Draft Problems

MacPHJr

Customer called and said there is flue gas spillage from her water heater according to the energy auditor. Spillage only occurs when either the boiler fires (shares vent) or gas range hood is turned on.



40k btu water heater with a 3'' gal vent connected to a 6'' vent connector. The 6'' vent handles both the water heater and the 140k btu boiler. 6'' vent runs 6ft to masaonary chimney. Chimney has 6.5''x6.5'' clay liner and is appx. 18ft tall. Has no obstructions. Very clean.



I checked my code book to make sure the sizing of the vent connectors and the clay liner was correct. But I couldnt find anything, yet.



Are the vent connectors and the liner sized correctly.



I have a -2'' of water column when just the water heater fires and -1'' when the boiler fires.

bill_105

To: Mac Jr.

The answer to the question (with no question mark) is NO!

Do this, google gas vent tables, like Hart and cooley or Amerivent, etc... Then get a grasp of multiple vent and vent connector concerns, all of it!  It's easy, just read it 25 times. Then throw in the clay liner thing, combustion air thing, range fan thing and anything else that might get in the way of a good draft. Fortunately it's cheap stuff

Tim McElwain

Is the boiler fan assisited?

The range hood issue could be because of the hood exhausting air from the building causing a negative pressure in the building.



As for the sizing I will get my tables out and check it later as I am on the way out of the office.



If the boiler is fan assisted that can sometimes be a problem with attached water heater using the same vent connector.



I would check this yourself, sometimes equipment will spill for a short time and then it will heat up and be okay. You should have a draft on equipment of -.01, -.02, -.03 is acceptable.



I would also do a combustion analysis on the equipment. Is there any evidence of Carbon Monoxide in the room where the equipment is located when the equipment is running?

MacPHJr

Am I reading my draft gauge correctly?

It is not fan assisted. I did use a CO analyzer. There was spillage at the water heater at 20ppm the entire time the boiler was running.



I use the Bacharach Draft Rite.It read -1 without the boiler running and was slightly less when the boiler was running. Is that the same as -.01?

MacPHJr

Am I reading my draft gauge correctly?

It is not fan assisted. I did use a CO analyzer. There was spillage at the water heater at 20ppm the entire time the boiler was running.



I use the Bacharach Draft Rite.It read -1 without the boiler running and was slightly less when the boiler was running. Is that the same as -.01?

Robert O'Brien

BPI

The Energy Auditor tested under worst case conditions,all windows and doors shut,dryer and all bath/kitchen fans on as as well as any air handlers,attic fans etc.

Tim McElwain

The equipment should be able to

maintain adequate draft at all times under all conditions.



The chimney is 6.5" x 6.5" which gives 42.25 square inches at 6500 BTU  per square inch it will handle around 270,000 BTU's at 4500 BTU per square inch around 190,000 which is very close to your two appliances.



Is this an inside or three sides exposed chimney?



Your draft should be reading a negative .01, .02 ,.03 not sure what you are reading and at what point you are taking the draft reading..



Here is the NFPA 54 code sizing for chimney and vents solution to you set up. with 2 or more appliances Category I single wall pipe vent connector into a masonry chimney it will handle with a 3 foot rise about 150,000 BTU's with 6" single wall vent connector so the connector is too small.A 7 inch with 3 foot connector rise will handle around 213,000 BTU's.



I would go to a double wall connector all the way and increase the water heater to 4" double wall.



What is the size of the space the equipment is installed in? Is there a dryer nearby? How many fans in the house? With all the fans running and the dryer along with all the equipment running at the same time check your gas pressure in and out, do a combustion test, test draft on both sides of the draft hoods, clock the burners to make sure they are all burning correct input according to the rating plate.



You also may need to bring in make up air to compensate for the air being removed by exhaust fans and other equipment.



Where is the air for combustion coming from?

Jim Davis_3

Draft problems

Any time draft goes down when another common vented appliance is turned on indicates the flue is too small or restricted.  One exception would be if the flue gets wet.  Draft can drop 2 or 3 times lower when a chimney is wet versus dry.  Does the flue have a rain cap??  If not, install one and the problem should go away once the chimney is dry.  There are ways to make equipment work in undersized chimneys without adding liner.  The worst thing to do at this point is make the chimney smaller.