Shop Furnace Problem

[Jeff Mackay]

I have a problem with the furnace in my shop. It's a Modine, gas-fired, ceiling hung furnace. It heats the shop just fine. My problem is with condensation. The flue runs up into the garage, takes a 90 degree turn for about 10 feet, and then another 90 degree turn through the roof. Condensation is collecting beneath the second 90 degree turn, where it goes through the roof. The space beneath the attic that the flue runs through is conditioned--I have R-38 insulation between the conditioned space and the attic.

Do we have any HVAC experts on SMC? I'd call my HVAC guys, but they're the ones who caused the problem to begin with. I'm afraid I won't have much choice but to catch the condensation and let it drain outside. Any other ideas?

Jeff

[Mike Reinholtz]

I do heating and air, might be able to help. I work for an equipment manufacture and handle escalations, my area being the west coast.

First, what's the efficiency of your furnace? (80%, 90%,...) Any info on the model or BTU capacity?

Some of this may seem basic, but I've seen some real disasters so please don't take offense, gotta start somewhere.

Was the exhaust run in PVC or sheetmetal? Is the pipe slightly pitched up toward the outlet? What size is the flue run in? Any chance you can measure the length and number of elbows used in the pipe? Most of us have pictures of projects and tools, but seldom for a furnace, any chance you can take a couple of the unit or the exhaust piping?

Some of this may seem silly, but once you know this stuff I can run the numbers and we can get a starting point.

[Jeff Mackay]

Hi Mike,

Thanks for the quick response. I attached a crude drawing of the whole system. Measurements are in circular callouts, and they are approximate. The ductwork is 4" metal, fed from the Modine by a 3" exhaust, that goes through a 4" to 3" reducer. There is about 40 feet of pipe, and a total of 5 90 degree bends. It takes a pretty convoluted path to exhaust.

The unit itself is a Modine HD. I can't find any info on the BTUs, but I think it's a 45,000 BTU. I checked the fan, and since it runs at 1550 RPM, it's either 30,000 BTU or 45,000 BTU.

Jeff

[Matt Meiser]

If I'm reading your drawing right, you have 35' of pipe and 5 90's?

The manual for mine is out in the shop and I couldn't find one to download, but I seem to recall a limit on how many feet of pip you can have. And the 90's add several feet each.

[Mike Reinholtz]

Matt is right, your exhaust is way too long.

Each 90 on average is calculated at adding 5' of pipe.

I looked up Modine and found the install manuals, based on what you stated for the unit it looks like the longest equivalent run should be 40'. Your system being at 65' (equivalent) means the exhaust gases slow down too much and cool off, allowing condensation. My advice would be to replace the existing pipe with a shorter/straighter run. The condensation building up will eventually rot out the pipe, allowing the CO gas to pour into your home.

Sorry to be the bearer of bad news...

Here is a link to modine's install manual:

http://www.modineheaters.com/Modine-PDP-Manual.pdf

[Josh Bowman]

I don't mean to hi jack this thread. But I have a simular problem. The furnace is a Goodman 100K btu furnace with power vent. (it came out of my house) See the attached photos of the vent which goes out the wall of my shop. If the furnace runs a couple of times a day, no problem, but in the Spring when only the pilot is on, I get quite a puddle of water in the furnace. I have to turn off the pilot to keep it from making mess. The dryer vents flapper is bent a little to allow some drafting while on pilot. Any thoughts on the matter?

[Mike Reinholtz]

Josh- high efficiency furnaces use draft inducers that have condensate drains built into them, I can't tell by the pictures if yours has this. (not close enough on the draft inducer). Having a pilot light has me leaning towards this not being the case.

There are a couple options to fix this:

First- Not only would this fix your problem, but even add to the efficiency. I would really consider adding a spark ignition kit or glow rod and get rid of that pilot light.

Second would be to look into the actual exhaust fitting, and looking into how much air is blowing into it. Insulating the exhaust is smart, but may also be contributing to your problem by keeping the air cold blowing down into the heat exchanger.

