Honeywell furnace flame detector - Page 1
Forum section: electronics / general

author: jmelson
author_rank: Super Contributor
timestamp: December 09, 2021, 03:43:22 am
content: Not sure this is the right place...Does anyone know how a Honeywell flame detector on a gas furnace works? I had one go flaky today. There's a single stainless-looking rod near the burner, in an area where flame fills the gap between the rod and the burner. It is obvious it is some kind of thermionic scheme where it detects the electrical conductivity of the flame. But, what sort of voltages are used and what currents flow?I cleaned the rod and moved it a bit closer to the flame and metal parts, and it seemed to work much better then. Before it got a flickering yellow "flame" LED,after cleaning, it got a couple blinks and then went to a solid-on LED, so I could clearly tell I had improved the operation.Thanks for any insight on how this works. Google searches didn't show any background theory.Jon

author: bdunham7
author_rank: Super Contributor
timestamp: December 09, 2021, 03:55:22 am
content: I believe the basic idea is that a small AC bias is put on the sensor rod and ionization allows a small DC current to flow. I don't remember the exact process, but the flame ionization acts as a sort of diode. The currents involved are single digit or tens of microamps--my HVAC-specific Fluke 116 has a high-impedance 600uA range for this specific purpose. This is a different device than the thermocouple flame sensors that you see on self-contained (no power) gas control valves.

author: ejeffrey
author_rank: Super Contributor
timestamp: December 09, 2021, 03:57:41 am
content: The phrase to search for is flame rectification sensor. They apply a current limited AC voltage, something like 50 VAC I think? Then measure the DC current flow.

author: WattsThat
author_rank: Frequent Contributor
timestamp: December 09, 2021, 04:05:49 am
content: Here is a decent description of a commercial system where the voltages and currents are much higher but the principal of operation is the same. Most YouTube/diy technical info is a joke.https://www.johnzinkhamworthy.com/smart-solutions/pilot-flame-rods/Residential sensors produce 5-10 uA from ~110 volts. Clean with ScotchBrite as sandpaper and similar abrasives may leave silica on the probe which melts and then insulates the tip, rendering it DOA.

author: floobydust
author_rank: Super Contributor
timestamp: December 09, 2021, 04:44:01 am
content: Generally speaking, you are applying HV AC to the kanthal rod and looking for the rectification property of the flame. This is thermionic.Excitation is around 50-100VAC and very low current under 10uA. Older equipment used mains sine-wave and modern gear uses a square-wave driven by MCU as safety codes require a lot of self-test for flame detection circuit.The rod must not have soot etc. buildup which is easily cleaned off with steel wool. Also, any leakage current to ground due to rust or soot, or shoddy/melted/wet wiring or cracked porcelain can be a problem. Note this leakage does not count as a false-positive for flame present because it's just an ohmic resistance seen and not rectification.If you're having problems, check the rod is clean with no leakage current path, and the pilot flame is centered on it. Not to the side or lifting up off the burner, which happens if the pilot burner is dirty or backdraft or chimney/negative pressure in the building issue. The flame detector can be working fine but if the flame is poor, flickering, off to the side of the rod etc. due to ash/soot buildup, it will constantly trip out.If you are very careful and need to spoof flame-present, then use a 1N4007+10MEG series resistor switched in at the moment when the controller needs to see flame, and of course manually light the pilot. I do this testing burner firmware. It's a dangerous procedure.There's two types, as far as a single rod - flame sensor, or igniter+flame sense function. If it's the combo function, the igniter will put many kV at the flame rod and the flame-sense circuit portion has clamp diodes so it does not get damaged during ignition.

author: james_s
author_rank: Super Contributor
timestamp: December 09, 2021, 06:55:25 am
content: As others have mentioned, the sensor is just a conductive probe that detects the flame electrically. The usual failure mode is soot or other contamination and the fix is to remove the probe and clean it up with something like a Scotchbrite pad.

author: jmelson
author_rank: Super Contributor
timestamp: December 09, 2021, 04:57:01 pm
content: Thanks, everyone! The symptom was fairly obvious, and I immediately recognized what was the sensor rod, cleaned it and it started working properly.I was just curious as to HOW it worked. I was surprised there was very little attention to keeping the ceramic rod it was mounted in clean from dust and debris.I did clean the ceramic mount, also.Jon

author: SoundTech-LG
author_rank: Frequent Contributor
timestamp: December 09, 2021, 05:11:00 pm
content: https://patents.google.com/patent/US20120288806A1/en

author: ejeffrey
author_rank: Super Contributor
timestamp: December 09, 2021, 05:55:02 pm
content: Since the sensor requires a conductive path from the rod, through the flame, to the burner you may also need to clean the burner in the neighborhood of the sensor.

