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Not understanding how Cv works in a valve like that is probably why so many radiant systems slip behind as you approach design conditions.
You will often see 2 or 3 circulators connected to 3 Cv thermostatic valve!
If this attaches it is a handy spreadsheet to help understand the effect of CV. Used for design or troubleshooting.
It applies to ANY valve, zone, ball, balance.
The radiant gets screwed any way you compare.
If you come off radiant and go to forced air the slab carries over btus as mentioned.
If you use forced air long enough for the slab to cool. Then go to radiant the slab has to work harder to get warmed back up.
A real comparison in comfort, and efficiency. Do a complete season comparison. One year with forced air, and one with radiant. I'm assuming the forced air is for ac. Or is it supplemental to the radiant.
Then I would use the degree day per sf formula as a base line to compare.
In reading your last post as @Rich_49
pointed out. When you turned down the radiant thermostat, and turned on the forced air the radiant slab carried part of the load for the forced air.
That heated slab will release energy for a few days.
Not a valid comparison at all.
also the boiler has to see below 130 degree return temps. If it's piped primary secondary it's possible the boiler is seeing higher temps if not pumped right.
Something is certainly not set up properly . The radiant slab also gave up heat to the space during your test , at least for a good period of time if the house is well insulated and air sealed . How many zones is this radiant system .T there is alot that can go wrong and it does in probably 75% of the installs , usually starts with design or lack of it , the rest comes from not understanding building science .
Suffice it to say that for a whole lot of reasons the comparison is completely invalid.
However, sadly, it's the kind of thing which gets published every day...
What he said^^^. Had the slab already been up to temp and provided there is at least 2" of foam underneath and around the perimeter of the slab, I find it hard to believe the radiant would take a drop more fuel than the scorched air while providing a more comfortable heat. Valid point with the DHW also. Was this boiler installed and tuned by a competent tech?
I would like to add, the OP is assuming the hydronic system was properly designed and set up.
What are your outdoor reset settings? Have you tuned them? Combi boilers most times are a compromise. To size for DHW load most times they are way oversized for the heating load, which will impact efficiency.
We would need a lot more information to even discuss this topic with respect to your system.