55' long OAK...Harman PF100

For the past couple weeks I've been looking into how long an OAK can be... Unfortunately all I found on the site were questions of how long an OAK can be, nothing particularly on how long an oak can be...unlike sizing supply lines using calculations/etc...intakes/suctions are a bit different.

I will be installing a 55' run of 6" 26gauge insulated wrapped galv. ductwork with a few adj. elbows...coming in through my basement bulkhead ( through an old wood workshop vent that was previously installed)...I did simple energy audit myself and found quite a bit of cold outdoor air pulling through this duct collector exhaust fan....Might as well use it and try it out as an OAK after removing the squirrel cage fan and a butterfly damper. Its going from 7" through the wall to a 6" reducer, 55' of 6", then a 6" to 3" reducer to 2ft of 3" flex duct into the intake on the Harman (pf100).
I came to the conclusion of a 6" run just because I thought 4" was a little too restricting...if it doesnt work, I'll tap into the opposite side of my basement wall which is about 35' away and try an OAK in that location.

Wish me luck. Pics to follow and I'll let you know if there are any adverse effects from the install regarding performance and temps. Should be a lot more efficient in the basement due to no conditioned air being pulled from the space for combustion or outside air being pulled in though the cracks/envelopes/etc..Basement is appx 68-70degrees as is, house is 73-75.. 2-6bags/day thus far .....
My previous post on the install of my pf100 - since installed a bypass type humidifier, burned 5 tons, wife and family happy....huge savings from oil... Thanks guys!

It works exactly like venting does with a very important difference you have nothing you are pushing against. The ideal is as short and as straight as is possible you want the run going uphill away from the device and you can handle a slightly longer run of OAK than venting. That 55' may work just fine.

Found this in the PF100 manual in the installation section:

The use of outside air is optional. Connecting to
outside air may be required by local codes, and is
strongly recommended. To install outside air, use 2¾”
I.D. metal flex pipe, part # 2-00-08544 (12’6”length)
or part #2-00-08545 (25’ length). There is a breakaway
hole on the rear panel which must be removed
before connecting the flex pipe. See Fig. 28. The pipe
should be run outside and terminate 3 feet or more
below or 1 foot or more to the side of the vent pipe
outlet. Never terminate the outside air above the vent
pipe outlet. The maximum length of this pipe is 25
feet. Inlet cover, part #1-10-09542 should be used to
keep birds, rodents etc.out of the inlet pipe.

The Harman manual also leaves a lot to be desired......Especially the technical info.

This is why I created a thread to see the good or bad with the install. 3" pipe is restrictive after such a length. Cross sectional area of 6" from 3 is obviously much larger. So we'll see. Pulling/inlet air is much different than discharge/suction. I'm pretty optimistic and want to make or bust all the OAK questions from/for the fans on the forum.

The 6" will probably get her done, good luck! Post your results.

Been kicking the same sort of idea around for the Revolution. From the manual .........




21
COMBUSTION AIR

1. Under certain conditions it is recommended that the Furnace be connected to an outside source of

combustion air to improve Furnace performance. Flexible metal hose, or rigid metal pipe,

(conduit), must be connected around (NOT INSIDE) the combustion air inlet tube (Figure 7). Be

careful not to pinch or bend the outside air pipe with too small a radius which may be terminated

flush with the outside wall but should be protected from wind and weather by a hood. Note: The

outside air pipe must terminate above the maximum snow line.

Figure 7

2. Outside air may be drawn from a semi-enclosed attic or crawl space or any semi-enclosed space.

Take care not to draw cold air past water pipes that may freeze.

3. Increase the outside air pipe diameter to 3” for runs over ten (10) feet and elevation over 4,000 feet.

Note: Long runs should be avoided.

4. Terminate the outside air pipe below the exhaust vent outlet.

5. 5. An open mesh screen should be placed over the outside air pipe opening to prevent birds or

rodents from nesting in the opening. Use an elbow or shield to prevent prevailing winds from

blowing directly into the outside air intake pipe. NOTE: Mesh screen should be no smaller than

one-fourth inch (1/4”) by one-fourth inch (1/4”).
WARNING: OUTSIDE COMBUSTION AIR IS REQUIRED FOR ALL MOBILE HOME

INSTALLATIONS!

