Rebar chair spacing.

[Raspy]

What's the verdict on in-slab thermostat heat sensors? I know my thick slab, the few times I have tried it, will overshoot considerably from the air temp sensed by the thermostat mounted on the wall reading air temp.

Slab temp thermostats are best where you have a radiant slab in a forced air zone, like a radiant bathroom floor in a forced air home. They will never control the room temp properly.

Overshoot will happen with a constant temperature setting on the room thermostat or when it is initially turned on after a shutdown. Also, it is caused by too much energy going into the slab at once. An oversized boiler with too high a delivery temp is a good example. A setback, room air sensing thermostat is much better than a simple, non programmable one. They will increase the comfort and decrease the heating bill, while making the temperature more stable. It seems counter-intuitive until you look at it the right way.

Air temp stats can be wildly affected by a breeze through a door, sun hitting it, lights and cooking, day vs. night temps, or many other factors. With radiant you have to be a bit more aware of how you run it and recognize what makes the house comfortable. Air temp is not the main factor that makes the house comfortable, it's the radiant floor, and that means a strategy designed to maximize that feature. It means you have to run the system before you need it and shut it off before you are done with it. Sometimes you only want the floor warm and not the entire room. You have to allow for the other heat inputs all homes have.

 

[dave1949]

Is the programmable setback t-stat you are talking about like any other prog. t-stat or is there something special about it?

I totally understand "run before needed and stop before done" concept. I could probably get pretty good results just forcing it to run twice a day through manual control. I think I would eventually zero-in on the time of the run required to get the results I want. Are you saying let a prog. t-stat do just that?

 

[Raspy]

Dave,

Yes, a normal setback thermostat with four settings per day is the best. You are right, to a certain extent, that you could get fairly good results by operating it manually twice a day.

A bad choice is a learning thermostat like the new NEST design. Poor for radiant.

There are a lot of factors. The better the insulation in the house, the less you need to program to dial out the overshoot/undershoot problem. But you still need to program to get the house to feel right and run economically. The programming also is affected by the input from the boiler.

The typical program turns the heat down to a minimum acceptable temp at night before you go to bed by about two to four hours. This lets the temp settle to a minimum level for comfortable sleep and good economy. Set to about 60 or 62 degrees. It also sets the house up for the next cycle.

Next, there is an aggressive call for heat that will always make the system go on. This happens about one or two hours before you get up and only in the bedroom/bathroom zone and the kitchen zone. A setting of about 70 or 75 degrees.

Next the setting goes to a daytime setting when you get up. Note: This actually when your feet hit the floor. Now the system begins to coast and find the daytime setting of about 65 degrees.

Next the evening setting of about 68 degrees or so. This is set for an hour before you get home or about 3-5 PM.

If you just leave the stat set at a given temp the house will be warm all night. Then in the morning the temp will go way up because the air outside warms up, lights go on, cooking starts and people are up walking around. This is overshoot and caused by running too much the night before. Instead we just run the system enough to heat the slab a bit. It greets you when you get up, but doesn't get hot for long enough to heat the whole house. Radiant heat is fine with cool air. By calling for an evening temp and leaving it on longer, the house is more comfortable for just sitting around reading in the evenings. Nighttime sleeping doesn't always require a very warm house, in fact, most prefer a cooler house. I, for instance, like the windows cracked.

A learning thermostat will keep turning the system on earlier and earlier in the morning trying to get the air warm by the time you've set. In other words, if you want the house warm at 7:00 AM, the stat might come on at 2:00 AM and you are back into the overshoot problem. The thermostat is measuring the air temp when we just need a timer to override and make the system go on for a short period.

It is a bit of a balance between boiler output and programming. But it's still the most comfortable of all heating methods. People sometimes tell me it's their favorite thing about the house and often say they will never have another house without it.

 

[dclynch]

All I can say is it was the best decision I made when adding a 24X40 garage to the house. I leave the thermostat at 50 degrees all winter and the wife an I love getting up in the morning and jumping into a warm car. I also found that I only need to kick it up to about 56-58 degress when we have our New years eve and Superbowl party. The only mistake I made is I let a contractor talk me into a electrin Mini boiler and the electric bill is huge for 3 months a year but I gladly pay it. I would like to eventually convert over to a hot water heater

 

[Redneck in training]

Ya dont use too many or the concrete flowing into the slab will just knock them out of position and float them away cause there is not enough weight on them to hold them in place.

That is my opinion too. If I do it next time I would use cement bricks.

 

[Redneck in training]

All I can say is it was the best decision I made when adding a 24X40 garage to the house. I leave the thermostat at 50 degrees all winter and the wife an I love getting up in the morning and jumping into a warm car. I also found that I only need to kick it up to about 56-58 degress when we have our New years eve and Superbowl party. The only mistake I made is I let a contractor talk me into a electric Mini boiler and the electric bill is huge for 3 months a year but I gladly pay it. I would like to eventually convert over to a hot water heater

As long as the hot water boiler is also electric the cost of energy to heat the garage will be the same.

 

[Redneck in training]

We have floor heating in our house. Every room has its own zone. We have a combination of zones with valves and zones with pumps. Three large rooms requiring single manifold have one pump each while three other rooms share single pump and manifold and are controlled by valves. Each valve has a limit switch that closes when the valve is completely open. The limit switches are used to start the pump.
All zones but kitchen have simple non programmable thermostats specifically designed for floor heating. Kitchen has a thermostat that can determine when to switch from heating to A/C and vice versa.
To give you some idea look at, in example, Tekmar. radiant

 

[wedge40]

I do plan on having two thermostats, but none in the cement. I redid the order with 3/4" pex and the pex alone was only $275 more. Not sure if it's worth extra expense.

Wedge

 

[dave1949]

Raspy,
Thanks for the info. I have radiant heat installed but don't use it. Our shower bathroom tends to be the coolest place in the house in winter. This brought complaints from guess who So, I tried using the radiant heat in the bathroom zone to take the chill off.

Since our slab is a minimum 8" thick, the boiler runs a long time to heat that enough to move the t-stat. Then it shuts off for 24-36 hours as the slab temp drifts down. It just repeats that cycle. Although it worked, it also added 100 gal. of propane use for the season. I don't consider that very efficient. I don't know if a programmable t-stat would cut my propane use significantly. I have found that running a small elec. space heater for 45 min. does the job for a lot less.

Now that guess who is retired, early morning showers are less important :laughing: