If you have an ERV doing its job you probably already have the "fan" part covered.

It's only a 12' X 20' sunroom with no inlet or outlet on the air exchange system. The air moves nicely into the house, probably because it is being heated in the sunroom and rising to the top. The sunroom is extremely tight. The natural flow is out of the sunroom at the top and into the sunrroom at the bottom with cooler air. The door has a tall transom window above it and I should have made it an opener. That would have increased the flow due to being higher.

as for the storage path though, you can't stop at "in the water"

This isn't about long-term storage, although part of the reason for the slab to be thicker is to allow it to soak up the maximum amount of sun we might see on a particular day and continue to radiate it into the night. Also to store the heat if the system isn't flowing at that particular minute.

If my system is flowing at 7 gpm with a return temp of 77F and I cut in the sunroom slab and the return temp goes to 78F, how much heat am I collecting? That doesn't include the warm air going directly into the house...

Note that the storage capacity of the 6" slab is more than 3,500 BTU/degF. If the working range is 90F - 75F, that yields potential storage of more than 50kBTU.

We're certainly not talking about long term storage absent the awesome phase change fun our friend here was having... that sounds like a blast to play with, but it's not a residential topic of course.

at 7 GPM with a 1 degree rise you're pulling about 3500 BTUs/hr out of the slab... but only if you have a place for that heat to go, otherwise you'll raise your water temp to slab temp and stop collecting pretty quickly, and all you've done is add the water volume of your system to your storage cap. that's something but not worth installing a system specifically to make happen in most cases... absent a very large water content system. IF you send it to otherwise colder concrete, then you add that concrete to your storage and it makes more sense EXCEPT for the argument I already made about "why bother" and the conductive path it takes to get that concrete into play.

concrete can store 26 BTUs per cubic foot per degree F.. about the same as 3 gallons of water. most radiant systems will have 20-30 gallons, but lets' even say you have a 100 gallon tank on the system... 33 cubic feet of concrete and no moving parts and you've got passive storage to equal that water right where the sun hits. if all you have is a small collector space, like sunroom, maybe the tank is better (but the fan is even better, or convection)... but in a passive solar open floorplan, thickening the concrete should be a no brainer instead.

Well, there is little reason to fret about concrete control breaks relative to active thermal mass floor assemblies. You can place the tube at the bottom of a 4-6” slab and you can remove as much passive solar heat as is required to accomplish the buffering. The thing about heat transfer…heat always takes the path of least resistance from hot to cold. If heat only hung out in the top of a slab, we wouldn’t need to add insulation to our slab floor assemblies.

We target placing the tube between 1/3 and 1/2 the slab thickness from bottom of slab…well out of the top 1/3 concrete control break danger zone. We are not fans of just stapling the tube to the insulation. Not so much because this creates a longer and less efficient heat transfer path (which it does), but more because we just don’t like to deliberately and unnecessarily introduce high stress concentration points in concrete slab assemblies. I suppose this is more engineering paranoia than a practical issue since this practice is fairly common, presumably because it is much quicker to accomplish leading to higher installation profit margin.

The reason we "bother" with active thermal mass floor assemblies and integrate our passive solar and hydronic radiant floor heating systems is to minimize our customer’s heating operational cost and system acquisition cost. If you actually do the engineering for a passive solar high heat gain design (i.e., one that targets 70-100% of your required heat load in a cold climate), you soon discover that just throwing more concrete at the problem does not adequately address the clear sky heat gain overheating issue normally associated with passive solar heating unless you compromise on your climatic heat gain objective. Sure, you can use fans to blow hot air around in what will turn out to be a failed attempt to address this. However, you can effectively distribute much more heat precisely where you need it by circulating water in 1/2” tube.

Regrettably, much of the information published about passive solar heating design is nonsensical rule of thumb stuff invented by the earlier pioneers in the 1960/70s before we had computers and could do a proper passive solar heat gain analysis. Unfortunately, much of this obsolete information is still propagated and still badly misused. My personal favorite is that “for every square foot of south wall window area in excess of 7% of the floor area, a passive solar building should have 5.5 square feet of 4 inch thick thermal mass material.”

