<!--[if gte mso 9]> Normal 0 false false false EN-US X-NONE X-NONE MicrosoftInternetExplorer4 <!-- /* Font Definitions */ @font-face {font-family:"Cambria Math"; panose-1:0 0 0 0 0 0 0 0 0 0; mso-font-charset:1; mso-generic-font-family:roman; mso-font-format:other; mso-font-pitch:variable; mso-font-signature:0 0 0 0 0 0;} @font-face {font-family:Tahoma; panose-1:2 11 6 4 3 5 4 4 2 4; mso-font-charset:0; mso-generic-font-family:swiss; mso-font-pitch:variable; mso-font-signature:1627400839 -2147483648 8 0 66047 0;} /* Style Definitions */ p.MsoNormal, li.MsoNormal, div.MsoNormal {mso-style-unhide:no; mso-style-qformat:yes; mso-style-parent:""; margin:0in; margin-bottom:.0001pt; mso-pagination:widow-orphan; font-size:12.0pt; font-family:"Tahoma","sans-serif"; mso-fareast-font-family:"Times New Roman";} .MsoChpDefault {mso-style-type:export-only; mso-default-props:yes; font-size:10.0pt; mso-ansi-font-size:10.0pt; mso-bidi-font-size:10.0pt;} @page Section1 {size:8.5in 11.0in; margin:1.0in 1.25in 1.0in 1.25in; mso-header-margin:.5in; mso-footer-margin:.5in; mso-paper-source:0;} div.Section1 {page:Section1;} --> <!--[if gte mso 10]> <!--[if gte mso 9]> Normal 0 false false false EN-US X-NONE X-NONE MicrosoftInternetExplorer4 <!-- /* Font Definitions */ @font-face {font-family:"Cambria Math"; panose-1:0 0 0 0 0 0 0 0 0 0; mso-font-charset:1; mso-generic-font-family:roman; mso-font-format:other; mso-font-pitch:variable; mso-font-signature:0 0 0 0 0 0;} @font-face {font-family:Tahoma; panose-1:2 11 6 4 3 5 4 4 2 4; mso-font-charset:0; mso-generic-font-family:swiss; mso-font-pitch:variable; mso-font-signature:1627400839 -2147483648 8 0 66047 0;} /* Style Definitions */ p.MsoNormal, li.MsoNormal, div.MsoNormal {mso-style-unhide:no; mso-style-qformat:yes; mso-style-parent:""; margin:0in; margin-bottom:.0001pt; mso-pagination:widow-orphan; font-size:12.0pt; font-family:"Tahoma","sans-serif"; mso-fareast-font-family:"Times New Roman";} .MsoChpDefault {mso-style-type:export-only; mso-default-props:yes; font-size:10.0pt; mso-ansi-font-size:10.0pt; mso-bidi-font-size:10.0pt;} @page Section1 {size:8.5in 11.0in; margin:1.0in 1.25in 1.0in 1.25in; mso-header-margin:.5in; mso-footer-margin:.5in; mso-paper-source:0;} div.Section1 {page:Section1;} --> <!--[if gte mso 10]> Hello,
My contractor recently installed radiant floor heating system retrofit in our basement.  I installed a Tekmar 508 thermostat with a floor sensor.  I drilled a 2 inch deep hole in the concrete in the middle of the basement below the thermostat just wide enough to accept the sensor. 

I wanted the floor sensor since the basement is also served by forced air heat, and i would like to maintain a warm floor in the future.

The slab is 4.5 feet below grade.

I am still finishing the basement so we haven't had the basement as warm as we will later on.  Currently the minimum temp is set at 16.5C.  The cycling setting is on auto.  The ambient temp sensor is turned off.

If i turn the minimum temp up to 17C, the zone turns on but it seems to turn off again before the sensor temp hits 17C, and it seems to run in short bursts.  The hot water is supplied by a tankless hot water heater, so I don’t believe short cycling would be a problem here.

 

My primary question is, what is the swing temp for a Tekmar thermostat?  I called them but it seemed like it was a kid who didn’t know much on the other end.

 

Thanks

that's a PID logic thermostat. It runs in cycles, and it shortens or lengthens those cycles based on how close to the target temperature you are.

This is why it controls overshoot in slab systems, because it doesn't wait until it is "satisfied" to turn off like a simpler thermostat does.

Short cycling is an issue unless that tankless is electric, though.

so you have it set up for "slab only" operation.
I would change the cycle rate to 2 or 3 (cycles per hour)

Normally I would view this as a 2 stage thermostat application. Stage 1 is the radiant, stage 2 is the air system.
Since you are already set up individually you will need to dial in the slab set point. Start with the slab set point near where the air set point is.
The higher that you move the slab setpoint, the more heating the slab will do, but the chance the the room will overheat will increase.

Dont hook up the air sensor because the air system will fight with the slab and shut it down. I assume that you want to do most of the heating with radiant.

Thank you for your responses. 
Rob, would you consider 15-20 minute run time to be 'short cycling'?  I haven't timed it but it seems to me that this is what it was, then off for about the same amount of time, then on again.  I have been testing it only for short periods at 17C or higher.  I would have thought that a tankless (this is a natural gas condensing) was meant to turn on and off regularly and would therefore be short cycling for domestic hot water all the time, but this is the logic of a novice. 
Bill, it is set up as "slab only".  I had anticipated the forced air/radiant battle and that is why i wanted a floor sensor - to keep the floor feeling comfortable no matter what the furnace was doing.
I am starting to think that going tankless on the advice of my contractor was wrong.  I have a Navien with the Heating Box and i have a feeling they are going be be running constanly this winter to satisfy a 120 sq ft addition with a suspended floor above grade over a crawl space with PEX under a plywood floor that will be tiled i.e. no thermal mass.  Even this fact alone wouldn't be so bad if they weren't so LOUD.  You always know when they are on.
I have been reading this and other forums and am thankful for the help in understanding they have given thanks to people like you. 
Cheers

that sounds pretty good to me in terms of cycle length.

Most ModCons are nearly silent while operating and made for heating space and domestic hot water indirectly.