Up the stack. When a furnace short cycles it is running in the in-efficient zone for more of the time. The heat goes up the stack instead of into the house.
see:
Hmmm. That's a serious concern. Thank you. How short do you think too short an ON portion of the cycle would be?
Also there should be some way to know if the furnace is turning off because the thermostat reached the desired temp, OR becaues the high limit switch turned it off. I realize now that I don't know why my furnace turns off.
Was it before or after you bought them that they converted to Mormonism?
Almost in the same way. I think?, that driving short distances and never letting a car's engine warm up to proper operating temperature. In the case of the furnace, it may get that warm, but because it turns off soon, a higher percentage of the On time is at the lower temperature.
I think I missed one word that changes everything. Sorry. People, forget that I mentioned DD's name here.
I've see it cause the over temp sensor shut down more than one furnace due to diminished airflow through the combustion chamber on NG fired heating systems. When the temperature dropped to unusually low levels here in Alabamastan, I left the rooms open to prevent the 7°F air from freezing the pipes in the crawl space under "The Crotchety Old Fart's Lair" because it's an older house with no insulation under the floors. The extra cost to heat the open unoccupied rooms is much lower than the cost of repairing freeze damage to the plumbing. ^_^
TDD
SM: So, the air handler blower is running, the gas is off, and the heat is going up the stack, you say? I'd think the heat is going into the building.
SM: Says decresed efficiency, but doesn't explain the process, and where the heat goes.
SM: This page is not available, or you have exceeded your limit for the day (my limit seems to be zero).
SM: I still don't see it, sorry.
SM: After, sadly we were not able to find Mormon furnaces. So, we gave them lots of natural gas, and now they blow hot air.
SM: With a cold return closed, the furnace would reach operating temp sooner, and spend more of the day at operating temp. Not sure the two can really be compared.
A few things. In any correctly designed and installed forced air furnace, you'd have to close down a lot more than one or two vents to have the furnace shut down due to exceeding the high temp limit. And in any recent vintage furnace, I don't think it's just going to restart. More likely it's going to stay off with a fault code flashing. If you choke off enough air flow, you also run the risk of burning out the blower motor, depending on what kind it is.
As to the loss in efficiency, exactly how much you would lose IDK. But you do lose efficiency if a furnace keeps going on and off instead of running continually. For one thing, modern draft inducer models run the blower to purge the system for like 30 secs before starting. Whatever heat there was, it's blowing cold air through it for that period and the heat is being vented outside with the air. At the very least, you're losing that heat. Also, if it does recycle, while it's sitting there cooling off from the over temp, some of that heat is also flowing out the combustion air path, losing heat. Or being lost to an unheated basement, etc.
If the furnace shuts down while the thermostat is still calling for heat, you have a problem, and it is short cycling.
What kind of thermostat? If it is an electronic digital stat it usually has an indicator telling when it is calling for heat.
Mine ( a 2 stage furnace - high stage really too big for the house) will occaisionally short cycle if it goes on high.
On Sat, 01 Mar 2014 06:45:14 -0500, Stormin Mormon wrote:
OK Stormey, I'll try to explain it in simple terms.
You start the cold furnace, and it has to warm up the stack to get a good draft going (unless it is a high efficiency burner - where you get a forced draft, but it still needs to warm up) When it gets up to operating temp, it is running at top effieniency. It warms up the heat exchanger and plenum, and at a certain plenum temperature the blower comes on to extract the heat from the heat exchanger and move it through the house. The Delta T between the return air and the heat exchanger temperature affects the efficiency of heat transfer - the goal is to keep the delta T (temperature difference) between the return air and the heat exchanger in the "sweet spot" where the maximum heat is extracted from the exchanger and delivered to the house. Now, if the airflow cannot extract enough heat, the heat exchanger/plenum gets too hot, and for safety reasons the furnace shuts down (short cycles). Now the firebox/burner/stack go into cooldown mode along with the heat exchanger. When the heat exchanger drops down to the "reset" temperature, the furnace refires and starts the cycle all over again. Much of this time the delta T between the exchanger and the return air is sub-optimal - dropping the heat exchanger efficiency. - and the furnace starts up in it's lower efficiency range untill it warms up again. Low airflow causes the heat exchanger/plenum temp to go above limits again, and the (short) cycle repeats itself. During this whole cycle, the efficiency is less than optimal. When the airflow is not removing the heat from the exchanger efficiently, heat goes up the stack.
