Incense & Peppermints

My alarm clock went off early this morning (welcome back to work!), and for some weird reason, I wanted to light some incense…

Settled for some Strawberry Alarm Clock instead. “Incense and Peppermints” hit #1 on the Billboard charts in November of ’67 and was co-written by band-member Ed King, who went on to later stardom with – wait for it…

Lynyrd Skynyrd.

The things you learn at FloPro!

Last week we talked about radiant floor heating in super-insulated, low-heat loss homes (click here and here).  You, dear reader, for the most part supported the idea – generally from a comfort standpoint.

No, the floors won’t feel “warm” with a max BTU/SF load in the 8-to-12 range, but most agreed there’s no contest between radiant floor and forced air when it comes to comfort, especially in rooms with high ceilings.

There was also some enthusiasm for heat-source options with low-temp radiant, including air-to-water heat pumps, geothermal and solar.  With supply water temperature requirements below 120, you don’t necessarily need a boiler.

It wasn’t all incense and peppermints, however.

A couple of readers expressed concern about shoulder-season overheating, suggesting an air-to-air heat pump be used for those periods.  Can’t argue with the logic.

But there is one quibble – nearly every reference to radiant included the word “slab,” as in “once the slab is heated it will take 2 days to drop a degree.”

Overheating is a legitimate concern.  But there’s one big “however” to consider.

Been saying this for years, but it bears repeating – unless you’re dealing with a basement (slab-below-grade) or a slab home (slab-on-grade), there’s no real reason to install radiant in a slab.

Slab-type jobs are large mass systems.  These slabs are like oil tankers – they don’t change direction very quickly or easily.  Gypsum concrete jobs are lower mass and respond a little more quickly, but still aren’t what you’d call “agile.”

Lots of folks say radiant slab installations work better because of the “mass.”  Folks, mass doesn’t equal comfort, nor does it necessarily equal efficiency.  At best, mass equals “thermal storage.”  A 4-inch concrete slab can store a truckload of BTU’s and can serve as a “damper.”  A slab can accept a ton of BTU’s without changing its surface temperature even one degree, and as we all know, a radiant system delivers heat based exclusively on its surface temperature.

Mass also equals forgiveness.  The more thermal mass you have, the bigger design/installation mistake you can make and still get a way with it.

But if you want responsive radiant, you gotta think low-mass – relatively speaking.  There are lots of low-mass radiant installation techniques/products out there that deliver excellent comfort, operate at very low water temperatures and respond very quickly.

Nearly all of them involve aluminum, which heats up fast and transfers heat like a champ.  If you install, control and zone the system properly, you’ll rarely, if ever, experience overheating simply because you “have radiant”  (key word here is “zone” – more on that in an upcoming post).

Any good radiant supplier offers low mass installation options.  Viega has Climate Panel, Mr. Pex has RetroPanel, Rehau has their Dry Panel System and  WattsRadiant offers Sub-Ray.  Then there’s Warmboard and Warmboard-R, heck, even extruded aluminum plates below the subfloor deliver BTU’s at very low water temperatures (check the output chart from this blog post).

The point is you don’t have to be such a concrete thinker when it comes to radiant floor heating.

Thermal mass isn’t good or bad, it just is.  In a super-tight house, it’s good to have friends in low-mass places..

Yep, that just happened!

2 Responses to “Incense & Peppermints”

  1. So I am new to your blog and trying to follow along… From what I read it seems that the concrete slab method is more suited as a heat sink for dissipating heat like with a geo-thermal heat pump unit, whereas the ‘radiant’ heat style methods utilize smaller/thinner media to actually transfer the heat into the space.

    Does this sound right?

  2. Yo Controls Freak –

    Sent an answer to this one on LinkedIn, but will post it here, too.

    I don’t think there’s a “best application” for slab. Radiant gets installed in a slab simply because the slab is there, like in a basement or a slab-on-grade building. It’s what you have, so it’s what you use. Slab has benefits — it requires VERY low water temps and has that “damper” effect discussed in the post. I has drawbacks, too…

    Low mass systems are used in stick frame applications because there’s no slab in which to put the tubing. Again, it’s what you have. The low mass has benefits and drawbacks, too. It basically comes down to using the best installation method for the project you’re doing. Hope this makes sense…if it’s a long answer to the wrong question, let me know. Thanks for reading!


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