 18 year old unit with a new 95% eff furnace. Checked everything. Nothing seemed to solve problem. My old thermostat was a Honeywell programmable on the wall for @ 6 years. I tried a new one and all is now working just fine.Worth a shot.Mike
Well, seems like maybe it's worth a shot....
I'm guessing the t-stat and furnace are the same age, which would be different than your case, but I have no way of knowing for sure, so maybe a little parts swapping is in order...
Thanks for the input.....
Years ago, I did a rather boneheaded thing. I installed a small condensing furnace, set up for NG and fired it with LPG. The heat exchanger sooted up and it would trip the roll out switch and the circulation fan would continue to run. I don't know the specific logic that allowed/caused this, either in circuitry or reasoning, b ut thats what happened. Just my personal experience.
On my previous furnace, the roll-out switch consisted of the thermostatic sensor which mechanically operated a microswitch. The switch was buried in the fiberglass insulation for the combustion chamber. Glass apparently would work its way into the unsealed switch. Without fail every two years it would fail to operate. (Found it out by using an voltmeter across the contacts; full voltage at a time when the contacts should have been closed.)
So, I had a replacement hanging by the furnace. I could change the thing out in 10 minutes. And then go buy a new one.
Side Note: That furnace died an exciting death. Something happened to clog the heat exchanger, and it started pushing heat out the front; melted all the wiring. I was home, so I could shut it down, but it still scares me.
Edited 12/23/2008 3:35 pm by JohnD1
We had an older Lenox and the fan ran constantly. It turned out to be a bad limit switch.
Family.....They're always there when they need you.
Man, I guess I'll have to get some schematics.
I see a simple t-stat control as....(ignoring all the safety functions) call for heat, turn on burner & fan.
Heat satisfied, turn off burner & fan.Of course, there are 2 speed fans, delays, etc.
But seems to me all the safety functions would shut down the fan, in operating and in failure modes. So I guess I'm not understanding what a limit switch does.
In many cases, the schematic is pasted on the inside of your furnace.
In other cases, the installer should have left the instruction manual with you. If not, the manufacturer's web site should have all the information you need. However, reading these manuals is a challenge; they cover all sorts of furnace varietys, and figuring out which paragraph refers to which part number is difficult.
The "limit switch" on an old "basic" furnace was actually a combo device.It had and over temp limit which would shut off the burner.But the other function was a set of contact which operated at a lower temperature. I think in the neighborhood of 120F.When the burner fired it it would heat the heat exchanger. When that got up to temp then it would start the blower so that it does no start by blowing cold air.When the thermostat was statisfied then it would turn the burner off. The fan control part of the limit switch would keep the fan blowing until it cooled off the heat exchanger. Thus getting the last bit of heat out of it.And because of the mechanical thermostats had an anticipater feature. That was a small heater under the thermostat so that it might turn off the burner slightly early..
William the Geezer, the sequel to Billy the Kid - Shoe
Thanks Bill....that's sort of what I thought, but I appreciate your putting it in words...If that switch got stuck closed, fan runs full time....
And I suppose if the fan was running all the time in a small bathroom it might tend to make the room feel cold.....Just curious, is there a parallel fan switch system for the AC?
AFAIK they AC don't use a thermostat for the fan.The either come on or off when as the house thermostat calls for cooling or they might have a time delay to keep the fan running for a minute or two after the AC stops to get the residual cooling off the cool.However, that cause the condensation on the coil to re-evaporate into the air. So it work as a dehumidifier is reduced.I think that they might play with that depending on the air that the unit is installed. AZ vs AL for example. But I really don't know..
William the Geezer, the sequel to Billy the Kid - Shoe
I'm glad I ran across this thread. During a remodel in the spring, the fan on my furnace would not shut off. Some one replaced a roll out switch which corrected the problem up until about a week ago. The fan is continually running again.
My question is, is this harming the furnace or is it dangerous in any way? There is not a lot I can do in the interim because I need the heat and I have not had good luck getting someone out to look at it since we just had a severe cold snap.
A previous poster mentioned the filter which I have not changed. Could this possibly be the problem? The furnace is very old and needs replaced, so I am told. Just hoping to get through the winter but now I'm concerned about the electric useage to run this fan.
Any input?
I really don't know.
In fact I was surprised to hear that the a rollout fault will cause the fan to stay on..
William the Geezer, the sequel to Billy the Kid - Shoe
"is this harming the furnace or is it dangerous in any way"?
No. There are some minor energy and possibly comfort consequences, but no safety or equipment issues with a contunually operating supply fan.
Other than wearing out the fan bearings a little faster.
The mark of the immature man is that he wants to die nobly for a cause, while the mark of a mature man is that he wants to live humbly for one. --Wilhelm Stekel
Many electronic ignition furnaces with a PC board in them have "the fan running all the time" as a result of some type of problem. It is a signal for the homeowner to go look at the furnace, especially if the "fan on" setting on the thermostat is not used.
Could be something like the ignition cycle failing 3 times, so the brain board locks out additional retries until the system is reset... some manufacturers have specific reset procedures (ie: turning it off doesn't reset it).
If the furnace has a draft inducer, the control board knows if the pressure switch is on/off when it isn't supposed to be, and that can trip the circulating fan.
Older Carriers have a board with a relay on it that goes bad, resulting in the fan being on all the time if the furnace has power (heat won't work).
Could be a stat wiring issue, too.
I've been nursing my 80% eff furnace for this heating season. A few times a day my furnace will try to turn on, but won't put out heat. I tried to troubleshoot myself and found that when the stat activates the furnace, the pilot will turn on as normal, but the main burner won't light. The air circulation fan does start though and slowly the house starts to cool. I must then shut the furnace down for a few seconds and turn it back on. Most times the furnace will start up normally, but sometimes I need to repeat this process.I found literature for my furnaces Honeywell gas valve and used it's troubleshooting sequence. I got to the stage where it recommends replacing the gas valve itself (they wear out). That's when I decided to get a new furnace.
Gas valves aren't too pricey if the rest is in good shape.