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I have 6 months to figure this.
While fixing ac, costomer wants me to fix problem before winter.
Carrier 80% power draft. HO complains furnace limit trips off during windy conditions. Claims this happened after reroof and replaced Bvent pipe.
8 feet of bvent with one 90 and cap.
Two service companies said termination cap needs to be extended higher. It is 12" off roof.
I cannot believe this is the solution with a power induced burner.
Anybody seen this as a reason to trip off limit.
you have flame rollout caused by venting or heat exchanger. If venting is sized properly and heat ex is good, than I agree with extending vent another couple feet. Also could be a weak rollout switch.
or you could have negitive pressure issues in the mech room when the wind blows.
ive seen that when the vertical vent is on the windy side of the house and the fresh air intake is on the back side of the house which will experience a lower pressure condition causeing your down draft.
he's lucky he has a furnace with that safty.
now does he also have a water heater common vented???
i reread you post...
12" above roof line is too low. i believe it needs to be 2 ft above anything 10 ft away horizontaly.
i think that rule has been amended. send a shout out to hearthman for clarification.
he's the resident expert on all things venting here on this site
Have you seen the limit trip or is it the homeowner that diagnosed the limit? If you think it through,an extreme vacuum, as would be created when it's very windy won't cause the heat exchanger to over heat nor will it cause the flame to roll back out of the heat exchanger. It will cause just the opposite effect, sucking the flame throught the heat exchanger too fast. If it were me I would be looking at flame lift off from the flame rod causing the flame to drop out and the furace to recycle. Is this what is happening, the furnace recycling? A roll out switch has to be manually reset so thats easy to rule out. If it is a flame rod lift off adding a little height to the chimney will help. Supposed to be 2ft taller than anything with in 10'.
If this is the typical 80% induced draft furnace, the vent system is still a natural draft system, that operates at a slight negative pressure. Once the furnace lights, the draft inducer is just dumping the combustion products into the vent system, it isn't blowing them down the vent pipe.
With an improperly terminated vent pipe, high winds can cause high or low pressure conditions at the vent pipe termination.
The draft pressure in the vent system should ideally only be in the -0.02 to -0.04"wc range, so adverse pressure conditions at the vent pipe termination can easily cause to much draft, or stop the venting altogether, depending on the conditions.
Either one can cause the furnace to shut down on one of several safety controls.
This is a power draft, not induced draft,system. You cannot tie this to any other vent.
Other service company showed HO how to reset safety. Unit is in attic.
If it is power draft vent terminatin won't make any differance. If it is induced draft like a normal 80% then yes it could. Tremination should be 2' level from the top of the pipe to the roof or higher. If the vent is with in 10' of any obstruction or roof peak thenit needs to be 2' above that obstruction or peak.
Are you sure you don't have an airflow problem? I would look at airflow before vent on this issue.
Questions regarding potential surfaces that could deflect a draft down the pipe-Quote:
Originally Posted by ch4man
Are there any walls within 10 feet of the vent termination?
Also what is the approximate pitch of the roof?
If it is truly power vented, then the termination should not make a difference but then it would also not make much sense that the problem started after re-roofing. Type B vent is also not the correct material for a system with a positive pressure in the vent.
If it is a Category 1, mid-efficiency appliance, then the 3/2/10 rule for chimneys doesn't apply either (2 foot above anything within 10 feet). For Type B vent, being a minimum of 8 feet from a vertical wall, the minimum height above the roof penetration for a flat up to 6/12 pitch is 1 foot. Reference Figure and Table 12.7.2 in the NFPA 54 book.
Most of the problems I have seen like this have more to to with where the termination is located than anything else. Is it located in a "pocket" on the roof? A "zone" that gets pressurized depending on which direction the prevailing wind comes from. Single story garage roofs beside a two-story house is a great example. Wind, most of the time, will help a vent system operate unless it creates a pressure zone on the roof that is at a greater pressure than the attic. So, it may be that increasing the height would help.
I've seen a couple of cases where roofers sat on or somehow crushed down the B vent cap restricting the venting. Worth checking.
BTW, are you referring to the high limit, a vent safety switch, or a roll-out switch?
Start with a manual for unit, make sure it's the way it should be. By change is house on water?
P.S. Something like this a while back on the water, winter storms, used a gas draft regulator to "solve" the problem as it was a draft inducer system.
please post the model number. ive never seen a carrier cat II or III furnace. id like to check out the I/O manual.
or does this have an add on power venter such a tjerland unit?
tia