40 gal amtrol tank w/ tankless water heater

Hans Andersen

I'am using an amtrol 40 gal tank for domestic hot water and the heat exchanging coil in the tank to supply my radiant floors. I use the tanks aquastat to turn on a grundflos circ. pump that circulates water from the cold inlet pipe on the tank to the hot water out on the tank. I use a taco mixing valve to set the domestic hot at 120. The tankless is set at 140 amtrol says 155 is max for this tank. I designed my radiant at 90 deg. and have no problems heating thru the radiant. The problem is that my domestic h/w fluctuates even thou i have the mixing valve. The water temp. starts out fine as i am using the stored h/w then when the tankless kicks in i get a slug of cold water then the water temp. raises. So i guess my question is whats the best way to heat my tank when domestic h/w is being drawn off and i olny have an inlet(cold) and outlet(hot) pipe.

Hans

Thanks for any input.

Jean-David Beyer

If it is a Taco 5000 series valve, ...

... then the hot water supply to the valve must be at least 20F greater than the mixed output you want. You are cutting it pretty close if you supply 140F and expect to get 120F out. (I am assuming the cold is considerably less than 100F.) The tolerance of these valves is +|- 5F, so if you set it to 120F, it will wander between 115F and 125F.



Many valves of this type require a minimum flow rate as well, or they drop out of regulation. I have a different kind of valve (small) that has 1/2 gallon per minute minimum flow to get proper regulation.