Help!! water heater venting issue


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Old 04-27-08, 09:22 PM
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Help!! water heater venting issue

I just installed a new nat. gas Rheem 50gal yesterday. This was close to a direct replacement as I was replacing a Rheem gas WH that was only 6 yrs old....but was a 40gal. The only difference was that I had to cut approx 3" off the vertical vent pipe that was approx 12" to start. And I had to adjust the vent pipe at a couple of elbows, turns things a bit and get it a little higher than it was...I'm still getting over the recommended 1/4" rise per foot as the OM states. The vent does run about 6 ft before it Y's in with the furnace pipe and then into the chimney.

After the install everything was fine, no leaks, venting good (did the match test).....install only took a little over an hour and commented to my neighbor, who helped me, that this was way too easy and with my luck something has to go wrong.....

So, I'm looking at it tonight and notice that the plastic cups around the water pipe inlet/outlets are melted, and realize that its not venting properly and now when I do the match test it blows the flame out....not that I needed that to show me....

Heres what I've discovered....keep in mind I had not a single venting issue with the old heater and the little plastic cups are fully intact on the old one. The new water heater vents fine until the small blower on the furnace kicks on (not sure what this is called) but I assume its clearing the combustion chamber of the furnace before the flame lights. When the furnace lights and the main fan starts the WH starts venting again. Sometimes this "small" blower on the furnace runs after the flame stops and sometimes it seems that its just running at odd times.....I'm not ruling out that the furnace may be acting up as far as this blower is concerned....but why does this matter with the new WH and why wasnt it screwing up the venting on the old one?? The only difference is a 3" shorter vertical and not quite as much rise. And should this really matter, I mean if that small furnace blower is running and the furnace vent and the WH vent "Y" together , it seems to me that should assist the WH venting....no?

I'm really at a loss on this.....any suggestions???

Thanks much
 
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Old 04-28-08, 06:39 AM
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Ok,
First off, can you answer a few questions?

1 Can you give me the model # of the new heater?
2 Is the Heating system gas or oil?
3 The little fan you are talking about. Is it the one for the burner if oil, or is it a power vented system?

I understand you never had problems with the old heater, but please bear with me. I am just going to give some examples until I get more info. Check them out.

Taking a stab in the dark I think you have a combination of issues here.

For starters, the new water heater may produce more BTU's. Check the rating plates of both heaters if you didn't dispose of the old W/H yet.

If the heating system is GAS then it must be a power vented unit or there would not be a blower when the burner starts, and should not even be tied into the chimney according to most manufacturers I have worked with.

If the heating system is oil, according to NFPA you are not supposed to mix fuels in the same chimney. I know, everyone does.

By the way is it a boiler or a furnace?

Next, possibly the fact that you shortened the vent pipe may have something to do with it, especially if the new heater produces more BTU.s. Maybe the old heater would have done the same thing if the pipe was cut to the same length.

Possibly the reason it has a positive pressure until the heating system lights is because that air is cold. Once the system lights the air is heated and causes the chimney to do its job. A warm chimney works much better than a cold one.

Another possibility is maybe the venting is undersized for both units. Does the vent increase in size at the connection of the W/H and the heating system? In other words, if you have a 6" vent coming from the heating system and a 3" vent coming from your W/H you should not be tied into just a 6" pipe going into the chimney. It should be increased to 7" or 8".

One more thing, maybe way off on this one but worth checking out, is the chimney built correct. It should be 2' above 10' across. in other words, draw a roof peak, you would need to measure horizontally from the chimney to the roof 10' across and the chimney should be 2' above that point.

Basically it could be a little of one or more of these items causing the problem.

If your water heater has a spill switch for the draft hood it should be shutting down the heater when it senses a positive pressure or excessive heat.


Hope this helps you out. Get back to me as you should not run the heater as is.
 
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Old 04-28-08, 08:58 AM
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Thanks for reply Plumbingods,

I'm posting links to pics that might clear things up a bit.

To answer some of your questions:

Ques 1- Rheem 22v50f1 (btu's 38000 Old one was 40000)
Ques 2-Furnace is gas, same as WH
Ques 3-I assume power vented system?

