Very cold climate, very well insulated home.   Are passive solar and hydronic floor heat complementary or counter productive?  I have heard good arguments on both sides of this question.  Your thoughts?

Posted By Garth Sproule on 11 Dec 2010 09:46 AM
Very cold climate, very well insulated home.   Are passive solar and hydronic floor heat complementary or counter productive?  I have heard good arguments on both sides of this question.  Your thoughts?

For myself, I would definitely say complementary, but it is good to design this way from the start if this is you objective. If when you say "very cold" you mean at least as cold as I have here near Calgary Alberta (-19c this morning) I don't think you can use only passive. So now you have to decide where the rest of the energy is coming from. Almost any energy source can be converted to hot water quite effectively so that would be my choice as it also adds mass to slow the swings. You will have to try to get the building to use as much of the passive solar without the hydronics getting in the way!

On sunny days you can circulate the water in the slab to distribute the heat to the entire slab, on cloudy days run the radiant heat input source as needed. Sounds like a win win situation to me.

-Rosalinda

Heating a slab that you later hope to be cool so you can heat it with passive solar is contradictory. IMO, active solar and a large, low mass radiator is a better combination.

The solar heat absorption by the slab in a sunny room won't change much by making it a radiant slab, but it can be nearly impossible to control if you go radiant + passive solar in the same thermal mass. Starting out several degrees above room temp when the sun first hits it, it'll rise in temp all day leading to overheating conditions fairly often.

jonr's solution is right- radiators of any reasonable mass or type, ribbed cast iron, flat steel, cast baseboard, anything, just not a massive concrete wall, will be very responsive to changes in heat load to the room as the passive solar heat is alternately soaked up & released by the slab. It needs to be controlled as it's own zone though, since it'll have peak and minimum heat loads separated in time from those of the rest of the house.

Of course in a very careful design it might still be made to work as both a radiant & passive solar slab using circulators to dump heat from one part to another, but it's not a no-brainer. The amount of heat (and ultimately the water temp you'd need) in the non-sunny rooms to keep them warm will be very different from that of the sunny room when it's sunny, but similar on cloudy days. From a design POV it's much simper to run the sun-room as a separate zone with radiators.

Harold Orr is one of Saskatchewan's great super insulation pioneers. He has stated many times that if a home is very well insulated and very tightly air sealed, that a heat source can be placed anywhere in the home and that differential temps between rooms will not be observed. With this in mind, I am thinking that a single zone radiant floor concrete slab heat source could combine with passive solar rather nicely. Even better if the floor circ pump can be made to run continously when the sun is shining. I really cannot see why this would be difficult to control. Surely there will be enough "unused" floor mass in the parts of the house that do not receive direct sun, to store all the solar heat that is collected?

If you're talking super-insulation, yep- doesn't much matter where the heat emitter is in the house. In a SUPERinsulated house like those Orr refers to the notion of running a hydronic radiant slab AT ALL is nothing but an expense, with essentially zero performance or comfort gain.

"Very well insulated" and "very cold climate" have pretty wide (and highly subjective) definitions- maybe it would help to define the U or R values, and the design-condition temps to make this a useful discussion. Are you talking R100 (RSI 17.6) clear wall values in Whitehorse Yukon or Churchill Manitoba, or less than half that somewhere else? It makes a difference on what actually makes sense. To some people R25 is considered highly insulated with a heating design-temp of -25C, but that has no bearing on what Orr is talking about.

if a home is very well insulated and very tightly air sealed, that a heat source can be placed anywhere in the home and that differential temps between rooms will not be observed.

I understand the theory - at some point even a closed off room has so much more heat transfer to adjacent rooms (through the uninsulated walls) as compared to outside, that it is close to the temperature of the other rooms. But in practice with any reasonable values, I'm skeptical. Plus you need ventilation anyway.

I live in a "well insulated" but not "super insulated" house in a heating-only environment, to use the "technical terms" as suggested by Dana1. :-) More specifically, it is R60 ceiling, R29 walls, conditioned crawl space with R19 crawl space walls, R5 foam under the crawl space floor, wood floors on the first floor with low thermal mass, and some passive solar but the long dimension of the house runs north-south. I am in a sunny part of Colorado (5.9 kWh/m^2/day) with 7355 heating degree days. With 4 high solar gain, triple-pane windows in the south-facing bedroom (3 south and one west window), I typically measure 72 F (22C) in that bedroom, and 67 F (19 C) on the other end of the house, and that is running my HRV in the recirculate mode for 67% of the time with fresh air the balance of the time. I can also run the hot-air furnace in the reciculate mode, but it usually increases air velocities enough that it makes the house feel cooler, so I avoid that. Therefore, in agreement with Dana1, I think that non-uniform distribution of solar heat gain could be an issue. I am going to add a reading area to that south bedroom to take advantage of the extra heat there.