Something I would be curious about is where the water is coming from, although it may seem straight forward I would really want to grab an inspection mirror and flashlight to look up above the burners and make sure there aren't any holes in the heat exchanger. That would be an immediate reason to replace that furnace (again with the threat of CO). Water condensing in this area is notorious for rotting out older heat exchangers. This is mostly avoided today because furnace manufactures stamp their exchangers out of stainless steel.

Hope that helps

[Dave Beauchesne]

Josh- high efficiency furnaces use draft inducers that have condensate drains built into them, I can't tell by the pictures if yours has this. (not close enough on the draft inducer). Having a pilot light has me leaning towards this not being the case.

There are a couple options to fix this:

First- Not only would this fix your problem, but even add to the efficiency. I would really consider adding a spark ignition kit or glow rod and get rid of that pilot light.

Second would be to look into the actual exhaust fitting, and looking into how much air is blowing into it. Insulating the exhaust is smart, but may also be contributing to your problem by keeping the air cold blowing down into the heat exchanger.

Something I would be curious about is where the water is coming from, although it may seem straight forward I would really want to grab an inspection mirror and flashlight to look up above the burners and make sure there aren't any holes in the heat exchanger. That would be an immediate reason to replace that furnace (again with the threat of CO). Water condensing in this area is notorious for rotting out older heat exchangers. This is mostly avoided today because furnace manufactures stamp their exchangers out of stainless steel.

Hope that helps

Mike / Josh -

I have been a HVAC guy / ticketed gas fitter for many years -

Mike called it right on all his info - Josh - if your furnace has a standing pilot light, it will be a 70ish % efficient unit - that being said, the venting you have would not pass any code up here in Canada - I have never seen a drier vent c/w flapper as part of a vent system ( I could be wrong though ) I really doubt that is legal

You say the drier vent flap is bent a little to allow the pilot ' fumes ' - it will condense for sure. Also, is the shop negative at all ( do you have a DC that is vented outside or something similar ? ) If so, it could be back drafting .

As for the Modine unit - too much vent - if a bunch of it is ' C ' vent ( galvanized only ) versus ' B ' vent ( galvanized pipe with an aluminum liner ), that will really cause trouble - but, you must not exceed the manufacturers recommendations as to flue ventilation requirements.

Goood luck to all!

Dave Beauchesne

[Mike Reinholtz]

That's funny Dave, I'm a fitter as well from local 597.

Josh-
I didn't open the second pic (traveling for work and stuck using a sloooow verizon card), but Dave is correct about the drier flap in your second pic, there is no way that would pass code inspection (at least not where I live). For starters, you could try replacing your exhaust with b-vent (home depot/ lowes sell it) and on the exterior use an exhaust vent that is open at the bottom. That would be a quick and cheap fix (much safer too).

[Josh Bowman]

That's funny Dave, I'm a fitter as well from local 597.

Josh-
I didn't open the second pic (traveling for work and stuck using a sloooow verizon card), but Dave is correct about the drier flap in your second pic, there is no way that would pass code inspection (at least not where I live). For starters, you could try replacing your exhaust with b-vent (home depot/ lowes sell it) and on the exterior use an exhaust vent that is open at the bottom. That would be a quick and cheap fix (much safer too).

I have no doubt that it's "not code". That was the only way I could think of to keep the critters out of it. If I put the "b-vent" on will it help with the condensation? What does a "b-vent" look like? I tried googling it and am not sure. Will it just replace the dryer vent? I feel that the pipe is not natural drafting enough to get rid of the moisture. The unit was in the basement of my house and vented to the roof, a story and attic away.

[Jeff Mackay]

I thought I posted a reply earler. Apparently not. I was afraid of that. So my original plan of just catching the condensation isn't a good idea.

The problem is that the unit is located under the garage. So there's too much garage, too much house, or too many windows and doors in the way to make it shorter. The unit is effectively sitting right beneath (albeit two floors down, with a garage in-between) from a window.

There is one possible solution that would use about 12 feet of duct and 2 90's: but it would require the vent to be placed about two feet away from a garage door (attached garage) and four feet from a fixed window. Do you think that would be a problem? Can I use a vent that is pretty much flush with the siding? There is driveway/sidewalk beneath that point, so I'll probably have to deal with ice in the winter, but better outside than inside.