There are many factors that can significantly affect thermal mass performance. For example, the maximum temperature that a thermal mass will reach during the daily irradiance time period depends on the initial temperature of the thermal mass, the daily irradiance magnitude (BTU/Hour), the daily irradiance time period (Hours/Day), the absorptivity of the thermal mass material, the specific heat capacity of the thermal mass material, the actual mass of the thermal mass, and the exposed floor heat loss. The heat gain provided by a thermal mass during the night time hours depends on this maximum temperature, the surface area of the thermal mass, the emissivity of the thermal mass material, the R-value of the thermal mass material, the convective heat transfer coefficient of the surrounding air, the specific heat capacity of the thermal mass material, the actual mass of the thermal mass, and the exposed floor heat loss. A thermal mass will also release heat during the daily irradiance time period when the thermal mass temperature exceeds the room temperature. Consequently, the passive solar heat gain that occurs during the daily irradiance time period can result from both the irradiance that enters the building that is NOT absorbed by the thermal mass PLUS any heat gain that is released by the thermal mass during the daily irradiance time period.

Anyhow, we have successfully used active thermal mass (high mass slabs) in our integrated passive solar and hydronic radiant floor heating system designs to eliminate the overheating issue and to minimize heating operational cost, and we are not going back to the old lame ways…

I can't imagine the physical process that would make this desirable compared to either more mass or simple air movement. I have never seen a redistribution scheme that ended up worthwhile in the end. But, I have not done passive solar engineering either, so it's entirely possible this is is ignorance on my part and/or I have only seen people flailing at passive solar ineffectually.

I would need to see math on why more mass wouldn't be as helpful as more mass connected by flowing water, though... that seems a bit odd. direct conduction downward through concrete must, as I noted, be more effective than adding a water transport link the middle of the conduction pathway.

otherwise you'll raise your water temp to slab temp and stop collecting pretty quickly, and all you've done is add the water volume of your system to your storage cap

I have the altherma set to deliver 84F water continuously. Because the return water is such a low temp, the slab was at a significantly higher temp all day. Without the sunroom operating, the water returns at 78F. When you cut the sunroom in, the return temp goes to 80F, actually at a rate of 9 gpm. That is 2F that the Altherma does not have to raise the water to send it out at 84F, representing 8,000 BTU per hour of energy. The water temp does not come fully up to slab temp because it is passing through at a certain rate. I took solar energy out of that slab all day and into the night and redistributed it to other portions of the house that were losing heat. Not sure why you keep bringing up the storage issue. We are using it as we collect it, except for the slab portion that coasts somewhat into the evening.

well out of the top 1/3 concrete control break danger zone.

Why is that a "danger zone"?

I can't imagine the physical process that would make this desirable compared to either more mass or simple air movement.

Slab mass is calculated according to total solar input at this location and the efficiency of that input. The sunroom is a defined size which defines the aperture and that, in turn, limits the amount of solar input that will reasonably be seen and collected. More mass doesn't do anything unless you make the sunroom (aperture) larger.

The extra energy needed to circulate through the sunroom is nil - a few watts added, according to the alpha pumps. 3,000 to 8,000 BTU per hour is a huge return in exchange for a few watts. AND, that only represents the energy removed through the radiant system. The warm air moving into the house represents additional benefit on top of that.

ok, white flag. I am definitely off base in some of my thought processes here.

ICF: I'd love to see/hear how you have your system set up if you have pics or sketches. I am not sure how you are getting around overheat if the altherma is in continuous demand mode... standard zoning? are those zones actually calling when you have the sunroom running, or are you forcing them open to receive solar heat?

sailaway: I have some learning to do about passive solar mechanics in concrete it seems.

I am remembering a driveway snowmelt we did once, dark concrete, where we were told to make it try to heat a pool. the water coming out of that driveway even on overcast days was surprisingly hot. I'm pretty sure that pipe was not elevated past the midpoint of the slab. It was a huge collector, but still, temperature is temperature. so I'm all wet on some of this: time to re-evaluate some assumptions.

thanks for sharing all.

Rob, perhaps the simplest way to think about the fundamental passive solar design problem is this way:

1) Maximum passive solar heat gain occurs during clear sky days (i.e., days with a clear atmosphere, no clouds, etc)…which we call Maximum Instantaneous Clear Sky Solar Heat Gain (BTU/Hour) and associated Maximum Monthly Clear Sky Solar Heat Gain (BTU/Day).

2) Often, the majority of days are NOT clear sky days and you get much less heat gain during these days…which we call Average Monthly Climatic Solar Heat Gain (BTU/Day).

3) Your design objective is to harvest and distribute as much Average Monthly Climatic Solar Heat Gain as possible for your climate/location to offset your actual building heating load requirement for that month.

4) You do NOT want to overheat the building during the clear sky days when you have Maximum Instantaneous Clear Sky Solar Heat Gain.

