I'll go even further and say that interior passive thermal mass is not something I would go out of my way to have. I may be unusual, but I like a nice steady indoor temperature (ok, somewhat cooler when sleeping) - where interior passive thermal mass doesn't help and can hurt.

I would pay something for exterior mass or active thermal storage. How much is very situation dependent.

Posted By jonr on 29 Nov 2011 11:44 AM
I'll go even further and say that interior passive thermal mass is not something I would go out of my way to have. I may be unusual, but I like a nice steady indoor temperature (ok, somewhat cooler when sleeping) - where interior passive thermal mass doesn't help and can hurt.

I would pay something for exterior mass or active thermal storage. How much is very situation dependent.

Having more thermal mass on the inside the thermal envelope always helps, and never hinders keeping indoor temps steady. 

Having more thermal mass only hurts if you want to be able to VARY the room temp over the course of the day.  If you want to keep your sleeping quarters cooler in a higher mass room, zone it separately, rather than counting on heat losses to cool it for you by the time you go to bed.

Whether (and how much) it's worth paying for more mass depends.

Whether or not passive solar and high mass are archaic is subjective, but both are clearly analog problems by their nature. Skepticism is warranted in my view on claims that they can be controlled precisely, or even simply and reliably.

My point on masonry heaters was merely to observe that, Dana's preference for low mass heat notwithstanding, high mass heat aux is not only common but considered the standard. There are three arguments why this should be so, none of which dates to the 18th century.

No. 1: Heating concrete by heating air is a pluperfect b*tch. This I know from experience. It takes a long time at air temps humans can tolerate. Delimit it further with a thermostat, and you an even longer bout of cycling that is not only frustrating but also quite likely inefficient. Hey, you answer, aux heat is about maintenance, about topping up, and it probably is in New Mexico. Until dana moved from the specific to the general, I had no problem with this thread. Which brings us to No. 2.

No. 2: In big chunks of the U.S. to approach aux heat as topping up is insanity. The critical factors here in the northeast are how many consecutive overcast days you might encounter, and how often these gray spells occur. By Day Three you will be quite cognizant of how many btus go missing per degree of heat loss in a high mass home. Efficiency and efficacy matter, too. What radiant slabs and masonry heaters have in common is the ability to pour in lots of heat in a manner that is both comfortable and lasting.

No. 3: A skeptic would liken the high mass in passive solar high mass to a giant flywheel that will accelerate and decelerate over 24- hour periods pretty much no matter what he does. A skeptic would make his peace with it by picking his spots: a nudge here and a tug there. A skeptic would say that electric baseboards are not up to the task.

To clarify wrt Dana's comment - yes, the "can hurt" part can be because you want to vary the temperature. For example, the mentioned sleeping or when you aren't home. Or any case where the thermal mass is part of the heating system.

Really guys - if we are talking a few heating days in the winter won't a few electric baseboard heaters or a small electric forced air unit be the answer? k.i.s. 'couse I agree with Rob's sentiment about 20 posts back. Passive floors by nature are either to hot or to cold and radiant floors are "just right." But hey , I'm north of 49 so really have no business talking about NM ;-)

Some food for thought... This quote is from an article in the Apr/May 2010 issue of Home Power magazine. It was written by Robert Riversong...a master housewright with 30 years of experience designing and building passive solar, super-insulated homes.

"Some building experts discourage mixing passive solar with radiant floor technology since the lag time can make it more challenging to maintain uniform indoor temperatures--especially when the floor is warming from below and then the sun comes out. Even if the thermostat shuts off the radiant circulation, the heat already in the floor will continue to emerge, while the sun is also heating the space. However, this can be an asset. Since the sun is raising the slab temperature, there will be less heat exchange from the radiant tubing in the south half of the slab and more heat available in the north half, which doesn't have the benefit of receiving solar gain. This selective heat redistribution function would be most efficient if the radiant tubing was looped in a north-south orientation."

Good point, Garth.

We have "solar zones" which we mapped out using sun angles in advance. Solar zones are portions of the slab which will see the sun. Of course, they are more concentrated up by the South windows and go away the farther North on the slab you go. I can't remember exactly how the calculations panned out, but the solar zones are probably about 1/3 to 1/4 the area of all the hydronic tubing. In many designs I looked at, the area of "solar zones" was something less than 20%, but we have a 4-season insulated sunroom of which nearly the entire floor area is considered "solar" and that boosts it up to more like 1/4 to as much as (1/3?). We can recirculate solar zones to other zones needing heat and redistribute heat for only the cost of pumping.

