How the 90% gas furnace works
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- čas přidán 18. 02. 2012
- Sequence of operation and effect of failures on the 90+% AFUE efficient gas furnace.
This video is part of the gas furnace series of training videos made to accompany my website: www.grayfurnaceman.com to pass on what I have learned in many years of service and repair. If you have suggestions or comments they are welcome.
If you are a homeowner looking to repair your own appliance, understand that the voltages and fuels used can be lethal and can cause a fire or explosion. Know your limits. - Jak na to + styl
I love finding ACTUAL experts like this on youtube. Excellent video.
Thanks for the support.
GFM
I know most of this stuff, but you are perhaps the best instructor I've heard so far. I hope you teach this for a living.
I did teach for 9 years before retirement.
GFM
Awesome description of the system controls. I have this same unit and wanted to get familiar with it for proper servicing.
nice to have the sequence of operation, thank you...
We just had a Trane XC80 gas furnace installed in the last two days. It's running beautifully. My boyfriend put the damn thing into high flame mode. It heated 3000 square feet of house in less than 15 minutes from 64 degrees to a comfortable 73
Any issues with it yet? 90%ers are starting to fade away.
just wanna say your videos are awesome and very well put together. thank you
I love how everything works in the order it supposed to work
What an awesome video. Great demonstration!
My 90 plus has been great for 20 years. i have had 2 issues. the pressure detect switch didnt detect fan coming on. Figured out with error code and tested switch with a meter. New switch installed (20 bucks for part). I also had the board go bad once. New board made entire house smell since a component burnt out. Scared us enough to call fire dept. New board was $200.00, i installed it in 10 min.
Another excellent video - thanks!
Happy holiday! Thank you for your great videos.
Merry Christmas
GFM
Terrific tutorial. Very informative on furnace operating sequence !!!
+Narang Arun Glad it helped.
GFM
Sorry about the late response. There appears to be some places that still do not have internet. These furnaces originally went into hard lockout after one trial for ignition. There were many nuisance shut downs. Probably due to power anomalies. So most furnaces have gone to using a soft lockout (meaning it will purge unburned gas then try again) two times and only hard lockout after 3 tries. Note it also will try again after 1 hour to see if conditions have changed.
what would happen if you had spiders web up orfices in the manifold for gas supply . I had this happen to me . It was simular to what flame rod not working right . But if you had a manometer hooked up to the burner manifold would you gat a high pressure reading indcating that you have a restriction in the gas supply form the gas valve . Would that be the proper way to test it without taking the manifold apart first .
Hi GFM, Wanted to ask on these percentages for the AFUE, The text I am reading says that plastic pipe can't be used on furnaces until 90% or higher. So is the lower percent number mean that the furnace runs hotter and has less chance of producing condensation?
Thanks for your help
Great videos!
very good sir!
We'll just call him "Teach" for teacher. Great demonstration!
I am a fitness equipment repair tech and licensed electrician, the equipment I deal with is not so far from HVAC in principle, motors, controls and air flow etc., thank you for these videos they are well done and interesting, you have made me seriously consider shifting my career. keep it up great videos I can watch them like a reality show lol
You mean maybe I'm like Survivor?
GFM
very much so
it's funny you say surviver, with how long you've been in the industry I'm sure you have seen many trend and code changes, have you made a video about that? the evolution of the industry
I have not. Sounds like one to do.
GFM
Right on, it would be very interesting. I have a question if I was to get into the industry, is a online school as useless as I think it will be or is there real potential to start a career from that point? I would imagine, as in fitness equipment repair and electrical, hands on experience is king
Thank you for the data.
+Gotchaplumber Gotchahvac Welcome
GFM
Awesome demo by the way! thank you for posting this!
but why didn't the furnace go into hard lock out after you unplugged the flame sensor? (first two times it didnt notice a flame at all) the third time it didnt notice a flame either...(which means that all three times the flame sensor was open) all three attempts had an open flame sensor connection... or is the flame circuit sensor always closed? or is there an element that senses if gas is traveling through the manifold?
Checkout my latest vid. Thanks for the idea. If you still don't understand, let me know.
GFM
The gas manifold pressure should be normal with a blocked orifice. If you have multiple burners, one or more will have no or a very low fire and the flame will not spread to the next burner. If the meter is clocked, the input will be low. As to flame sense, most furnaces sense on the the last to fire burner, so there will be no flame sense. I would be looking close at each burner flame to note difference in flame size. Hope this helps.
