A few over generalizations here like tankless are not going to be efficient. Just saw an efficient unit on This Old House that was vented with PVC pipe (very efficient) and would handle about 15 GPM so they are out there. I didn't see the brand but this is going to be expensive. Not your average tankless unit. You really have to look at your particular situation and needs when looking at units. I think the claim that tankless is more efficient without looking at other factors is bogus too.
One problem with a lot of tankless units is the limited flow rate, especially with low incoming water temp as noted before.
I installed a large condensing, tank type water heater to supply heat for radiant floor and large shower. PVC for air supply from outside and flue vent. I plan to interlock the floor control so it doesn't come on during showers - it pulls the temp down some and affects SAF. Otherwise it is great.
I also installed a drain line heat recovery device. Interesting unit with no moving parts. The drain line from the first floor goes through the basement right by the water heater so plumbing was easy. It goes in the drain line and has copper tubes wrapped around the drain pipe with water from the street going to the water heater. The water is preheated by the drain water. They claim up to 60% recovery and it does recover a lot of heat. I got mine just before copper prices started shooting up about 8 years ago and they have switched distribution but they look to still be available:
http://gfxtechnology.com/
I have no connection with the company other than buying one of their units and being happy with it.
One problem with a lot of tankless units is the limited flow rate, especially with low incoming water temp as noted before.
I installed a large condensing, tank type water heater to supply heat for radiant floor and large shower. PVC for air supply from outside and flue vent. I plan to interlock the floor control so it doesn't come on during showers - it pulls the temp down some and affects SAF. Otherwise it is great.
I also installed a drain line heat recovery device. Interesting unit with no moving parts. The drain line from the first floor goes through the basement right by the water heater so plumbing was easy. It goes in the drain line and has copper tubes wrapped around the drain pipe with water from the street going to the water heater. The water is preheated by the drain water. They claim up to 60% recovery and it does recover a lot of heat. I got mine just before copper prices started shooting up about 8 years ago and they have switched distribution but they look to still be available:
http://gfxtechnology.com/
I have no connection with the company other than buying one of their units and being happy with it.