Which brings up another point. When the sun floods onto a passive slab, it is highly unlikely that it will be affecting the whole slab surface. Aperture (window) limitations and furniture, throw rugs, etc help see to that. Even if the slab is up to temp, the loss of heating plant input in the shaded areas soon turns it from radiant slab into buffering mass. Initial ambient temperature rises are best handled by ventilation, anyway. Your HRV can go into recirculate mode and move the warmer air to other areas.

Seems like a bit more thought needs to be put into how passive solar and radiant go together.

Riversong is correct that if you're doing it as a single-zone slab with the t-stats in the north side, and looping the tubing from the direct-sun portions of the slab to the sunless portions would lessen the overshoot, and it may even be designable in a more-rigorous fashion (albeit a much more complex model than most heating systems.)

Posted By Garth Sproule on 04 Dec 2011 12:28 PM
Some food for thought... This quote is from an article in the Apr/May 2010 issue of Home Power magazine. It was written by Robert Riversong...a master housewright with 30 years of experience designing and building passive solar, super-insulated homes.

"Some building experts discourage mixing passive solar with radiant floor technology since the lag time can make it more challenging to maintain uniform indoor temperatures--especially when the floor is warming from below and then the sun comes out. Even if the thermostat shuts off the radiant circulation, the heat already in the floor will continue to emerge, while the sun is also heating the space. However, this can be an asset. Since the sun is raising the slab temperature, there will be less heat exchange from the radiant tubing in the south half of the slab and more heat available in the north half, which doesn't have the benefit of receiving solar gain. This selective heat redistribution function would be most efficient if the radiant tubing was looped in a north-south orientation."

Would you not then have to keep the whole house in constant circulation? Normal once the air temp is reached the zone shuts down. If you keep everything in motion, would you not have to go with very low mix and boiler temps? What happens on sunny winter days when you have high solar gain on the south but high heat loss on the north? Again it might work if you can put up with high temperature swings and cold floors but doesn't that defeat the comfort level normally associated with radiant floor slab?

I think that what RR is trying to point out is that where the direct sunlight is hitting the concrete, that the surface of the floor will be much warmer than the depth level in the floor where the tubes are located. Therefore the heat energy from the sun will be moving down through the concrete, whether or not the circulator is running.  Heat can be emitting in cooler parts of the house while heat is being collected where the sun is shining.  Also remember that in a super insulated home that the temperature differential between rooms tends to be small.

What happens on sunny winter days when you have high solar gain on the south but high heat loss on the north?

In that case, the only substantial gain is still through windows. The roof, the walls, the foundation and of course, the heat lost through infiltration or your heat exchanger are all still high due to the high delta T. Zones on the North are going to be calling for heat. So might the solar zones, just less frequently.

it might work if you can put up with high temperature swings and cold floors but doesn't that defeat the comfort level normally associated with radiant floor slab?

I think that passive solar fans are usually willing to put up with a little more variation than the average radiant heating customer. How many times did this happen during the laying of the radiant tubing?

INSTALLER: (Struggling to fit tubing into a tight pocket) "Whew!"
HOMEOWNER: "I don't think you need to worry about getting tubing in there."
INSTALLER: "There might be a cold spot on the floor."
HOMEOWNER:"I don't think I can get my foot on there now, much less after it is built and with furniture..."

Keep my runways and places I am most likely to drop foot on warm and I'm happy, even if there are a few places I can find that feel cold. To that end we paid special attention to having bathroom floors warm. If the rest of the house is warm enough that the floors aren't getting charged, at least we will have warm floors during bathroom duties. Bathrooms are rarely solar space.

It is easy to incorporate some passive solar principles and get a 15% or 20% boost in heating. If you want more, say 30% or 40% or higher, it is going to be more difficult to work in and you will have to put up with more variation.

Solar spaces with small volumes and large glazing are the ones most likely to be overwhelmed by a surprise visit by the sun. Ventilation remains the best strategy for buffering that sort of temp rise.