I haven’t found that my outdoor reset helps all that much with my modulating boiler.
Biggest issue with these systems is simply that the high thermal mass of the floor just doesn’t allow respond fast enough for anything but stable outdoor conditions. Think thermal inertia. The outdoor reset simply gives the boiler a head start on changing conditions. It can spool up the boiler as temperatures drop outside, or cut back the propane as it warms up, but, in the end, it’s no match for thermal inertia.
The floor sensors seem to be more like limit sensors to keep from overheating the slab than something to control ambient temperature with large fluctuations in outdoor temperature, although I suppose a lower limit set slightly below the room air target could be useful to keep the slab temperature up to improve response slightly.
Like my hvac guy who’s probably installed hundreds of systems says, they’re ok for tweaking, but don’t expect miracles.
In my case, I have, from memory, a 100 or 110 k btu boiler with an 8:1 or 10:1 turn down ratio, and a slab somewhat larger that the op has. I think part of his problem is simply that the water heater is undersized, and will take a long tome to bring the slab to the target temperature.
Patients, Grasshopper.
Unless your building leaks and is poorly insulated 100k Btu should be enough .. if not too much.
My new house project (if I can ever get it done) is a stone church from 1873 w/ new addition on the back ---- it's spray foamed. Some exposed stone walls for cool factor. Obviously -- tall ceilings. Foamed roof deck under the slate -- about 4". 4000sf + over 5 levels. The boiler is a Viessmann with a net of around 60k. It had no problem in PA at sub 10 degrees (coldest last year) keeping the place above 70.
With radiant you always have to think about response time. In areas where there is greater regular temp swings in 24 hours -- you have to plan for that. Same when doing radiant in more temperate climates because the system will be asked to supply fewer BTU's in the middle of a sunny day. Doing radiant in very cold climates is actually easier -- the BTU loss is constant.
If you think about my above house there are many times in the spring and fall when that space does not need the 12k minimum -- so the boiler will cycle on the primary. If you don't have a primary/ secondary w/ constant circulation the boiler has to stop.
ODR can't work w/o constant circulation and some way to modulate the input. The Germans first came out with Cast Iron that was more forgiving of lower temps. They designed these high mass boilers to fire on and off to keep a set temp in the primary -- it was an easy jump to condensing boilers and then full modulating. The idea stayed the same ,,,, I did a full ODR over 30 years ago.
The next thing people don't fully grasp is the floor. 1/2 Pex 12" apart in a 4" slab is not going to evenly heat the slab -- there will be bands of heat. If you double the Pex and place it 6" apart the slab is going to respond much quicker and with lower water. A 12" spacing in a cold climate will be easier to work with because over time the slab can stabilize and the variations needed are less. If the floor is 6" thick the problem is greater. Also when you try and keep the interior of a garage at 60 -- the input is going to be more stable and the discomfort of over shooting the target temp less of a problem.
This is why I always tell people to think about what they are heating. In a big garage it's often advisable to plan on making some areas warmer then others -- this can easily be done with more tube in one area -- or with flow control. I like to add loops to those areas -- with both Pex and manifold spaces cheap it's easy to double up the areas where you will be. This naturally makes those spaces faster to respond.
The better boilers have additional controls. My Viessmann boiler has an interior wall control -- I can change this temp and the boiler will alter it's internal curve to either make the space warmer or colder. It will also allow a space to better adapt to sun gain.
Using a wall thermostat to turn a system off can be problematic. They work when it's an odd day .... but if common you have to plen better and lower the curve ... constant circulation is often needed to get this close.
I have had to use slab sensors in some situations. I built an addition off the back of my suburban house years ago .... lots of glass with a limestone floor. Trying to control using the temp of the air was not possible throughout the winter -- the sun would heat the space and turn off the floor. The floor could not respond fast enough and it was always cold as the sun went down. The solution was to fix the floor temp with a slab and have some supplemental heat. In that case I had two tower Runtal units made that would flood the space with heat when needed. In the cold of the winter the floor was fine on ODR.
There are just too many variables. Looking at a space before you put the tubes down is very important - especially in larger or odd shaped spaces.
