3 minutes is 6 gallons. Put a bucket on n there to capture if you don’t want to waste water. The energy to recirculate will cost more than the water.

You could install an on-demand recirculation pump so you're not wasting energy circulating hot water when it's not needed. When you want hot water you press a button to activate the pump. It's installed under the sink and it's loud but only runs for about a minute or less. It can be installed anytime even after construction is finished. Here's one example that I use in my home: link

Tankless water heaters are much less efficient than tanked heaters when running, but infinitely more efficient when not running (0 vs something). A 24/7 recirculating system will nibble you to death with a tankless heater.

What North Texan stated isn't really correct. Gas tankless heaters are more efficient than standard gas tanked heaters period.

A tankless heater will increase the amount of time it takes to get hot water by about 10-15 seconds.

If you want recirculation, install the return line now while things are open. Don't do it later.

Insulate all of your hot water lines, including the return. This is code in California, but it's worth saying for everyone else.

This tankless is getting expensive - a $2000 recirc line, plus the cost of the heater and install. Do you really want to do that? (and I say that as someone who likes tankless heaters)

A hybrid electric could work very well for you - and save you install costs.

Patricia Colwell is correct - in that a tankless heater can often mean that you can place the heater closer to the point of use, which can reduce wait time.

A recerc pump that keeps water in the line hot does waste energy. I'm not a big fan. An "on demand" recerc system can save water and not use much energy. But it's also not quite as convenient.

An on demand recirculating system?? More to investigate!

Absolutely love mine!!!!! You can set a schedule of when you want the recirculated to work. I set ours from 6:00am-10:00am and 6:00pm -10:00pm. It's one of the best things I've ever done!!!

Jake, you are the one that’s incorrect, possibly because you didn’t note my main point, number one below:

1) Minute by minute, a tankless water heater costs more to run than a tanked heater...but it only runs when water is demanded, hence the issue with a recirculating system that isn’t on-demand.

2) Generally, it is calculated that households that use more than 41 gallons of heated water a day would save money by using a tanked system. Households that use 41 gallons or less will generally save money (26-41% is the usual figure quoted), but....

3] There is a significant additional cost to first install a tankless water heater, and the break even point can be between 12 and 20 years for an electric tankless and 21-27 years for a gas model.

4) The average tanked heater needs replaced after between 10-15 years, while a tankless model can have some parts changed and last 20 years or more. Also, the replacement of a tankless heater is less expensive than the first install.

5) Electric tankless models are less expensive and more efficient than gas models, but the cost to run them is higher at current, and foreseeable, natural gas and electrical rates.

North Texan,

I'm sorry, but your point #1 is simply not correct.

A gas tankless is about 92 % efficient (condensing model) any time it is running. It is that efficient because it extracts enough heat from the flue gasses that it takes the gases below the point of condensing the water vapor - and thus even gets the latent heat in the water vapor. That's true any time it's running. It's not an average between "low efficiency and off" as you suggested. Condensing gas tankless water heaters extract so much heat that they vent with plastic vent. There are 82% non-condensing units that vent with stainless steel, but the vent is so expensive it makes more sense to use a condensing unit and vent with PVC.

BTW: Condensing gas furnaces work on exactly the same principle.

Standard gas tank heaters are about 60% efficient and can't get much better than that. The reason is that they must leave the flue gases hot enough to rise up through the atmospheric vent (hot gases rise). Atmospherically vented (standard) gas water heaters also have higher standby losses and air infiltration / exfiltration issues. There is a 3" flue up the middle of the tank that is open to the sky 24/7. They also take their combustion air from the conditioned air in the house and dump it outside requiring makeup air. Most gas tankless units get their combustion air from an outside intake.

I think that you are confusing the issue of input requirements for a tankless heater. A tankless heater (gas or electric) has to heat water very quickly - and thus the input requirements are massive (a typical gas tankless is 200k btu/h vs 40k btu/h for standard gas tank).

An electric water heater is almost 100% efficient already - and an electric tankless doesn't improve on that any. Electric tankless saves about 3% on standby losses.

I have no idea where you got point #2 - but since point #1 isn't right, Point #2 is also not right.

There is a significant cost to retrofit to gas tankless. In new construction the cost difference should be about $500 or less. I can't really comment on the payback because the install costs are so out of whack. In my view, atmospherically vented gas water heaters should be off the table - just as atmospherically vented gas furnaces are no longer available.

I would never recommend whole house electric tankless. The operating cost savings (3%) aren't enough to justify any of the additional service requirements. There is no discernible payback. They don't generally produce enough GPM either. There are much better solutions for homes without gas - such as hybrid heaters. Electric Point of Use units can make sense in some situations, but not whole house electric tankless.

Jake, you are talking about the unit efficiency, not the systemic efficiency. The cost savings, for a tankless unit, will seldom be over $100 a year, and the retrofit installation will usually cost at least $1000-1500 more than a replacement tanked unit Install. Add the premium of $300-700 for the unit, and you can see the payback takes time.

North Texan,

Thanks, but I understand exactly what I'm talking about - I don't need you to try to clarify that,

I also know that the payback for a retrofit - based only on gas savings over a tank unit - isn't really reasonable.

Nonetheless, if a tankless unit can avoid lining a masonry chimney - or can avoid significant rehab of a masonry chimney, or avoids a installing a two story metal flue, then the calculations all change.

If saving space is an issue or continuous hot water is an issue that can also change things.

If there is a basement or a garage - the electric hybrid units are pretty attractive and there are some significant rebates on them.

I have used tankless water heaters in our last 2 builds & will do so again in this next build. The difference being that I've used electric in the past but will be using natural gas this time around. I will also be placing it on the exterior of the home & not on the interior. Personally, I would not consider anything but tankless. I've lived with both and far prefer tankless over anything else.