Most cost efficient water temperature.

Benheating

Hello Heating Helpers.
In 2014 I installed a simple single zone slab radiant system installed in my new garage slab. about 1000 sq ft slab. Slab has under-slab and perimeter insulation. Garage is insulated well, located in Michigan. I heat the water using a 40gal natural gas tank water heater that has a 40,000 btu burner. I normally keep the garage at 55 deg. through the winter, and the system can maintain that adequately. ( a heat load calculation was performed during the design phase, and was determined to need 29,000 btu) This is a "Closed" system, and the water heater only supplies heated water for the floor. I keep the water heater set to maintain about 100 degree water temperature. The taco 007 circulator feeds five, 300 ft loops of 1/2 pex in the floor. Thermostat control is from a standard wall mounted thermostat sensing the air temperature. There is no floor sensor. I included a picture of the setup for the visually enticed readers.


The system has been functioning fine for the past 6 yrs. However I am wondering about a few things regarding the water temperature. Is there an ideal temperature that I should keep my water tank at, to minimize cost? My thought in keeping at 100 degrees was that I would have less standby losses when system isn't calling for heat. When the system IS calling for heat, the 40gallon tank cannot "maintain" 100 degree water for the duration of the cycle, until the circulator shuts off. After a while, water going in does reduce to 80-90 degrees, as the stored supply is used up. The burner runs for a while after the circulator has stopped, in order to bring the tank back up to 100 degrees.
Should I have instead increased the water tank to 135-ish degrees and installed a mixing valve to mix return water with the tank water to get 100 degrees going into the slab? Would this make my water heater have shorter burn cycles when reheating the water? Or is the energy required to heat the slab the same, and higher water temperatures just mean more standby losses?
I also have recently learned about condensation concerns at water storage temperatures under 120 degrees. I dont "see" a condensation issue currently in my exhaust stack (naturally vented water heater through the roof. Should I keep the water hotter to minimize condensation in the flu?

Thanks in advance for your time!
Ben

hot_rod

The lowest possible temperature to maintain the space is the most efficient. But on cold days that temperature requirement is higher. Ideally a control that would modulate the temperature would be most efficient.

I'd look inside that water heater burner area. If you see a lot of rust, scale, soot that indicate the tank is running too cold and condensing in the flue piping.

Cranking up the tank temperature and adding a mix valve would help prevent that, but fuel costs will go up with higher temperatures and standby loss..

A condensing boiler would be a much better appliance, they love those low temperatures.
There is just so much you can do with a basic tank water heater, it's probably 78% or so efficient. But the upfront cost makes them common for small systems.
Modern condensing boilers can run 90% plus efficiencies.

Are your fuel costs acceptable?

EBEBRATT-Ed

I would thing low return water temp will not prolong the water heaters life. I would boost the temperature to 120 and mix down to 100 with a mixing valve

Larry Weingarten

Hi, As Hot Rod said, here is something you don't want to see in the combustion chamber of your heater. All the rust is a bad sign. Also, if you saw wet rust on the walls or lower head of the tank that would be bad. If you saw much white scale left by water, that would be bad too. If there were any soot build-up, that too would be un-good. You can use a mirror to look at the underside of the flue, which, along with these other things, can give a good idea of tank condition and how it's been operating.



Yours, Larry