Gas Valve Pressure Setting Question

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Old 11-28-07, 08:55 AM
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Gas Valve Pressure Setting Question

I recently overhauled my mid-80's Gaffers & Sattler natural gas furnace by installing a new ignition control unit, pilot, ignitor/sensor assembly, gas valve and bi-metal fan limit switch. This was done after pulling out the burner array and cleaning accumulated scale from the burner tubes (all eight of 'em), the manifold pipe and cleaning the burner orofices with a fine wire. A repair was made to the air shutter to keep it in place because one of the retaining clips keeping the shutter against the manifold had broken and lighting/operation had become a problem. Both clips were replaced.

Well, lighting/ignition is no longer a problem and everything is working smoothly, except the furnace is short cycling. My guess is that the heat exchanger does not stay hot enough to keep the furnace running until the thermostatic call for heat is satisfied. The heat call is made, and the unit fan cycles on for about two or three minutes, then off for a minute or two, then back on. The burner remains lit while the fan is off.

When adjusting the air shutter after installing the new ICU and gas valve, I noticed that only five or six burner tubes light and carry flames after ignition and that flame does not settle in on all eight tubes until after the firebox has heated up and gone through one or two fan cycles. The flame seated on all eight tubes appears about 1/4" high when the firebox is HOT.

The short flames, short cycling and failure of all eight burners to light on ignition make me suspect that the gas pressure setting on the valve might be a little low...

What do y'all think?

Can anyone help me out with information on what the factory specs were for gas pressure on this furnace unit? I don't have a Preston's Technical Guide and the information on the factory plate has bleached off (UV).

Here are the gory details:

Unit:
Gaffers & Sattlerroof mounteddual pack
withMagic Chef label reefer unit;
MODEL NO. 3D-60-OH-1-2

ICU: Robertshaw Controls/Uniline 780-715

Gas Valve: Robertshaw Controls/Uniline 720-079
Factory set 3.5" W.C.

Limit Switch: Honeywell L4064B1451
Set at: 80 F fan OFF; 140 F fan ON, 190 F GAS OFF

Thanks in advance for your help!

MM20
 
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Old 11-28-07, 12:17 PM
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Originally Posted by MechanicalMan20 View Post
Well, lighting/ignition is no longer a problem and everything is working smoothly, except the furnace is short cycling. My guess is that the heat exchanger does not stay hot enough to keep the furnace running until the thermostatic call for heat is satisfied. The heat call is made, and the unit fan cycles on for about two or three minutes, then off for a minute or two, then back on. The burner remains lit while the fan is off.
How odd (coincidence). I was at an old duplex under renovation (for the last 15 years ), and vacant, and landlord wanted me to get the furnace going as it was going to be 5 degrees out 2 nights ago and it was in the teens yesterday. I came here and first inspected furnace, gas lines, etc.

Turned it on and she fired up. The blower came on in a couple minutes after the burners fired up, as this furnace has the same type of bimetal fan switch as you mention. Then while I was stalling and walking through the house, the blower motor shut off. So I went back in there to take a look and to my surprise, the burners were still going! At first, I thought someone tampered with wires and wired the safety so the blower went out instead of the burner(under the initial suspicion of high limiting due to modification to rooms and the duct work).

The blower motor then came back on, in (edit) about a minute?. After doing this a couple more times, it dawned on me: The temperature in the heat exchanger was dropping everytime the blower came on because I was drawing in 36 degree air through the cold air return! As time went on, the duration the blower ran for increased to my satisfaction and I left.

Just minutes ago, I rechecked on the duplex and it as at set temp 50 degrees and all is good. ALL that was "wrong" (nothing in actuality) was the drop in heat exchanger temp from the cold house.

So what was YOUR house temp at while you were getting yours going?

And it does sound in your particular case that gas pressure is not as good as it should be - and first one must make sure everything is clear before getting into pressure adjustments. That is an area I have never got into, as I feel I may be crossing the line and getting into a liability risk there - as opposed to being simply a diagnotician and parts changer.
 

