I'm planning on building a passive solar home in Placitas, NM in about 2 1/2 years. If we design and build the house correctly, the heat should not be needed very often. While I would like to use radiant heat, embedding the coils in the concrete slab does not seem practical. Should I plan on using electric radiant coils? Is Warmboard a better solution? Thanks in advance for the advice. Bill

What are you planning for cooling?

Do you have any local data on a structure that might be similar in passive function which would give you a pretty confident estimate of when heat might be needed, in terms of daily and seasonally? And how much heat might be needed during those times?

What is impractical about hydronic tubes in concrete? But I would use active solar and keep any extra passive thermal mass, if any, on the outside.

Placitas is located just north of Albuquerque. It's about the same elevation as Denver, with 300+ days of sunshine a year. The summers reach the high 90's The winters get to freezing. With a high thermal mass wall and roof, I hope that we can go most of the year with only turning on the heat for a few cold nights during the winter. The reason radiant heat embedded in concrete is inefficient is it will take 8 to 12 hours to heat the slab which doesn't help me heat the house quickly when I need it.

What are you planning for cooling? Won't a passive solar in Albuquerque get absolutely smoked in the Summer?

Currently I plan on providing an air conditioner for the house. I'd like to find out more about the Coolerado system (if I've spelled that correctly). But, what I've been reading is that if I design the home correctly, my cooling costs will not be high. The thermal mass of the walls will keep the home cool in the summer and warm in the winters. That assumes I have the correct amount of southern glazing and overhang from the roof.

I've looked into the coolerado system recently. Be aware that the system design requires very high (700+) CFM basically blowing out of your building, so it's really amazing efficiency numbers are at least significantly offset by that wasted CFM. compare to other methods carefully. It boasts really high efficiency numbers but then wastes a bunch of it so against other solutions it may, or may not be a net winner.

Using the same slab for passive solar and radiant heat isn't always compatible. During the overnight hours if heat kicks on you've pre-warmed the slab to above room temp, so the slab won't be soaking up much of the passive solar gain until you've overheated the place. (WarmBoard would be even worse, since it isolates the mass of he slab from the room with insulation, rendering that mass useless from passive solar point of view.) With active solar and slab radiant it can be better controlled, but it's a far more expensive proposition doing active solar vs. passive.

In a passive solar design radiant ceiling makes more sense- it's low mass and won't pre-heat the slab. But whether it's really "worth it" to spend ten grand on radiant for the modestly higher comfort levels on an intermittent-use system in a high-R passive solar house is questionable. Low temp panel radiators are probably the better approach for most.]

Passive solar works GREAT in Albuquerque if you design in the gains & thermal mass right.

Dana,
Thanks for the input.  That's what I was thinking based upon my what I've been reading.  I'm currently looking for a good architect or builder to work with.  Do have any recommendations in the Albuquerque area?

Sounds like a good application for electric radiant ceiling.

Depends on the locals utility & fuel costs, and just how "PassiveHouse" you're going to go on it. At a lower solar fraction with only R25-ish walls and subsequently higher heat loads, something like Daikin Altherma air-to-water heat pump for both domestic hot water and panel radiators might have a better long-term cost/benefit and would really rock on efficiency in Albuquerque, using less than 1/3 the amount of power for space heating.

A ductless mini-split would likely handle the heating & cooling loads, but not the hot water heating, but you could do it all with the Altherma. If you're on the gas grid, a tank-type gas-fired combi would be even cheaper to operate than a heat pump for the space heating, but you'd probably want a mini/multi-split for the AC.

heh... what is a "good" application for electric radiant ceiling? it's good for floors but unless they have figured out modulation control for electric ceiling mats, the hot head issue is real...

some passive houses are using their IAQ ductwork and a duct heater to boost temps. 200 CFM can do about 8kBTU/hr or so.

If you need more, or don't have a ducted IAQ system (silly, I know.. you DO have a ducted air solution for breathing, right?), or think you'll need the warmth more often, then I am not sure I agree that radiant is a poor match for the passive slab. Sure, technically you're losing some BTUs you could have harvested, but hey, think of all the BTUs you could harvest opening the doors at night to cool down the mass even further prior to solar gain... .kind of misses the point that you don't really want the floor to get cold. What good is the BTU harvest if you're not even meeting the basic goal of comfort, and if you heat the room at all, the slab will pick up temperature as well.. less than if heated directly, of course, I'm not suggesting they are equal, but moving away from the slab isn't "free" from the passive solar perspective either.

with slab sensing and minimum temp options slab radiant could work fine. you're only going to heat it a few degrees above room temp most likely and letting the slab fall below room temp means it's not releasing heat so it's not really "gaining" BTUs until you heat it OVER your desired room temp... there is more to it than this but I think this way of thinking of the solution is basically sound. if the slab cools to 65 and your room temp target is 68 you can't get ahead slurping up BTUs to get to 68. You're just choosing to ride out the swing to save energy. That's part of how passive houses work, and it's fine, but if you're deciding it's time to heat, then it's time to heat.

