Water Heaters - Water heater venting problem

In my utility room (ground floor on slab) I have a 13 year old gas boiler and a 2 year old gas water heater (atmospheric draft). The boiler (which has a power damper in the flue) has a 6" vent which runs vertical for about a foot, then approx 45% to a union. The water heater has a 3" vent which runs approx 8" vertical, then some very minor angles to the union. The union merges the 3" and 6" ducts into a 7" duct that runs vertical into the attic and then approx 45% in the attic until near the roof line, where it goes vertical out the roof. I have had this set-up for 15 years (through 2 water heaters) with no downdraft problems. Recently I noticed a significant downdraft under the water heater draft hood when it fires up and escalated levels of carbon monoxide on my alarms. I checked the vent from the roof and the utility room fresh air feeds (one 12 inches from the floor and one on the ceiling - both get air from attic) and found no obstructions. The roof vent cap is also ok. I tried smoke and match tests under the water heater vent hood with the boiler on/off and the water heater on/off and it is hit or miss if the smoke/flame goes up the vent. When the unit does backdraft, there is a very acrid smell and you can see some condensation around the draft hood. Any suggestions. Note that I have more problems with downdraft on windy days.

I suggest you contact a pro ASAP.

Also, local codes need to be examined. My preference would be to vent the water heater separately with its own flue. Another possible solution is a vent damper, if allowed. This device is generally used on the atmospheric vented appliance. The damper is closed when the appliance is off and opens when on.

If you're to the point where you're seeing readings on the CO alarms you have an immediate deadly situation at hand. I would shut those appliances down and get a pro in ASAP! It's beyond DIY!

Note that the last two days, we had extremely high winds 50-70 mph. I spoke to a heating supply store and the salesman said that he even had downdraft coming in his fireplace due to the high winds. Since the high winds dissipated, my CO detector has had very low readings (max of 14, but usually 0). The salesman indicated that the configuration of the venting is ok, but stated that there is a vent cap that only allows air out and not back in. I also checked with the town and the manufacturer of the water heater. The town codes allow the merging of the vents and the manufacturer advised that although it is acceptable and legal for them to merge, they recommend a separate vent for the water heater. Has anyone ever heard of this type of vent cap?

There are some other caps out that you can put on a gas B vent pipe. They do help. Might also add a 3 ft piece of B vent on the top of the vent pipe there now. It will sure help
But lots of times I have found that over time a tree can grow up around the home and screw up the draft from a oil or gas vent pipe.

ED ;)