We have them and are very happy. (Rennai)

We hooked ours into our geothermal system which preheats the hot water when it's running. So the heaters use less gas to heat. (Don't think about electric ones as they aren't fast enough at heating water.)

We have a wide ranch house so we ended up putting in 2...one on the east side of the house to run the laundry, PR, guest bath and kitchen...and one on the west side to run the master bath. We wanted energy efficiency so didn't want a recirculating loop to keep the water hot 24/7 for the master bath which is only used for maybe 30 min a day, but didn't want to wait forever for the hot water to get to the tap. So we put in a tankless to serve the bath so it only comes on as demanded, the hot water gets there faster and I don't have to worry if the laundry is running or anything else when I want to take a shower...no hot water interruptions.

Not an issue, but quirks given how they work, if there is more hot water demanded than the unit can supply, it will reduce the volume of output, but maintain temperature. The units are not happy with "trickle" volume demand as it doesn't demand enough to make it come on. So in the old house, I would trickle in hot water to keep my bath tub warm when bathing. With this unit, I have to turn the water on and off to add hot water as needed.

We have one on each side (exterior) of our house, close to the points of use. They are Rinnais. They have been installed since our major remodel/rebuild in 2006. We are very happy with them, they take up no real estate, are trouble free, and supply endless hot water whether there are just the two of us or a house full of guests. We have very hard water so we have 'descaled' ours twice since we installed them just to keep them in peak condition otherwise they have performed effortlessly.
If we ever build again we will certainly choose tankless.

I would suggest researching the new Heat Pump Water Heaters as a better alternative to tankless for many situations. Tankless can be a good choice for the right situation but dont be fooled into thinking you will save energy. Several studies including the one below find that even Natural gas units are not cost effective.

Designing a house with a small plumbing footprint is one of the most cost effective ways of conserving water and energy.

Here is a link that might be useful: Universtiy of MN study finding Tankless are NOT cost effective

Thanks for the reply's!

We have two gas-powered Rennai tankless heaters and mostly love them. It is nice when I have a dozen people visiting that everyone can shower and bathe one right after the other, using all three bathrooms, and never worry about running out of hot water.

However, you should be aware that one problem with having tankless water heaters is that many of the newer water-saver appliances (like dishwashers and washing machines) use so little water that they often don't pull enough flow to start the tankless heaters going.

My water miser dishwasher, for example, is located fairly close to my tankless heater but there is still about 25 to 30 feet or so of water line between the two, plus it takes a few seconds once water starts flowing for the heater to kick in. That means, when I first turn the hot water tap on at the sink, close to a gallon of water has to flow out before I get HOT water. So, if I just run my dishwasher all by itself, that gallon of tepid water has to go into the dishwasher before any HOT water enters it. Problem is, according to my dishwasher specs, it uses less than 6 gallons of water TOTAL for the pre-rinse, wash, and final rinse cycles. So, I figure it probably uses maybe 2.5 gallons each for the wash and final rinse cycles... which means that at least 20% of the water entering the dishwasher is tepid before any hot water flows in. Worse tho is that the DW doesn't seem to pull all 2.5 gallons at once. Instead, it sounds like the water flow cuts on and off several times during the cycle with several minutes between each burst of water spray coming into the dishwasher. When the water flow cuts on/off quickly, the tankless water heater either never cuts on at all or cuts back off too quickly to deliver any hot water.

Needless to say, our dishwasher was NOT getting our dishes at all clean and I thought we had gotten a bad dishwasher. I was going to have it taken out and returned but then had a bunch of family members visit right after we moved in. First day they were here, several of them happened to take showers in the evening while I was running the dishwasher. VOILA, nice clean dishes! So I figured out that the problem had to do with the dishwasher and my tankless water heater not playing nicely together!

Eventually we intend to install a small 2 gallon electric water tank under the sink and put it on a timer so that it only operates for a few hours each evening. Just long enough to supply hot water for the dishwasher after dinner. We haven't gotten around to doing that yet.

So for now, I wait to run the DW in the mornings while DH and I get our showers. That seems to keep enough water flowing thru the tankless water heater for a long enough period of time that my dishes are clean when I go to pull them out of the dishwasher. Still, it is kind of a PITA to have to run downstairs, run enough water at the sink to get hot water going, start the dishwasher, and then run back upstairs to shower... and then make DH go ahead and get up and get his shower - even when he'd rather sleep in - just to keep the hot water flowing to the dishwasher. Maybe I should make getting that small electric tank installed a higher priority. LOL!

We shut off the hot water supply to our front loading washing machine. The washer uses such a small amount of water that it is more economical for it to heat the water rather than drawing through the tankless heater. Our Bosch dishwashers also use very little water and are designed to draw only cold water so there is no problem there. I didn't know that some dishwashers draw hot water rather than heat it fully themselves. If your dishwasher is intended to pull from the hot water system, bevangel, the simple solution would be to disconnect the hot water feed so that it draws only cold. Problem solved!

