Radiant / Hydronic hybrid - 2 thermostats, which one wins ?

Al Letellier

I would definitely use a slab sensor and a Tstat combo to control this space. If it's a open area to the other parts of the same zone, you'll probably have trouble controlling it with anything less. The key here is that the kitchen needs "floor conditioning", warm enough for comfort while not overheating the space when used in tandem with the forced air.

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S. Orbine

Radiant / Hydronic hybrid - 2 thermostats, which one wins ?

I'm building a 4000 sq ft, 2-story home. Primary heat is warm air with one air handler / coil per floor. So the main floor (about 2000 sq ft) is one zone warm air. There is supplemental staple-up radiant under about 300 sq ft of kitchen. The same kitchen area is already served by the main warm air heat, but the owner wants the warm floor effect in the kitchen.

Question: The radiant has it's own thermostat / zone valve. Where do I place the thermostat? Any special type of temperature sensor required? Floor (slab) sensor?

hydronicsmike

Why not 2-stage?

The first stage should be the Staple up and the second would be the warm air. Two stage heating thermostat with slab sensor would in my opinion be the best ticket. Slab Sensor could maintain a minimum slab surface temperature and also protect the surface by having a maximum limit.
I don't like talking specific products here, but thermostats as such are available with another remote air sensor input. In other words, one thermostat at one location with the slab sensor wired to it (doesn't really matter where, as the temperature is sensed at the floor) for the radiant slab in the kitchen, then the thermostats internal air sensor and optionally one more remote air sensor for air temperature averaging throughout the zone. Stage 1 is the Zone Valve for the radiant and Stage 2 is the Air Handler.

Mike

Don Walsh

Remote stat/ sensor

Can the sensor be installed under the floor in a staple up? It is far too late to get a sensor in the marble floor?

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hot_rod

This is tricky

It becomes hard to keep the floor loop running as the FA stats will tend to warm the space first. Of course shutting down any FA registers in the radiant area will help, make sure the furnace can handle this however.

Two stage stats, in this case would not be a good match as you don't have enough radiant surface to warm the rest of that room.

The tekmar stats with air and floor sensors help a lot, also available as a Wirsbo item, although it is still hard to maintain a warm floor without overheating the space.

There will be times when the FA stats are satisfied, the room is at setpoint, and the floor zone is off, you will notice a cooler floor.

Any additional heat from the radiant will overshoot the FA stat setting if the floor is above 72F or whatever the FA stat is set for.

Yes, you can mount the sensor below the floor. Try to use a piece of heat transfer plate, or flatten a piece of copper tube, to get a better contact area on the sensor. It will take some fiddling to get it all set, as the sensor will see a cooler temperature below the sub floor than embedded in the mudset or in contact with the tile. In other words if you want a max. surface temperature of 80F you may need to read 95 or so at the bottom of the subfloor.

You could carefully chisel some of the subfloor from below to get the sensor in contact with the tile or backer, probably not worth the effort, to correct the sensor error.

hot rod

hydronicsmike

In his case...

...I thought he is controlling an entire floow with one stat. If he was to use a 512 tekmar 2-stage Thermostat and add an 079 Slab Sensor (3/16" OD), the Slab Sensor would operate Relay 1 for the radiant and Relay 2 for the Air Handler. Although his radiant portion is rather small compared to the entire floor, this is where the slab sensor would be located. I would propose a Slab Minimum of 70°F and Slab Max of 82°F. The thermostat would then maintain the surface of the slab anywhere inbetween the 70 and 82°F while trying to satisfy the air temperature setpoint. Then, the thermostat would have its own internal air sensor and then I would add another (076/077) for temperature averaging throughout the whole floor. The air handler would be cycled (Relay 2)to maintain the air temperature at desired setpoint. I have done many of those before and those worked well. It usually ends up maintaining the surface of the radiantly heated floor (in the kitchen in this case) at the Slab Max setpoint. If you feel that 82°F is too warm, lower it.
Since the Slab Sensor 079 is so small in diameter, it is commonly installed between the tiles in the grout (right term?). - Can easily be done after the fact.