GFM
My only question is the small drain on the furnace that goes into my floor drain. Is that for condensation? Does my furnace only have condensation during the heating season? Not during the cooling season? Great video and thanks.
I have a goodman 90%+, after the thermostat is satisfied the burners shut off than about 30 seconds later the blower shuts off but the inducer motor never shuts off. anyone have any ideas?
Hey Grey can you do a video explaining the thermostat terminals on the furnace board which is which and how they work and so on I never really did understand them. Thanks and keep up the good work.
I thought how a gas furnace operates is its has 2 sides , the microturbine side and the fan side, i thought the gas like the cng that comes from the gas company and it has a electric starter to turn over the gas turbine and fuel comes in and plugs ïignite it and as its running on the gas, the exhaust heat is used to heat the heat conductor which is directed by the fan system by a heat thermostat. And when the gas turbine is up to speed i thought the clutch activates the blower fan and both run 1:1 until the thermostat determines the set house temp .
do 90 plus furnaces seem like they short cycle naturally? if I set it to 70, it goes to 70 not 71, I'm used to furnaces going 1 degree warmer than set temp?
The high efficiency furnaces cycle like all others. If the anticipation of the thermostat is correctly set, the temp in the structure should not vary.
GFM
Is this your furnace in your house? The trane you told me about
I smell a little gas when our heater/gas furnace is turned on so should i be worried and it kinda sounds weird
It should never smell like gas. Get someone out to look at it.
GFM
I love how you didn't say even one thing about how this unit condensates in heat mode
thank you for posting, a big help walking me through. when you say power do you mean from the breaker box, or the on off at the gass Junction box, my hubby left last night and the heater messing up, now. thought it was good whent to bed, then woke up to it making that noise of trying to and then stopping. clean the heat bar, and its like no power? and of course the first snow of the year here. :(
You can start by the light type switch on the furnace. That will reboot the electronics.
GFM
grayfurnaceman thank you. it ended up being the fan motor got it fixed last night. i am thankful for you postings the video it was a big help, and was able to do it myself and didn't have to spend lots of cash i don't really have to spend right now. thank you again.
Welcome
GFM
Just had a 96% furnace installed seems to run great, but when the furnace is off I can smell a very tiny bit of gas around the furnace, the installer said that is OK. Once running I don't smell anything, could this be normal? The furnace cabinet has rubber grommets on the side I guess to run stuff in and out, not all of them are used and one has a small hole in it and this is where I can smell the gas coming from. But it's very faint and not all the time. Is this OK.
+Rays RCs I would say not. The problem with small gas leaks is you get used to the smell and may not recognize a bigger one. I would want it checked out with a flammable gas detector or a very careful soap bubble test.
GFM
Thank for the info, called the installer he is coming back to do a pressure test on the system. To work out if leak or unused fuel Im smelling. But it's from the two stage valve I smell it (took the side panel of yesterday)
+Rays RCs Sounds good to me.
GFM
Quick question: I have a 90% furnace. Hot surface ignitor came on, but it did not fire up. I checked the voltage going to the Gas valve and it was reading 9.2 volts. I changed the board, checked all the roll put switches and high limit switch. I have continuaty but it is still not firing up. What is next, the gas valve itself???
First, is gas available to the valve? Before changing the gas valve, I would check the voltage at the wires to the gas valve. Wires must be disconnected from the valve. This must be done during a trial for ignition. That is, when the HSI is glowing for 10 to 45 seconds, there will be power to the gas valve for 4 to 7 seconds.
GFM
@grayfurnaceman Thanx for the reply. I did check the voltage across the 2 stage gas valve. I was getting 9.2 volts. I had Nest thermostat installed 6 months ago and now the heat is not coming on. W1 is connected and w2 is not connected on the terminal board. Do I need to jump w1 and w2 for the 2 stage gas valve to work?
The W1 and W2 connections will not stop the valve from working. The way it is now connected, will just lock out the 2d stage of heat.
You can try jumping from R to W1. This will eliminate the thermostat as the problem.
GFM@@copkhan007
What if inducer comes on and then turns off after maybe a minute or so
Sir Murdock The pressure switch is probably not making. Look for blockages in venting.
GFM
yup.
GFM
why are there two pressure switches?
hendrikslv The unit is a 2 stage furnace. Different inducer speeds for the stages.
