ICF, I am afraid I am struggling right now in finding a way to better help you understand this. The sun is the ultimate radiant heat source. Like hydronic radiant floor heating, this solar radiant heat is transmitted by infrared waves and this radiant heat is NOT absorbed by air. The only way that irradiance (solar radiant heat) will cause air to get warm is by first striking a solid surface, causing this solid surface to get warmer than the room temp, which will then allow this solid surface to transmit heat into the room via both radiation and convection. It is convection that causes air to get warm. This is why air gets colder as you move further away from the surface of the earth and go up in altitude. In fact, if you cause the air to move more around this heated solid surface (e.g., ceiling fans, etc), you will actually increase the convection heat gain into the room. An intelligent controller doesn't let the solid surfaces (i.e., thermal mass) that are exposed to irradiance ever reach a temp that would cause the room heat gain to ever exceed the room heat loss.

Good passive solar thermal mass has an absorptivity of about 85% which means that 85% of the irradiance heat BTUs that enter the room are immediately absorbed by the thermal mass. So, by "some", I was referring to the 15% of heat BTUs that are NOT immediately absorbed by the thermal mass. The 85% of heat BTUs that are immediately absorbed by the thermal mass are immediately conveyed by the hydronic system from the irradiated thermal mass to other rooms in the building where this heat is needed. The 15% of heat BTUs that are NOT absorbed have already been planned for, are used to heat this room, and the thermal mass temp in this room has already been configured by the hydronic system to provide correspondingly less heat gain than it normally would so that the total room heat gain is maintained equal to the total room heat loss resulting in no change to the room indoor temp.

The only way that irradiance (solar radiant heat) will cause air to get warm is by first striking a solid surface, causing this solid surface to get warmer than the room temp, which will then allow this solid surface to transmit heat into the room via both radiation and convection. It is convection that causes air to get warm. This is why air gets colder as you move further away from the surface of the earth

OMG. What a trainwreck.

What happens when you compress a gas?

Gotta say I can't see how slabs can be used dynamically as opposed to passively. You have to assume an hour plus of thermal lag per inch of concrete. In other words, if irradiance starts at 10 in a corner of the room, it could be 1 in the afternoon before the first btus reach pex, assuming center placement. Then for a few hours after that, you have the issue of a modest amount of pex, a whole lot of concrete and not much delta T. Then the sun goes down. I have to admit that my experience is colored by the absence of rays in the NE where, alas, if can be cloudy AND cold. But if I did have major insolation I can't see how pex on 8 inch centers under 2 inches of concrete could stop the room from overheating.

Even so, my passive solar house took a dramatic turn for the better after I installed nuclear winter shades. I cribbed the idea from Doug Kalmer on builditsolar.com, a pretty simple one of gluing foil faced bubble wrap to roller shades. I went Doug one better though by using HD outdoor patio shades.http://www.amazon.com/Cabo-Sand-Roll-Up-Shade/dp/B00CTIR9IW/ref=sr_1_1?ie=UTF8&qid=1387924057&sr=8-1&keywords=cabo+sand+roller+shades $50/window and much more attractive than you would guess. I thought I'd save them for cold spells, but I've been dropping them every night. I also have motorized cellular shades, so I'm figuring somewhere between R6 and R8i counting uncoated double pane as R2. I have modest condensation, an inch or two on the bottom of the windows, when ambient drops into the teens.

You have to assume an hour plus of thermal lag per inch of concrete.

Yup. Takes, I dunno, 90 minutes to see an effect in the circulation. I'm the one who puts tubing towards the top like it is supposed to be. Because it is a sunroom with full sides and overhead exposed, it is possible. There are very few other places in a passive solar where you get insolation that long due to the apparent motion of the sun.
It is also possible because of the low temperature system. Think about it. If you are circulating at 100F, you aren't going to get much heat from a slab at 95F. On the other hand, if you are circulating at 75F.....

And finally, circulating a hot slab isn't about preventing overheat in the room. You have to have another strategy for that. What it does is to move SOME heat from the sunroom slab to the system, allowing it to distribute throughout the rest of the house.

When our temps dropped into the low teens, there was no condensation. In fact, as previously noted, we used the sunroom during that period of low outdoor temps and sun to thoroughly dry some wet glulams that we were trying to mill up for stair treads.

Exactly right Todd, the thermal lag of standard concrete is 42.1 minutes/inch. You can easily reduce this by 50% by using high conduction rate additives which is a beneficial approach for the irradiated thermal mass zones of buildings using integrated passive solar and HR heating systems.