In a properly operating system, when the furnace gets the plenum up to temperature and the blower comes on, the correct amount of air passes the exchanger to transfer enough heat from the exchanger to keep the delta T in the "sweet spot" where maximum heat energy is removed from the heat exchanger, and the minimum required heat escapes up the stack. On variable speed blowers, the system is set up for a specific temperature size across the heat exchanger. On my furnace it is spec'd at between 30-60 degree F temperature rize across the heat exchanger with 28000-40000btu output The bigger model of this furnace is 45-75F at 89-72000 btu output. My blower is set at about 650 CFM
At high limit, the delta T will be higher than spec, so more heat goes into the house. Then, it goes through the cycle. I can't see this being a major loss of efficiency.
There is no "sweet spot" for the heat exchanger efficiency. Setting the blower on HIGH blowing more air over the heat exchanger will always be more efficient then lass air.
It's only a sweet spot in terms of comfort and not feeling a draft in your home, not in terms of heat exchanger efficiency.
You're not suggesting that Clare is mistaken?
You have it backwards. With a lot of the air handler air flow blocked off, you have less air moving, the air in the plenum get *hotter* and the temperature delta across the heat exchanger is lower, ie the plenum temp is closer to the temp of the combustion gases. Hence, you recover less heat and more of it goes out the exhaust.
Also as pointed out previously, if the furnace then kicks off and recycles on and off due to the high limit being hit, during the period it's cooling down, most of that heat is being lost up the vent, or could be lost to an unfinished basement, attic, etc. I think that's largely a moot point, because I believe any modern furnace is going to trip and stay off with a fault code if the high limit is exceeded. But if it does continue to cycle on and off, no question you're taking a hit on energy loss.
FTR, I thought we were talking about closing the warm air supply to some rooms, but actually I don't think there is much difference.
But they don't run all day. It might be 10 or 20 minutes. Yeah I see your point that, assuming as was said that they warm faster, they get to operating temp sooner than normal, but they might then zoom though that temp range on to overheated soon thereafter.
I think they can but maybe there is no general rule. Maybe each similar furnace setup is the same but ....
Well I don't know what the but is, but I've been bothered since yesterday, per my previous post, wondering if that was why my furnace was turning off. In the short period between replacing the igniton transformer and having the next problem, I was sitting at my computer in the room next to the furnace and for about an hour I logged when it started, when the blower started, when the fire stopped, and when the blower stopped. The notes are somewhere in my Agent outbox so I have to find them and read them.
The rest of this post is about my 1979 furnace and may not interest most people, but though many things have changed over the years, I doubt the final actions described here of the control panel, etc. have changed.
I've also been trying some more to understand the furance wiring. There are a couple omissions to the wiring diagram (2 resistors), and one or two more things** I haven't figured out yet, but most I do understand and when the heat limit switch opens, that interrupts the 120 volt black wire to the furnace. which will turn off the burner motor, the ignition transformer, and the control board power transformer (which powers the furnace relay so that too turns off the furnace) . The fan wiring I find complicated but I don't think the fan turns off because it's needed to cool off the plenum etc. which we just said had overheated. Anyhow, after the house reached 168, the furnace never ran for more than 20 minutes iirc and that started to worry me.
Since I've had trouble with the control panel, what I've been doing is turning the furnace on by hand (with a stick*** holding down the relay armature) for 2 or 3 hours when I wake up in the morning, and for 2 or 3 hours just before I got to bed. (Less time during the couple days it was warmer out) I get the second floor of the house up to 178 or even
180 before I got to bed (where I often listen to the radio for a couple hours and then sleep 8 hours) and it's 164 to 168 in the morning. Under the covers it's always warm enough, but a couple days, I've had to dress quickly and go turn the furnace on.If my furnace could overheat in 20 minutes, surely it would overheat in
3 hours and the high limit swtich would turn off the whole furnace. But it doesn't. I have to turn it off by taking the stick out. So it must not be overheating. ***The stick was originally used to post an illegal advertising poster on public land. Baltimore County law now allows anyone to remove such signs. Many are held up by wire things, but when it's wood, I save the wood. About 2 feet long by 1 by 1/2" with a point at one end. **Things I don't yet understand about the schematic. The switch that turns the furnace off when there is no flame (or maybe too much black smoke??) has four wires going into it, but the schematic shows nothing about how they are connected inside -- it's a black box -- and that makes it hard to understnd the main relay's operation, since two of those four wires are connected straight to the main relay coil. Also the fan relay, which is in the fan & limit control enclosure, I think, I have just started to figure out. The schematic does not use the same exact symbols that radio and tv schematics do. I think I've found a double throw switch but I'm not sure. (Maybe I'll post the diagrams) It also doesn't match my own fan, which has 3 speeds but only one is connected, the same fan speed for everything. This one from the web for my model may have high speed for AC and low or medium for heat.It also appears that 24 volts are provided to close the main relay, when the thermostat calls for heat, but maybe that's lowered to 12 volts to keep the relay closed once it has closed. Can't tell because of the black box.
