Big Maxx tubes getting red, where to get manometer?

[bobs409]

I was running mine today to hopefully burn off some the smell as it's new and noticed the tubes at the first bend were getting red so I turned it off to be safe.

A search here sends mixed signals, some say it's bad, some say it's normal. The flames are blue and centered but do have orange at the ends and I can see the flames are reaching the curved bends so maybe too much flame from too much pressure?? I did convert this to propane using their kit as it was for NG. (only one available at last years end of season sale!) I did exactly as it said to do.

One thing I did not do was to put a manometer on so that's next on my list. Where is the best place to get one of these? (as in cheap!) lol I'll use it once and never need it again.

 

[brewchief]

http://www.amazon.com/dp/B001TOHT78/?tag=atomicindus08-20

Simple as it gets

 

[bobs409]

Thank you! Ordered and will be here in 2 days.

 

[dave67fd]

The tubes may exhibit a slight glow on the bends with the lights out. If you can see it glowing in the light it's probably excessive.

Confirm your gas line and manifold pressure.

 

[bobs409]

Ok so now I got this $25 dollar plastic tube with numbers on it but no instructions. (gee, ya think somebody is making some profit on these???)

It shows where to fill with water but I have no idea how full it needs to be.

HELP!

 

[txst]

For LP, you should be in the neighborhood of 11" water column at the gas valve, so fill the tube so that it allows a difference in the height of the water around 12". You need to find the pressure port on the outlet of the gas valve. If the pressure is too high, then you will need to adjust the gas valve.

Tubes that glow red are beyond the safe point of the aluminized tubes - the aluminized coating will start to oxidize around 1000F, which is below the visable limit. Tubes that constantly glow during operation will fail prematurely (crack).

 

[bobs409]

I just don't know how much water to put in this. I read the reviews on amazon and someone said to the 0 mark but want to confirm here first.

I ran my heater for a bit today and had no red showing in the tubes this time. ??

 

[brewchief]

Fill it so water is at the zero mark, hook it up and you will read the pressure on the inner tube, if you fill it up with water and gently blow into the hose you will see it pretty easily

 

[bobs409]

Thanks brew!

 

[bobs409]

Ok finally got to check this with the manometer. It read about 8.5 and crept up close to 9 while running it. It did not move no matter what I did with the adjustment screw. I have it all the way screwed in so I'm thinking maybe the line pressure isn't high enough?

What would happen running it with this lower reading? (should be 10)

BTW, the tubes are looking good, no red anymore.

What kind of gauge do you use to test line pressure?


Thanks,

Bob

 

[Andamo]

HVAC companies use instruments that read the pressure in inches of water too, but are easier to transport and use. You might want to get the gas company to bump up your regulator a bit, but if they're going to charge you to do this, you're ok with the slightly lower pressure. It might take the heater slightly longer to bring the temp. in the room up though.

 

[txst]

Low pressure results in less heating output. The appliances are designed for 10-12" water column. It's completely safe, as they are tested at 1/3 the rated input for CO emissions, ignition and flame extinguishing. Many manufactures atually send the units out de-rated, as it is conservative.

Do you have any other appliances on the LP line? You should check the pressure with all appliances running that are connected to the same gas line. Honestly, I wouldn't worry about it - the difference in output is probably not noticeable.

 

[AlbertaSS]

I just got off the phone with tech support at Mr. Heater. I was having the same problems with glowing tubes. Set the manifold pressure down to 3.6 " H20 from 4.0 on my 80K btu unit and was still having a slight red glow. Finally was able to talk to someone there this morning and was told this was normal.

 

[A_Pmech]

When I installed my heaters I found it necessary to increase the supply pressure at the low pressure regulator on the house. Mine was set right at the minimum appliance input pressure of 11" of water. When a demand is placed on the regulator, the pressure sags slightly. Increasing the pressure solved the supply pressure problem and allowed me to set the heaters to the correct burner supply pressure.

 

[jdcompman]

The tubes may exhibit a slight glow on the bends with the lights out. If you can see it glowing in the light it's probably excessive.

