Hydronic heating issues with precast slabs

[shobroco]

I'm a structural guy working on a 3 storey building design for someone who has built a couple of houses & knows everything. He has done his own hydronic heat & wants to do it again, & I want to use precast hollowcore floors with a topping for the tubing; the precast would form the finished ceiling for the spaces below. He says he has to insulate the underside of the slabs & drywall them or the rooms below will overheat & the rooms above will be cold, I say it's all in the controls & insulation is not necessary. He said he couldn't control his house heating properly until he insulated the floors (composite open web joist/concrete system). Does anyone have direct experience with this?

 

[willard3]

Start with the fact that heat rises....

 

[shobroco]

Yes, exactly. He claimed that his slabs in his house radiated the heat downward, however the water temperature is only 30 or 35C so I find it hard to believe.

 

[Ziggypump]

Heat doesn't rise, hot air does. Think infrared heaters. Without some insulation, the spaces below will be heated, but not to the point of the space above not heating. And yes, controls will take care of his worries.

If you were installing the coils below the slab, and no insulation, a lot of heat would be transferred to the space below. Insulation would be needed.

 

[moltenmetal]

I have no direct experience with this exact situation, so take what I say with the appropriate measure of salt.

Somewhat surprisingly, most of the heating done by these systems is by radiation, not by free convection. Infrared light is just as good at shining downward as it is at shining upward, so the "heat rises" comment, while true, is also a bit misleading.

Insulating per se is not required, unlike in a situation where you have tubing mounted UNDER a wooden floor and hence you need very high temperatures for the floors to work at all. Insulation is less necessary with a really excellent thermal coupling between the tubing and the floor surface you want to heat (i.e. what you'll get when you embed the tubes in a floor topping such as lightweight concrete or gyp-crete). The slabs are hollow so the top-to-bottom thermal coupling isn't going to be perfect even without insuation. Insulation becomes more necessary any time the client wants to top your gyp-crete covering with something of poor thermal conductivity, i.e. hardwood or cork etc.

Insulating the underside of the top floor will definitely make the system easier to control to the client's satisfaction on a floor by floor basis- the top floor frequently having the MOST heating demand when you actually run the calcs- but so would the inclusion of some radiant walls or a couple flat panel rads in the design.

The big challenge with these high mass radiative heating systems is temperature overshoot during spring and fall. These systems are not good at tracking setpoint changes. A few faster-responding elements such as rads in the design, plus a good outdoor reset temeprature controller, help a great deal with making these systems work satisfactorally.

 

[MiketheEngineer]

Call a n HVAC guy who knows what he is doing

 

[shobroco]

The owner has been talking to a mechanical engineer (whom I have worked with and who is very good) however he has his own ideas about things and likes to be his own designer. He doesn't interfere with the structural end so I have no issues, but I'm trying to demonstrate that my system choices are efficient and economical and he drags in things like the supposed need for insulation negating the advantage of the precast slabs as both structure and finished ceiling.