Furthermore, you have to understand that a concrete thermal mass still immediately absorbs about 85% of the passive solar irradiance BTUs, even though the heat conduction through the thermal mass thickness experiences this thermal lag before reaching the pex. And you also have to understand that the physics/math associated with all of this is fully deterministic. So there is no need for the pex to immediately transfer the increased passive solar heat gain that was not immediately absorbed.  The thermal mass temp only needs to be at the right reduced temp at the right time so as to provide the right reduced heat gain to compensate for the increased passive solar heat gain that was not immediately absorbed.

Nevertheless, a standard HR control system will not be able to properly control the temps of a building having significant passive solar heating. This is why ICF has to move hot air from his hotter zones to his colder zones in order to maintain adequate temp control. This is also why an intelligent controller that provides sufficient predictive control system phase lead as previously described is required for buildings using integrated passive solar and HR heating systems if you want to avoid having to move hot air in order to maintain adequate temp control.

No amount of prediction or "intelligent" control can stop a passive solar space from overheating, particularly in a residence, and especially not through circulation.

Sorry ICF, but you couldn't be more wrong. You still seem to be basing your thinking on the common misconception that radiant heat sources (e.g., passive solar or HR) directly heat air in buildings. Radiant heat sources do NOT directly heat air in buildings. Radiant heat sources can only heat air in buildings if convection heat gain is first allowed to occur. Convection heat gain can only occur in buildings when air is allowed to move in close proximity to a surface that has been heated to a higher temp than the air. If a passive solar irradiated surface is maintained at a lower temp than the desired air temp, convection heat gain can't occur and the air will NOT be overheated.

If you think about it, this is really no different from how a "passive" thermal mass approach INITIALLY works. A "passive" thermal mass approach initially reduces passive solar overheating via immediate irradiance absorption and convection heat loss until the thermal mass temp heats up to the desired air temp. Once a "passive" thermal mass reaches and exceeds the desired air temp, convection heat gain occurs and the "passive" thermal mass stops working. A "passive" thermal mass approach also can't do anything about the irradiance BTUs that were NOT immediately absorbed. Only an "active" thermal mass having sufficient area operating at a sufficient lower temp than the desired air temp (i.e., having sufficient heat transfer control authority) can address this issue by immediately creating a BTU convection heat loss equal to this unabsorbed irradiance BTU heat gain. You simply can't accomplish this using a standard HR controller and a standard HR zoning approach.

Sailor - if I understand what you are saying correctly, you need to have the slab at a lower temp then the air so it can reabsorb the btu's that bounce of it.

We have radiant floors because we don't like cold floors.

Sort of FBBP. It isn't so much a matter of absorption performance as a concrete slab will absorb the same irradiance BTUs at most any temp. It is more a matter of creating a convective heat loss to compensate for the unabsorbed passive solar heat gain.

I suppose whether or not you consider the floor to be cold depends on your perception when you are actually standing barefoot on the floor. In a well insulated building without significant passive solar heating, you only need a floor surface temp of about 74F to maintain a comfortable HR level at the outdoor design temp for the building. For a floor zone receiving passive solar irradiance, you only need to keep the floor surface temp a couple degrees below your desired indoor temp to create the required compensating convective heat loss. You also need to keep the core of the thermal mass a couple of degrees cooler than the floor surface temp to create the required heat transfer rate from the irradiated surface to the pex so the floor surface temp does not increase given the irradiance heating load. So if your desired indoor temp is 70F, the floor surface temp of your floor zone receiving passive irradiance may be about 68F.

I am standing barefoot on a sunny floor having a surface temp of 68F right now and it feels quite lovely to me. The supply temp to this irradiated zone is 62F and the return temp is about 86F on average (it ranges from 82F to 90F depending on circulator speed as aggressively limited/controlled by the controller). The supply temps for the primary zones not receiving irradiance are about 84F and their return temps are about 74F. The supply temp for the building perimeter zones are about 74F and their return temps are 60F, which is used as the low temp supply for the irradiated zones when the controller calls for passive solar irradiance bypass mode.

***I am standing barefoot on a sunny floor having a surface temp of 68F right now and it feels quite lovely to me.***

Nah - I'm not buying it. The sun is shining, you're thinking about all the money your are saving and that is giving you a warm fuzzy feeling. It is in fact over ruling your mind so that you don't know that your feet are cold.

So the supply is 62. Would that not mean that at least some of the floor is close to that temp?

The return is 86 so some of the floor would be that hot on the surface. This would be radiating to the air as well as conducting to the pex would it not?

Did you indicate in an earlier post that you had programmed the controller to seek out the coldest zone or do you just assume certain ones will be cooler and tell it to open those?

You still seem to be basing your thinking on the common misconception that radiant heat sources (e.g., passive solar or HR) directly heat air in buildings.

Interestingly enough, my thermostats measure the air temperature directly. They don't "look" at the slabs.

Have you figured out what "adiabatic lapse rate" means and why the air gets colder as you go away from the surface of the earth?