If the Air temperature is above the desired setpoint, the thermostat would disable Relay 2 completely and Relay 1 would only be operated to maintain the slab at the minimum limit (70°F). Radiant is such a beautiful thing. If the surface temperature is equal to the air temperature, the slab does not produce any heat output. This allows for maximal flexibility and you are hitting 2 flies with one stone.

I hate being product specific on here, but the products I know most about are tekmars.

Hope this helps.

Mike

Mike T., Swampeast MO

The battle...

Hot Rod's comments seem on the money.

I'm guessing the the warm air thermostat is not located in the kitchen.

Something you might consider:

Install a Modultrol or some similar device in one? both? of the air ducts feeding the kitchen. With 300 square feet there's likely more than one...

Operate the Modultrol off of its own thermostat such that it closes when the room heat is satisfied. Don't use that thermostat to control the air system itself--just the suppl(ies) to the kitchen.

I used such a strategy with the A/C in my office so that I could counter the intentional design that kept that room about 3° warmer than the rest of the space served by that system. It allows more temperature fluctuation than normal in some circumstance but for the most part works quite well. The radiant floor in the kitchen should help to "temper" any higher than normal air temperature fluctuation.

Of course you'll have to verify that your air system can deal with such "monkeying".

Bill G

Steve,
Didn't I tell you this is the best spot to get meaningful input?! These are some of the best designers/installers in the business. Keep me posted!
Bill

hot_rod

What I have found, Mike

is a minimun slab setpoint of 70F is much to cool to control a radiant floor. Heck it will be 70 just from the FA system. 70F feels ice cold on bare feet to radiant worshipers!

Lets say the heating stat is set to 72F. This is the temperature at which the homeowners agree, and feel most comfortable. Any floor temperature above that number WILL add heat and slowly overheat the space.

A max of 80 on the slab, or tile surface certainly will overheat that space, as you still add heat to a space even with a seemingly small 8 degree delta T.

In this case the very small square footage of radiant compared to the whole square footage, lets say 2000 on the main, radiant zone level, will not allow that stat to stay in stage one very long on a heating day. As soon as stage two kicks on the FA overpowers the radiant and satisfies the call for heat. The floor starts cooling off until the next heat call.

Forcing the radiant on with a slab surface setpoint above the ambiant air setpoint, will overheat.

It really gets exciting when you throw a morning solar gain into the equasion, when the floor has a belly full of heat

hot rod

Unknown

Hey hot rod,

What temperature would you suggest if he was to install a surface setpoint sensor?

Seems that this is a pretty tricky condition. Talk about ballancing act!

Gary

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hydronicsmike

I do agree...

...but you will always get what you want. He wants warm feet. Set your slab minimum to something that makes you happy. Sure enough, if the surface of the slab is higher than ambient, it will put out heat. No question. But is it really going to overheat the entire floor level by that much considering it being really only a very small portion of the zone?

Its really only a question of either or, unless he adds cooling to the system. What is it that makes the customer happier?

hot_rod

It will always come down

to the homeowners idea of comfort. I have found a surface temperature of around 74-75 is about as low as I can go and still detect a warm floor surface. In the winter months I'll bump my setpoint bath floor control to 80 or 82. I use a setpoint control with and adjustable diff. set to 2 degrees. It does take some seasonal adjusting to maintain an ideal temperature, at least for me.

To answer the original question it would be helpful to see the job. You need to know where the forced air stat is mounted in relation to the radiant floor zone. What is the rest of the main floor layout. If bedrooms with closed doors are on this level, etc, etc.

It is always a balancing act. You need to adjust to thee customers wants, needs, and desires. Surprisingly, often times a simple, inexpensive setpoint control with a slab or surface sensor is just what the homeowner wants! Mount it in the mechanical room with an easy to understand and adjust control and away you go.

Often times homeowners really want a simple adjustable control. Like in the days of the "round Honeywells" Turn clockwise for more heat, counterclockwise for less.

I've found complicated, multifunction, programable, hi-tech, controls with ten pages of instructions confuse the homeowner. For them them the rotating "knob" works best. It puts them back in the drivers seat, in control

Ask your customer, or look at their VCR, they know their technology comfort range.

hot rod