author: SoundTech-LG
author_rank: Frequent Contributor
timestamp: December 09, 2021, 06:09:28 pm
content: Determine the path of least resistance for your furnace/application, and clean.

author: floobydust
author_rank: Super Contributor
timestamp: December 09, 2021, 07:52:18 pm
content: Quote from: jmelson on December 09, 2021, 04:57:01 pmA home's furnace is mission critical up north. Within hours people and pets get really cold.I've had a few friends phone me when it's -30°C outside and their furnace has conked out.Most of the time it's a dirty flame rod. Furnace repair guys charge out $135/hr and will dink around to try stretch it to a 2 hour callout, for something that takes a few minutes.I've also replaced hot-wire igniters, a $20 part that they markup to $125 and install in a few minutes. Worth keeping a spare around actually.

author: james_s
author_rank: Super Contributor
timestamp: December 09, 2021, 11:15:02 pm
content: I think if I lived somewhere that cold I'd install dual redundant furnaces, you could even go with smaller units and run one of them most of the time and both when it gets really cold out, a distributed 2-stage furnace effectively, and if one failed you'd still have a backup. Electric baseboard heaters or wood or gas fireplace inserts would be another viable option.I hate the HVAC industry, there is a great deal of protectionism and from what I've seen it's very common for those guys to milk people and take advantage of emergencies. I know several people who have called a tech out only to have them pushing hard to replace the entire system and then they end up fixing it themselves and realizing it was a trivial repair.

author: bdunham7
author_rank: Super Contributor
timestamp: December 09, 2021, 11:34:57 pm
content: Quote from: james_s on December 09, 2021, 11:15:02 pmNot a bad plan. It's common to have electric or gas backup for heat pump systems, but a dual small furnace rather than a two stage would be good. Unfortunately I don't know of any very small furnaces and I don't think code would allow you to have them both feeding the same ductwork.QuoteWhere I used to live in the Midwest I had a business neighbor in the HVAC business and I fixed his vehicles, he did my HVAC stuff. I never realized how well he treated me until I moved and tried to deal with HVAC contractors as a retail customer. They seem to think that they are entitled to your money on a regular basis. I'm crawling around in my attic for the next month or so replacing and resealing all my ductwork because the first thing the contractors do when they see my perfectly good 20 year old furnace is quote me a whole new system including slapping in new flex-ducts (I have rigid and I want to keep them) for $20K. The other option is unlicensed hacks that will do terrible work. Fortunately where I live it doesn't freeze and a few space heaters and a gas fireplace will keep us warm enough.

author: james_s
author_rank: Super Contributor
timestamp: December 10, 2021, 08:54:32 am
content: Quote from: bdunham7 on December 09, 2021, 11:34:57 pmI can fix vehicles and I can do HVAC. I have noticed that entitlement attitude, and I think that's why they are so hostile toward any kind of DIY, they're afraid people who are handy will figure out that it isn't nearly as hard as they pretend it is and not out of reach of a competent and experienced DIYer. It's just a mix of plumbing, electrical, fabrication, physics and a little engineering, it's not rocket science. The hardest part is finding good information, the HVAC forums I'm familiar with strictly forbid giving DIY advice and will immediately lock any thread where it comes up. That attitude ensures that come hell or high water I will do it myself and never pay them a dime even though I could afford to.

author: tom66
author_rank: Super Contributor
timestamp: December 10, 2021, 09:51:28 am
content: Quote from: james_s on December 10, 2021, 08:54:32 amSimilar story with AC units in the UK. You can now legally buy mini-split units that contain propane as a refrigerant. It's not a gas that has a high GWP (global warming potential) or ODP (ozone depletion potential), so it's considered safe for consumers to handle with effectively zero training. Yes, it is flammable, but that is no different from a consumer setting up a gas barbeque/grill. Yet, reading on the AC repair/installers forums, they're completely up in arms over this clearly unsafe technology being accessible to every Joe with a hammer and drill. I will certainly be installing my own HVAC in my new home.

author: SoundTech-LG
author_rank: Frequent Contributor
timestamp: December 10, 2021, 04:22:46 pm
content: The HVAC "pros" seem similarly determined to protect any possible "secrets" of the new "smart" thermostat installs. The manufacturers seem to refer customers to "pros" as well, rather than provide any details that might help a DIYer gain any insights. I have never dealt with any HVAC pro, and hopefully will never have to...

