Cold flue Pipe causing intermittant backdraft???

[LilHulk1962]

Hello All;
Here's a weird one. I am having an intermittent CO problem. I have two induced draft furnaces and a natural draft Hot water heater with combined flues.
The FAU's are 100k BTU's each and the Water heater is a State 75 gal with 75k BTU's. They are all located in a basement, (finished), mechanical room.
The problem we are having is an intermittent rise in CO. We cannot reproduce the problem at will. The Water heater has a 4 inch flue with a length of 2 ft before it connects to the common WYE. It has a 10 rise. It is brand new. The FAU's have 3 inch induced outlets increased to 6 inches at a combined tee. Then just 2 ft to the combined WYE. The flue pipe is 7 inches and rises 30 feet to the cap. The combustion air is 2 - 6 inch rounds, half to top and half to bottom of the mechanical room.
All engineering data I have seen shows we are with in spec.
The CO rise only occurs when the flue pipe is cold, (i.e. the FAU's or Water heater haven't been used in awhile).
I have done everything I can think of. I have raised the flue to even with the ridge of the roof and installed a high wind cap. I have done extensive CO reading around the whole house. I don't want to switch to an induced draft water heater because I have no way to run a new flue and combustion air pipe. I am thinking it's a negative pressure problem but I need some more help before I foot the bill to have a house envelope and smoke test done.
Any advise, my customer is running out of patience.

[spotts]

Welcome!

[spotts]

Call the Dave at
and let him diagnose this for you. He is very knowlegeable. Tell him Spotts sent ya.

Other than that you'll need to read rule #2 from this site.

2. Do-It-Yourselfers - not here.

This site is for industry professionals and folks seeking HVAC/R advice and knowledge. Please do not ask for step by step instructions on purchasing, installing or repairing your own HVAC equipment, that is our job and our livelihood. We are generous, but not to a fault.

Questions of this type will not be answered and may be deleted.

Let us know what Dave says!

[LilHulk1962]

Yeah I know, Actually I am a professional NATE certified, NCI certified HVAC service tech with 12 years field experience in HVAC, Boilers, Chillers, etc. Sadly a job comes across every now and then that can really eat your lunch. This one is such a job. I have never come across such persistent one.

[spotts]

Once you get Pro Membership theres a forum for this stuff. Did you get the wind we did in the flatlands last night? It was a tree splitter.

[LilHulk1962]

I noticed, I just moved to Boulder in October from California and you guys some nasty wind out here. Although I lived in the mountains for three years I forgot how bad it can get.

[spotts]

AAnnnd do you have a high wind top on that waterheater?

[LilHulk1962]

Got a high wind cap on top of the flue. Hey by the way trying to figure out how to register for professional membership. This is my first post on this sight, although I've read many in the past years.

[spotts]

Got a high wind cap on top of the flue. Hey by the way trying to figure out how to register for professional membership. This is my first post on this sight, although I've read many in the past years.

http://www.hvac-talk.com/vbb/showthread.php?t=116113

Get yer post count up.

[LilHulk1962]

Got it. I just found the application. Thanks bud.

[jodimart]

Check this out

http://www.cpsc.gov/cpscpub/prerel/prhtml08/08241.html

It might help

[LilHulk1962]

Very helpful. The water heater is the same model but the serial numbers are not with in the recall. I have opened a dialog with the manufacturer as the problem is the same. HVAC-TALK RULES and thank you for your post.

[beenthere]

How far is the ridge from the cap.

Yours is not a uncommon problem. The chimney gets cold, and won't draft till it warms up.
Its not a neg pressure problem. Cold chimneys don't draft well.
Does this room get cold?