Venting Question for b-vent and single wall
So I have a question concerning a b-vent chimney and single wall common vent for a 150k 80% furnace and a 75k water heater.
In my career, I've been very fortunate to rip out a lot of the chimney vented appliances for high efficiency and the units I've serviced were always common vented correctly (lower BTU higher than furnace/boiler). So I haven't come across too many situations where the venting was wrong, but that appears to be the case here.
I did a combustion test on a furnace the other day and the CO level was very high (well over 400ppm undiluted). The unit is only two years old, and I pulled the blower to look at heat exchanger and stuffed a camera in there and that looked fined. Pressure drop was .75 IWC on manometer so good there (even with water heater firing too). They do need make up air in the room as it is confined space. The chimney is also not tall enough (about 20') and there is a roof line right next to termination thus not following the 10 and 2 rule (2' higher than the adjacent roof).
When I measure amperage of draft inducer, it's running about 2 amps higher than the FLA. It seems like it's not drafting properly, like it's encountering resistance.
The chimney is a 6" b vent and there is a horizontal common vent that enters the chimney. The common vent is 6" with a 6" tee that has a 5" x 4" reducer to pick up the water heater first on the branch (it is on the flat pointed slightly down to get 1/4" pitch). There is about 4' of 4" going to water heater. The water heater has no rise (sliver of a piece and then right into a 90, which is wrong and will be corrected). On the run of the tee on the common vent is the furnace. The tee has a 6" x 5" reducer on it and a couple of feet of 5", a 90, another 90 down to furnace, a 5" x 4" reducer to about 18" of 4" off the furnace. Furnace has a rise of about 3'. All piping is pitched correctly. When I looked this all up in gas code for sizing the 6" for chimney and 5" for the appliances checked out. (I used 20' and the chimney will ultimately be higher)
Once we add a section of b-vent at the roof and a new cap, the horizontal is less than the 75% of chimney height, so we will be good there. Currently the run is 16' from chimney to the furthest appliance, the furnace, which is too long based on 75% rule.
So my question is I usually see the water heater enter up top from horizontal to vertical of chimney. That's how the code book shows to common vent the two. Would the water heater with the lowest BTU being picked up on a horizontal be correct entering first? It is technically higher because of pitch right? I've heard the furnace wants to be first so as to not push exhuast out of the WH flue collar, but I can't seem to find the referenced code. Any thoughts would be much appreciated. We are planning on installing a fan in a can for the make up air due to the confined space issue, along with adding another piece of b-vent to the top of chimney. Any thoughts would be much appreciated. Thanks!
Comments
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I can't help but notice that you posted this query in OIL HEATING. Is the furnace or water heater a Gas fired or Oil fired appliance? The reason I ask is that you can not use B-vent on an oil fired appliance. It says so in the installation instructions on every brand of B-vent ever made. If you have an oil fired appliance connected to a B-vent SHUT IT OFF AS SOON AS POSSIBLE. Then get a proper vent system for it.
L-vent or class A manufactured chimney is acceptable for fuel oil. If it is a gas furnace and a gas water heater then we can asl to have this discussion moved to Chimneys and Flues then we can address your query.
Edward Young Retired
After you make that expensive repair and you still have the same problem, What will you check next?
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Yeah if it is oil it must be type L as @EdTheHeaterMan mentioned
If gas and because it is a metal vent (not masonry) I believe the furnace and water heater can enter at the same height but a Y fitting is better than a tee. Extending the height of the "B" vent will help.
If the furnace is induced draft and the WH is natural draft you have to use the correct chart in the code (fan & nat) and you have to check to see if the vent connectors can be single wall or if B vent must be used.
It is hard do decide what it needs without being on site.
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No this is natural gas and I'm aware of that for oil.
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EBEBRATT-Ed I was planning on swapping tee for wye, and I did look it up in Nfpa 54 which is what we use. I used the chart for b vent chimney, more than one appliance, single wall, and this setup can be used with current sizes. I think the big issues are the chimney not being tall enough (doesn't have 2' over adjacent roof), the horizontal is greater than the 75% rule, and the space being confined space and not enough air for combustion.
What are your thoughts on where it enters common vent though? Nfpa 54 says lower BTU appliance higher, which if it is the first on horizontal common vent it is. But I've heard contradictions from plumbers/hvac techs that the furnace would want to be first so as not to create issues with water heater.
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I'm no expert on the finer points of common venting. Higher than FLA on the inducer motor suggests there may be a mechanical issue with the motor (like bad bearings). Current on centrifugal blowers with forward curves blades as typically seen in inducer motors tends to go down as airflow decreases. Underperforming draft inducer can definitely cause high CO.
Did you measure draft at the furnace vent outlet when you did the combustion analysis?
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RPK I didn't measure that because the chimney wasn't high enough (2' past adjacent roof line, it was below it) and the horizontal was past the 75% rule so that right there told me it had drafting issues. Hindsight being what it is I should've measured that too. With the venting not being right but size being correct, I figured that was the reasoning for poor draft.
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On venting I go to the Mechanical Code and or International Mechanical Code as to sizing and placement.
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HomerJSmith we use NFPA 54 in MA
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NFPA is correct for MA. Just follow what the code says regarding flue placement into the chimney
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Chimney height and common vent fix was the issue. Thanks guys.
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