I think my vent is 4", I have a Bryant Plus-80 furnace, and I just replaced my hw heater, (I'm in BC Canada so YMMV) - the flue hat for the hw heater is sized for the vent that's there, I hadn't thought of that aspect - if I replace my furnace with a HE one - since I just shelled out on the new hw I wouldn't want to change it just yet (but I guess I would run the plumbing for a he hw heater when I do the furnace- Both furnace and hw heater Y into a single vent of course, if it was hw only, would there be a reducer that runs off the flue hat? I could certainly see a difference if it was only one appliance using the vent, but I've never seen any condensation coming back down....I wonder if OP's issue might also be some negative pressure....which could be dangerous. jmnoelle13, can you try a test for us? Get a stubby candle like a tealight, and place it right next to the flue hat on top of your hw heater, when the heater's at work, and see if the flame of the candle is drawn towards the flue hat? If it is, all is well in terms of airflow, if not, your system or parts of the ventilation system in your house is drawing combustion products back down the vent, which is very dangerous. Your new furnace should have two side vents, one for exhaust, one for combustion air. You probably didn't have direct combustion air feed before, so you should be better off in that regard. As for reducing the vent size, can you drop a sleeve down inside the vent from above?

Thank you both so much for sharing your knowledge. Randy427: I would have thought our units were enough to run only two hot water outlets at the same time but the Rinnai RL94 significantly reduces hot water flow from the sink when the HE washing machine is running. The Noritz significantly reduces hot water flow from the shower when the bathroom sink faucet is turned on. Aliceinwonderland: We are in Southern California and I have wondered about scale deposits but it seems odd that it would begin affecting both water heaters at exactly the same time. The 3 year old Rinnai manual says that it does not need to be flushed if using a scale inhibitor. The 10 year old Noritz was installed before we owned the home and doesn't have a valve to flush (not sure why not). We had a plumber come to look at it about a year ago and he said his maintenance procedure was just to clean out the dust and dirt that might have accumulated under the cover.

skyclad- Let me again point out that a Pressure Reduction Valve (PRV) should not be buried and the city telling you that lines would have to be dug up sounds entirely bogus. PRV's do not last forever because they have an internal diaphragm that needs to be refurbished periodically (typically every 10 years or so). There is also an adjustment screw that you have to set to get the correct pressure, and sometimes you need to reset it if the supply pressure changes significantly. I'm not saying it's impossible that someone buried it -- in the concrete or behind some sheetrock -- but only an imbecile would do so. You may have a PRV somewhere and just be unaware of it. Or there might not be one, and you will need to have one installed. Finally, when the employee told you 130 PSI was "normal," all that means is that could be the normal supply pressure, not that it would be ok to have that pressure inside the house. Regardless of whether it's 150, or 130 or even 100 psi, as Jake said, you've got to get it adjusted down to no more than 80 psi.

@Karen Williams This is the control panel on my tankless water heater. Water Set Point ... the output temperature the unit feeds into the hot faucets ... can be set in 1°F increments between 50°F and 140°F. Temp Up and Temp Down buttons adjust the Water Set Point temperature. Then press Temp Set to make it the normal at-all-times temperature if desired ... otherwise the adjustment is temporary and will revert back after a number of minutes. I have 103°F as the "normal" temperature and raise it when needed for washing dishes or clothes, without pressing Temp Set, and it reverts back to 103°F after 30 minutes so I don't have to remember to do that. That's just how my unit works, they're not all the same. Setting the temperature "at the faucet" means I set the water heater to the temperature of heated water I want/need "at the faucet" ... such as 103°F for a shower, instead of 120°F or 135°F which is much hotter than needed for a shower ... and I turn on only the hot faucet (or set a single-lever faucet to full-hot with no cold mixed-in). Thus, the tankless unit has a high-enough flow rate passing through it for the duration of the shower to insure it doesn't shut off due to too-low flow. 120°F, 135°F, 140°F output temperature requires mixing in cold water and turning down the hot flow at the faucet to get a comfortable shower, which reduces the flow through tankless possibly below the required rate which can cause it to shut off completely and the shower goes cold. Remote control panel means exactly that ... some tankless water heaters can have an optional extra temperature control panel installed at a convenient location such as bathroom or kitchen so the homeowner doesn't have to go down to a basement or out to a garage or wherever the heater is located to adjust the temperature. My heater doesn't have a remote control but it's located in a broom closet in my laundry room behind the kitchen near the center of the house so is easy to access any time I want to change the setting.