Is it possible to boost the exhaust somehow? Or to insulate the duct--by wrapping it with something nonflammable but insulating?

Thanks for the help!

Jeff

[Josh Bowman]

Josh- high efficiency furnaces use draft inducers that have condensate drains built into them, I can't tell by the pictures if yours has this. (not close enough on the draft inducer). Having a pilot light has me leaning towards this not being the case.

There are a couple options to fix this:

First- Not only would this fix your problem, but even add to the efficiency. I would really consider adding a spark ignition kit or glow rod and get rid of that pilot light.

Second would be to look into the actual exhaust fitting, and looking into how much air is blowing into it. Insulating the exhaust is smart, but may also be contributing to your problem by keeping the air cold blowing down into the heat exchanger.

Something I would be curious about is where the water is coming from, although it may seem straight forward I would really want to grab an inspection mirror and flashlight to look up above the burners and make sure there aren't any holes in the heat exchanger. That would be an immediate reason to replace that furnace (again with the threat of CO). Water condensing in this area is notorious for rotting out older heat exchangers. This is mostly avoided today because furnace manufactures stamp their exchangers out of stainless steel.

Hope that helps

Mike and All,
The way I acquired this furnace is that I declared it unfit for the house and had a new American Standard unit put in. This one is about 15 years old. I was in the process of stripping the motor out before it went to the land fill, when I was able to see the heat exchanger fully. It was pristine! I got the HVAC guy to look at it and aside from the burners looking kind of worn, the rest looked great to him and he suggested the shop. I've been wondering if the double wall pipe would do better than the single wall and hi temperature insulation I have on it. How do I go about putting a glow rod or sparker on it? That would fix the problem. Is the way I ran to pipe acceptable? I began thinking that I should pursue running it out the roof to help it draft.
Jeff, if I'm getting this to far off topic to solve your problem.....please let me know and I'll bow out....

[Josh Bowman]

I thought I posted a reply earler. Apparently not. I was afraid of that. So my original plan of just catching the condensation isn't a good idea.

The problem is that the unit is located under the garage. So there's too much garage, too much house, or too many windows and doors in the way to make it shorter. The unit is effectively sitting right beneath (albeit two floors down, with a garage in-between) from a window.

There is one possible solution that would use about 12 feet of duct and 2 90's: but it would require the vent to be placed about two feet away from a garage door (attached garage) and four feet from a fixed window. Do you think that would be a problem? Can I use a vent that is pretty much flush with the siding? There is driveway/sidewalk beneath that point, so I'll probably have to deal with ice in the winter, but better outside than inside.

Is it possible to boost the exhaust somehow? Or to insulate the duct--by wrapping it with something nonflammable but insulating?

Thanks for the help!

Jeff

Jeff since you and I are having the same problem, I'm riding along. I can't see though when your furnace begins to condense water. Is it during long idle periods or while it being used? I am defenately not an HVAC expert, but I suspect that a power vent would only help if the unit makes water while it's being used.

[Dave Beauchesne]

Josh / Jeff:

I suggest both of you get a decent gas fitter involved - carbon monoxide / fire / rotten heat exchangers / code violations ( etc. ) are all serious and potential problems in both cases.

Josh: There is no way what you have is to code. Just upgrading to B vent will not fix your problem; get a manufacturers installation instructions to start.

Jeff: Shorter vent is likely what is required - however, depending on the jurisdiction, the termination has to be XXX number of feet from a building opening.

Good luck -

Dave Beauchesne

[Mike Reinholtz]

I agree with Dave, at this point I think both issues should be resolved by a licensed tech.

Josh- The spark ignitor (or glow rod) is pretty straight forward to install and shouldn't take the tech too long. After installing that the exhaust could be resolved as well.

Jeff- Your situation really needs a professional to take a look. I know it's not cheap and today we all try to save money where ever we can, but this isn't a place to cut corners. Although it would have probably been easier to do at the time of installation, generally if the run is going to be too long we look to install the furnace elsewhere and duct the air where it's needed (such as placing the furnace upstairs or in the garage).

Good luck to you both!

Mike