GFM
whats the difference between a high efficient gas furnace and a reg furnace? Do all high efficient gas furnaces have A/C.
There are furnaces that are rated at 80% AFUE (annualized fuel usage efficiency). Then there are 95% AFUE. These last are high efficiency furnaces. A/C is an option with a lll furnaces. Hope this helps.
GFM
Very informative! My furnace runs, pilot light is on but there is no flame in burner. I suspect for some reasons the gas valve isn't open. What would causing that?
Dan If the blower runs there is power to the furnace. Is this a standing pilot furnace?
GFM
grayfurnaceman yes. The pilot is always light up. I manually relight the pilot the 1st times in a year. Sound like the pilot waits for gas to kick in but that took up to 20 minutes. This happen 1-2 times a day.
Dan I am assuming the is a thermocouple system rather than a thermopile. If so, there gas valve is probably failing. If power is available to the gas valve, it should star immediately.
GFM
grayfurnaceman look like thermal couple to me because I see cooper wire is burn the pilot light. What I dont understand is why it only happen a few times a day. When it happens I just turn off/on the furnace hope it will burn. Really waste electricity here.
really need help here. have a 90% furnace where the flame at the spreader between tubes is very high and getting burner box red hot constantly tripping roll out switch. doesnt seem to have a cracked exchanger but flame spreaders are deterierated a bit making a small space inbetween burners. flame in heat exchanger tubes looks good just high at flame spreaders.
If the spreaders are rusted out, you need to replace the burners. What brand and model#?
GFM
hot service igniter usage?
Are you asking if it uses a hot surface igniter? If so, yes.
GFM
Installed a new furnace. It makes same sounds as one is video but are so freaking loud. Old furnace was quieter
Do you have a favorite brand model of Natural Gas Furnace? I am considering replace my GMP 80,000 80% Natural Gas furnace.
+Todd S I generally use Trane. Still plenty of problems but ok.
GFM
Lennox makes awesome efficient 96 percent efficient gas furnaces
My inducer motor overheated and started to fire up and shutoff so I went to CZcams to see why it was doing that and tested the pressure switch with a paper clip and it fired up but the inducer did not come on , so I took out the inducer and the motor was burnt out. I checked Amazon for the inducer motor and the earliest they can have it was in 20days and the temperature in my house dropped to eleven degrees,what am I going to do. So I was walking along the street in my neighbourhood and low to my surprise some one put out a Lennox furnace it was not the same model as mine but the inducer was the same except mine had brackets on it and thi one didn’t so I said no problem, I cut the plastic brackets out of my old motor and plastic weld them unto the new one and low and behold it worked. Then I called repair clinic for the pressure switch and furnace runs like new
Glad you got it going.
GFM
Still confused if the inducer motor proves the vacuum for the pressure switch. Or if the pressure switch proves vacuum for the inducer?
Inducer produces the vacuum that triggers the pressure switch to flip (close circuit). It's the closed circuit that "tells" the ignitor that draft induction worked properly so it will turn on, and the gas valve will then let in gas. In essence, the flip of the pressure switch is "proving" the draft inducer is working properly. Otherwise, the sequence will not continue.
We have a new high efficiency gas furnace and recently once or twice in the evening we get a 2 to 3 second gas explosion in the furnace "Very Load and then you can feel a tremor in the house, however the furnace does not come on. What could be the problem. The make of the furnace is "HEIL"
It sounds like a delayed ignition. This is pretty serious. I would get someone out as soon as I could. If you can't get a service tech out right away, check with the gas supplier to see if they can help.
GFM
Does anyone know if a 40 year old furnace is safe?I rent and I need to know but landlord does not care.
They are safe if they are maintained.
GFM
THANK YOU SO MUCH! I needed to hear that.
If maintained they are safe. But they cost hell lot of money for gas and you have to check them and operate manually, Depends on country where you live and gas code + gas prices. Difference in price for gas could be quite huge when used to heat a house. Even by factor of 3. In europe appliances have to be checked on annual basis and maintained. In absurd cases gas company can disconnect you and request papers from gas inspector. Also appliances without safety valve(thermocell gas lock shutoff gas if there is no fire) are banned in here more than 20 years. But in the US there is no such code. Also you can still use pipes with threading for installations. Here only a pressed, soldered or welded pipes are allowed here for a new installations.
Did you know that a 80% furnace works exactly the same way?
Not quite. The sequence is the same, but the pressure switch must tap into the combustion chamber.
GFM