Vent size reduction from appliance to chimney

Edski26

Hi everyone,

I have a similar question as one posted before; https://forum.heatinghelp.com/discussion/160930/vent-size-reduction-from-appliances-to-connector-and-chimney-liner, but mine does not include a chimney liner.

I recently bought a building and the boiler has a 7" exhaust, but it has a reducer to a 6" 30-Gauge galvanized steel pipe. The pipe connects to my brick chimney that has a 6" hole and nothing else is connected to the pipe nor the chimney.

Last week I called in a furnace guy to get the boiler ready for the winter and he said that the boiler was very good but he recommended that I change the entire exhaust to a 7" B-Type pipe because that is what the boiler's exhaust is and I should also make the hole bigger in the chimney to 7 inches. He said that I cannot use a reducer because it is not correct.

Is he correct and the exhaust cannot be reduced 1" and if he is, why do they sell them, or is he just trying to make $2,300 dollars? I also find it odd that when I bought the building, the building inspector told me that I had to replace the relieve valve because it was leaking water, which I did, but he did not say anything about replacing the size or type of vent pipes. Is it ok to leave it as it is, or do I have to replace it?

Now, if it is dangerous, I would gladly spend the money, but I kind of feel like he is trying to make more money.

Thank you!

EdTheHeaterMan

Not necessarily. What is the model number of your heating unit.

Some manufacturers (of boilers mostly) use the same rear section for all the different sizes on that model boiler. so the largest 9 section boiler might need a 7" diameter vent while the smallest 3 section boiler may be able to use a 5" vent connector. You need to look at the installation manual for you model unit. If the Manufacturer says that your unit can be vented with 6" than you do not need to use a 7" flue connector.

As a standard rule of thumb, your contractor is correct, that is why you should have the original I/O manual on site. Sometimes older versions may be different than newer versions. That is why you need the one that came with the equipment when it was installed.

What is the Make and Model number? That I/O manual may be easily found on the " interweb". Or is that the "World Wide Net"?

EBEBRATT-Ed

What @EdTheHeaterMan said. Read the boiler install manual. It will tell you if you can reduce the flue size or not.

EdTheHeaterMan

If you look at this old boiler I/O manual you can clearly see that every boiler size from the 268 size to the 968 size has a rear flue opening of 7" regardless of the input. See page 18 for flue diameter. If you used the top flue modes of the same exact cast iron block, you can use a 5" flue connector pipe on the 2 and 3 section boilers with lower input. You can use a 6" vent connector pipe on the 4 and 5 section boilers and on the larger boilers you must ise the 7" flue connector pipe. see page 19. https://www.weil-mclain.com/sites/default/files/field-file/68-manual_1.pdf

I also remember installing one of those boilers where the chimney crock accepted a 6" flue connector pipe and the back of the boiler had a 7" opening. Although I knew it was just fine, the plumbing inspector failed the install based on the vent pipe connector having a 7" to 6" sheet metal reducer as the first fitting on the breach of the boiler. I said OK and that I would fix it.

Did you know that you could insert a 6" pipe inside the cast iron 7" breach connector and it fits pretty snug. There is a hole in the cast iron to accept a sheet metal screw from the 7" pipe that you must lign up just right when doing maintenance. (a real PITA). You can put a 1" long sheet metal screw thru that hole to grab the 6" pipe on the interior of that breach opening.

I did just that and called for a re-inspection. The reducer was gone and it passed. I don't even think he looked at the other stuff, he must have thought that I has a 6 to 5 reducer and replaced all the parts to 6"

After he left I put the reducer back the way it was supposed to be. I think it is stronger that way in case someone does a lousy tune up some day and there is a delayed ignition, I didn't want the flue pipe to blast off the back.

Mad Dog_2

A picture would be nice, but that being said, It's never a good idea to reduce flue size in the direction of air flow.  You'd be better off with an insulated SS liner up the chimney instead of B Vent. Home inspectors ("engineers" reports ) generally have an extremely limited base of knowledge, especially of the mechanical trades.  They are often former G C.s, not plumbers, Steamfitters or mechanical contractors. They often pick at non essential things and miss Life Safety ones.  Mad Dog 🐕 

ethicalpaul

Peerless ships a reducer with some of their boilers, it definitely depends on the boiler

Edski26

I do not have the original manual, the previous owner did not give it to me, but here is all the info from the metal tag on the boiler. I hope this helps?