Unfortunately, my dishwasher is NOT designed to draw only cold water. In fact, the specs say that one should make sure that water supplied to the dishwasher is at least 120 degrees. Although the dishwasher does have a supplemental heater to boost the water temp on up to over 145, it definitely will not heat room temperature or tepid water up enough to clean the dishes.

Bevangel - have you considered that a new dishwasher is a cheaper option both in ongoing costs and install. A hot water heater requires a new circuit so install is expensive.

The ongoing costs aren't much of course but also you have to consider would a 2 gallon unit be enough? The recovery time would need to be better than the cycle time on the dishwasher - easy enough to check.

Tankless water heaters are good except for the draw and delay issues. People install 2 because of the delay issues and then you could have installed solar with a recirc loop and have more instant hot water and lower ongoing costs.

We paid $6k for solar that was down to $1.5k with incentives. Factor in $800 for gas tanked unit + install and the difference was only $700. The recirc loop was about $1000. Compare to 2 tankless units with install and it was probably cheaper.

I am very fussy about hot water and would never rely on solar for hot water, esp in our area where, eg, winter of 2011, our PV panels didn't work for nearly 8 weeks as they were buried under 3' or more of snow. However, by having the geothermal unit preheat the hot water in winter, and use the heat from the A/C to heat the water in the summer does save us money and is more reliable.

Cousin is in the HVAC business in Mass. and he switched to tankless and said his hot water costs dropped by about 50%.

I took a quick look at the study, and a lot depends on your relative energy costs. For example, they used an electricity cost of 6 cents/kwh...we are in the highest electricity costs in the country at about 22 cents/kwh so the cost savings would be significantly more dramatic than in the study referenced.

Annie - higher electricity costs make tankless look worse since tankless use more electricity than a tanked unit. They both are heating with gas but a tankless uses electricity for freeze protection and ignition. So increasing the electricity cost only makes tankless look worse.

NG rates are much more uniform across the country.

Solar always has a backup so snow is not a problem. Also solar thermal panels can operate in partial sun conditions unlike a PV panel. They work pretty well being partially shaded by trees when a PV would shut down. I've had my backup off for months but I wouldn't get to do that in the winter. That being said, a full coat of snow would shut them down - it would be possible to melt the snow pretty easily by running the coolant through the panel but I'm not sure that is done.

I can see that if the heaters needed electricity for freeze protection (in which case that would be a real, "Yeah, why bother?") but ours are in the basement and need no such thing. The electricity used for ignition is minimal. Also, in our area, we are on LP...no nat gas available.

So this really points out that each region and situation is unique and therefore one needs to consider all the factors that affect their specific situation (alternative energies, climate zones, subsidies, etc.) rather than rely on a single study.

One study not good enough? Though less scientific, Consumer Reports research and lab tests indicate that Tankless water heaters are not a cost-effective investment.

LP (propane) is one of the most expensive fuels for water heating and is rarely a good choice. Basements tend to be great for heat pump water heaters in most US climates.

Here is a link that might be useful: Consumer reports says tankless not worthwhile investment

Now I suppose if you have to use LP, the payoff is pretty quick but the right answer would be not to use LP.

Annie - don't you have geothermal? Why not use that or do you just use the tankless to boost the temps?

We do have geothermal with a desuperheater and use the tankless to boost temps in the winter, or for the spring and the fall when the geothermal is turned off....during the summer we only have it on for AC as necessary, but of course that heats the hot water too.

It's hard to parse our LP costs because we also use it to run our 20kw generator which runs 20 min self tests every 2 weeks and we've had to use it quite a bit over the last year given storms and power outages (6 days one storm, 3 days another, a few short-term outages in between). We also use our LP for our gas fireplace, but we don't do that too often...instead we usually hang out by the wood stove.

And the stinky part of it all is that we can't use solar to generate power when the power is off...

Ah, I see. (Confess I didn't have the time or patience to pour through the real study, but this summary does it nicely.)

They include the much higher installation costs for a retrofit from tanked to tankless. We were building new so were able to plan and accommodate gas hook ups and venting as part of the building plan vs. trying to figure it out in an existing structure.

Also, I would want to compare the cost to a tanked system with a hot water recirculator as that's what our alternative would've been in order to get hot water to the master in any reasonable amount of time.

Here is a link that might be useful: Are tankless a waste of money?

Very happy to find this thread.

We have a Rinnai on tankless water heater and have been very happy with it. But going to be installing a geothermal system soon for heating and cooling the house.

Annie - does your geothermal system feed hot water into a "holding" tank" before it goes to the tankless water heater ?