I will say I am not a passive solar expert though... so if there is rigor to disprove this, I'll listen. I am just not seeing a big issue here.

if your need is truly intermittent, than heating mass won't make sense and I concur that ceiling (but, absent a new product I dont' know, not electric ceiling) is a fine choice, and panel radiators are similar in cost to some radiant ceiling solutions, so either would work. cathedral area, use radiator, flat ceiling use some wider on center radiant or radiator. Electric boiler is cheap and can be offset with PV later if you wish for truly occasional use heat especially, and/or heat pumps.

for intermittant use... while I love the altherma... I'm not sure the economics would make sense. for continuous duty I would wholeheartedly recommend it.

I have the guy for passive solar with radiant floors in Albuquerque.

During the overnight hours if heat kicks on you've pre-warmed the slab to above room temp

Passive solar control systems need a certain amount of ability to look forward into the future. You wouldn't have your heating system come on towards morning unless you really, really needed the heat in your thermal mass. If that truly was the case, it is less likely that you would be wasting solar gain in the upcoming daylight period.

The thermal mass of the walls will keep the home cool in the summer

Something has to get the thermal mass cool in the first place. In Albuquerque I presume it will be overnight low temperatures, even in Summer?

Something has to get the thermal mass cool in the first place. In Albuquerque I presume it will be overnight low temperatures, even in Summer?

The summertime overnight temperatures range between the mid 50's to the low 60s. So I think we should be fairly comfortable at night. My major concern is still having quick, reliable occasional heating when I need it in the winter. I still like the idea of floor radiant heating. What do people think of having electric radiant heat in the floors? From what I'm reading, it could be quick and relatively inexpensive.

Radiant in floor heat is not quick at any time.

What do people think of having electric radiant heat in the floors?

It sounds ideal for your situation, providing you can keep the electric radiant needs fairly small and focused. The more extensive you get, the more the initial cost and operating inefficiencies of electric radiant begin to point to the usage of something else... like a ductless minisplit. That's why I kept asking about cooling. Provided you can keep your cooling needs low you are still in the ballpark with a minimal electric radiant system, but if you need a lot of cooling, you might as well start looking at something "bigger".

Actually, the Coolerado might help you. They want to say it isn't a "swamp cooler", but it looks to be the offspring of a mating between a swamp cooler and an energy recovery ventilator to me. It might actually work well, assuming you will have a tight house and need to have a ventilation system anyway. It might also be able to assist in nightime cooling through a function which brings in cool outside air. to flush the house in the Summer.

the coolerado is not a ventilation solution. you need ventilation in the winter too.

If you want quick response to your supplemental heat, you can't put the heating elements inside the thermal mass.

The economics of the Altherma are admittedly not ideal, but if you're not on the gas grid and you're getting both hot water and space-cooling out of it (chilled slabs work well for sensible cooling in low-humidity climates) there may be an argument. The economics of ANY radiant are pretty poor in an intermittent use situation, but maybe BadgerBoilerMN's passive solar/radiant guy would be able to assess the particular design with a sharp pencil.

In many/most high-R passive solar houses panel-radiators or mini-splits are easier to rationalize. Radiant is all about comfort, but if it's only running a significant duty cycle 8 days a year, it's pretty expensive. Bumping the temp up a couple degrees on low-flow tepid-air system such a mini-split with a continuously variable speed blower (and not blowing directly on your kitchen table or something) is comfort enough for those 8 days, and the same unit can be used for air conditioning. Heating design temps in that area are 15-25F, a range where pretty-good mini-splits will run a COP of 2.5-3.0 and that's at DESIGN temp. On a snowy sunless day with temps in the mid to high 20s you'd be in the 3+ range if sized reasonably. (Mini-splits are more efficient when sized 2-2.5x the actual design load, since they are far more efficient at low to mid-speed on the compressor than they are at high speed, and usually have at least a 3:1 turn down ratio, but that's a whole 'nuther subject.) At a COP of 3 it's using 1/3 the power that electric radiant would to support the same heating load.

The Coolerado is a decent cooling system in a dry-air climate, but they're pretty oversized for a high-R building that's been designed/operated for minimal solar gain during the cooling season. A mini-split is cheaper and provides more function (but is also not a ventilation system.)