author: jmelson
author_rank: Super Contributor
timestamp: December 10, 2021, 05:44:12 pm
content: Quote from: SoundTech-LG on December 10, 2021, 04:22:46 pmYes, they want LAWS to make it illegal to even change the furnace filter.There was a proposed law that would require an electrician to change a burned-out light bulb and he would have to file for a building permit to do the work!Amazingly, the state legislature didn't fall for it!Jon

author: james_s
author_rank: Super Contributor
timestamp: December 10, 2021, 08:18:02 pm
content: Quote from: tom66 on December 10, 2021, 09:51:28 amThe thing that blows a big hole in the hysterical rantings about safety is the fact that any consumer can walk into a hardware store, home center or plumbing wholesaler and buy a gas fired water heater off the shelf no questions asked. You cannot buy a gas furnace or boiler anywhere though, the hardware stores don't carry them and the HVAC wholesalers will refuse to sell to anyone who is not a licensed contractor because of "safety" and any number of other silly claims. They moan about hacks doing improper work, well duh, it's so hard to get hold of the parts and information to do the job properly so people make due with what they can get. Fortunately there are online dealers now that will sell to anybody, though information is still lacking. You can can easily buy a book that will teach you enough about nearly any trade to do the job properly yourself, except for HVAC. I have several good plumbing, electrical and carpentry books written by professional tradesmen that show a lot of the tricks and techniques. I am not aware of any good HVAC how-to books.

author: electr_peter
author_rank: Unknown
timestamp: December 11, 2021, 11:47:16 pm
content: This video is relevant to discussion on flame sensorThermopile Testing, Construction, Current and Energy HarvestingActual power delivery and resistance is measured on good and faulty thermopile, teardown is done.

author: NiHaoMike
author_rank: Super Contributor
timestamp: December 12, 2021, 02:53:41 am
content: Quote from: james_s on December 10, 2021, 08:18:02 pmWhat license is needed to buy a residential gas furnace? Split heat pumps and A/C units just need EPA 608, which is pretty easy to get. A good book would be "Modern Refrigeration and Air Conditioning".

author: WattsThat
author_rank: Frequent Contributor
timestamp: December 12, 2021, 04:44:13 am
content: It might be by state here in the US.As mentioned, there are online dealers that sell boilers and other gas oil fired devices that don’t appear to have any licensing requirements, I’ve had no issues buying boilers in PA. Not shilling for them but I buy often from SupplyHouse dot com as service and prices are great and UPS ground has stuff here next day. Ordered a new ignition transformer for my oil burner last Friday at 5pm, they said it would be delivered Monday. UPS dropped it off Saturday due to the holiday work load. Gotta luv it when online commerce works well.The walk-up plumbing and hvac supply houses can all go suck an egg, they’ve done it to themselves by gouging non-trade walk-ins. My local Ace hardware has 90% of what I need most of the time and their prices are in line with the big box stores so that’s where most of my local biz goes, to someone in my community that I know and he employs 20+ full time people. The odd ball stuff goes to the specialty online retailers.

author: james_s
author_rank: Super Contributor
timestamp: December 12, 2021, 06:56:20 am
content: Quote from: NiHaoMike on December 12, 2021, 02:53:41 amWalk into any HVAC supply house with your EPA 608 card (got mine 20 years ago) and ask to buy a furnace or split system, let me know when you find one that will sell to you.Every local HVAC supply house I'm aware of requires you to have a contractor license and be a professional HVAC installer. They will not sell to individuals, period.

author: jmelson
author_rank: Super Contributor
timestamp: December 13, 2021, 12:12:03 am
content: Quote from: floobydust on December 09, 2021, 04:44:01 amFirst time out there, I cleaned the rod with what I had available (cloth), and put it back in. Worked fine for a week. Now, armed with knowledge of what to do, when it started going flaky again, I cleaned the rod with sandpaper. Before, the flame LED blinked twice before going solid on when the flame started. After sandpaper cleaning, the LED goes to solid on immediately when flame starts, no blinks.Thanks, guys, for the help.Jon

author: NiHaoMike
author_rank: Super Contributor
timestamp: December 13, 2021, 03:45:50 am
content: Quote from: james_s on December 12, 2021, 06:56:20 amThere's plenty of split systems sold at Home Depot, some even include gas heating. Look at the details for one and I don't see any mention of license needed to buy.https://www.homedepot.com/b/Heating-Venting-Cooling-Air-Conditioners-Whole-House-Air-Conditioners/N-5yc1vZc4kd

author: james_s
author_rank: Super Contributor
timestamp: December 13, 2021, 05:25:50 am
content: Interesting, I wonder if it varies by state? I've certainly never seen any on the shelves in the stores and the ones you can order are the same off-brands some of the online dealers sell cheaper. They do have a small HVAC display in the Home Depot near me but it's just a front for a network of contractors that install it. You can't just walk in and buy the equipment they have on display. The situation may be improving since the internet has made it possible to get this stuff, used to be the only places that sold it were HVAC supply houses. 20 years ago it was very difficult just to get a part like a gas valve or control board, now that part is easy at least.

