Slab sensor conduit

[hellspcangel911]

Hi All,
I'm getting ready to pour my slab Thursday, and realized I should add conduit for a slab sensor. I was originally going to run a wood burning boiler but now probably a propane boiler. Needless to say I havent chosen a thermostat so I'm not sure which sensor I need to accommodate. I'm leaning towards The Honeywell T6 but would prefer something Wi-Fi.
What do you guys run for conduit for the slab sensor? I was going to do the same 90 degree 1" electric conduit with 1/2" pex... does that work? Any concern using Pex will cause a delayed temp reading?

Thanks
George

 

[rlitman]

The usual solution is to bury two sensors (one spare) directly embedded in the slab - no conduit makes for more intimate contact.

 

[hellspcangel911]

Thanks Rlitman, are the sensors universal enough to accommodate different model thermostats?
Would thermostats fit through a 90 degree 1” conduit into 3/4 copper?

 

[yatg]

Would thermostats fit through a 90 degree 1” conduit into 3/4 copper?

the sensors are about 1/4" diameter and 1-1/2" long

use an electrical sweep, not a hard 90

 

[hellspcangel911]

thank you, thats very helpful.
this is what I used

Carlon 1 in. 90-Degree Schedule 40 Standard Radius Belled End Elbow UA9AFB-CTN - The Home Depot

Find the Carlon 1 in. 90-Degree Belled Standard-Radius Elbow UA9AFB-CTN, gray crafted with Sch. 40 PVC and is sunlight resistant for use in outdoor and exposed areas at The Home Depot

www.homedepot.com

 

[PCustoms]

Can you run the conduit to something (cutoff bucket, coffee can etc) to leave a void in the floor for now?

Then when you get the probe figured out, feed it back from the hole and patch the small spot with a bag of quickcrete.

 

[hellspcangel911]

Can you run the conduit to something (cutoff bucket, coffee can etc) to leave a void in the floor for now?

Then when you get the probe figured out, feed it back from the hole and patch the small spot with a bag of quickcrete.

I can, but what is the benefit of the void?

 

[PCustoms]

I can, but what is the benefit of the void?

Gives you a hole to put the sensor into and then feed the wire back to the monitor.

It also puts the sensor in direct contact with the slab. I assume these are meant to be embedded, but I might be overlooking something

 

[hellspcangel911]

I misunderstood, I thought you meant the container would be fully encased. Thats a good idea, I'll put a skoal tin at the end of the pex run, and mark it off the ceiling. If I cant get the sender through the pex, I cant drill it out off the plumb mark and thread it through.
Thanks guys for the fast responses.

 

[rlitman]

Thanks Rlitman, are the sensors universal enough to accommodate different model thermostats?
Would thermostats fit through a 90 degree 1” conduit into 3/4 copper?

So far as I've seen, they're pretty universally 10K Type II thermistors. Those come in a many formats, but for floor sensors, they're typically potted in a 1/4" OD tube of various relatively short lengths. If you have an ohmmeter, you can confirm your sensor matches using this table:

https://www.bapihvac.com/wp-content/uploads/2010/11/Thermistor_10K-2.pdf

The wire on these sensors is pretty thin, so I would expect it to be about as easy to push down a conduit as a piece of twine. I'm not saying the 1" PVC sweep won't work (man is that overkill), but if it were me, I'd probably just use a piece of 1/2" PEX with the end taped over as a makeshift probe well. You shouldn't have any trouble using a piece of say copper wire to push the sensor down a single piece of PEX, and being uninterrupted, you won't risk getting snagged on a joint.

The plumb mark on the ceiling is a great idea!

 

[hellspcangel911]

Thanks for the advice and the link!

 

[PoorUB]

When I did HVAC I did a lot of floor heat and all we ever did was run a length of PEX tubing with the end tapped over. We would run the sensor down the PEX with a plug of paper and a metal fish tape. We have never had any concern with the temp probe touching the PEX. Never saw a reason to worry about it. Never had a complaint. As the floor heats up, the PEX heats up and the air inside the PEX too. If it didn't, the whole concept of floor heat wouldn't work!

If it gets too hot in the space, run the floor cooler, if you need it warmer, turn up the floor temp. Who cars what the actual number might be? The big answer is if you are comfortable.

 

[road_king]

Conduit used for my Viega sensor was just a length of pex tubing

 

[hellspcangel911]

Thanks guys, I ran a 12’ length of 1/2” pex

 

[rlitman]

...As the floor heats up, the PEX heats up and the air inside the PEX too. If it didn't, the whole concept of floor heat wouldn't work!...

That's what I meant by a "probe well". It's a sealed hole you shove a sensor into, and yes, the temperature inside the buried tube should equalize with the slab pretty quickly, but there's going to be a delay that adds to the hysteresis. In a heated poured slab that has such an enormous thermal mass, it's damn near impossible to overshoot, so it should be good enough. In electrically heated thin surfaces such as Ditra Heat, the very same sensor is in intimate contact with the thinset, partly because there isn't enough thickness for a PEX sleeve, and partly because overshoot is a bigger issue.

 

[PoorUB]

We did a job where we forgot to put in PEX for a floor temp sensor. I ended up carefully mounting an electrical box to the floor and drilled a hole just large enough for the sensor. I shoved the sensor in the hole and covered it with some fiberglass insulation. It worked fine.

 

[Juiced06GTO]

I also just ran a piece of pex with the end taped over and fed my sensor into it once I figured out what I was using for a tstat. I ended up with a Tekmar 562, it is Wifi enabled, accepts the floor sensor input and will control cooling as well for when I once day add a mini split.