Item 1) can be accurately determined for any given location (ASHRAE has details).

Item 2) can be forecast with fairly good accuracy if averaged over a specific month…similar to degree heating day data.

Item 3) can be accomplished by accurately designing the building roof overhang and solar fenestration (SHGC and area) for the given location and properly accounting for any local terrain obstacles (see our website software).

Item 4) gets to the real nature of the problem…

What happens when you have a clear sky day? If you did your design to accomplish item 3) and you don’t have any thermal mass, you will have a severely overheated building. As you begin to add passive thermal mass, you begin to reduce the severity of the overheating. If you can actually get enough passive thermal mass area into the building…which can be very challenging because of the magnitude of the Maximum Instantaneous Clear Sky Solar Heat Gain and the requirement that the irradiance needs to strike the thermal mass to be absorbed (which can be additionally challenging for most floor plans that people like to live in)…yes, you can theoretically eliminate the overheating.

But even if you can actually get this much passive thermal mass into your building, what happens during item 2) which may be the majority of the time? The passive thermal mass, which is now excessive for this condition, will absorb much of the heat gain that you need during the irradiance period to offset your actual building heating load requirement. In fact, if you use this approach, there would be many days that this excessive passive thermal mass would not even get to a sufficient surface temp to convect/radiate/release any of the absorbed heat gain back to the building after the irradiance period. In short, you will fail to meet your item 3) primary design objective.

So the only way around this dilemma is to have a “variable” thermal mass capacity to address the variable nature of passive solar heating. If a hydronic radiant floor heating system is also part of your design, it is relatively easy to also include isolated active thermal mass zones (areas near the solar fenestration, sun rooms, etc.) to accomplish this. In our simplest designs, this excess solar heat gain just reduces the heat that the boiler would normally have to provide and is redistributed to the other non-passive solar zones or is simply rejected from the building. For residential designs, if the climate is accommodating, this excessive heat can just be dumped to an outdoor slab (e.g., patio, walk-way, pool, etc). If the building is in a location where there are significant days without significant solar heat gain, the advantages of storing the excess heat may outweigh the added system complexity. However, I am a follower of KISS and low maintenance systems (especially for residential designs), and we prefer not to go down this path unless there is a very compelling reason to do so.

We have both passive solar heat gain and passive thermal mass performance software on our website should anyone want to explore this more.  I have learned a lot from hanging out here a few months.  Sharing and learning is a good thing.  Just wish we could be little more gentler to each other sometimes... 

I am not sure how you are getting around overheat if the altherma is in continuous demand mode... standard zoning?

Because we are dry walling right now, mudding, priming, texturing, etc., there is no point in setting the control system. We brought up the radiant slab temperatures slowly, over a couple week period from 77F to 84F, which might have been about right for the mid-20F weather we were having. Once the weather hit the mid 30s, I was going to knock it back to a constant supply of 83F, but we had some heavy mud days and needed the extra heat to keep the air moving, so I didn't adjust it. Now, the temp has "skyrocketed" into the mid 40s, and the supply temp definitely needs to go down, but we had a moist environment yesterday, thanks to 65 gallons of primer shot all at once, so I am keeping it high yet another day. SO, the short of the long is that we have open windows to vent the extraordinary humidity being generated right now and the overheating condition just facilitates that.

I do have to say, however, that I am struck by how steady this system is. You could almost set a steady supply temp based on outside temperature. Maybe that is the benefit of interior mass.

Posted By ICFHybrid on 26 Mar 2013 02:27 PM
I do have to say, however, that I am struck by how steady this system is. You could almost set a steady supply temp based on outside temperature. Maybe that is the benefit of interior mass.

YES, YES, YES, and why we are so excited about ICF that has the majority (if not all) the foam on the exterior side and why we use both actual and derived/forecast outdoor temp in the PID control algorithm!

" we just don’t like to deliberately and unnecessarily introduce high stress concentration points in concrete slab assemblies. I suppose this is more engineering paranoia than a practical issue since this practice is fairly common, presumably because it is much quicker to accomplish leading to higher installation profit margin"

Yes and no.

Yes it is paranoia, (perhaps if you define high stress and how the a residential concrete slab might suffer same) and yes, it is "quicker"-- we call it; efficient use of labor-- and no, it is not primarily to gain a higher profit, but to martial human resources (we consider the most precious) in a judicious manner. Reductionism if you will. I appreciate the your depth of knowledge in passive solar design having kept up with this common practice in the South West, but bristle at the cavalier use of labor.