I have also found that sometimes when the house is cold enough that the thermostat should call for heat, when I start to close the relay by hand, when the armature is half-way there, it's pulled shut magnetically****. But it didn't shut without my help. That goes with something I think you said, that there's a problem with the main relay coil. So I've decided to replace the control board (including the main relay coil) first and worry about the thermostat later.
****Other times when the house is cold, there is no magnetism in the relay. How can that be? Maybe that's a problem with the thermostat or maybe it's gremlins.(ONE DAY WHEN I GUESS I HAD THE STICK just right, the furnace went on and off by itself for 24 hours or so, and the house was always 68. I guess the stick wasn't holding the armature closed but it was holding it close enough to the relay coil that the coil could pull it the rest of the way. Nothing else makes sense.)
I also remembered that my thermostat is electronic or semi-electronic, not like the round honeywell thermostat that was used initially in 1979 and is in the schematic, and disconnecting the stat and expecting to find zero ohms across the red and white wires when the house is cold might be wrong. Maybe it needs its 24 volts to work right.
Well, right now I'm not using the thermostat, but now that this issue has come up, I'm going to up the priority of the control board replacement and I should know soon.
Although with a day to think about it, it seems to me from theory and looking at the wiring diagram for my furnace installation that if the high limit switch opens, the furnace would shut down, except for the blower needed to cool it off, and it never shuts down until I do it manually, even after 3 hours. That's certainly not a short cycle.
I have maybe the first residential setback thermostat sold, from 1984 maybe. It's electronic or semielectronic, The clock is digital but the time and temp settings are mechanical switches. Anyhow, no indicator.
But when I replace the control board on the furnace, I'll put in a couple wires that extend beyond the furnace, so I'll be able to use a voltmeter to check without having to get my hands in among hot wires.
Isn't the loss of efficiency in large part because of unburned fuel, because of the wrong temperature for the fire chamber. And it's the unburned fuel, gas or oil, that goes up the chimney.
In the case of oil, with a cracked heat exchanger and apparently even with a good heat exchanger, doesn't the unburned oil get on the walls and ceilings and anything else that attracts it, and make the house dirty. And if that's not true all the time because of a misadjusted furnace, it's stil true during the startup time when the firebox is not hot enough. Isn't that what they're referring to when they say oil is dirtier than gas?
Why does it blow it outside? Why not inside? Were my old furnace to run the blower before ignition, it would just blow the heat into the heating vents and out the other end if it ran long enough.
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No, because it has little effect on the fuel burning. Close to 100% of the fuel is burned, no matter what.
Irrelevant.
And if that's not true all the time because of a misadjusted
Because combustion gases go outside, not into the house.
Were my old furnace to
Really? It blew combustion gasses into the house? That explains a lot.
You obviously don't understand the difference between an air handler blower and an inducer blower. And I've never seen a furnace run the air handler blower *before* ignition. Everyone has a delay, so that it doesn't blow cold air.
Majority of furnaces in todays housing stock are oversized, so restricting the airflow can very easily overheat the plenum. And restricting airflow REDUCES the load on the blower motor, so you are unlikely to burn out the blower motor.
You are talking the "combustion chamber". The combustion chamber is purged to make sure there is not an explosive mixture of gas and air in the chamber when the ignitor comes on. . You most certainly do NOT want to purge the combustion chamber to the "conditioned space" - it MUST be purged to the outside.. This is NOT the blower that circulates the heat in your house. You must not have ever seen a gas furnace newer than about 20 years.
The air circulation blower does not EVER move combustion air,
Over sizing the *system* in a house has nothing to do with the ability to overheat the system. If you install a furnace that is too big for the *sytem*, then yes it could overheat easier if you restrict the air flow. But good grief. Stormin is talking about shutting off a couple of vents. I've never heard of that causing a furnace to go over temp and shut down. And if it does, something more is wrong than closing a couple of vents.
And
You might want to re-think that in view of that fact that a lot of new furnaces have ECM motors that are designed to maintain a given CFM of airflow.
Can't you follow a thread? Nothing I said ever stated or implied that the air handler moves combustion air. Yet you're replying to the part that I wrote.
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