WARNING

USE THE AIR INTAKE

DAMPER FOR

ADJUSTING

COMBUSTION INLET

AIR ONLY! OTHER

METHODS OF

RESTRICTING OR

BLOCKING

COMBUSTION INLET

AIR ARE STRICTLY

PROHIBITED

Hey STB,,
can you please expand on this? Why do you want the OAK run to slope up from the stove? Assuming it has to do with condensation but I'm nit sure. Thanks!

Cold air has a natural tendency to go down hill

Hoss summed it up quite well, always work with nature whenever possible. This is especially true when you are attempting to do things that otherwise might not work well.

If you look at the entire combustion air path as a system you want to aid the hot and ash laden air to leave the venting so you make certain that the vent side is always going up slope, the other side of the combustion air path (which if push came to shove could also be handling exhaust) you want to aid the cooler air's desire to sink.

Everyone also needs to remember that the OAK inlet must be below the exhaust vent termination.

Basically the OP's OAK is likely to work well even though it exceeds the manuals maximum run for a 2.75" OAK because he is going to have over 4 times the cross sectional area with that 6" duct.

The proof of the pudding is to try it and see.

here it is....
Kind of a messy transition to the 2-7/8" harman intake....
Have a damper right before the intake goes out the bulkhead.
No adverse effects on system...flame is at a good angle and not starving for air at all, same as before.

I also insulated my plenum, started to burn a bit better pellets - got a 7 degree increase at all my floor registers....going to be insulating all the main trunk lines, hoping to get an even higher raise in temp.

SO ...................Bottom line, NO difference with the OAK ....???????????????????? Or am I reading more into this than you're sayin' ????????

Bottom line - Runs great, Huge difference installing the OAK ...... no issues or starving going 55ft long (and with SEVEN 90's) with 6" duct for an OAK.

Enough volume in the 6" duct inlet to supply necessary combustion air. The pipe is cold - only from the bulkhead to about 5 feet away.Not going to insulate it, small surface area, no issues of condensation so its not that bad of a loss from cooling the room area it is in, basement is 74 degrees--outside air is 5-30degrees the past few days/nights.
I closed the damper (summer mode to prevent unnecessary airflow/bugs going in) just to see the effects and obviously it bogged down the flame - I could hear a slight whistle from one fitting -I missed foil taping one fitting, solved the whistling issue. In the pics above, nothing is taped, it is now. Opened the damper back up and the flame went roaring. I'm happy, wife is happy - $120 for all the fittings and duct lengths. Well worth it, inexpensive and no brainer in my book. basement is much warmer now from not using the conditioned/heated air nor pulling air in from cracks/seams along the rim joist and windows and reheating the area.

Thanks for the comments/input..... Makes me wonder if 4" would have worked for the entire run - Glad 6" was beyond adequate.

Sounds good Alain.

Nice Alain...I'm thinking of putting an OAK on mine over the summer. I keep reading on the forum that the connection to the PF100 is 2 - 7/8". The manual says it's 2 - 3/4".

I believe ID is 2-3/4, OD is 2-7/8" .... Its a little funky trying to keep the lower cover still removable for maintenance while hardpiping or even using some flex for the intake.
I've noticed a huge difference in temps in my basement. My only concern was the air being too cold for proper firing the pellets...haven't seen or felt any adverse effects of 'too cold' combustion air. I went a long ways with my run, I ran it next to the supply, so by the time it gets to the furnace its a bit warmer....no issues with condensation either. I used uninsulated ductwork. Good luck with your install!

It's got to be 2-3/4" OD because the metal flex pipe Harman sells is 2-3/4" ID. I'll have to put my digital caliper on it and measure it. Then I'll know for sure.

Cold air is denser than warm air according to my pilot training, so I would say the cold air would make the fire really hum.

Glad it worked out for you!

I use -20* air frequently with no problems. -40* the stove does not like manual mode at all, so I just keep it in auto mode.

Usually if your OAK is condensating/freezing it will be right where it comes into the home against the wall. Both my OAK (Boiler/Stove) do it right where it comes in. Our house is pretty dry though!