Raising the tube in a slab will raise the potential output, but raising it for any other reason, including tube contact with the concrete, will return no measurable gain. As for output, once you have enough and confirm that design water temperatures are met by the heat source, in a reasonably efficient manner, the rest is parsing amphibian cilia.




http://www.ijee.ie/articles/120504/article.html

What happened to the OP? Did it get too "deep" in here for them? (grin)

Here's what I would do in Minnesota:
ICF basement and first with QuadDeck main.

I'd put radiant in basement and main and polish the concrete floors

Use proper orientation and solar-appropriate windows and design on the South.

ETA: Sample cost for 30' X 50' footprint utilizing ICF and concrete deck.

ICF Walls @ $15/sf wall:  (30' X 2) + (50' X 2) = 160' X 19' height = 3,040 sf X $15 = $45,600
Concrete deck @$12/sf : (30' X 50') X $12 = $18,000
Basement Slab @ $5/sf : (30' X 50') X $5 = $7,500

TOTAL = $71,100 / 3,000 sf livable = $25/sf

Of course, there are excavation costs, and I didn't include footings or a lid, windows and doors or any utilities or finishes, etc., etc., but that's the start of a mighty fine energy efficient home.

Barely keeping my head up myself and I have taken more solar system out then most have seen.

I would live in a house like that...

Badger, there is a pretty good description of stress concentration here:

http://en.wikipedia.org/wiki/Stress_concentration

Introducing a stress concentration point basically means disturbing the smooth, uniform surface of a structure so as to create more localized stress than there would be normally (i.e., perhaps by drilling, cutting, adding tube in this area). Structural failure will often occur at stress concentration points. This shouldn’t be a problem for slab-on-grade assemblies because the loading is largely compressive. Concrete is great for compressive loading, but is not good for tension loading…which is why we reinforce it with steel rebar (steel is great for tension loading).

For a first-story elevated slab assembly, the bottom-most portion of the slab will be under maximum tension loading. So placing tube at the bottom of this slab assembly raises the hairs on back of my paranoid mechanical engineer neck. I would want the tube dead center in the slab where the stress is minimal. Designing safe building structures is a complicated subject onto itself. Licensed civil or mechanical engineers need to take a separate vigorous exam to receive a special structural endorsement in order to design building structures.

Perhaps the most famous stress concentration failure was the de Havilland Comet:

http://en.wikipedia.org/wiki/De_Havilland_Comet

I appreciate your references, but my reference was to residential, slab-on-ground installations, for the most part immune to the consequent failures you submit. I am not an engineer, but have fought with a skeptical public-- both layman and professional-- over the completely innocuous addition of radiant floor tubing to the average slab-on-ground installation. We nearly always elevate the tube on more demanding i.e. thicker slabs where heavy equipment may tread, or machinery is placed. As you say; for structural more than thermal performance. As you know, the thicker the slab the smaller the concern.

We had only experience to back up our claims of radiant floor tubing having no ill-effects on the structural integrity of slab-on-ground installation until we designed (circa 1993) a 100,000 s.f. building that housed heavy machinery including several 40 ton presses as I recall. They were anchored several feet into the ground independent of the floating slab and we went around them. Long story short, big cracks everywhere, structural engineers called in with concrete engineer at the lead. Conclusions; bad concrete, uneven pour, pull it out do it over, no more problems.

As for suspended pre-stressed concrete, mechanical contractors know that you don't drill through them with out the benefit of a stamped work order...heheehee
I think large spans of elevated concrete may be beyond the scope of this enlightening discussion. Thank you for the tutorial.

Agreed, designing concrete building structure is definitely a specialty and must be done properly…and we don’t offer that service either…way too stressful… If anyone is looking for a great concrete structure civil engineer in CA or OR, we would highly recommend Eric Snyder:

http://www.snyderengineer.com/

Sorry you didn’t like my airplane example. I also manage Flight Operations Engineering at Boeing, so I often tend to mix airplane and construction topics when discussing general engineering design approaches.

So this is your part-time gig?

Right now, more like multiple full-time gigs providing vision and engineering/project management oversight to very capable teams. While the end products are vastly different, the processes used to create innovative and successful products are remarkably the same (e.g., research, conceptualization, feasibility, requirements, design, and production/construction). Like I use to tell my scouts back when I was Scoutmaster, if you can make your career something that you really love, it isn't really work and you won't be able to get enough of it.

Besides, we all know how tentative the Lazy B is as a financial concern.