Forced-air Furnaces: The What, Why, and How technology connections heat pump

by alsaCEMusic



Let’s not get too heated, this is after an inflammable subject. Frankly it’s exhausting, but we gotta do it. We gotta explain how furnaces work! And soon, we’ll talk about what will replace them. I’ll try to remember to put a link here when that time comes!

Links!
Technology Connextras (the second channel that stuff goes on sometimes):

Technology Connections on Twitter:

The TC Subreddit

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Forced-air Furnaces: The What, Why, and How

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29 comments

Technology Connections 29/12/2021 - 4:32 Chiều

**A fascinating correction about the flame sensor!**
It turns out that fire, as in literal actual flames, acts something like a diode. This means that the furnace is able to detect flames by putting an AC voltage on the flame sensor and monitoring for a voltage drop. The fire will actually conduct some current to the chassis ground of the furnace! This also allows it to detect if a flame has gone out mid-run. Pretty neat!

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ethreldur 29/12/2021 - 4:32 Chiều

"I've made an embarrassing mistake: I misstated the efficiency by a whole 1%!"

Glad you take your accuracy so seriously XD

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Bearded gaming 29/12/2021 - 4:32 Chiều

my blower inducer went out. tried to buy the motor (mfg no longer makes it separate from the ENTIRE ASSEMBLY). turned out it was 1100.00 for JUST THE PART. so i amazon primed a new 96% 2 stage, cost me 1300.00…. i hate our throw away economy

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Twerkingfish 29/12/2021 - 4:32 Chiều

A brief comment on the furnace igniter: my family had a ~25-30? year old furnace (we don't know because the previous homeowner was not really mentally with it anymore) that was either 78% or 80% efficient. It had a flint based starter block that cracked and wouldn't ignite the furnace anymore. We found this out (fortunately in the fall, and not the dead of winter) because our home smelled strongly of natural gas from many failed starting attempts after a school break where we went on a small vacation. It would cycle on and go through all the steps, then fail to ignite (making many clicking sounds, run for a while, and shut off).

If I may for a bit, I still do miss the sounds of that furnace. It's weird but whatever. There was something deeply comforting about the whirring and flamethrower sounds, and it was really cool (especially when I was younger) watching the flames start up and go from orange to a nice blue color. This nostalgic feeling was probably why I was listening to the furnace from my basement bedroom and found out that it was trying to light and failing.

Anyway, what I'm saying is that a "sparky thing" such as a flint starter or an electric arc generator may be less reliable than a simple hot bit of metal that starts a fire, and this could've been the reason why they fell out of fashion.

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ᛋᛒᛖ‍ᚱᚫᛞᚻᛏ 29/12/2021 - 4:32 Chiều

As I was sitting there watching this, at the exact moment that you mentioned the smaller blower motor (draft inducer) I heard mine turn on.

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Psi Q 29/12/2021 - 4:32 Chiều

I never realized that there is a difference between a water boiler or water heater apart from the name/country, since i am living in a house from 1960 which back then had a pump free density difference driven central (water) heater system.
which also did not heat up to "boiling" the water but had a closed loop with expansion tank (air bubble) at the top.

The problem with the heat exchanger corroding was also known for the VW Beetle. The passenger room air was heated using exhaust pipe/gas heat, since the engine was aircooled. Now the wall on the heat exchanger corroded and the engine exhaust could mix into the passenger air.
You of course will smell that because its not only CO… but people are lazy and cheap on maintenance.

Biggest benefit of having a burner heat up water (or porous stone blocks..) is that you can modulate the produced heat so it is a bit above the needed heat.
The water (thermal mass, concrete) can buffer that stored heat. While the water is really hot you only need to trickle water through the water:air exchanger, the colder it gets the higher the flow. When temps get too low even at max flow you restart the burner.

Biggest heat loss happens on flushing before start, the longer the burner can stay on, running at optimal air:fuel ratio, running at the lowest output needed, the more efficient it gets.

The lower your (water) temps needed to properly heat your house/air/room the more energy you get out of the condensation.
(Large floor heating panels running at around 35°C are a lot better there over wall/window mounted heaters running at 50-70°C)

I think at around 65°C non freedom units it starts going into getting unusable because the exhaust temp needs to be a lot higher for the exchanger to work.
But this means even old houses without doing any isolation/renovation, as long as its not really freezing cold outside, can get use out of condensation burners. Usually the roomheaters are way oversized and can be run with lower temps and still get enough output into the room.
When it gets really cold outside and higher flow/volume isnt enough the temperature needs to get higher, meaning less condensation.
Any modern burner will adjust a temp curve accordingly and if possible modulate pump and heater power in multiple stages.

The most efficient heater would run at the lowest sufficient power non stop. Best air/fuel/heat output. (Which is kinda easy to do for gas (not gas-oline) burners.

Final note: i am surprised the burners get only hyped to 95%, beeing used to the US overselling things in Ads. Around here condensation optimized burners get sold at "up to 150% efficiency" because they compare numbers to the worst "all heat out through the exhaust" old style burners…

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Richard Patty 29/12/2021 - 4:32 Chiều

Correction. When the thermostat calls for heat it does not instantly turn on the draft inducer. The first thing the control board does is check that the pressure switch is open. If it’s closed, the furnace will not go through the lighting sequence. The switch could be closed cause it’s faulty or because there is something in the exhaust that is preventing the switch from opening after a heating cycle.
The draft inducer is creating a negative air draw on the switch. If there’s a blockage, the exhaust air could remain in a negative pressure keeping the switch from opening after a heating cycle.
Once the board recognizes the switch is open, then it will send power to the draft inducer motor. And wait for the switch to close.
Also the flame sensor doesn’t detect heat. The board is looking for a DC signal through the flame rod. Flame rectification. The flame completes that circuit to ground. Anything less than 1ua (1 micro amp) will cause the gas valve to shut off.
New furnaces will not operate without a good ground. Cause it’s through the ground circuit that the flame rectification occurs.