Last edited by ecman51; 11-28-07 at 12:47 PM. Reason: Added last paragraph
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Old 11-28-07, 01:05 PM
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Originally Posted by ecman51` View Post
<SNIP>

The blower motor then came back on, in (edit) about a minute?. After doing this a couple more times, it dawned on me: The temperature in the heat exchanger was dropping everytime the blower came on because I was drawing in 36 degree air through the cold air return! As time went on, the duration the blower ran for increased to my satisfaction and I left.

Just minutes ago, I rechecked on the duplex and it as at set temp 50 degrees and all is good. ALL that was "wrong" (nothing in actuality) was the drop in heat exchanger temp from the cold house.

So what was YOUR house temp at while you were getting yours going?

And it does sound in your particular case that gas pressure is not as good as it should be - and first one must make sure everything is clear before getting into pressure adjustments. That is an area I have never got into, as I feel I may be crossing the line and getting into a liability risk there - as opposed to being simply a diagnotician and parts changer.
The WAKE SETTING on the Honeywell digital t-stat is set for 70 F. When it went on this morning, the temp in the house was about 66 F. Air returns are located at the ceiling level, not the floor.
 
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Old 11-28-07, 01:31 PM
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So much for the first theory, then.

But what you say about weak flame could cause same thing to occur, alright. Technically the furnace will heat up to cause the fan switch to come on first. Generally the temp rises in time, to a certaintemp and remains there under ideal designed conditions. But if the flame is weak though, as yo claim, and you have 66 degree return air (or probably somewhat higher since up at ceiling) passing over the exchanger, the drop could send the temp below the fan switch's "off" setting.
 
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Old 11-28-07, 02:22 PM
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Hi MM:

All the burners should lite at the same time. It sounds like they are ribbon burners? [long burner with small slots on the top] Make sure they are CLEAN. Sometimes you have to run a hacksaw blade [gently] thru each and every slot. Also, make sure the heat exchanger shell above each burner is not sooted up. Next, make sure all the crossover tubes are in alignment. Look at the burners; see how the flame carries from burner to burner? Those are called crossovers. They want to be clean and aligned with each other. I agree, the fan is cycling because of insufficient heat. The gas valve comes preset for 3.5" wc [at the outlet] which is the norm for natural gas. You need to see what the gas pressure is upstream of the valve. Also, take the gas valve out and see if some debris didn't get dislodged and is blocking the inlet screen to the valve. The gas valve inlet is at the inlet side of the piping, right? Let us know how you make out. Thanks.
 
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Old 11-28-07, 03:53 PM
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Originally Posted by daddyjohn View Post
Hi MM:

All the burners should lite at the same time. It sounds like they are ribbon burners? [long burner with small slots on the top] Make sure they are CLEAN. Sometimes you have to run a hacksaw blade [gently] thru each and every slot. Also, make sure the heat exchanger shell above each burner is not sooted up. Next, make sure all the crossover tubes are in alignment. Look at the burners; see how the flame carries from burner to burner? Those are called crossovers. They want to be clean and aligned with each other. I agree, the fan is cycling because of insufficient heat. The gas valve comes preset for 3.5" wc [at the outlet] which is the norm for natural gas. You need to see what the gas pressure is upstream of the valve. Also, take the gas valve out and see if some debris didn't get dislodged and is blocking the inlet screen to the valve. The gas valve inlet is at the inlet side of the piping, right? Let us know how you make out. Thanks.
Thank you.

The burner tubes were cleaned of scale with a light wire brushing when I had the array disassembled. All tubes were tapped and emptied, then the tops were vacuumed with a stiff bristle plastic brush. I think the tubes are clean. (-:

Burners are 1" diameter pipes with a line of narrow slots side by side on top. The flame spreader is attached to the manifold frame and lights the burners at the manifold side of the burner tube. The pilot lights the first tube at the end furthest from the manifold and the flame spreads backwards, then across the array. Everything is cleanly aligned, the unit design does not allow otherwise if reassembled properly.

Gas valve inlet side is debris-free.

Would soot build-up be best removed with a vacuum bristle brush or a stiff wire brush, followed by vacuum?

Thanks again!
 
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Old 11-28-07, 08:31 PM
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brush and vacuum
you have any other gas appliances in the house?
 