Confirm your gas line and manifold pressure.

^^^This! Completely normal. I was worried as well and built my own manometer to check it. Pressure was spot on.

 

[brewchief]

It's not uncommon to have to make a slight adjustment to the outside regulator at the house.

There will be a port on the inlet side of the gas valve the same as the outlet.

Is this the only appliance on the regulator or are there others?

 

[bobs409]

Thanks for the help guys. This is the only thing that runs on this tank.

I ran it again yesterday and noticed a little red in the curved tubes again. As long as it's normal then. I just don't want to burn a hole through and wipe out everything I own some night because of it!

With the side cover off and running, if you look at the flames going into the tubes, how far do yours go? Mine seem to go slightly around the curve a bit and are orange on the end. That's right where the tubes glow too.

I can't imagine them making tubes that couldn't handle the flame going through them.

 

[dogdas]

You are definitely NOT getting enough air across the heat exchanger. The odds of your gas supply pressure being too high are a million to one, the meter has a discharge for that. You fan is not producing what is needed to remove the proper amount of heat off the burner tubes. Did you call the factory support line and ask them? The main problem you will have is fatigue on the heat exchanger from lets say 50 degrees to 750 degrees. The bottom line this is NOT normal.

 

[AlbertaSS]

dogdas, as I stated above I called Mr. Heater tech support about the tubes getting slightly red on the bends, was told this is normal if the manifold pressure is set properly for your altitude.

 

[txst]

Guys:

I have designed furnaces for 20 yrs with Carrier and with Trane. I am telling you that with aluminized steel heat exchangers, glowing red at the bends will significantly reduce their life due to two reasons: (1) Thermal fatigue (the expansion and contraction of the steel); and (2) Thermal oxidation ("rusting" of the tubes due to temperature). Also, anything over 1000 degrees F is going to bake off the aluminized coating. Even with 409 series stainless steel ( a higher end material, which is not used in the Big Maxx heaters), the maximum safe temperature is around 1150F or so. Another thing that significantly affects the heat exchanger life life is thermal gradients - i.e. the termperature change per inch of the tube. More extreme gradiants will caust the material to fail more quickly. There is actually an ANSI test for this, of which I have run many heat exchangers on.

As mentioned, low or misdirected airflow over the heat exchanger or "over firing" (too high fuel presure or oversized orifices) will cause the tubes to overheat.

There is no way that I would allow this to continue in my home. I would lower the manifold pressure.

 

[AlbertaSS]

txst- I took your advice and hooked up the manometer again to the heater. I adjusted the manifold pressure down to 2.4 from the recommended 3.6 inches H20 and got all but one of the tube bends to stop glowing red. I will play with it a little more this afternoon. Quite a difference from what they recommend. lol. we are at about 2300 ft elevation. Just hope it can keep up when it gets really cold with the reduced out put.

 

[txst]

AlbertaSS:
I'm surprised that even at 2.4"wc that you have a tube still glowing red - that's a pretty significant de-rate. You don't want to go too low, as there will be a point where you can introduce ignition difficulties. Typically, manufacturers recommend smaller gas orifices above 2000ft due to the lower air density for combustion, and that also affects the amount of air blowing over the heat exchanger as well. The rule of thumb is 4% de-rate for every 1000ft above 2000ft.

Do you have a way to check the RPM of the fan?

 

[AlbertaSS]

txst- Actually to get the last tube to stop glowing I had to reduce the manifold pressure down to 2.0"wc. Not sure what to think. I am a steam engineer and have been working with high pressure boilers for almost 20 years and can appreciate what overheating the tubes can do. The metal quality of the tubes must be inferior. When I called Mr heater tech support I was put on hold while the person helping had to ask someone else for an answer. Made me wonder. I bought an 80,000 btu unit to heat a 1350 sq ft shop but if it has to be run at reduced capacity it will probably not keep up. Makes me wish I would have spent more money on a better unit. lol. Anyway, I think I can get a laser timing light from the millwrights at work that they use to check rpm on fan couplers to combustion fans. Would you know what rpm the fan should run at? Thanks for your input.

