My wife and I are considering a variation of solar thermal with radiant floor heating/cooling design. This is the system as the engineer/architect described it to me: The heating tubes are immersed in a suspended concrete floor over Pan Deck and a non-vented crawl space - what he calls the ‘plenum’. The crawl space has ICFs over the perimeter footings below grade and a thin layer of concrete on the bottom for rodent/moisture proofing. The hot water from the solar collector units flow to a holding tank and then to the floor. The residential water heater has coils going into the floor heating storage tank and thus pre-heats the house water. The system can be run in reverse on snowy or cloudy days when the solar collectors heat is insufficient. In this case the gas water heater pre-heats the radiant floor system. The engineer also uses evaporative coolers to push air through the plenum/crawl space and up through floor vents. I’d love some feedback on the merits and draw backs of this system.

Yes, I have a similar system myself (including the ICF and rat-proof crawl corridor) except that the concrete slab floor adjacent to my south fenestration is used to collect the heat from solar irradiance in lieu of solar collectors on the roof. With integrated passive solar and hydronic radiant floor heating systems, the devil is always in the details. The fact that you have an engineer sorting out the design is goodness and the system should work great. Solar collectors do add additional system complexity and cost, but they are also more efficient at capturing the solar irradiance BTUs (i.e., they require less area to accomplish the job). If you have an agressive passive solar house design and hydronic radiant floor heating, it may make more sense to NOT use solar collectors and instead have a hydronic radiant floor zone accomplish this.

Where will this be located?

California's central Sierra foothills: Moderately arid. Winter nighttime lows around freezing, summer daytime highs in the mid 90s. Good south facing exposure.

Can you better describe the "plenum"?
I would be concerned with the airflow over the rat slab. Using the whole crawlspace as a "plenum" will likely lead to inefficiencies even if the rat slab and footing are well insulated. The scrub effect of air moving through the crawlspace will increase heat loss.
This might be good during the cooling season but not so much during the heating season.
Could also lead to condensate issues and mould.

Just as FBBP said. You need to really know what you are doing before you start playing with all the air movement and humidity issues you have mentioned. Is there some reason "plain old" methods of heating and cooling are not being considered?

Presumably the floor is very well insulated from the crawl "plenum" so as to keep the downward heat flux to well under 10% of the upward heat flux. Our crawl "corridor" is vented and is either R50 or R60 (10” or 12” EPS). There is nothing worse than rats in a crawl corridor/space and I get light-headed just thinking about it.

I expect that you would sometimes need forced circulation to keep the air in the crawlspace at the same safe levels of humidity as the room above. Or put a dehumidifier down there.

Some people have used a crawlspace plenum heated by forced air and achieved warm floor radiant comfort without any hydronic system.

Yes, the Romans were the first to heat their buildings that way. It works, but you can more easily convey heat through a building and more efficiently transfer heat to the floor by running hot water through a small tube than by running hot air through a large duct.

Even if it was ducted, it would be a lot more efficient then using the whole crawlspace for a supply plenum or an attic space for a return plenum. But you are right, it can't compare with radiant.
Why is it that sometimes I get the impression there are two people posting as one???

Regarding the question of insulation, the perimeter is insulated with ICFs to a depth of about three feet. The bottom of the suspended concrete floor is not. It is just concrete over corrugated metal sheeting (pan deck). The engineer said this is not a problem because with the cold surface dirt mitigated by the ICF perimeter, the plenum floor should be near the geothermal base-line temperature (58ish degrees?) Also the heat transfer would be only by radiation, not conduction like with as slab. You can't insulate under a slab can you? I came across information about ICF suspended floors at this link: http://www.quadlock.com/images/insulated_concrete_forms/insulated-concrete-building-envelope.jpg I asked the engineer about this. He said that this could be done but the gain in efficiency probably would not justify the increase in cost. I didn't really understand how EPS would be that much more than pan deck unless it is more labor intensive to install. Anyway,that's what he told me.

