I installed a closed hydronic system in my new garage. It has 5 250' runs of 1/2" pex. The pump was a taco 007.The heat source is a takagi tankless water heater that supplies output and return temps as well as GPM info. I fired it up and it took about 24 hrs to heat up as expected.it got nice and warm in there and cycled on and off within reason. My concern was the manifold return temp. It output 122* and return maxed out at about 67* flowing about 1.1gpm. This seemed to be a low flow to me as well as too large of a temp difference. So I changed out the pump to a taco 0015 3 spd pump. On high This increased flow to 1.8 gpm and return temp of 77*. Should I be concerned with that low of a return temp and step up the pump size yet again? Or let it go since the temp inside the building is satisfactory and pretty consistent? This is a simple system just for a garage but I would like it to be efficient as possible. It seems to me that if the return temp is too close to supply temp then not as much heat is being transfered to the slab but I am no expert. Again this is a simple garage setup not a house, it just needs to work good enough for a garage. I know all about the debate of not using a boiler so im not looking to get beat up over that. I feel like if I didnt know these numbers I wouldnt be care because it warmed up nicely but im open to do a little tweaking to gain efficiency. Thanks

10 to 15 deg F is target temp drop across circuit for residential living space. For a garage you can certainly increase this to twice this value or more. You can determine the proper flow rate and size a Taco pump or any other pump using the free DIY software on our website. In addition to the desired temp drop across the circuitd, you will also need to enter the garage heat loss numbers that get you to that high a supply temp.

Did you expect this garage to require 30,000 BTU/hr?

What is the insulation under the slab?
Dan

The insulation under the slab is 2 in foam board. I did initially anticipate it requiring around 30k btu/hr but I honestly dont recall how I got to that number. It sounds like maybe I am putting the water in too hot at 122*. Do I need to lower it and if so by how much? I was in garage all day this weekend and it was 66* air temp inside garage with the outside starting at about 9* ending the day at 28* roughly.. It was very comfortable and evenly heated throughout the building. The system came on only 3 times in 8 hrs running for around 10 min each time. I am leaning to the question of is a supply and return temp that are close together (low delta t) more for comfort and the most evenly heated surface or more for efficiency of the system? I dont plan on any barefoot travel in my garage.

Reduced temp drop across a given circuit requires increased circulator pump flow rate. Increased circulator pump flow rate results in increased pex friction head which requires increased power usage and hence results in reduced pumping efficiency. So you really don't need to reduce the temp drop across the circuits any more than is necessary. In other words, just run the circulator pump at the lowest flow rate that gets the job done. For residential barefoot friendly floors, 10-15F is the typical circuit temp drop design goal. For your garage, you will likely be happy with a circuit temp drop of 20-30F or perhaps even more. The only way to determine the right supply temp is to run the hydronic radiant floor heating design software as previously described. We have detailed instructions for all the software on our website.