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Old 11-28-07, 10:29 PM
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Originally Posted by daddyjohn View Post
brush and vacuum
you have any other gas appliances in the house?
okay

4 burner gas range and Takagi TKjr
tankless water heater

both are working fine with no
noticeable drops in pressure

gas supply to meter is 1"

gas supply from meter to furnace is 3/4"
shut off valve is 3/4" and flex line is 3/4"
reduced by bushing to 1/2" iron pipe into
gas valve

Will try to get simple manometer in
next day or two and read pressure
on both sides of valve.
 
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Old 12-02-07, 06:03 PM
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Okay, here's an update... I picked up a Ritchie -2" to 15" W.C manometer and measured both sides of the gas valve with the furnace running and all burners lit. Here are the pressures:

Inlet side fluctuates slowly between 7 3/4" W.C.and 8" W.C.

Manifold side is steady @ 3 3/4" W.C.

Checked and brushed surfaces above burners and encountered virtually no soot.

All burners still not lighting on ignition and start up.

So, whaddya think guys... should I try ratchetingup the gas pressure a quarter turn, or does Preston's Guide showing that manifold side pressure should be greater than 3 3/4" water column?

Thanks for all your help and suggestions.

MM20

Oh, one other thing daddyjohn, I also have a gas clothes dryer in house.
 
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Old 12-03-07, 05:16 PM
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Yeah, try 4 inches. The numbers you have now are very good. Gas valves are usually sized/rated at about 7" wc at the inlet. The norm for the outlet side is 3.5" wc. The other thing you might do is remove the gas orifices from the manifold and make sure there's no rust/scale inside the manifold. All the burners should lite off the first time around. Before the upgrade, did they lite off correctly?
 
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Old 12-03-07, 05:28 PM
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Was this furnace ever working right? If it wasn't, it could almost sound like it was orificed down for propane.

And is this open or closed combustion?
 
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Old 12-03-07, 06:54 PM
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Burners not lighting

Usually on the manifold end of each burner there is a small tube or wing which extends to a similar part on the next burner. These tubes or wings are sometimes refered to as crosslighters. Their purpose is to carry flame from one burner to the next. There is usually a tiny slot in each one to accomplish this process. Remove each burner & check & clean the crosslighters. I've found a feeler gauge to work quite well.
 
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Old 12-03-07, 10:32 PM
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Thanks for the posts folks! I'll try to answer all your questions as best I can.

I boosted the pressure to 4" W.C. and noticed very little change in burner behavior and flame height so I goosed it up to 4 1/2" W.C. where it is now. There is a noticeable increase in heat output and the short cycling stopped.

The burner flames are not as sharply defined as before and I had to open the air shutter a bit more to settle the flames into the burners. The third and fifth burner tubes are not burning as cleanly as I would like (flames dancing - no yellow). It appears that they should be pulled and the orofices removed from the manifold. The tubes are clean.

The furnace renovation involved replacing a previous set of parts (valve, ICU and pilot) with the same part numbers by Robertshaw, only brand new. The burners did not light off correctly due to the broken air shutter clip. Prior to that I have no knowledge other than when it was new, many years ago, it worked like a charm and I assume all burners worked correctly.

The cross lighting is handled by a single, half round bracket positioned over the manifold end of the tubes. Everything there is clean.

The furnace will run through a WAKE up cycle in the morning and I'll post a blow by blow account of what happens afterwards. The two cycles I ran it through after upping the pressure were uneventful in a good way.

ecman51: What is the difference between open and closed combustion please?

I ran my pressure tests and made adjustments with the service shroud off the machine, but operational observations (lighting, running, etc.) are made with the shroud in place and the small observation door in front of the firebox/pilot open. Operational testing is done with the observation doors screwed shut.

Thanks again folks. I feel as though great progress is being made.
 
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Old 12-04-07, 10:07 AM
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Wink Morning Report

The furnace lit and ran without problems this morning upon the t-stat WAKE cycle. The outside temperature was about 38 F and the inside temperature was in the low 60's F.

The fan and burner ran continuously for over two hours until the heat call was satisfied.

