Storage capacity of electric tank heater matter for a radiant floor?

Seems to me it doesn't, and here's why.

I have a 30 gallon electric domestic water heater as the heat source for my heated slab. You can see the building and system description in "Radiant Slab Recent Experience" here http://www.garagejournal.com/forum/showthread.php?t=185015&highlight=jrfiero .

It takes about ten minutes to empty the initial 30 gallons of ~100* water, then an element stays on until the system is no longer calling for heat and the tank is back up to 100. It took 15+ hours to raise the air temp from 50 to 60 last weekend, so having 10 minutes of preheated water doesn’t seem to mean much. During most of the time it is acting as a point-of-use heater. So heat output in BTU or watts and the physical size would seem to be the determining factors in choosing a heater, not the storage capacity.

When the slab is around 50* and starting to heat, the heater puts mid-80s water into the slab, I assume running on the upper 3800W element. At some point when the return fluid gets warm enough so the bottom 5500 element again kicks in and the heater puts mid-90s fluid into the slab. These are assumptions about the elements, I just received two 240V indicator lights and a switch for the convertible bottom element, so I'll know more later.

Thoughts?

Jonas

I put my floor heat on a 2x day timer for about 1.5hrs. You may want to force the circulator on periodically.

You are exactly correct. In your application you have a tankless heater with a 3800 watt element, or a very low btu boiler. The tank is doing you no good but it is not supposed to and really, it is just a convenient way to get a boiler.

As you discovered, the wattage of the upper element is really your working number. I would switch the elements so that the higher wattage element is on top. You don't even need to hook up the lower element.

There are some stories of people hooking up the elements so that they both run when the stat call for heat, I wouldn't do it. 15 hours at 3800 watts to go from 50 to 60 is actually pretty good. It will be shorter with the higher watt element.

If you ever get tired of working with that tank heater you ought to consider a regular tankless electric heater from a company like rheem that is a 7000 watt boiler but gets by somehow with the same 10 gauge wire that you likely fed your tank heater with. It is cheap and hangs on the wall.

The volume of the tank is worthless. In fact, I am amused that people use 50 gallon tank heaters when a cheaper 30 gallon tank heater is available with the same size elements.

Highbeam, thanks for confirming my suspicions.

My heater has a lower element that is "convertible" from 3800W to 5500W by attaching a bus bar between two terminals.

I have a 30A 240V SPST toggle with which I'm going to replace the bus bar so I can easily change between the two. After I get used to the system I'll probably leave it on one or the other, but I'm still playing with it now.

I'm going to install an indicator lamp on each element so I can tell which one is operating when.

And then I might do the “redundant thermostat” rewiring like this http://waterheatertimer.org/How-to-w...ermostats.html so only the lower element runs. It still uses the upper thermostat, and does not run both elements at the same time.

Jonas

that's a great link, thanks .. i do this stuff for a living and i learned a thing or two from that site

the only benefit i can see from having a high capacity storage tank in an electrically 'fueled' radiant heating system is if you were heating huge amounts of water at off peak hours to garner savings from cheaper electricity prices .. such a set up would need to be thousands of gallons because as you point out your tank full of btu's are gone in no time and it takes many hours to recover the heat loss in your garage

in a gas fired application we'd use a storage tank to increase system volume and prevent short cycling, which is basically having the boiler experience frequent firing and cool down which shorten it's life and kill efficiency

Thanks for the great link on water heaters!

Cheers,
Tim