This topic has been covered A LOT on the Plumbing Forum. Do a search on that Forum for "gas tankless heater" and you will be reading threads for hours. But in the meanwhile, I'll give you a little summary: the top manufacturer is Rinnai, with pricing to match. There are other good tankless manufacturers also, you'll see them discussed on the Plumbing Forum. Here are pros and cons of tankless: Cons: - you will need to install a larger gas line - you will need to install special venting specific to a tankless - you will need to make sure that whoever installs it, REALLY knows what they are doing. A lot of people are saying they can install them, but don't really have the expertise. Rinnai requires its installers to go through a training program. - upfront costs to purchase and install a tankless are significantly higher than traditional tank. - it is important to know what climate you live in for this decision. If a cold climate, your incoming water temperature in winter (about 40 degrees for me) will require a significant amount of gas to heat the water "instantly". If you live in a mild climate, this is not an issue. Some argue that a tank heater, meanwhile, is heating the water 24/7 which is a waste. However, today the tanks are so well-insulated and efficient, this is not as much of an issue. Pros: - very small footprint. - Endless hot water. You will not run out of hot water the way you might with a tank heater. For many people, this pro outweighs ALL the above-listed cons. Here is a link that might be useful: Gardenweb's Plumbing Forum

"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, thanks for all the good input. I didn't intend it to become a debate about solar, but found that interesting as well. I'm not familiar with the lease program that is mentioned. I'm looking to have an array installed that will be owned completely by me. I can get a 20kw system installed for about $60,000. There is a new Duke Energy program offering $1 rebate for each kw installed, so subtract $20,000. Then there are 55% state and federal tax credits, reducing the investment further to $18,000. The system should generate about $2,500 in energy per year (assuming 9 cents per kwh), resulting in about a 7 year payback. There is also potentially a benefit I can derive from depreciating the asset, but am not sure that is allowed, so need to check with my tax expert. I think the main different between now and before is the $1 Duke rebate. Without that, I agree the payback period would be too long (about 11 years), but a 7 year payback seems reasonable to me. There is always the possibility of repair costs in the first 7 years which could lengthen the period a bit, but the warranties on the panels at least are much longer than that. In any case, I think it is a worth a try. With regard to the water heater question, I agree that two heat pump units might be too many, as the one in the basement might be idle for long times, so am not considering doing 1 heat pump (for the upstairs which will get used daily, but shouldn't use more than 80 gallons per hour as there is only 1 shower), and then a gas powered tankless for the basement. We have natural gas that will be already piped to the house for the stove/grille. Any other thoughts or suggestions are welcome, thanks for the lively debate!

riellebee So I'm going to rough-in some numbers: Range: 15k btu/h Fireplace: 80k btu/h Furnace: 75k btu/h (x2) = 150kbtu/h water heater 200k btu/h (x2) = 400k btu/h Total potential load: 645k btu/h - again, an estimate. You can check your furnaces if you want to be be sure. is it likely that you would have enough things on at the same time to exceed your 425k btu/h service? - Yes, it's quite likely. What happens when you exceed the capacity of the service? The gas pressure drops and the range and the gas logs will just not run at full capacity - no big deal. But the things like the furnace and water heaters will throw error codes showing low gas pressure. The water heaters are likely to have that problem anyway, because at 30' of 3/4" pipe they are only getting about enough gas to run one of those heaters full tilt (the water heaters are variable input). The meter may be 20' away, but in actual length of pipe it's probably close to 30' including elbows. I can see close to 10' of pipe in the photo. The fact that the return line for the circulation isn't warm suggests it's not working. Fix that and insulate your pipes and your immediate problem will probably go away. BTW: Insulating the pipes is not expensive or difficult. Foam pipe insulation comes in 4' sections that just snaps around the pipes and uses self adhesive strips to hold it closed. It cuts with scissors. Even if you just did the exposed pipes in the basement you would reduce a lot of heat loss. My guess is that the builder can't really fix the "over-subscribed" gas service at a reasonable cost. One solution is to use a different water heater. A hybrid tank/tankless would solve the problem. The link goes to an AO Smith Vertex 75 gal water heater 100k btu/h and is 92% efficient and vents with the PVC vent pipe you have. I'm not particularly endorsing that brand - just using it as an example. It would provide you with plenty of hot water (you could run 1 shower continuously) and would reduce your BTU load by 300k BTU/h (gas service problem AND undersized gas pipe to water heater solved). In addition, because it's a tank of hot water - along with about 6' of hot water in the pipe - your wait would be significantly reduced even without a circulation pump. It sucks for your builder because it would cost them about $3500-4000 to fix it. They could probably realize some salvage value from the tankless heaters - say $1000-1500. But If they are only out $2-3k total I'd say they should consider themselves lucky.