Author Topic: question trouble shooting goodman propane furnace  (Read 1901 times)

Offline mattmctree

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question trouble shooting goodman propane furnace
« on: February 10, 2017, 01:39:44 PM »
moved in to a house and had a look at the furnace. its a 2006 goodman gmvc950704cxab with a propane conversion kit. i noticed that the condensate water was black and upon pulling the drain tube for the venter motor and secondary heat exchanger have found  black particle ( soot) i took a combustion reading at the vent outlet and it was reading 142 ppm co. after contacting the manufacturer they said up to 400ppm was acceptable. that seems quite high to me. i checked the gas pressures and set them to 6 "wc for low fire and 9.8" wc for high fire (gas valve wont get right up to 10") when i fire the unit some times the gas pressure pulsates in and i cant get a steady reading) im guessing this is the reason for soot up. i checked that i had 24 v at low fire terminal on board and that i had it at both side s or panel pressure switch and at the low fir terminal on the gas valve. 24v is present and the voltage is not flucuating which could cause the strange gas pressure. i am thinking that it it a faulty gas valve . has anyone had any experience with this. also is there a way you clean out the carbon or soot from the secondary heat exchanger?  i thought i might add that the pressure for the pressure switch it more than ample and it not making and breaking contact continuously. i took ambient co readings in the house and am at 0 ppm

Offline Admin

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Re: question trouble shooting goodman propane furnace
« Reply #1 on: February 10, 2017, 02:52:55 PM »
Usually you can increase the regulator pressure outside.  Make sure it's set between 11-13" wc.  It may be set too low which is why you can't increase the manifold pressure.  You're manifold pressures seem fine to me though.

Are you talking about the pressure switch that opens when the propane tanks are close to being empty?  It's possible the previous owner was running the tanks empty.  This will sometimes cause sludge to enter the gas piping and gas valve.  I would probably replace the low gas pressure switch and the gas valve to be safe.

Offline slo-115

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Re: question trouble shooting goodman propane furnace
« Reply #2 on: February 10, 2017, 03:23:37 PM »
Generally those goodmans, when converted to lp dont have a lps. Which can be problematic. "You cant get more then 9.8"wc". Put your manometer on the incoming side of the gas valve and test the incoming pressure and the working pressure (with the burner running on low and then high fire) More then likely the reg isnt keeping up and is sending low pressure into the gas valve (soot) Be sure to test the "lock up" pressure as well. With these colder February days propane regulators can get stuck and even can freeze up a little (single reg setup). In the past ive taken them off and inside to warm up for 15mins. Is the propane tank a 500gal, or 420lb tank? Is it using 2 regs (high pressure at the tank, low pressure at the house) or A two stage integral setup (low pressure from tank to house 1 reg)?

Offline mattmctree

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Re: question trouble shooting goodman propane furnace
« Reply #3 on: February 10, 2017, 03:42:33 PM »
i will get more details shortly. the manufacturer said that these units can run up to 400ppm . that seems incredibly high. i dont really work on furnaces more larger equipment where my co is usually under 10ppm. is there anything to worry about running the unit as is as far as a saftey aspect?

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Re: question trouble shooting goodman propane furnace
« Reply #4 on: February 10, 2017, 04:11:29 PM »
If you're not reading CO inside the house I wouldn't shut it down but I would address the problem sooner than later.  I have seen people red tag the furnace for having a dirty heat exchanger like yours.  The definition of defective is, imperfect in form or function.  They can argue the dirty heat exchanger is imperfect in form and therefore defective.

Quote
Clause 4.21.1 - Where the heat exchanger of a furnace installed in a dwelling unit is found to be defective, it shall be replaced.

If the secondary clogs up completely, the vent pressure switch should open.  Or the roll-out switch will trip.  Goodman is used to processing a lot of warranty heat exchanger claims.  It probably wouldn't be very hard to get a new primary and secondary at no charge.

Offline slo-115

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Re: question trouble shooting goodman propane furnace
« Reply #5 on: February 10, 2017, 05:02:27 PM »
I agree the secondary is likely a little plugged up a from soot causing the higher then expected c0 readings. Once it's sorted out it may wash itself clean. Like admin said the limit on the burner box will tripped if things get to restrictive and its a manual reset

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Re: question trouble shooting goodman propane furnace
« Reply #6 on: February 10, 2017, 06:19:29 PM »
I just noticed the GMVC95 manual has procedures for inspecting and cleaning the heat exchangers.

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FLUE PASSAGES (QUALIFIED SERVICER ONLY)
The heat exchanger flue passageways should be inspected at the beginning of each heating season. If necessary, clean the passageways as outlined below.
1. Turn OFF the electrical power and gas supply to the furnace.
2. Disconnect the gas line and remove the burner/ manifold assembly by removing the screws securing the assembly to the partition panel.
3. Disconnect the flue pipe system from the induced draft blower.
4. Remove the induced draft blower, drain and pressure tap hoses from the recuperator coil front cover.
5. Remove the recuperator coil front cover to expose the coil tubes and turbulators.
6. Remove the recuperator coil turbulators individually by slowly pulling each turbulator forward firmly.
7. Clean the recuperator coil tubes using a long handle wire brush, such as a gun cleaning brush.
8. Clean the primary heat exchanger tubes using a wire brush attached to a length of high grade stainless steel cable, such as drain cleanout cable. Attach a variable speed reversible drill to the other end of the cable. Slowly rotate the cable with the drill and insert it into one of the heat exchanger tubes. While reversing the drill, work the cable in and out several times to obtain sufficiemt cleaning.  Repeat for each tube.
9. Clean residue from furnace using a vacuum cleaner.
10. Replace the parts removed in the previous steps in reverse order.
11. Turn on electrical power and gas to furnace. Check for leaks and proper unit operation.
12. Severe heat exchanger fouling is an indication of an operational problem. Perform the checks listed in Startup Procedure and Adjustments to reduce the chances of repeated fouling.

