I had to run a new flue when I upgrade my furance to a 90% model. The combined 4" flue never did draft properly with just the WH pilot light. Replaced with 3" flue and it drafts properly now.

You say you have negative pressure because of the building code but don't have the required duct going into the vicinity of where a combustion burning appliance exists to break the negative pressure?

I do have a fresh air inlet duct that leads into the boiler room on the lower storey of the building. My house is over 30ft tall, and the chimney (exhaust ducting) doesn't get warm enough for the exhaust gas to rise up (updraft).

Q.

Service tech may of provided your fix , It is worth looking into to , there may be a mathematical formula to find out if a smaller diameter pipe would fix the venting. Seems they have formula for near everything to do with ducts and Hvac.

good stuff! I shall consider what servicetech has RE: calculations.

@servicetech: please advise.

Q.

Just looked over some old notes and paradoxically insulating the exhaust duct has a significant effect on venting. Metal pipe cools off quickly and, once cool, does not support temperature dependent draft.

Interesting that many high efficient water heaters require PVC as the vent pipe. Inherent insulation in PVC?

Steve

Also might look into shorting the run , more math I suspect.

GAMA venting tables
http://0323c7c.netsolhost.com/docs/gama%20venting.pdf

I'm not certified or licensed in handling gas appliances, but I figure the Gas Appliance Manufacturing Association and American Gas Association should have it right. ..I noticed the revision is from 1991 but I figure you would at least know if you are way oversized with the vent and would therefore possibly require a duct sleeve to be added.
Also be sure you have double-wall B-vent pipe and not a single wall pipe.

Hope this helps.

Sorry for the delay in posting pics.

Here's the deal:

http://i1167.photobucket.com/albums/...r/DSCN1189.jpg

http://i1167.photobucket.com/albums/...r/DSCN1190.jpg

http://i1167.photobucket.com/albums/...r/DSCN1192.jpg

fresh air inlet:

http://i1167.photobucket.com/albums/...r/DSCN1194.jpg

Notice: both the hot water tank and the hydronic boiler runs off of 4" dia and then into a 5" pipe into the B-vent that goes into the wall..

Q.

Those elbows and reducers are ridiculous .. in my opinion it should of been connected with more care to air flow, elbows and reducers by their nature cause large amounts of restriction.

If it was piped by a *talented* person you would not see such a contraption of fittings.

talented workers are far and few between , even when safety is a factor you often end up with monkeys doing the Job..

quote:
Every PVC fitting has an airflow resistance that is approximately equal to the footage given in the table below. For example each 4" 90 degree sweep elbow adds the equivalent of 6 feet of straight pipe resistance while a 4" 90 degree sharp edged elbow adds 15 feet of equivalent straight pipe resistance

Pipe size restriction

4 inch Reducer cause the the vent to perform as if it is 30 ft longer
6 inch reducer causes the pipe to perform as if it is 60 ft longer


How to measure airflow in PVC piping requires careful measuring of airflow with a pitot tube or calibrtated anenometer to determine CFM