I know, the plates would work better, I am not debating that at all. I am just putting it out there that with 140 degree water, I raised my house from 58 to 70 in less than 2 hours this week when it was only 6 degrees and 20-30 MPH winds outside.

I had hydro air heat on my first floor for 7 years. So i decided to to remove it because the duct work was too intrusive to the ceiling.
I spent about a month researching radiant heat for my first floor so I could make a clean look for the ceiling in the basement. I have a strict budget...but bent it a bit too change the first floor from Unico high velocity hydro air to strictly radiant heat.

I didn't use plates, they were just way too much money. Instead I ran 2 loops (4 pipes) in each bay. Each loop is a separate zone. So what i did is use a two stage thermostat. On loop runs when just a little heat is needed to maintain. The second loop runs when the first can't keep up or if someone cranks up the thermostat more than 2 degrees higher than the current temp. The water temp is between 135 and 145, a little high, but I can lower that next fall when I move and repipe the boiler with an outdoor reset valve. i also placed the R19 insulation back into the bays once the job was done

It is amazing. We set it back to 58 at night then at 7:00 am it turns on to 70. Even on that cold windy 6 degree night this week there was no problem, we never ran the wood stove either, we wanted to test the system out before selling the Unico unit. Also I only have it installed in a little more than 1/2 of the first floor, the bath/living/office areas have no heat right now, so its a bit cooler there.

I notice it takes about an hour to get the floor warm, i imagine the plates would heat it up much faster, but from there it is perfectly warm. the heat surrounds you. We don't notice the draft windows as much either. I used the staples that push the pipe right against the floor, hoping not too much noise would be head. When the system turns on after sitting a while with cold water, there is some noise heard, but it sounds just like water dripping of the roof, like snowmelt on a warm winter day, drip drip drip. Thats all, it is almost relaxing, and only lasts for 5 minutes.

So now back to finishong the basement, which is getting a 3/8 pex run between the sleepers.

if you do subfloor tubing like this, you should always suspend the tubing about an inch from the subfloor and don't use staples, use hangers of some kind. with high temp expansion and contraction staples can shave away the pipe over time and wear through.

Also, the second run of tubing *shouldn't* be doing much for you, though perhaps it is speeding up your response time a little. Are you seeing anything that would contradict that, such as a cycling of the second stage under regular conditions?

with 140 average temp you should be producing somethign like 16 to 18 BTUs/sq ft or so. what is the heat load for this space?

Just curious, but do you have a data-logger or minimum-capturing digital thermometer to verify that it actually got DOWN to 58 at night? You can put the setback to 45F if you like, but there's no guarantee that it'll hit that unless you leave the windows open, eh? ;-)

My house is neither super-tight nor super-insulated, but even on nights when outdoor temps bottomed out near 0F in the pre-dawn hours it takes longer than 10hours of overnight setback to drop a full 10F, let alone 12F. Last year running it on a hydro-air system maintaining 68F daytime temps with 60F nighttime setback the system never fired overnight, and the lowest temp logged in the coldest room in the house was 61F.