What were the manifold pressures before you adjusted them?  How do the burner flames look?
Is this direct vent using 2" venting?  I attached the vent sizing table below.  Make sure the slope is good.  Make sure the drainage system is good and nothing can cause water to backup.

Also disconnect the drain line off the rubber elbow attached to the exhaust vent.  Stick the wand up inside there and see how much CO there is versus at the vent outlet.

Goodman released a bulletin and seemed to mention 200ppm but wasn't very clear on what to do.

Goodman Bulletin SF-041

Offline mattmctree

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Re: question trouble shooting goodman propane furnace
« Reply #7 on: February 11, 2017, 11:49:55 AM »
i will have a look over everything later. one thing that confuses me is that the gas pressure only pulsates on low fire and not on high fire. also all my other appliances have good gas pressure going to them. being that i am getting proper voltage to the gas valve could it not also be a faulty gas valve? the furnace draws air from indoors for combustion which is clear and the exhaust vent is clear as well. being that i am not very farmiliar with combustion analysis and am still learning what do most people see for readings on high efficiency equipment running on nat gas and lp ( co. stack temp co2 o2 etc)? goodman tech support said it can run up to 400 ppm does that not seem extremly high? the tech support mentioned that the low stage gas pressure being all over the map would definatley cause the soot up. any help in this matter would greatly help. im trying to study up more on combustion analysis and take some courses. its a shame most companies have apprentices as filter changers and not with journeyman and then throw them to the wolves. seems like an actual apprenticeship doesnt exist any more.

Offline mattmctree

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Re: question trouble shooting goodman propane furnace
« Reply #8 on: February 12, 2017, 12:20:48 PM »
ok . so today i checked my pressure and my gas pressure was good coming from the tank. the unit only pulsates on low fire and not on high fire. i have good voltage going to my low fire terminal on the circuit. i isolated the gas and ran the lp dryer and cook stove and there was no pulsating in gas pressure in either of them. you could also hear the regulator on the propane tank throttle up and down when the furnace was in low fire and not high and when the other gas appliances were running without the furnace that pulsation/throttling was also not present. im going to give the heat exchanger a good cleaning. it actually says in the service manual that it should be done every year using a gun brush for the secondary. would i be safe in deeming the gas valve faulty in the furnace its a two stage honeywell. sorry for all the questions but im just trying to wrap my head around everything. most service calls i get are for hot surface igniters and the gas pressure is usually bang on

Offline harshal

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Re: question trouble shooting goodman propane furnace
« Reply #9 on: February 12, 2017, 03:54:16 PM »
The co reading is fine as per their bulletin if it under 400 ppm.I would inspect the burner sometime they get corrosion from the condensation especially in propane.

Offline Jaybran

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Re: question trouble shooting goodman propane furnace
« Reply #10 on: February 14, 2017, 07:34:18 PM »
Have you verified that the correct conversion kit was used and have the correct offices installed. I found one that had multiple size orfices and the manifold pressure was set too low causing sooting. The heat exchangers will need to be cleaned or replaced. The furnace will never burn properly if the heat exchangers are restricted. It's a dirty job!

Offline Porcupinepuffer

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Re: question trouble shooting goodman propane furnace
« Reply #11 on: February 15, 2017, 02:12:20 PM »
I've never worked on a goodman but assume it's the same procedure when converting. I would make sure orifice's are correct for LP, and confirm springs in the gas valve have been changed to the proper LP springs. Does it appear all coversion to LP stickers are placed correctly on the door and gas valve of the furnace?

Offline Sergroum

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Re: question trouble shooting goodman propane furnace
« Reply #12 on: February 17, 2017, 03:43:22 AM »
Never had to work on goodman propane. But I have encountered a lot of Carrier Nt to LP conversions when the conversion was not done properly and caused excessive sooting. The installer would change the gas valve and change the orifices (sometimes). But fail to ... change the jumper on the side of the gas valve, as well as drill in screws into the ventricles of the burner.  One thing that I learned to notice is when the gas valve has a sticker "lp" somewhere, but it also has a sticker "nt" left behind. That sticker is actually hiding something important behind it, and if the installer never bothered to remove it, then the installer never installed the conversion properly.

Of course that's with carrier, not goodman. But it's something to keep an eye out as well. That sooting could cause a lot of serious crap. It could plug up the heat exchanger. It could plug up the burners, causing the flame to go backwards and actually burn down the wire for the HSI.  It could plug up the orifices even, preventing ignition completely.