author: AlbertL
author_rank: Regular Contributor
timestamp: December 13, 2021, 09:32:18 pm
content: Quote from: WattsThat on December 12, 2021, 04:44:13 amThe traditional supply houses don't want DIY business. Or more precisely, their contractor customers don't want them selling to DIYs. They're obsessed with it - I used to read some trade publications, where contractors would talk about not doing business with supply houses that "sell to anyone", and suppliers would smugly boast about turning away "home-moaners" who showed up at the counter.I've bought parts from SupplyHouse.com and have been very pleased with their service. But for buying equipment, my concern as a DIY is getting warranty parts and support. Online vendors point out that manufacturers' warranties require installation by a licensed contractor and advise buyers to hire one. I suspect this limitation could be challenged in court, since merely being licensed does not guarantee that the equipment was selected and installed in accordance with the manufacturer's instructions.

author: james_s
author_rank: Super Contributor
timestamp: December 13, 2021, 09:44:59 pm
content: Quote from: AlbertL on December 13, 2021, 09:32:18 pmAll they've done by having that attitude is ensure that I will never give them a cent. Now that there are online suppliers that will sell to anybody I'm happy to do business with those places.The warranty is something worth considering and a friend of mine brought that up, then he got a quote to have a mini split installed and realized we could install it ourselves, screw it up, throw it all away and buy everything again, then have that break down and throw it away and buy a whole new one yet again and STILL come out ahead vs what he was quoted so whatever. If my furnace that I installed ever breaks I don't care about the warranty, I can buy a whole new furnace and still still save money. Really though it isn't even about saving money, even if there were no savings at all I'd still do it just to screw those smug assholes out of the profit they feel so entitled to.I've also bought from supplyhouse.com, they're great for small parts and accessories, but there are other dealers that have a much better selection of big stuff like furnaces, air handlers and condensing units.

author: AlbertL
author_rank: Regular Contributor
timestamp: December 13, 2021, 09:56:39 pm
content: Agree 100%. And as far as the legitimate concern about the manufacturer having to do warranty replacements on equipment damaged by improper installation - that could be addressed by requiring an inspection of the completed system by a manufacturer's representative (at the customer's expense for a reasonable fee) to initiate warranty coverage. I believe it's done that way in the roofing industry where, as in HVAC, most failures come from poor installation workmanship rather than defects in materials.

author: james_s
author_rank: Super Contributor
timestamp: December 13, 2021, 10:07:26 pm
content: Also if you do it right, failures are in my experience quite rare. Between furnaces and heat pumps I've installed about 10 of them so far starting more than 16 years ago. At this time every single one of them is still going strong, one I'm getting ready to replace just because I want something better and more efficient, not because there's anything wrong with it. The only problems I've had are the motor run capacitors in two of the heat pumps failed several years ago, that's a $10 part that is a common failure item. It's not rocket science, the equipment is not particularly complex and it is designed to last decades. All you really need is some attention to detail, do neat and tidy work and in the case of anything involving refrigeration, keep all parts of the hermetic system absolutely clean and dry. Handle it like a body you are doing surgery on, not a spec of dust or contamination inside. If you can follow instructions and do really nice quality silver soldered or brazed sweat connections on copper pipes chances are it will just work.

author: AlbertL
author_rank: Regular Contributor
timestamp: December 13, 2021, 10:58:58 pm
content: My only other concerns about a DIY install is the risk that manufacturers may start shipping equipment that requires entering an activation code, or may stop making manuals publicly available online. All to "protect the consumer from the dangers of unauthorized installation", of course.

author: floobydust
author_rank: Super Contributor
timestamp: December 14, 2021, 01:36:45 am
content: Thanks for giving them ideas on locking products to pissants pretending to add value.Here you can purchase electrical goods and wire a house yourself DIY, and the gasfitting Home Depot sells the fittings and pipe, hot water tank, fireplace etc.It's just the final inspection that gets done by the municipality to close the permit.One friend had a furnace moved and the contractor used his teenage son and friend to do the sheet metal work. It looked so bad the work, I called regulatory and caused a shit storm because a gas vent is not something kids do.Unfortunately, anyone can claim to be a plumber.