Two things to remember about the TINY heat pump on top of the hot water heater. It only removes heat from the room to replace the BTUs that are removed from the space via hot water going out the pipe. Example. You go on vacation for a week. ZERO net heat is removed from the room. In other words, heat transfers from the stagnate hot water back into the room and then the heater kicks in and removes it from the space and transfers it back into the water... not free, but very inexpensive and absolutely no need for the home's heating plant to make up any heat - as I said, no NET heat loss or gain. Now take the case where you take a hot shower upstairs. ANY heat removed from the basement by the hot water piping remains inside the conditioned household envelope except that which exits via the plumbing drain in the shower. Also, presumably cold replacement water from the street is brought into the envelope to replace that drained water. I have NEVER seen a change in my basement temperature due to the heat pump hot water heater because my subterranean basement is simply a massive heat sink that resists temperature change big time. While I do remove like a pint of water from the air per day in the summer, I have never been able to measure a change in humidity - just the nuisance of having to deal with disposing of the condensate. You're creating an apples and oranges situation when you compare heating with gas and "pumping heat" from the room into the water. With your gas heater, the heat that 'flows' back into the room is not recycled back into the hot water heater, rather it is "replenished" by bring NG into the room and igniting it in a non-recoverable manner. The net effect is to heat the room while you are on vacation... as I suggested there is ZERO net effect with the heat pump hot water heater in my example. I have documented in this forum that my electrical savings for my family of two is approximately $40 per month due to the hot water heater as compared to the electric hot water heater I previously owned. I also have tried, with mixed result, to explain that taking the basement as a system and drawing a diagram show heat in and heat out of that system is helpful. Too many folks that have never owned one of these and have made false assumptions have reached conclusions that are absolute nonsense. My unit is a GE model... I had a compressor leak after 11 months that was repaired under warranty. Reportedly this problem was not an isolated case... and the Model was discontinued. If you have the time, the final math problem (actually rather simple) is to assume some water usage and calculate the enthalpy rise in the heater times the gallons and determine how much heat is being removed from the basement... that is the amount of heat that your heat pump would need to replace in the basement to maintain equilibrium. If I were the OP, I suspect I would opt for the Gas heater because the response time is faster, because it is quieter, and because he will not have to mess with disposing of a pint of water every day during the summer. But, I suspect that you need to caution the OP that his heater is heating the inside of the house during cooling season. :-) I played loose with my terminology re Heat Transfer in the above in order to attempt to better communicate the concepts - by background and education I am capable of using proper terms and terminology, but choose this approach.

"Does the Navien have a bypass? That's good info to know. I hadn't run into the min flow issues until they were reported in the plumbing forum - I don't live where water inlet temps would ever get to 80*. You are about the condensing units having EF's in the .90's" Jake, The Navien has no mixing bypass in the unit like the other flash units that superheat the water in the exchanger to 170-190 and then remix water downstream for temp control. That is why the copper heat exchangers with the intense heat are subject to liming. So they do have difficuties firing and staying on in minimum fire when the cold mix water is to high in temp. The Navien has 100% flow through the #436 stainless steel heat exchangers. Since the unit's heat exchanger is capable of the flow, erosion and due to velocity, along with lower operating temps it is not susceptible to liming & calcification. The pitfall of the design is a slightly wider temp fluctuation than the Jap platform heaters, but this is not noticed so much in the "A" model that buffers that with the internal tank. IMO, the "A" model is the only way to go for the approx 125-150 cost upgrade. The lack of mixing valve downstream, makes the Navien a perfect heater to add in-line to the output of a solar tank to raise the water temp only when tank is low or depleted due to usage or lack of solar input. Rinnai has just released some info on their condensing water heater, 95%. Pitfalls are still polypropylene concentric venting and a Nicaloy primary heat exchanger which to me tells me both the install and unit cost will remain high. Only one left now without a unit above 90% is the Paloma/Rheem/Ruud line.

Wow... "power direct vent" was a new one for me. I had to look it up. It looks like a power vent (uses fan to exhaust) with a coaxial outside air intake to draw combustion air from outside. Of the two, I would use a power direct vent since it's getting air from outside instead of inside the heated space. The answer to your question about hot water if power goes out is "no". The odd thing is that the efficiency ratings aren't great (.67). Consider a condensing gas tankless for the same amount of money and .92 energy efficiency.

Powervent water heaters do not have a dedicated air inlet . They still require free air for combustion. The difference is, they dont have a draft hood. If your house is tight or in a negative, a conventional vent water heater will not vent properly, and the products of combustion could spill out the draft hood. A way to combat this is by bringing fresh air in from outside .ether through an hrv or a duct. The other option is direct vent (2 pipe, brings air from outside .) And the price sounds high, but navien is a very cheap brand and they also come with a lot of different options. My navien has a 6 L tank with an internal recirc pump. I also needed to run a new gas line as well as upsize my water supplies to accomodate. I dont quote retro fits, but navien installs usually come out to between 2800 - 3000. They are time consuming . Also consider hidden costs . Mark up to cover warrenty . Valve kits. Copper. T and p. Gas line. Set up. 636 vent . Drilling through the house. Mounting it. And following every step of a 75 page install manual. As not to voide the warranty. The time and fuel to get the unit and bring it to your house. Also the first $30 of my hourly rate goes to operating costs and government agencies. You get what you pay for. Make sure your installer is tssa certified. Has a gas licence. Plumbing licence and insurance. There is an insane amount of misinformation on this forum. I am a licensed plumber, gas fitter and back flow tester . As well as a small business owner, if you told me to do a job for x dollars , i would tell you to have a nice day. Be more respectful of professional trades people. Not everyone is a crook . And if someone quotes a job low, they will cut every corner they can. Hence why i would never buy a new house.