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No Touchy 29/12/2021 - 4:32 Chiều

oh, dude… you don't know what 'the middle of nowhere' is. i lived 40yrs in your area (just outside Chicago)
about 5yrs ago i moves to rural southeast Idaho. THAT is the 'middle of nowhere'. (in a good way though)

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captaincrazyhat 29/12/2021 - 4:32 Chiều

I heat my home with wood lol

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Nick Larocco 29/12/2021 - 4:32 Chiều

Just a short story about carbon monoxide poisoning.

One winter my family were all experiencing headaches, and me migraines. None of our three carbon monoxide detectors were going off, but we called the fire department to check it anyway since they have more sensitive instruments. None of theirs went off either. We must have spent a few weeks like this, eventually we called them back and they registered DANGEROUS levels of carbon monoxide, especially in my room which was directly next to the furnace! Fortunately none of us suffered severe harm, but if everyone if your house is suffering from headaches have someone check your carbon monoxide levels, preferably twice, it might save your life. The fire fighters said they were surprised none of us (especially me) was dead, or seriously injured by the carbon monoxide.

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Mike D 29/12/2021 - 4:32 Chiều

Instead of creating "negative pressure", using an air exchanger to replace air in a space is a better idea.
Especially since it won't be directly outside air and you can recover heat/enthalpy (depending on the unit).

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Rigel Botts 29/12/2021 - 4:32 Chiều

We have the the same furnace and stove.

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Sampel 29/12/2021 - 4:32 Chiều

All lies its just 8 Cubes of stone in a box type formation :/
Can be built with a crafting table

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Whitbypoppers 29/12/2021 - 4:32 Chiều

Electric resistance heating of whole houses became quite common in Atlantic Canada after the seventies oil crises. This area had no access to natural gas till 2000. This was replacing oil-fired heat at the time. Today, heat pumps are becoming more popular.

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Dennis Maeder 29/12/2021 - 4:32 Chiều

Sensible explanation. Thanks!

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Johnny Fo 29/12/2021 - 4:32 Chiều

I was hoping you'd explain why sparking igniters went out of favor for Hot plate igniters. However, having had to mess with lots of igniters, gapping them is a constant struggle especially if you have to maintain them often. The hot plate seems like it might not need as much maintenance.

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Michael Molter 29/12/2021 - 4:32 Chiều

Furnace wouldn’t light this winter, but after watching this video earlier this year, I was able to diagnose and fix it myself same day. Dirty igniter.

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Holden Hoover 29/12/2021 - 4:32 Chiều

Where i live, a forced air wood furnace is actually quite popular. However its starting to change, as now people are starting to have an oil furnace and wood.

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Allan Wakefield 29/12/2021 - 4:32 Chiều

I've got 2 carbon monoxide on every floor of my house, along with 2 smoke detectors and 2 fire extinguishers… every floor, labeled, and checked regularly. Bedrooms are almost 3 stories up I also have emergency ladders in all upstairs rooms. Fires are no freaking joke.

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James Dickens 29/12/2021 - 4:32 Chiều

There is also the flame roll out sensor.

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CJLinOHIO 29/12/2021 - 4:32 Chiều

Thanks for posting, love your videos.

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chara dremur 29/12/2021 - 4:32 Chiều

There should be a co and smoke detector in the duct that will shut the furnace down, and lock it out.

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Doc King 29/12/2021 - 4:32 Chiều

Nice video. Thanks. SWMBO thinks you ought to do a Force-Air Furnace VS Geothermal in-floor heating video. I'd 2nd that.

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M PB 29/12/2021 - 4:32 Chiều

I salute your attention to detail very well made video

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Chris Moody 29/12/2021 - 4:32 Chiều

From a HVAC tech you did bang up job explaining the modern furnace. Now if people would just not ignore their aging equipment. Yeah even if it still puts out heat, a leaking cracked or rusted thru heat exchange can make you dead in no time at all. If it’s bad enough it can be in 10 minutes.

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David M 29/12/2021 - 4:32 Chiều

The sparky thing as you call it has NOT gone out of fashion. I had a new furnace installed last year (2020) and it has a spark igniter.

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kleerude 29/12/2021 - 4:32 Chiều

Watching this while hiding in my bedroom while the building manager fixes my heater (it was the flame sensor!)

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Garry Billick 29/12/2021 - 4:32 Chiều

Except that this stable genius doesn’t understand how flame rectification works in this application. Might want to learn the difference between a flame sensor and a thermocouple before you continue to give bad information. You also ignore the most important reason for an induced draft motor. You really should leave this to someone that actually knows what they are doing.

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Humphrey Winnebago 29/12/2021 - 4:32 Chiều

Negative pressure may be a good thing, eh? Makes sense to me. Could this be a small point for the single-hose portable AC? Maybe…or maybe not for people like me with allergies.

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