Re: Thermal Collectors / Mirror Concentrators

Dan Lepinski, P.E.

"Thermal mass" works like a storage and slow-release system. Its gradual and steady release of heat evens out the temperature swings (variation) associated with conventional heat sources.

The method you're suggesting defeats the entire purpose of thermal mass. Installing a heat source on top of the slab directly beneath the flooring is no different than baseboard or other forms of heat, except it's more evenly distributed. There may be minor thermal coupling between the hydronic heat system and the slab, but not nearly to the same extent as using hydronic plumbing embedded in the slab.

Used properly, thermal mass is an excellent means of controlling ambient temperature in cooler climates. Problem is, people aren't patient and want immediate results .. which thermal mass won't provide. It's slow to heat and slow to cool. That's what makes it so effective.


On 6/1/21 9:20 PM, Jim Duncan wrote:
Installing hydronic heating to a slab should only be done on the top of the slab. Flooring panels with indented tracks for the heating hoses goes down on the concrete and under the finished flooring. The heat is less than an inch below and the heat goes into the room and not the concrete. Makes an easy retrofit for existing slab foundations.

Jim Duncan Solar Acres Farm 817.917.0527

On 6/1/2021 2:21 PM, Donald wrote:

A subcontractor specializing in Hydronic heat installed both systems & I confirmed they were installed per manufactures guidelines.


They both had insulation between the concrete slab and the dirt below.




*From:* <> *On Behalf Of *Joachim Saupe via
*Sent:* Tuesday, June 1, 2021 2:09 PM
*Subject:* Re: [NTREG] Thermal Collectors / Mirror Concentrators


So it sounds as if the hydronic radiant heat you installed used all the concrete in the slab plus the dirt under the slab as a huge heatsink. 

IF the hydronic system would have been installed in the top, say,  2" of concrete with insulation to the rest of the slab, the heat retention would not be as long.


Hydronic radiant or Electric radiant, the heat source doesn't make any difference.




*“The test of our progress is not whether we add more to the abundance of those who have much; it is whether we provide enough for those who have too little”*

Franklin D. Roosevelt




On Tuesday, June 1, 2021, 01:33:20 PM CDT, Donald < <>> wrote:



Dan and all,

I have installed full house Hydronic radiant heat in two custom home concrete slabs. One around 2000 & the other around 1996. Both cost about $5,000 total cost. The dirt pad was prepared, pluming roughed in, grade beams dug, poly installed and rebar tied. The radiant heat crew then came in and tied their flexible piping (and there was a lot of piping) to the rebar. Then the slab was poured.

Both owners regret installing the systems. Their main reason is that North Texas experiences a cold snap and then 10 days or so later it warms up so much that the retained heat in the slab keeps the house too warm. Occasionally they resorted to turning on the AC to cool the house temps down. Both clients grew tired of attempting to figure out how to manage this. Both felt it was not worth the investment.

I am a Builder Member of the Dept of Energy's "Building America" program, <>, with 10 or so, "Building Science" independent teams. Their purpose is to decrease the energy consumed in residential homes. They have 10,000+ Case Studies and they recommend Hydronic Radiant heat above Tulsa, but not below it. They consider it not cost effective below Tulsa, as we have so few really cold days.

The Building America group also maintains that with the lower cost of PV systems, it is less expensive to install electric radiant heat systems as you generate your own energy. Based on this and other input, we stopped installing solar hot water systems about 10 years ago.

I do regularly install electric radiant heat in master baths or master suites. There is not much better feeling when arising on a cold morning than stepping on a warm floor.

Thanks - Donald Ferrier <>

-----Original Message-----
From: <> < <>> On Behalf Of Dan Lepinski, P.E. via
Sent: Monday, May 31, 2021 4:26 PM
To: <>
Subject: Re: [NTREG] Thermal Collectors / Mirror Concentrators

Thermal mass heating systems built into the slab are very low pressure -- not city water pressure.  The plumbing is also flexible to accommodate twists and turns.  Subsequently, chances of a leak are essentially zero.

Moreover, when a slab is used as thermal mass, it's insulated from the earth during construction by installing a layer of insulating material before the slab is poured.  This prevents warmth from the slab from migrating into the soil.  It also cushions the slab from earth movement.