 

[jawnd393]

Why did no one recommend a simple manometer made out of a dollar worth of plastic tubing before the OP spent $25 on one?

 

[txst]

AlbertaSS:
2.0" wc of gas pressure is pretty low. I wouldn't operate there due to the risk of having igntion issues and the reduced output.

I'm not familar with your model. I have a 60K Hot Dawg in my garage and it's a 1625RPM motor, so it will be slightly less than that.

I'm surprised by these problems and Mr. Heater's response.

 

[AlbertaSS]

txst:
Yes its getting frustrating for sure. I am going to bring home a laser heat sensing gun from work tonight and see what the temperature of the tubes are at on their exterior. Hopefully I can tweak the manifold pressure up. I will try call Mr. Heater again and ask for a technician. Not being sexist but when I called I talked to a woman who had no idea what I was talking about , put me on hold, then a few minutes later responded. The fan does seem to move a lot of air. The shop is 45 feet long and standing on the floor can feel air currents at least 20 feet from the heater. The millwrights are not working this weekend but will try and get the RPM strobe from them early this coming week.

 

[AlbertaSS]

Well I slowly moved the manifold pressure up tonight as I measured the tube temp. At 2"wc I saw around 450 to 475 degrees on the bends. At 3.6 " wc there were two tubes with a dull glow on the bends and the hottest I saw was around 800 degrees. Safe? I don't know. From what I researched the aluminized steel pipe is good till 1022 degrees. Still have to measure fan rpm.

 

[txst]

AlbertaSS:
Are you seeing the tubes glow in daylight or in the dark? The reason I ask is that it makes a difference in the temperature. The tube temperatures vary quite a bit around the tube and inbetween the tubes due to air patterns and radiant effects. You shouldn't be able to see tubes glow at 800F. The higher the pressure, the longer the flame is inside the tube is, which explains your temperature measurements. Typically, the flame is extinguished just prior to the bend in the tube. If it goes into the bend, the flame can impinge on the sides of the tube and can cause high CO emissions.

From what you are telling me, if your fan is putting out the corrct CFM (airflow is not restricted and the fan is at the correct RPM), I would adjust the gas pressure down to around 2.8-3" w.c. If the furnace is designed and built properly, there are only 5 things that can cause this:
1) Low airflow - Check for things restricting air from entering the furnace (shelf, wall, etc) or slow fan speed.
2) High manifiold pressure - Obviously, the higher the pressure, the more flow through the gas orifices.
3) Wrong orifices in the gas manifold - Each burner has an orifice. NG orifices are larger than LP because LP has roughly 2.5x the energy content (BTU/Cu ft.) than natural gas. At higher altitutudes, you must replace the orifices with smaller ones to compensate for the reduction in air density.
4) High energy content in the natural gas. While the BTU/Cu Ft does change significantly over time, typically utilities keep the value on the low side so we use more gas. This is highly unlikely.
5) Very high inlet temperatures - Sometimes people mount these in a shop with a high ceiling. If the furnace is mounted high and there is a lot of air stratification (significantly warmer where the furnce is mounted compared the the work area), it can cause these problems, along with limit switch trips.

Do those furnaces come with LP conversion kits? One thing you could do is drill them out to a size slightly smaller than the NG orifices you have have in there. Again, you shouldn't have to do that.

Hope this helps.

 

[AlbertaSS]

txst:
Thanks for the info, really appreciated. The heater is mounted in a cooler spot in the building beside the overhead door. It is mounted the recommended 18" from the wall. I was wondering about the orifices because shouldn't all the tubes be red, just not two of five currently? Will check today. Will try and get the strobe from work today to check the fan rpm. The glowing is definitely more noticeable with the lights off, just a dull glow and on the upper part of the tube bend about the size of a golf ball. I did notice the temps jumping around quite a bit when measuring the bends depending on the spot.