Brand: Bryant
Type: Gas Boiler
Model: 234A 7PW
Series: E
Product No: 234AACOO2OO70EO9
Input BTU: 240,000
Output BTU: 192,000
Cap Lbs Per Hour: 192

Thank you All...!

Bob Harper

The chimney must be suitable for the class of service. That means its going to need a listed liner properly sized to the appliance. Liners can be smaller than the appliance collar if the sizing allows. The vent connector on gas must be sized per the GAMA tables found in the gas code and the listed instructions. The more stringent typically applies. Often, a vent connector will have to be increased over to the chimney breaching where it might be allowed to reduce into the liner. A 6" vent connector must be a minimum of 24ga./ 7" must be 22 ga. When in doubt, conact the mfr.

EdTheHeaterMan

This looks like your manual and on page 5 is the information relating to flue connector pipe

https://manualzz.com/doc/2028198/bryant-234-d-boiler-user-manual

That said, your existing chimney may be adequate. Worst case is that you may need to remove the 6" crock and put a 7" crock in its place. A decent chimney man can do that easly. What does the inside of the chimney look like? Is there a clay liner? is it 8" x 8" which is pretty standard? How tall is the chimney from the point where the crock is located to the top? With that info and the info in the manual It should be easy top determine if you can keep it as is, or if you need to change anything.

captainco

Why does everyone just guess, guess, guess, when you could test test test A combustion test and functional flue test makes it simple.

Edski26

Thank you all so much for your recommendations...!

EdTheHeaterMan

So, based on your I/O manual and the National Fuel Gas Code NFPA 54 table 13.1 (d) Masonry Chimney What you have will work if your chimney is 50 ft tall. (probably not) Look at the Red stared numbers on the chart for a 6” Single-wall metal connector diameter (the connector is the horizontal galvanized smoke pipe you are referencing in your original question)


If you have a 30 ft tall chimney (not out of the question… 2 story home with heater in the basement) then you can use a 7” connector with the heater up to 15 feet away. See the Blue stars.

If you have a 20 ft tall chimney then you will need a 7” connector but the boiler needs to be within 5 ft of the chimney. See the green stars.

If your chimney is only 15 ft tall, from the base of the chimney where the connector pipe enters the chimney, to the top of the chimney, then the connector needs to be only 2 feet long

To be clear the connector is the 6” pipe on your system that goes from the heater to the base of the chimney. And that is probably not the correct size.

You may need to enlarge the base of the chimney opening. And have new 7” crock installed.

I actually wonder if you really need that size heater? Perhaps that heater was too big for you home and that is why the chimney only has a 6” opening. The original heater was most likely smaller and a 6” connector pipe would have been just fine.

$2300.00 sounds a lot cheaper than the price of a new boiler now, doesn’t it?

I think all you need is a larger opening.

Edski26

Wow, EdTheHeaterMan, I like the way you broke that down. The chart is very helpful. Now having all the information, the furnace guy was right all along. I feel better making the decision and going forward and upgrading to a 7" B-Type connector. Thank you so much...

EdTheHeaterMan

There more than 10 of those charts in the code book, you need to know which one to select to get the info you want that apples to your situation. here is the one just above it for a Masonry chimney using a B-Vent connector. The numbers change slightly

EdTheHeaterMan

@captainco has a point, these charts and tables are way over kill for most situations. You could have a combustion analysis completed to determine if you actually have a problem. The test should be completed under the least favorable conditions. Like a relatively mild day (above 50"F) where the barometric pressure is low ( like a rainy day). If your system operates properly under those conditions, then you may not need to do the work.

Your contractor is going by the book. I think @captainco actually wrote the book. I know he teaches the book and lets you know what parts of the book are wrong!

captainco

Those venting tables were created by GAMA in the 80's, They are based on science and math but has never been tested. If we look at flues and see how many have rust on them then we know how often those tables are accurate. They are merely guidelines to start.