Slab Heat temperature sensor placement

SteamNow · 3:13PM

I have a job where I will be installing a new cast iron boiler for the customer but his entire house is all slab heat. The previous boiler never had any kind of mixing valve. But I would like to install a 3 way valve with an outsode stat and have it done properly. I watched Dan's video on how to properly pipe it but I'm wondering should I put the water temperature sensor at the inlet to the floor or at the outlet of the floor and control from there?

HVACNUT · 10:11PM

In slab radiant? Oil or gas fired? What make and model boiler? How much water does it hold? "Outside stat" meaning outdoor reset? Will it need a buffer tank? What will the water sensor be connected to on the other end? How old is the piping in the slab and do you know the material?

hot_rod · 10:32PM

If it is a concrete slab, getting a sensor in a mid point location may not be an option? Strap on the return piping to get a close approximation of the slab temperature, adjust accordingly.

I would use a thermostat that read both slab and air temperature. If you do slab only they can over-heat on a mild day. With a dual sensor you can set a not to exceed room air temperature.

outdoor reset helps a lot with slab radiant to prevent over-shooting. As well as energy savings. If this is just skab radiant, no high temperature zones? Then with a cast iron non condensing boiler a 4 way valve is ideal, if you add the motor and controller it has ORD and protects the boiler from cold return. Tekmar is one brand.

SteamNow · 10:42PM

https://forum.heatinghelp.com/discussion/comment/1904905#Comment_1904905

Yes, 1/2" copper tube as old as the house, I estimate about 40gals of volume, with a gas-fired green mountain GMGW-K 175Kbtu/hr input. Yes, I mean outside reset. I would connect it to the copper return where all the lines come back out of the slab. I've never seen one with a ci boiler that just heats the slab, I've seen them with a mix of radiant and slab, but never just one. The customer very explicitly does not want a condensing boiler.

hot_rod · 10:46PM

Back in the 80- and early 90's most all radiant systems were cast or copper boilers. It just takes a valve of pumping logic (injection mixing) to make it work, safely.

SteamNow · 10:59PM

https://forum.heatinghelp.com/discussion/comment/1904910#Comment_1904910

Thank you, I hadn't heard of those 4 way valves. I had an idea of running it like the picture below, but what you mentioned takes care of all the possible issues at once. I didn't know how to ensure the boiler wasn't getting too cold water back. The green boxes were the possible water temperature sensors. Right now all that is there is an old CGA that is pumped directly into the slab and was last set to 165.

SteamNow · 11:00PM

https://forum.heatinghelp.com/discussion/comment/1904915#Comment_1904915

There's none of that here. Just pumped out and comes back in.

hot_rod · 11:52PM

it would be best go lower the temperature to the slab and protect the boiler

Rarely do you need more than 120 to the slab unless it has a very high heat load or very wide tube spacing ?

Slab Heat temperature sensor placement

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