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?

I have exactly the same problem. I have no idea how the air gets into my hot water lines . I recenlty installed a new hot water tank and its a simple one for one.I never had this problem before but now each morning I have nasty burps at the faucet I first use . Its great for the rest of the day but only in the morning do I get the trapped sputtering air. I do have an existing Vacuum relief valve attached to the cold intake line to the Hot water tank and itys probably 25 to 30 years old there is no leakage but could this somehow be allowing air into my hot water tank.? I am also scratching my head.

You have to understand the physical properties of water on this one and while it is technically true that you have a leak, I doubt very seriously if it is a leak that requires any action on your part. Water reaches maximum density (8.34lbs/gallon)at 39degF and if heated or cooled from that temperature it expands. When heated from maximum density temp of 39degF water undergoes a fairly linear expansion up to about 10% by volume at the boiling temperature and when it flashes to steam it immediately expands another 1628 times by volume (1sq.in of water produces 1sq.ft of steam). The actual boiling temperature and rate of expansion is affected by the working pressure however I don't have the pressure temperature tables immediately at hand, and understanding that the working pressure can vary considerably from one location to another, for the sake of illustration allow us to use numbers that we are all familiar with. Water expands approximately 10% by volume from maximum density (39degF) to boiling temperature, which is 212degF at Standard Atmospheric Pressure) for a total differential of 173degF. Per code, we set our water heaters to 125degF therefore the temperature differential between cold water coming in at maximum density, again 39degF to the set point of 125 = 86degF. From this we can see that the differential between maximum density and the working temperature of our hot water system is approximately 50% of the total differential between maximum density and maximum expansion, therefore if we have a maximum expansion of 10% by volume, we could then say that the expansion in our hot water system is approximately 1/2 the maximum or 5% by volume. No let us put this all in perspective. If you have a 50gallon water heater and you fill it from a cold start with water at 39degF as the water heats up to the 125degF calling temperature the water will expand to approximately 55gal by volume. Under normal circumstances as the water expands the additional volume is compensated by water backflowing through the supply line to the municipal main or the well pressure tank. (This also explains why we are required to install an expansion tank near your water heater whenever we install a pressure reducing valve or a check valve on your water main.) Now in relation to the question at hand, how does air get in the lines? Under normal circumstances if we turn the water heater off the expanded hot water will slowly cool to room temperature thus the expanded hot water will contract. As the water contracts additional water will come in the supply line to maintain the total volume in the hot water system however, the post states that they turn the water heater off and close the house main water valve. Under these circumstances as the expanded hot water cools the supply has been closed off therefore as the hot water contracts it leaves a void in the pipes. We previously concluded that the amount of expansion was approximately 5gal by volume. A 1/2" pipe contains 1 gallon for every 98 linear feet of pipe and a 3/4" pipe contains 1 gallon for every 43.4 feet of pipe thus in all likelyhood 5gal by volume is greater than the total volume of all your hot water pipes in the structure. Now given that the water will contract and flow back to the water heater, the lines are then left with a vacuum and nature abhors a vacuum. Under these circumstances air will enter the pipes through microscopic pores or crevices or through faucet & valve mating surfaces which are no doubt to small to even pass a molecule of water. In this case it is enough to just understand what is happening and there is no further action required.

Wise questions and encourage you to carve out the proper space for the HPWH. Most experts are not going to be ok with the pantry-door-open approach though it would probably perform fine if you actually left it open all the time. For noise, expect something like loud fridge. Plenty loud for most people to keep the pantry door closed, which could lead to problems. Yes, air in a room will be cooler and drier with HPWH, the smaller the more noticeable. I think AirTap's model is the only one that can be ducted at this time and don't know anything else about that brand. A smaller closet could work with homework and good HVAC craftsmanship. Conditioned crawlspaces and basements are ideal locations for HPWH. I feel if topography is suitable for a crawlspace, then you are approaching headroom necessary for HPWH. You could maybe dig an extra couple feet to create a pit for the appliance but would ensure the depression has adequate daylight drainage provisions. The extra cost of this work could be balanced by skipping the dehumidification needed for most basements and crawlspaces but it still might not be enough to totally eliminate conditioning needs. Electric demand water heaters can make sense in some situations but I don't think 23' is going to warrant them. You might be better off finding a plumber well versed in smart distribution design. I think homeruns (and close-to) of 3/8" or 1/2" tubing would be better in most situations. For those still in design stage, consider making small plumbing footprints a top priority.