Yes FBBP, it may all be perception as standing in the irradiance zone of a well done integrated passive solar and HR floor heated space is indeed a beautiful thing and the utility cost savings is very comforting too. Seriously though, I don't discern any difference in stepping from the 74F non-irradiated area to the 68F irradiated area. I do notice a difference when stepping into the master bathroom which operates at about 82F.

You are correct that if the hydronic fluid temp is a given temp, then the slab temp in close proximity to the pex will tend to be close to this temp too. However the surface temp of the slab can and will often be vastly different than the pex fluid temp because the slab thermal lag effect and the slab heat transfer effects.

The surface temp of a HR slab depends on three heat transfer effects: 1) the heat gain/loss from the pex circuit which is primarily a function of the pex spacing, the supply temp, and the return temp (which is itself a function of circuit flow rate provided by the circulator pump and the temp of the slab in close proximity to the pex), 2) the slab downward/outward heat loss which is a function of the local climate and soil conditions, and the R-value used to insulate the slab, and 3) the slab upward heat loss (or heat gain if there is significant passive solar irradiance) which is a function of heat radiation and heat convection into the living space. This is why a slab not receiving any passive solar irradiance may have a supply temp of 84F and a return temp of 74F while still maintaining an average slab surface temp of 74F. If you take a Harbor Freight IR temp gun and measure the floor surface temps across this slab, you will find that the area where hydronic fluid enters the area (i.e., where the hydronic fluid is close to the 84F supply temp) will be a couple degrees higher than 74F and the area where the hydronic fluid exits the area (i.e., where the hydronic fluid is close to the 74F return temp) will be a couple degrees below 74F. However, the good news according to Sieg and from our experience is that as long as you keep the supply/return temp delta less than about 15F, your bare feet will not be able to discern this either.

In December, our irradiance area is about 18' into the southern rooms which is the maximum value for our passive solar design given the roof overhang and fenestration geometry. This irradiance area is divided into four zones for the reasons previously described. The first 2' of area from the building perimeter is also zoned because this is where the maximum slab downward/outward heat loss occurs which allows us to use these zones as the low temp supply to the irradiance zones when required. This perimeter area also benefits from having independent control given that it doesn't experience irradiance (i.e., it is shaded by the wall below the fenestration) and to allow better addressing localized fenestration heat loss effects. All the remainder of the standard HR zones simply operate normally, but significantly benefit from all the harvested free solar heat gain. There is no need for these standard zones to do otherwise as the air temps in all the zones do not deviate from the desired temps.

Actually ICF, that is another common misconception. Your thermostats are measuring more the result of the HR radiation striking and heating the temp sensor than measuring the actual air temp. The actual air temp is typically several degrees colder than what a temp sensor in the proximity of a HR floor indicates.

You can prove this to yourself by shielding the thermostat from your HR floor and observing what happens. Just loosely cover it with some aluminum foil for about 40 minutes and then notice what the temp reading is after removing the foil. This is also the reason why you can sometimes feel perfectly comfortable while standing or moving about in a building/room with a HR heated floor, but you notice that when you lay down on your comfy couch or on top of your bed you soon get a chill and somehow feel colder. Of course, this is also one of the reasons why HR floor heating systems are so energy efficient. Your air temp can be significantly lower than it would have to be for a heated forced air system and you will still be perfectly comfortable.

Given that it is the surface temp of the floor that determines the BTU heat gain of the space, you really need to be directly controlling this temp too if you want to ensure precise indoor temp control. This becomes even more important when you have significant passive solar heating. Just closing the control loop on an inaccurate room temp sensor is akin to just treating a symptom instead of addressing the root cause. If I had to rate the importance of HR temp sensors from most to least, it would be slab, outdoor and indoor. If you have accurate slab and outdoor temps, an intelligent controller almost doesn't need to use indoor temp. I would highly recommend that you take advantage of your slab temp sensors.

Your thermostats are measuring more the result of the HR radiation striking and heating the temp sensor than measuring the actual air temp.

No amount of hair splitting will facilitate this discussion. I can cover the thermostats with a box impenetrable to "HR radiation" and flush warm air into the box and the thermostats will register the increased air temp. Your contentions are just becoming ludicrous from a real world point of view.

Sorry you feel this way ICF and that we are unable to come to a meeting of the minds on this subject. Having successfully done several commercial/residential integrated passive solar/HR design/builds using our preventive medicine approach, we would never go back to the old approach of allowing air to overheat in some zones and then having to distribute it to other zones that would otherwise be underheated. We would only consider that approach if we were pressed into a situation having to intergrate passive solar and ducted hot air systems...and these days we would recommend mini splits before going down that trail. As I said before, if your approach works for you, rejoice and be happy.  This discussion is concluded.

I suggest installing a WEL system and putting real-time data online.