See pics, but I think venting is correct 3" from WH, 4" from furnace both into about a 6" into chimney.

http://i284.photobucket.com/albums/l...5/100_3457.jpg
http://i284.photobucket.com/albums/l...5/100_3458.jpg
http://i284.photobucket.com/albums/l...5/100_3459.jpg
http://i284.photobucket.com/albums/l...5/100_3460.jpg
http://i284.photobucket.com/albums/l...5/100_3461.jpg
http://i284.photobucket.com/albums/l...5/100_3462.jpg
http://i284.photobucket.com/albums/l...5/100_3463.jpg

Not sure about the power vented furnace.....what I know is if I turn thermostat up to start furnace, this "small" blower starts and runs for about 30 secs before flame ignites....It is during this 30 secs that the WH doesn't vent. Now this small blower seems to run at different times.....I think it runs on after main furnace fan stops.....but last night I turned thermostat down to shut furnace off....when furnace stopped everything just shut down....no small blower running on..

Thing is when I noticed the melted caps on the WH....this small blower was just sitting there running.....it wasn't at the beginning of a heat cycle....may (must) have been at the end of heat cycle....it was just running and seems to run at odd times.

The house chimney is good

The whole thing has me confused.....That 3 in I cut off and a slightly more horizontal run just doesn't seem to me that would be the issue....and with the new WH being less btu's??

see the pics, let me know what you think.
 
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Old 04-28-08, 09:04 AM
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Btw,

I'm running the WH but have the furnace turned off....been checking it and its venting fine.

There is no sensor that shuts the WH off if its not venting
 
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Old 04-28-08, 09:49 AM
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Allrighty then,

Thanks for sending the pics., they do help.

OK, the venting looked fine to me.

You must have a built in power venter, but it must not be a high efficiency condensing furnace that is why they can tie it into the chimney.

The blower coming on before the burners fire is called pre-purge. It is intended to purge any gases before firing the burner.
The blower coming on after the burners fire is called post-purge, also to clear out any flue gases after the burner shuts down.
some systems use only one, some use both, and some use none at all.

It sounds like the problem is happening only when you use both the furnace and water heater.


TRY THIS
...

Make sure the water heater is up to temp or turn the gas valve on the tank to pilot so it wont fire but you don't need to relight the pilot later.
Now turn up your heat and do your match test without the w/h running and see what happens. If it blows out your match we know it is from the power vent. You could have a partially plugged vent or chimney. Hey, I have found squirrels in the smokepipe before. Of coarse it was dead by the time I found it.

I hate to say it but it is possible that the 3" you cut off could be making the difference between the old and new water heaters. Maybe on the old one it was about to come out or even did but not hot enough to melt the plastic rings around the pipe nipples. Or maybe the plastic is just a different kind and melts easier.
What I'm saying is maybe the problem has always been there but it just didn't show up as well.

Trust me, I am racking my brain out on this one trying to come up with solutions. If I think of something else I will add it.
 
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Old 04-28-08, 11:13 AM
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Why is the vent from the furnace immediately increased in size as it leaves the furnace?

Yes, the lack of "rise" in the vent from the water heater is definitely a contributing factor with this problem. You could shorten the vertical from the water heater some to correct this problem.

I would prefer to see that Y at the chimney turned so that the branch is below the horizontal. If there is no good reason (and I cannot think of one) why the vent from the furnace is increased in size then the furnace could be re-piped to enter into the branch of the Y and the water heater vented to the run.
 
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Old 04-28-08, 11:26 AM
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Furd,

I was having furnace issues a few year back it was "overheating" and kicking off some sensor and was told that my venting needed to be re-piped....The furnace used to have an elbow directly connected and venting was quite cobbled. The furnace guy that re-routed everything told me I needed (I think) double walled pipe from the furnace.....So I think that the pipe coming from the furnace is actually 3" ID and OD is 4"....does that make any sense??
 
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Old 04-28-08, 11:45 AM
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Yes, it makes sense but that does not look like double-wall pipe to me. I think it is just the next step up in size single-wall pipe.

Plumbingods may be on the right track with a partially blocked chimney. Is this a metal chimney all the way to the roof and does it have a draft hood (cap) on top?

Picture of the top of the chimney, please?
 
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Old 04-28-08, 11:55 AM
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OK,

Give me a little bit cause I'm back at work now....I'll get a pic of that as soon as I get home.

It does have a metal cap on top....was planning to get up there and check it all out tonight. The only reason I hadnt was the whole old to new wh and wasnt thinking anything could have changed there.....
 
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Old 04-28-08, 12:08 PM
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A picture from the ground is fine, I want to see if you have anything that may be causing a downdraft. Include pictures showing the roof and any structures within ten feet of the top of the chimney.
 
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Old 04-28-08, 01:42 PM
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Old 04-28-08, 04:38 PM
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It is possible that the surrounding trees are causing a minor downdraft under some wind conditions but I rather doubt it.

It is more likely that particular cap is not allowing a sufficient amount of gases escape from the flue. If so, it would require a high capacity vent cap be installed in its place.


