I am currently working on a small 720 SF home with a peak heating load of ~18,000 BTUs/hr, and I am wondering if anyone has ever combined a electric tankless (eemax ex280 28.5kw) for DHW with radiant heating in concrete floors (highly insulated slab). Please see attachment for System Diagram that I recieved from our installer. It currently uses 6 gallon electric tank to connect the radiant slab with the tankless unit. Do I have to worry about the radiant floors overheating since they usually operate at 100 degree F? Will this burn up the elements if used for extended periods of time or limit longevity of unit? I am wondering if there are other "better" options or ways to configure the unit. The space will have a 200 amp service, and will loaded to 191 amps according to NEC 220.82. Should I be worried about tripping the main breaker? Note, with such a small space, we want to avoid large storage tanks and minimize the mechanical equipment. Let me know. Thanks!!

18KBTU/hr is only 5.5 kw- you won't be overtaxing a 28kw tankless maintaining the slab at temp.

Slabs are self-buffering- it's a lot of thermal mass, so you don't really need a tank to keep the system from short-cycling. Are you using the 6 gallon tank to mix down to 100F or ???

Pumping 100F water directly into the slab won't hurt it. Control the pump with a zone relay and a PID-algorithm thermostat and you'll be fine- the slab won't over heat. I'm not sure you can always get over 25btu/ft^2 out of a slab at 100F though (18k/720'=25BTU/ft). Depending on floor covering, closets, appliances, furniture & everything else you may be looking at 30btu/ft+. I suspect you're really looking at 120-130F water on design day.

The real issue is the lack of heat exchangers- don't run potable water in the radiant, since it'll spend significant time stagnating at legionella temps. For the cost of a heat exchanger and another pump you rather avoid the situation. Design your flows to deliver your design-day heat load with minimal overcapacity, and let the thermostat decide when and how long to run it. Run times will be long, but that means fewer/longer cycles (=less thermal cycling of the tankless)- it'll last longer that way. The return water on design day will likely be ~80F or more, so if you're looking at a 20 degree rise in the heat exchanger to deliver 100F out, that's ~1.8gpm to deliver 18KBTU/hr @ 100F. You could also run the radiation side at 2gpm & lower delta-T if the heat seems uneven, which it might, depending on how the tubing is layed out. (10-20F delta-Ts are typical in radiant systems.)

But I'm just a hack at this- why not post it to the radiant heating forum?

http://www.greenbuildingtalk.com/Forums/tabid/53/view/topics/forumid/12/Default.aspx

Dana1. Thanks for the quick reply and the reference the the radiant heating forum, where I will repost the data. I do agree that a heat exchanger should be used to aviod running potable water in the radiant, and this diagram does not specify that. Furthermore, I do understand that the tankless unit has more than enough output to heat such a small space; however, I want to make sure that we do not burn out the tankless elements. My understanding is that tankless are meant to have cold running water into them "cooling them off." When we cycle hot water 100+ degrees on the return side for long periods of time, could this cause harm to the tankless unit?

PS. The tank is placed to cool the water down to a reasonable temp... possible do the same thing with a mixing valve?

Use some thing like this http://www.titanheater.com/proddetail.php?prod=SCR2-N120 For your infloor heat. I use these units in smaller homes and work realy good and cheap to run. I take it your on off peak?
It has a temp dial on it so you can set it for 100F or what ever temp your looking for. I Dont go over a 108F