BEST CONTROL FOR HIGH MASS SLAB

[frank t]

FIRST TIME POST:
I am looking for suggestions to improve temperature control of high mass slab. I am currently using a Harmon 260 Trident boiler with a digital air thermostat. The problem is the slab overshoots room temperature at times and will wait too long before it cycles on. Do I need a slab sensor? How do I install one later? Something on the return line possibly? I have a 3/4 inch conduit that runs into the center of the slab for the kitchen island power. There is a piece of romex in the conduit -- can it also share a slab sensor?
All suggestions welcome. Thanx in advance.

 

[pybyr]

FIRST TIME POST:
I am looking for suggestions to improve temperature control of high mass slab. I am currently using a Harmon 260 Trident boiler with a digital air thermostat. The problem is the slab overshoots room temperature at times and will wait too long before it cycles on. Do I need a slab sensor? How do I install one later? Something on the return line possibly? I have a 3/4 inch conduit that runs into the center of the slab for the kitchen island power. There is a piece of romex in the conduit -- can it also share a slab sensor?
All suggestions welcome. Thanx in advance.

I'm fairly sure that most codes say that you cannot have 120VAC wiring and low voltage signal (thermostats, sensors, etc.)wiring in the same conduit.

Sensor wires also tend to pick up false signals from induced current or voltage when they are placed close to/ in parallel with AC wiring.

I'll be glad to be wrong on any of the above if someone else can suggest a way to use your conduit; maybe someone can come up with some other approach.

Good luck

 

[in hot water]

Lower the supply temperature to the slab as much as possible, high mass is like a large flywheel, it doesn't like to start or stop quickly.

The very best control is an outdoor reset. This control adjusts the supply temperature going to the slab based on outdoor and in some cases indoor input. So the slab gets just the correct amount of heat to cover the load. Once you get them set up correctly you end up with constant circulation of the perfect supply temperature, over and undershoots are greatly minimized. tekmar gas some of the best reset controls that I have used.

A slab sensor alone, or with an air sensor will not adjust the supply temperature going to the system. They are a nice control for bathrooms where you want to idle the slab warmer than the rest of the home, but it needs to be zoned properly to do that.

High mass is tough to control in areas where you have frequent or wide temperatures swings in the winter, they just don't respond quickly.

hr

 

[Gooserider]

FIRST TIME POST:
I am looking for suggestions to improve temperature control of high mass slab. I am currently using a Harmon 260 Trident boiler with a digital air thermostat. The problem is the slab overshoots room temperature at times and will wait too long before it cycles on. Do I need a slab sensor? How do I install one later? Something on the return line possibly? I have a 3/4 inch conduit that runs into the center of the slab for the kitchen island power. There is a piece of romex in the conduit -- can it also share a slab sensor?
All suggestions welcome. Thanx in advance.

I'm fairly sure that most codes say that you cannot have 120VAC wiring and low voltage signal (thermostats, sensors, etc.)wiring in the same conduit.

Sensor wires also tend to pick up false signals from induced current or voltage when they are placed close to/ in parallel with AC wiring.

I'll be glad to be wrong on any of the above if someone else can suggest a way to use your conduit; maybe someone can come up with some other approach.

Good luck

That is very much correct, there are lots of reasons to not put low voltage and AC in the same conduit, and Code is just one of them... Obviously there is concern about the remote chance of a short somehow connecting the AC to the low voltage, and the problems of signal degradation from induced currents that you mention... One of the other BIG problems that isn't mentioned, is that you have a potential when you have two wires running in close proximity, to get a transformer effect where the hot wire can induce a voltage and current potential in the other wire - so that all of a sudden that "dead" piece of low voltage wire has potentially dangerous levels of voltage and current on it - which can both smoke the connected equipment, and possibly deliver dangerous shocks to the person working on it...

This is something they make a big deal about in all the network wiring books as a potential danger, and why properly done network cabling goes to great lengths to avoid sharing real estate with AC...

Gooserider

 

[ISeeDeadBTUs]

Paraphrasing hr, slabs are a b!tch to change temps in. I think I have decided that it is best to keep the {air} stat up so that the tubes don't sit idle for too long. My basement slab will not run unless I keep the stat at 67 or higher. This is because the stat picks up standby losses from the boiler.

Hr or Caldwell can prolly correct me where I'm wrong here, but I think most Euro systems are set up with constant circulation in mind. The Viessmann uses outdoor reset control and desired indoor temp setings (on the boiler control, not thermostats) to determine supply temps. I think my system is a bit bass turd eyezed in that we use thermostats to turn on/off circulators.

I am not ready to try this 'higher temp theory' in the garage slab yet. But I can tell you that currently, when that zone comes on, it puts a serious hurting on boiler temps. Even though it is only 576[], that mass robs every bit of warmth I can send it. I would try turning the slab up to 60 °F if I wouldn't keep finding that people leave the door open! :ahhh:

I had asked my heating contractor about slab sensors and he said no so I never investigated it further.