I thought a bit more about your previous furnace problem and I don't think that increasing the size of the vent piping was the correct solution. Overheating is usually caused by either overfiring the furnace or having too low a blower speed.
 
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Old 04-29-08, 05:03 AM
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Originally Posted by furd

It is more likely that particular cap is not allowing a sufficient amount of gases escape from the flue. If so, it would require a high capacity vent cap be installed in its place.


I just don't understand why this can be the case....The WH vents fine on its own, vents fine when furnace is running....the WH is 2000 btus less than what I had.....chimney is clear.....really how couldnt it be given the fact it vents both the furnace and WH together just fine.

This has something to do with that small blower and when "it" runs and "it" only running is when the WH doesn't vent....its like the small blower air is turbulating at the elbow and not allowing the WH to vent when its running......I tryed plumbinngods idea and held a flame (lighter) at the WH hood when just the sm blower was running (no WH burning).....and it didnt blow flame out....but it didnt draw it in either....just kind of static.

So....could a guy get some kind of "in line" fan that could assist the WH and wire it into this sm blower so it comes on when this blower does?? Out of the question???


Thanks
 
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Old 04-29-08, 07:04 AM
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I see this problem from time to time at installations with assisted draft Cat I furnaces and natural draft water heaters. Likely, the previous water heater was also backdrafting to some extent, but the problem was not noted. IMO, this arrangement should be prohibited, but at moment ANSI standards allow this type of spillage to occur for 45 seconds. One solution is a barometric damper with a spill switch. See this thread at Inspection News.
 

Last edited by Michael Thomas; 04-29-08 at 08:16 AM.
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Old 04-29-08, 08:24 AM
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As I stated previously I don't think that installing a larger vent pipe on the furnace was the proper fix for the overheating issue. More likely that was a problem due to insufficient airflow across the heat exchanger and by increasing the flue size you have actually decreased the efficiency of the furnace.

But back to the present problem. Your problem started when you installed (had installed?) the new water heater AND changed the slope of the vent for the water heater. As Michael Thomas noted, you probably had a borderline situation before and this new water heater, even though it has a lower BTU rating, is having a draft problem.

What has changed from the old to the new? The slope of the vent piping.

You have several choices as I see it. One is to increase the draft from the chimney by installing a different cap. Another is to change the way that the water heater and furnace vent piping are connected to the chimney. A third is to determine if the problem you originally had with the furnace was properly fixed or just masked by increasing the vent size. The easiest would be to shorten the vertical portion of the vent from the water heater to allow a greater slope on the vent to the chimney.

It may take a combination of these to effect a real cure.
 
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Old 04-29-08, 08:32 AM
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Looking at Michael's link I learn that the 12 inch vertical riser from the water heater draft hood is a code issue.

I also saw (and I should have picked up on this previously) that the power vented appliance (the furnace) should ALWAYS be connected closer to the stack than the natural drafted appliance.

So, with this knowledge I would recommend that the wye fitting at the chimney be re-oriented and the furnace vent to the branch with the water heater venting to the run. It's too bad that the chimney was originally constructed with the inlet as low as it is.
 
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Old 04-29-08, 06:44 PM
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I've tryed removing the vertical, connecting the elbow direct to the draft hood.....made no difference.

I'm probably going to re-route the ducting per Furd's suggestion.
Although, I do like the idea of the spill switch.

Thank you for the help...I started a thread with some of my posts over in the ducting forum. Just trying to get maybe some more suggestions before I start tearing into things....In a day or so I wont need the furnace and that will buy me a little time. Right now I'm running the WH,no furnace, during the night and thru the day. Then run the furnace, no WH when I get home from work.
 
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Old 05-03-08, 10:52 AM
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I would check the install manual for the furnace. I suspect it may not allow an induced draft furnace to share a flue with a water heater.


edit: I checked a manual for a typical ( Goodman) 80% furnace with induced draft. They DO allow sharing a flue with a WH if sized correctly. There is a caveat that the flue must rise vertically by at least as many feet as it runs horizontally.

In general, the other issue sometimes is if the first elbow is too close to the WH. I think they like to run several inches vertical before the first elbow,.
 
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Old 05-03-08, 11:13 AM
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Looks like the duct work needs to be reworked so that the fan for the furnace causes a suction on the heater vent rather than back pressure.

Same principle as water hose attachment for applying pestisides & fetilizer.

A larger opening above the inlet of the W/H vent should causes a drop in pressure creating a suction. The vent for the furnace will need to extend past the opening for the W/H inlet.
 
 

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