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Old 03-22-2012, 12:58 PM   #1
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Rheem white powder near exhaust vent?


I have a Rheem II heater that is from 1996 forget the model but I think its 80k BTU. It has an exhaust stack that has no vent and then went up an T-ed of to a water heater that had a vent. I installed a instant water heater that has its own exhaust system. So I removed the T and just stuck a straight section of pipe to replace the T.

What I suspect is that the water heater is what was being used to drain the condensation from the pipe. Seems kinda odd to me but that is what I suspect.

Is my assumption correct and can I just get a straight section of pipe that has a condensation drain or does the heater also need a vent?




Last edited by mikebau; 03-22-2012 at 01:18 PM.
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Old 03-22-2012, 01:30 PM   #2
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Rheem white powder near exhaust vent?


If its only white powder, then that's just normal precipitate and nothing to worry about.
BUT..It's probably condensate looking at your set up.. Your pictures were not up when I wrote this.
The vent pipe is supposed to be correctly sized according to the total BTU's (and other facters) and it looks too large. (What size is it?)
You also now have 30- 40,000 less btu's to vent through it with the HWT change.


Last edited by how; 03-22-2012 at 01:45 PM.
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Old 03-22-2012, 01:36 PM   #3
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Rheem white powder near exhaust vent?


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If its only white powder, then that's just normal precipitate and nothing to worry about.
If you are getting water dripping down the exhaust vent then that's a different issue. It sounds as if you might just have a single walled vent for your furnace instead of B-vent which helps reduce the condensing of the flue gas. The vent pipe is also supposed to be correctly sized according to the total BTU's which is now probably 36,000 btu's lower than what it was designed for.
It's definately a B-vent pipe all the tubing states B-pipe and was installed when the house was built. There is only one section of tube I replaced. If you look at the picture it's the small section right after the 90 degree bend that leaves the furnace. Its a little bit shinier then the rest maybe 6-8 inches long. That is where the T was that led over to the water heater. I also used a B type pipe to replace the T.

The sizing of the pipe you could be correct. But I would assume that even in the old configuration it was possible for the heater to run without the water heater. So I suspect that either could have run by themselves with that pipe system. But I could be wrong. I suspect I need a condensation drain and that currently the water is running back into the heater.

No dripping water. Just white powder near the base of the stack that enters the heater. If I open the heater I see white powder inside the heater not huge quantities. But white powder that looks like its in the exhaust fan and then also near the burners. I'll get some pictures of the powder.

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Old 03-22-2012, 02:08 PM   #4
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Rheem white powder near exhaust vent?


The vent size is chosen to be able to channel the total btus that could ever be produced at one time . The first worry is CO spillage.
Anything that cools down the flue gases like long lateral runs, oversized venting, venting through cold areas, etc will produce condensate dripping issues.

You now have roughly 30% less btus so the vent gases are often going to be cooler again and potentially produce more condensate than you had.

Plus I believe your original venting may have been oversized to begin with.

What is your vent size, the distance from the furnace to chimney section that exits straight up, the height of your vent termination?

OK, The additions to your last post caught me again. White powder is no big deal as long as you are not seeing water stains above or below the ID assy..

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Old 03-22-2012, 02:49 PM   #5
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Old 03-22-2012, 04:17 PM   #6
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Rheem white powder near exhaust vent?


Your B-vent is not staying warm enough to vent furnace fumes.

It is back drafting thru the burner section.


Does you instant hot water tank take air from inside the home to burn the gas?

A pick of the exhaust would be helpful.

Last edited by hvac5646; 03-22-2012 at 04:29 PM.
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Old 03-22-2012, 04:24 PM   #7
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Rheem white powder near exhaust vent?


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Your B-vent is not staying warm enough to vent furnace fumes.

It is back drafting thru the burner section.
If I remeber correctly my vent tube is 5 inch. So what's the answer smaller tube? I didn't design this system it's how the house came built back in 96.

I guess what I dont understand is a water heater doesn't run all the time. So the furnace would have been cycling on an off multiple times with OUT the water heater. So why is the pipe to big now vs when it was running connected to a water heater that wasn't running?
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Old 03-22-2012, 04:32 PM   #8
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Rheem white powder near exhaust vent?


Read my last post. I edited it.
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Old 03-22-2012, 04:35 PM   #9
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Rheem white powder near exhaust vent?


If your water heater takes air from inside home for combustion you are drawing the B-vent into a neg-pressure and fumes won't vent.
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Old 03-22-2012, 04:35 PM   #10
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Rheem white powder near exhaust vent?


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Read my last post. I edited it.
No my tankless water heater is a fully contained system. It pulls cold air from outside. It's a GE profile tankless water heater. It's a single pipe but the "outer" pipe is plastic and is where the intake air is brought in. The iner steel pipe the exhaust goes out.
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Old 03-22-2012, 04:39 PM   #11
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Rheem white powder near exhaust vent?


Furnace is in garage?
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Old 03-22-2012, 04:43 PM   #12
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Rheem white powder near exhaust vent?


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Furnace is in garage?
Yes furnace is in the garage.

Not the best picture this was still during the installation but all I have on my phone at this time.
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Old 03-22-2012, 04:50 PM   #13
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Rheem white powder near exhaust vent?


Your flue may be plugged.

If the burners look "lazy" or wavering with a lot of yelow in them, than you probably have a plugged flue. Might even be a cracked heat exchanger.


I would normally say a lack of air is your problem but being in the garage I don't think you have an air exchange prob, unless you insulated the dickens and and installed a high R-value garage door.
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Old 03-22-2012, 04:54 PM   #14
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Rheem white powder near exhaust vent?


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Your flue may be plugged.

If the burners look "lazy" or wavering with a lot of yelow in them, than you probably have a plugged flue. Might even be a cracked heat exchanger.


I would normally say a lack of air is your problem but being in the garage I don't think you have an air exchange prob, unless you insulated the dickens and and installed a high R-value garage door.
I looked at the three burners and they looked pretty good when it was on. Every once in a geat while I will see a tinge of an orange flame but 99.9% of the time they are blue all the way to the tip. Same garage door that has always been there is on there. I have someone scheduled to come out and do a yearly service on it next Thursday. Anything specific I should ask them to do?

It's been about 1.5 years since I installed the tankless heater and this is just something I noticed recently.

Oh and thanks everyone for the tips and suggestions.

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Old 03-22-2012, 08:09 PM   #15
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Rheem white powder near exhaust vent?


The lack of any signs of scorch marks and the fact that the roll out switches (in the picture) have not tripped make backdrafting an unlikely diagnosis. A badly cracked exchanger could help cool down the vent gases but once again neither the PS or the roll out sw have been affected.
I would recommend that the tech check out the exchanger problem possibility
BUT...
If that B vent is only 5" then his furnace vent would have to be half that size which is not possible. It's more likely that he has an oversized vent, with an long lateral run, 30% less btus with the HWT change, all in a cool garage, hense the condensate powder vent production.
Let us know what your tech finds>


Last edited by how; 03-22-2012 at 08:19 PM.
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