Home > HVAC > Fluepipe design, Part 2

Fluepipe design, Part 2


Feature Story

By George Lanthier


OK, if you remember in the last article we looked at fluepipe design and what it really takes to do it right. Last month we looked at single units, this month it’s multiple units. Keep in mind that I’m continuing to follow the NFPA Standards, NFPA31 and 211, for this subject, and both NFPA and I can’t stress enough that any manufacturer’s requirement supersedes anything either of us have to say. No surprise there; any code regulator or instructor always give the manufacturer of a listed, certified or accredited product more value than the code. Nobody knows the product better than them, FACT!

Although I told you to hang onto it, here’s that new chart from Field Controls again.
In this article, we’re going to do just two examples using tees and 90s to make it bad, and Y-connectors and 45s to make it, as Goldilocks always says, ‘just right.” We’re also going to keep the parameters the same. Our chimney is still 35 feet high and our flue pipe on our heating unit is 6 inches, but now it’s a furnace and to that we’re going to add a water heater with a 5-inch flue. Sound familiar?

Just like last month, let’s first do it the traditional way. In Figure 1, we have a drawing that shows a simple install of the oil-fired furnace and the water heater.

We have six pieces that lead from the furnace to the chimney. The lengths are 11, 13, 12.5 and 14 inches, and two 24-inch pieces that total 88.5 inches, or about 7.5 feet, and then we add two 90-degree elbows at 11 feet each and a tee at 38 feet for a grand total of 67.5 feet of TEPL. Amazing. I’m not even going to worry about the water heater in the winter, but in the summer it will have to overcome 50.5 feet of TEPL just by itself, hmmmmm.

In Figure 2, I’m using 45-degree elbows again and this time I’ve used a Y-connector to make the changes. From the furnace I now have a run of 4, 24, 7, 24 and 21.5 inches for 80.5 inches, or about 6.5 feet of run. Add 20 feet for the Y and you end up with a grand total of 26.5 feet of TEPL, or roughly one-third of doing it the conventional way. Again, if I just look at the water heater, the burner there will only have to overcome about 17.5 feet of TEPL, or about one-third of the other method.

But what about that transition, does that count? Oh, sure. Again, a great chart from Field Controls can be found in their literature. This time there’s a tricky part, unless it’s only one pipe size, then you use the .5 d/D dimension. For our job that means adding 3 feet to the 17.5 feet, bringing the total TEPL for the water heater to 20.5 feet, again well below the chimney design of 22.5 feet we established for the chimney in the last article. Just to refresh your memory, we said that we had a 35-foot chimney. We deducted 5 feet for the height of where the fluepipe goes into the chimney, leaving us an actual chimney height of 30 feet. Seventy-five percent of that leaves us a working dimension for a fluepipe of no more than 22.5 feet, which is the maximum allowed by both of our standards.

For any other job, you should follow this procedure as outlined by Field Controls and Figure 3.

The reducer or increaser ratio (d/D) small diameter divided reducer ratio is d/D=4/8=1/2. To estimate the equivalent foot-length for the fitting, use the smaller pipe diameter for the equivalent length figure.
Example: 4-inch to 8-inch reducer, the reducer ratio is .5 and the smaller pipe diameter is 4 inches. So, from the chart the equivalent would be 7 feet.

OK, that’s all out of the way. Now, I want to take you through how I would do a multiple unit. I don’t go against OEM specs on a new install, ever. But, when a job is a few years old and it has been a problem since day one it’s time to try something else and numbers and good simple math just don’t lie.

Both of these jobs have 7-inch fluepipe coming out of the breeching of the boilers and we’ll do the sizing of the combination flues in the next article. In Figure 4 we have another one of those jobs loaded with 90-degree elbows, four to be precise.

InFigure 5 we have the same job converted to 45-degree elbows. The tees have become Y-connectors and I just want to focus on those since the pipe lengths and transitions will be just about the same and you know how to figure those now, right?

The four 90-degree elbows equal 12 feet each and the two tees are worth 44 feet each for a total of 136 feet of TEPL, and remember that doesn’t include the transitions and piping. But, if I switch to four 45-degree elbows at 6 feet each and add the two Y-connectors at 23 feet each, my new TEPL just for fittings is now 70 feet of TEPL, or roughly half, getting the idea? By the way, just to vent that 90-degree job without the piping figured in you need a chimney 180-feet high, think about it! Excuse my sarcasm, I’m famous for it, but are some of your draft problems starting to make sense to you?

In our final article we’ll look at the transitions and multiple vent pipes. l FON


See ya.


George Lanthier is the owner of the Firedragon Academy, a teaching, publishing and consulting firm. He is a proctor and trainer for the industry’s certification programs and is the author of nine books on oilheating and HVAC subjects. He can be reached at 132 Lowell Street, Arlington, MA, 02474-2756. His phone is (781) 646-2584, fax (781) 641-7099 and his Web site featuring the industry’s only ‘trade-only” discussion board can be found at www.FiredragonEnt.com.


Leave a Reply

Your email address will not be published. Required fields are marked *

*