Guess the short cycling problem is fixed.

When time permits this weekend I will take out the burner array, disassemble, check and re-clean all the orofices to see what difference in the lighting and running behavior of the burners that makes.

I will report back then. Feel free to make suggestions in the meantime. Thanks!
 
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Old 12-05-07, 05:50 PM
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Smile If at first you don't succeed, try again!

Well, all of the good advice received has been put to use effectively. In anticipation of colder, inclement weather over the weekend, I decided to get after the burners this afternoon. Here is what I did, and the result.

Pulled the complete burner array and disassembled almost every part of it (did not take gas valve off of manifold). Removed each burner tube and re-brushed the vent slits to get the last bits of visible scale. One of the problem burners still had quite a bit of scale built up on the slits.

Removed each of the burner orofices from the manifold and re-poked them with the fine stainless steel wire I used before. They were still clean, but when I checked the manifold areas behind each orofice, I discovered a machining shaving still attached where the fifth burner tube was. Just to be safe, I removed the potential blockage with needle nose pliers.

A careful reassembly of the manifold and burner array put all tubes in perfect alignment.

The manifold array was returned to the firebox, securely anchored and reconnected to the gas line.

A test firing lit all eight burners at the same time and they stayed lit (hooray!). After letting the firebox heat, I readjusted the air shutter for good measure and no dancing flames were noted. All flames had seated properly on the burner.

After climbing down from the roof, I checked the temperature display for an electronic thermometer remote sensor I had placed in the outlet register furthest from the furnace and noted a maximum achieved reading of 112 F after about five minutes of running.

Thanks again to all of you for your assistance.

BTW, if you're wondering why I went to all this trouble for a 20+ year old furnace, it's because I had recently bid replacing the unit and got quotes from $10,000 to $12,000, excluding the California duct testing, permit, etc. Granted, this was for a top quality high SEER rated two-stage furnace dual-pack but the price made me think about what would actually be saved.

The AC in the present unit works great. The furnace did not so I decided to open it up and have a look. After discovering/fixing the broken air shutter, it seemed that I could make it like new for a few hundred bucks in parts.

My plan is to invest the $12 G's saved on a solar photovoltaic installation atop my garage (always in the sun). The long term gains will be much greater than the minor savings a more energy efficient dual-pack might provide. When the furnace dies, the savings from solar power will probably have paid for a new, high efficiency unit by then.

Bye all! And again, thanks very much!
 
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Old 12-05-07, 08:04 PM
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Thumbs up Good Job

Sounds like you did a really good job in going over & repairing the furnace. Glad to hear all is well. With all of the CA requirements, that replacement could have easily gone up by 25-50%. Also good to hear you are going to make use of solar energy. It's free & zero pollutants. We are just here on this earth for a short while & it would be nice to pass it along to the next generation in better condition than when we got it.
 
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Old 12-05-07, 09:18 PM
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Thanks for the feedback.
 
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Old 12-05-07, 10:24 PM
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One last comment - I promise

The other repair I made to the furnace but had not previously mentioned due to its irrelevancy is that I reinsulated the machine's plenum space because much of the original fiberglass insulation had come loose or deteriorated. I reinsulated it with 1 1/2" thick fiberglass duct insulation board which provided a much higher R value and better sound attenuation. This was done after vacuuming and washing out the plenum space; and vacuuming the fan gear and squirrel cage. The insul board was anchored with stickum backed nails and pressure washers and 3M #80 industrial contact cement (because of its high temperature rating). The reinsulation of the plenum area added about $100 to the total renovation cost but I believe it will be well worth it since beforehand, the plenum floor was not insulated at all and it sits above a very hot roof in the Summer.

The other oddity, noticeable as making for quieter running, is that I changed out the originally installed insulation blocks (3/4" comp board withneoprene padding on both sides) which the machine was resting on and substituted neoprene motor mounts for a '55 Chevy. These motor mounts are flat doughnuts, available at any good auto parts store, are cheap - about $1.50 each, and are big enough to do the job (1" thick by 3" diameter with a 3/8" bolt hole through the center).

Thanks once again daddyjohn, ecman51 and grady
 
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