 

[AlbertaSS]

The saga continues- talked to someone again today who seemed knowledgeable at Mr. Heater. I had noticed the aluminum coating had broken down on the heat exchanger tubes where the tube bends are glowing red and would rub off with your finger. He told me not to worry about that. Really? He also told me the manifold pressure can be run as low as 3"H20 column. Okay that seems reasonable except a couple tubes will still glow red at that pressure. Oh that's ok he said, you have a 10 year warranty on the exchanger if anything happens. Great, burn the shop down if a tube pops but I will get a new exchanger.

 

[txst]

AlbertaSS:

That confirms everything - the tubes are definitely getting over 1000F on those tubes.

If it's any consolation, there is little safety risk with an induced draft system like this. If the tubes do fail, either the pressure switch or the thermal limit/rollout switch will trip before there is any danger (if the system is designed properly). Regardless, that's a pretty pathetic response by a company rep and I wouldn't be happy either.

 

[A_Pmech]

I would expect the chance of a flame getting outside the heat exchanger would be fairly difficult. The heat exchanger on my heaters operates under negative pressure. The draft induction fan is in the burner exhaust.

FWIW, the aluminized coating has burned off in the curves on both of my heaters. (Beacon Morris)

 

[AlbertaSS]

Thanks guys, that makes sense with the exchanger being under negative pressure. I'm just going to run it with the man pressure at what they recommend and keep an eye in it. I did check the orifices they are all the same-074 is what is stamped on them. Thanks again

 

[brewchief]

Out of the hundreds of gas furnaces I've removed over the years I bet 98-99% had some spots where the aluminizing had gotten hot enough to discolor and rub off. Many have cracks but often nowhere near the overheated spots.

 

[txst]

Just to be clear - cracks normally occur due to thermal fatigue (they can be a result of mechanical stress, but that is rare). Thermal fatigue is from the expansion of the alumized steel due to increased temperature during the heating cycle and the contraction during the cool down cycle. The stress increases as the thermal gradiants increase across the surface of the tube (the surface temperature variation over a given area) and as the ultimate temperature increases. Also, tubes fail due to thermal corrosion - the temperature gets hot enough to oxidize the aluminized steel. These failures look just like the tubes were in a salt bath and they just rust away.

 

[Mark Mr.Oz]

Well I guess I am to the operational point of asking another question....Fired up the heater today (on propane as it has had the conversion)..I noted that the 3 burner jets flame out into the tubes but there is "wasted" flame in between the tubes? Is the water column or pressure to low? I also noted (in the dark) that only the center tube has a light red glow (at the first bend only). Am I not running the heater hot enough? I don't know the water column setting for sure as it was only leak tested by the installer. The unit seems to be keeping its own but not performing as well as I imagine it should be. Any input? What do you fellows see on your Big Maxx units? Also is the water column adjusted by the pressure screw under the plastic cap? Thanks in advance!!

 

[D.J.]

In my opinion some of you are not using proper terminology. Water column is measured in inches and pressure is rated in pounds per square inches. These are two totally different things. LPG is normally checked by using a manometer and usually 11 inches of water column. Not trying to be an *** or anything!

 

[Mark Mr.Oz]

I'm learning..so your not being an ***....this is valued information to us newbies!!

 

[ForceFed70]

In my opinion some of you are not using proper terminology. Water column is measured in inches and pressure is rated in pounds per square inches. These are two totally different things. LPG is normally checked by using a manometer and usually 11 inches of water column. Not trying to be an *** or anything!

Inches of water column is a way to measure pressure. Pounds per square inch is another way. So is Kilopascal. They are all units of pressure.

When we talk gas appliances, inches of water column is used because the pressure is so low. Otherwise we'd be saying things like "Drop it down to 0.11 PSI"

Here's a calculator that can be used to convert between WC and PSI. That wouldn't be possible if they both weren't measuring the same thing. https://www.convertunits.com/from/PSI/to/water+column+[inch]

 

[ForceFed70]

OP: I have the same heater. Mine also glows red in the corners when the lights are off and she's been running for a while. I think what you're seeing is normal for the heater. When you look at the top tube, the aluminized coating is discolored (and probably broken down) about 1/2 way across the front/exposed side. Each tube lower it's a little less, and the bottom tube is just discolored at the bend.

On it's 4th winter - no problems so far.