Regarding the issue of condensation: This system has no forced air (besides the swamp cooler fan). Granted, sustained periods of high humidity are rare where we are but I hear your concern. Is it possible that the low volume of plenum air adjacent to a the large heat source (radiant floor) keeps the air warm enough to prevent a dew point?

You can't insulate under a slab can you?

It's ALL about insulating under the slab.

(besides the swamp cooler fan)

I'd be a bit worried about turning your crawlspace into a giant swamp cooler. Can the engineer show you other buildings where this is done, and what recommends it over plain old A/C?

Heat transfer from a suspended slab will be by both convection and radiation. The heat convection coefficient for still air (e.g., an unvented crawl space) would be 1.56 which, if you inverse, is equivalent to the still inside air film R-value of 0.64. You need to insulate a suspended slab, especially one that will have hydronic radiant heating as the delta T will be significantly higher than an unheated floor. Failing to insulate will result in your downward heat flux being greater than your upward heat flux (i.e., because 58F is still colder than 70F room temp and heat moves from hot to cold)...when you should keep the downward heat flux to less than 10% of the upward heat flux.  Sieg recommends a minimum of R30 for vented crawl spaces. You will likely need a minimum of 10R for your unvented crawl corridor.  As mentioned previously, we used R50-R60 under our vented crawl corridor slab for our own personal home.

So if I understand this right
- the rat slab will be at about 58º (soil temp)
- the crawlspace has no additional heat so it would then be 58º plus the heat loss from the suspended slab (which should be substantial given it will be around 85º during heating season)
- there is no air flow over the rat slab during the heating season.
- air flow is induced by the swamp coolers during the cooling season.

just because its designed by an engineer, it should work??

I share your concern FBBP and I should have originally stated "an actual licensed and experienced engineer". Based on what has been stated by the OP, I would be looking into the credentials of this "engineer".  Like most states, it is illegal in CA to say you are an engineer or to practice engineering without first having a professional engineering license issued by the state that you are designing a project for.  Unfortunately, enforcement is lax in most states and consumers are often clueless.  There are also some companies who hang out on this forum who get highly offended anytime we use the "license" word relative to "engineers" or "HVAC Pros" because they lack these credentials...and we really don't want to offend anyone.

Posted By sailawayrb on 03 Dec 2013 10:20 AM
I share your concern FBBP and I should have stated "an actual licensed and experienced engineer". Based on what has been stated by the OP, I would be looking into the credentials of this "engineer".  Like most states, it is illegal in CA to say you are an engineer or to practice engineering without first having a professional engineering license issued by the state that you are designing a project for.  Unfortunately, enforcement is lax in most states and consumers are often clueless.  There are also some companies who hang out on this forum who get highly offended anytime we use the "license" word relative to "engineers" or "HVAC Pros" because they lack these credentials...and we really don't want to offend anyone.

Sorry Borst, that was a little low on my part. Just couldn't help myself ;-)
The problem is not so much as to wither he is an p.eng. or not. The problem is if he has enough experience in this field. My experience is that I get the best results when my p.eng. works with the experience my hydronic designs/installers bring to the table. There are a number of very experienced installers that have knowledge that no engineer will ever get from books and some of them would not recognize a room by room if it was staring them in the face. If they play nice together, you can get amazing results.

No offense taken FPPB. Yes, experience is very important. However, without first having the book knowledge, a HVAC designer/installer will be learning on the job as they go and they will often keep making the same mistakes over and over again until they create a big enough problem that they actually learn from it or go out of business...so just hope it isn't your project...

So what do you recommend as best practice for floor insulation?

For a suspended concrete slab with hydronic radiant floor heating exposed to a vented crawl chamber, we like EPS as previously described. For a crawl chamber application, EPS blocks can be hung, secured, and air-sealed to provided the required R-value relatively easily and affordably. EPS is also a more green product than many insulation products. Dana likely has the best handle on insulation buildups on this forum and can give you good recommendations for your unvented crawl chamber.

Thanks all for your insight :-)