I have radiant heated wood floors which are controlled by tekmar thermostats(519). It has a slab sensor in the floor. They have been working great but this morning the floor temp had exceeded the set maximum of 85 degrees, it was 90 degrees. Today it still seems to exceed the max temp I have set. What is going on?

I see that it has different modes - air only, floor only and both (with a min and max for the floor). You are definitely using the third mode? And the sensor is in an appropriate position?

Yes, I am using the floor min(set at 70) and max. The room temp is set at 73. Maybe it was just a fluke but the floor temp had never gone up to 90 before. It wasn't unusually cold out and I hadn't changed anything. I did lower the max temp to 81 and it still goes a little above that so maybe that's the way it works. I just didn't think it would go way over max.

Where are you mesuring your floor temp in relation to the floor temp sensor, or are the floor temps you are relating comming from the thermotat/floor sensor? What is your boiler temp running at? Do you use outdoor reset? IF the boiler temp is running way hot, I could see it short cycling to accomidate the floor temp requirement, but shutting off to soon to deliver enough heat to satisfy the room temp requiremnts, so the thermostat may be overiding the floor temp limits to satisfy room temp...

Where is this “slab” sensor located? How and where did you determine this “slab” temperature? Please keep in mind that it is the actual floor surface temp that determines the room heat gain rate. Is your sensor located in the concrete slab below the hardwood floor or is your sensor located within the hardwood floor near the surface? The concrete slab temp will be significantly higher than the hardwood floor surface temp. 85F is often considered the floor surface temp upper limit for most residential rooms. 90F is often considered the floor surface temp upper limit for residential bathrooms.