Water Heaters - Tankless as a pre-heater/post-pre-heater

I have radiant floor heating in my house, I share a 50 gal. natual gas water heater for both heating and domestic use. The 50 gal. heater is a standard off the shelf. What I am wanting to do id see about puting a tankless heater to pre-heat the water on the return back into the 50 gal. heater.

Has anyone heard or have done this. If so, should I get a whole house or a smaller tankless heater, since it will only be pre-heating the return from the floor...

My house is 1450 sq. ft.

also any suggestions on a model and brand of tankless heater...

beatnik, Welcome to the DIY Forums.
Personal opinion: I would get another standard water heater.(it would not have to be another 50 gallon one) You didn't say if you had priced tankless heaters. They are quite expensive and not all plumbers will work on them. This means that if you have a problem later, you will pay for a "specialist" to correct it. You will also have to check the gas supply piping to make sure it is sized correctly for any additional usage. Otherwise you could be starving one or both of the heaters which is not a good idea and also dangerous.
Good luck and watch this post for other opinions/answers.

I have looked arougn an it seem that a small gas water heater is close to the price of a point of use tankless...

Bosch AquaStar 125HX NG: $599
Bosch AquaStar 125B NG : $497

any suggestion an a small natural gas water heater,

Only thing I can recommend is to check the energy efficiency stickers on different brand water heaters and the warranties that come with them. Some cover the tank, some the parts, some the parts and labor. The prices in your post seem high to me. I KNOW I can buy a good 40 gallon water heater for less than those prices and you may not need one that big for what you need to do. I am partial to the name brands that have been around a while. Many are made by the same manufacturer but sold under different names.

Are you wanting to add the pre-heater because you are running out of domestic hot water?

Jon

yes and no, it depends on how cold the weather. an example would be last night, washing some dishes and did not have hot water, just had warm. the pump for the floor was running and the water temp at the pump was about 100 F. The shower this morning was warm but verry comfortable. I am also looking into trying to reduce my heating bill, but do not have a lot to spend...

I have came accross the Bosch AquaStar 125BL and AquaStar 125BS which looks like they are designed for what I want to do. Looks like they are designed to take preheated water. the 125BS is designed for solar preheated water and the 125BL is designed for circulating and reheating water from a storage tank. They are between $500 and $600.

here are some exaple:
http://www.houseneeds.com/shop/HeatingProducts/WaterHeating/AquaStar/AquaStarApplications/125BSsolarApp.htm

http://www.houseneeds.com/shop/HeatingProducts/WaterHeating/AquaStar/AquaStarApplications/IndirectWaterHeating125BL.htm

I was also looking at american waterheater's Polaris, but the price jumped to $2900 (was about $1400), way out of my range. But, I heard lennex is trying to buy it....

If it was my house . . . ( I am heating the basement floor and bathroom floors with radiant heat off of my 60 gallon electric hot water heater tank)

I would buy the most efficent hot water tank as possible, place it in series after the hot water tank you have now. Continue to use the first tank for the floor heat. This way the radiant heat will not use up all of the hot water available and leave you with lots of domestic hot water from the second tank.

Or, you can set it up to let the radiant floor heat circulate through both tanks and pull your domestic hot water from the second tank. Then set up the piping so that if one tanks ever fails, you could easily by-pass it and still have hot water for you radiant use.

Good luck

Jon

forgot to mention i have a spacing factor. I have a loft type house, the bottom floor is concrete with the radiant heat, kitchen, living-room, dinning room; second floor two bedrooms with a bathroom between them; and a third mechanical room with the ac unit and water heater which is above the upper bathroom. I already have a 5O gallon tank and adding another tank makes uncomfortable, not to mention space....

Since space is the limiting factor, place the tankless hot heater on the output of the hot water tank. Then go and enjoy a hot shower. :)

Jon

the only problem I see with putting it on the output is the water might be to hot and not cold enough to make the tankless turn on and it only heats the shower and domestic.. this is why I was thinking about the return from the floor and reheat the used water as it returns back into the tank. it would be colder and will keep a nice hot amount of hot water for both domestic and floor.