Van, thanks for your very prompt reply. I have looked up the codes, and am still working my way through them. Right now, after reading the definition of a Category 1 device, I can't determine if my WH is in fact,a Category 1 device. It does, however, have a 4" vent, which is connected to a 5" (nominal diameter of the inside wall). From the furnace/WH to the outlet on the roof, everything is b vent. With the exception of the outdoor portion (about 4 ft on roof), the entire run in in unconditioned space. This includes the basement. Where the furnace & WH link up, there is about a 3 ft run to a vertical portion, followed by an upwardly pitched run of about 12 ft, then a final vertical portion through the roof. I estmate the entire length to be about 30 feet. If you consider the basement as conditioned, then it would be abou 1/2 unconditioned. Are you by any chance, a NJ contractor? V

Do not use 8" under any circumstances. NFPA 54 regulates the venting capacities for your Category I appliances. The tables, as pointed out are very specific as to length of vent connector, height of chimney and btu capacity of the system. You may actually be able to run 5" all the way. Is this LP or NG? What is the make of the furnace? That manuf should be able to provide you with information on vent recommendations. You will need the model numbers of the appliances, btu capacity of each. Distance from the appliance breach to the vertical chimney and the total height of the flue. Termination should be 3'above anything within 10'. You should use b-vent for your vent connector. It will really help the operation of the system. Look at Dura-vent or Selkirks site for information on their products. They are the two top manuf of B-vent.

Mike, Do some calculations just for grins and see if a tankless would work for you. A Rheem 74 will do 4.7 gpm at a 70 degree temp rise (35 degree inlet to 105 for a shower). There are some heaters that have a higher BTU input that will do a little more gpm if you need it. The Rheem is about $1200 with the vent kit and the valve kit. It would save you about 41% in fuel costs over a tank and you would get a tax credit. The only tankless units that had problems that I'm aware of were the Bosch units that used the water to power the ignition. Very lame. I think there were other people, particularly with electric units, that worked properly but weren't sized properly. If it were me, the choice would be between the normally aspirated sidewall vented (commonly called direct vent) tank heater or the tankless. The largest direct vent tank heater I normally see is 50 gal and they have crappy energy factors (in the high 50's). I think you will find them for around $700. The powervent costs about $1200 bucks for a 50 gal and are about as complicated as the tankless and about as noisy. I think the early models were more noisy that later ones. If you can purchase a tankless that will serve your needs for between $1200-1400, including the vent kit, and you have adequate gas service to the home, you will pay back the additional cost of the tankless over the direct vent tank heater in less than 5 years assuming you save $120 a year and take advantage of the $300 tax credit. The Rheem tankless has an energy factor of .82 while the Rheem direct vent 50 gal has an energy factor of .58 That's a 41% improvement and would easily net $10 a month difference in energy savings. If you go with the direct vent tank, it will pull air from the outside. If you go with a tankless, I would get the direct vent option so that you don't have to worry about venting the utility room (and letting the noise into the rest of the house). BTW: Page 12 and 13 of the link below do a good job of explaining the restrictions on side vents. Of course check with your inspector, but if you are looking for guidelines, these are pretty good. Here is a link that might be useful: Installation manual for tankless side venting

You are correct, when using the mathematical formula for determining whether the space is a confined or unconfined space we must first total the BTU's of all combustion devices in the space in question by example, a furnace will typically range from 50K to 200K BTU so for illustration let us assume 100K/BTU for the furnace plus 38K/BTU for the water heater for a combined total of 138K/BTU. In this case a confined space would be any room less than 138K/btu x 50Cu.ft/kBTU = 6900cu.ft. Again allowing an 8' ceiling we divide 6900/8 = 862.5sq.ft. This means any room with less than 862.5sq.ft would be considered a confined space. We could then use the square root of 862.5 to get an estimate of the required room size which would be 29.36' x 29.36'. From this illustration we can see that a typical unfinished full basement would meet the requirements of an unconfined space but if the furnace and water heater are then enclosed in a utility room within the basement that enclosure would probably be a confined space. The codes then offer numerous options for venting a confined space. If the total basement area would meet the dimension as an unconfined space we could then install vent grilles through the partitioning wall from the utility room to the remainder of the basement. Code would require two grilles, one within 12" of the ceiling and the second within 12" of the floor and each grille would be required to have 1sq.in. for each 1K BTU but not less than 100sq.in. In this example we would need grilles that provide 138sq.in. 11.7" x 11.7" therefore you could install two 12" x 12" grilles or you could install a louvered door between the utility room and the basement. Another option would be to install fresh air intake grilles through the outside wall. In this case you need 1sq.in for each 4K/BTU. 138K / 4 = 34.5sq.in. A 7" round duct has a cross sectional area of 38.48sq.in so one 7" round duct from the utility room to outside air would provide the necessary combustion air. There are a number of other options as well, such as venting combustion air from the attic space. While i offer these figures as an example it must also be understood that the actual code requirements can vary dramatically from one jurisdiction to another so you would need to check your local codes. If you are installing a power vent water heater most of them have a provision for directly ducting in the combustion air as well so you would not need to be concerned with the formulas for that type of water heater. I would also agree wholeheartedly with Pinoke. Some jurisdictions do not permit a homeowner to make any gas connections but if it is permitted in your jurisdiction a connection such as you propose would be a straight forward job and should not be beyond the ability of a skilled DIY'er. There are some code requirements that you must be aware of, but they are not at all difficult.