Home builders don't install such efficiency measures because: 1) They don't know how to do it.  2) Training in the work costs $.  3) It adds to the overall cost of the home (not significantly .. but "some" cost just the same, which typical builders are loathe to do).

Mike Renner (NTREG member) has thermal heating built into the slab of his home.  It's been trouble-free from the day it was built.  Hopefully Mike will chime in and give your more information.


On 5/31/21 3:50 PM, Mellen West wrote:
I would never run hot water floor heat piping in a concrete foundation in North Texas.    The soil has high clay content and is “expansive”.   On e little leak can cause a lot of grief and expense.

 I speak from my experience as warranty coordinator for Coleman Homes.  Coleman used post tension to hold the slabs together and slab leaks were fairly rare events but when they happen it was an expensive misery for all.  Just because the Romans did it does not make it a good idea.  Makes great sense for a greenhouse, but not a whole house.  My opinion.



*From:* <> < <>> *On Behalf Of *Philip Timmons via
*Sent:* Monday, 31 May, 2021 1:29 PM
*To:* <>
*Subject:* Re: [NTREG] Thermal Collectors / Mirror Concentrators


Thanks, Dan.


Understood and agreed.  I think Solar Thermal heating actively heating a concrete slab (usually concrete embedded PEX or other tubing circulating the hot water) would be a GREAT practice here in North Texas -- but I see very little.  Folks with that stay warm even when the power goes out for days.  


Did you look at Saul GriffITH's (et al) "Rewiring America?"   (Very sharp former DARPA guy -- The Sankey Flow Chart at his level detail is worth the read, alone).  They pretty much by-passed any Solar Thermal aspects.  Or maybe I am reading too much towards this -- but it seems like (to me) we are missing a big part of the big picture with US all just going Electricity!    Here is a link if you or anyone is not familiar.  Free Download, donation / membership optional. >>>  



On Monday, May 31, 2021, 12:53:13 PM CDT, Dan Lepinski, P.E. < <> < <>>> wrote:



We have a solar hot water system on our house.  I installed it before putting in the solar electric equipment.  Both systems are 16 years old.

Solar hot water isn't "sexy" like photovoltaics.  For instance, we can't give "excess" hot water to our neighbors as happens with excess solar electricity flowing backwards through the utility meter.  When the water gets hot (per the thermostat max setting I programmed in), the system turns off to prevent overheating the water in the storage tank.

Conversely, solar hot water is much more efficient at converting solar energy into heat and hot water than systems that convert sunlight into electricity.  Reports from the Dept of Energy indicate up to 85% of the available solar energy is converted into heating the water.  Newer systems may be even better.  We have two 4'x10' "collectors" that are sufficient to heat water for five people, and still have some hot water left.

Yes, solar hot water systems are still being designed and installed.  Not so much here in north Texas that I've observed, but more so in other parts of the country.

Where electricity costs are high, and natural gas is prohibited (there are places in the US where natural gas is no longer allowed for use in new subdivisions), solar hot water is often combined with solar electricity.  California is at the top of that list.

Current "state of the art" in solar hot water hasn't changed as rapidly as solar electric equipment.  We have two primary topologies: 1) "Flat plate collectors" that to an extent look like solar panels, and; 2) "Evacuated tube" collectors that resemble large test tubes.  Evacuated tube collectors are more expensive than flat-plate systems, but are also more efficient, particularly in cloudy areas and cold locations.  Because of this latter trait, evacuated tube systems are more common in Europe than flat-plate systems.  Flat-plate systems are the majority in America.


On 5/31/21 12:07 PM, Philip Timmons via wrote:
Anyone in the group still do anything in the Solar Thermal Collector / Mirror Concentrators domain?  

Or has that kind of went the way of tail fins and large cars? 

Talking small-ish scale, single house, small business, small process sized operation.  NOT talking about acres of mirrors.  

Seems most things are going the way of Renewable Electricity ONLY, and then using Electricity to heat water, heat (and Cool) building space, as well as heat for small process.   Looked like that is where things were heading in Saul Griffin's  "Rewiring America," and also in practice?   I am not saying that is fact, just what I am observing.  

Is anyone here still using or designing Solar Thermal?  Was looking at reviewing some of what is the current state of the art. 


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