In this environment, which I have described above, I know one family whose house has some passive solar and radiant floor heating (with significant thermal mass) that is coupled to an active solar hot water system, and they said control of the heat was a difficult problem that they had not solved. It essentially overheated during the afternoon, and was cold in the early morning.

The key words here are very cold climate, which obviates the need to "hope [the slab] will be cool so you can heat it with passive solar..." as jonr writes. It will be cool and you will need to heat it some way. It is not just DD heat loss; it is also daily insolation given shorter days and more cloud cover. Think Michigan. Or Pennsylvania. Or Massachusetts.
The trick is anticipate how much heat to add. In the classic passive solar house, a masonry heater provides a fudge factor by storing heat during the burn and releasing it slowly over time. A hydronic slab is an excellent stand-in -- superior in fact because it takes up no extra room and represents more mass than a fireplace.
No disrespect, Lee, but your south facing bedroom and my main floor -- 1k sf of concrete and 40 linear feet of windows -- are not equivalent. And I'm placing my 220cfm fifth port HRV in a vertical chase designed to move air between floors. We need to distinguish here what's serious passive solar and what's not.
Not that I am shouting Eureka yet. Two more months....

toddm-

220 cfm of air flow that is essentially at room temperature. Stand clear of the registers. brrrr!

Lee

Fan speed on a Greentek is adjustable.

The climate where I live has 9500 HDD (Southern Sask) but has lots of sunshine year round. It can get as cold as -40°C in winter. The home I am hoping to build will have R20 under the slab, R50 walls, R5 high SHGC windows, R 80 ceiling. Most windows will be facing south.
I understand that radiant slab heating is overkill and that the floor will not have that warm toes effect, but it just seems so easy to add the relatively cheap pex tubing at the time of the floor pour.

Let me restate some things that may be obvious to most but are worth repeating
The better insulated the house, the less solar gain you need or want.
The more thermal mass you have, the more solar gain you can handle without overheating.
In Smaller well insulated houses, single source heat can be effective at heating without big differences in temps room to room. There have been several studies that show this. This breaks down in larger houses.
The term Passive solar usually refers to a design where the majority of the heat for a dwelling is designed to come from solar energy, and the energy is transferred directly to the thermal mass.
“Sun Tempered” is the term usually used for a design where a significant portion, but not the majority of the energy comes from solar.
It seems to me that a system that combined active and passive solar heating in the same thermal mass would be quite hard to regulate. I would think about doing the following for active/passive. Passive solar direct gain in a concrete slab, active solar either flat plate or evacuated tube going to a large well insulated water storage tank (with electric back up). One coil in it heats the dhw, another coil goes to a fan coil heater centrally located in the house.


The house I live in is definitely Sun Tempered, not passive Solar. I figure between direct solar gain, a solar airheater, and a flat plate collector for dhw, solar provides about ½ my total energy needs. My secondary heat source is a small wood stove and a ceiling fan in a fairly open design. (propane boiler for back up heat and DHW). The interior rooms are not insulated between rooms. The interior temps stay about the same in all rooms if I leave the doors open, bedrooms are cooler if I leave the doors shut when the woodstove on. If I were doing it again, I would have just used a propane direct vent heater as my back up heat source. The boiler was overkill.
By contrast what toddm is building is truly designed to be a passive solar house. This requires a bit more thought and planning to do correctly.

How much heat can you store in a plain slab that has cooled to 65F overnight and can be heated to 80F by the sun? How much heat can you store in a slab that has been heated to 75F by a hydronic heater and can be heated to 80F by the sun? Don't heat your slab with other sources if you intend to use it for solar heat storage.

If you are looking at using hydronics to spread the solar heat around (more like an active solar storage system), that is a different thing.

Thanks for all the reponses...lots to chew on here. Here is what Robert Riversong, a well known Vermont buider had to say regarding this subject.

"A properly-designed passive solar home, with between 7% and 12% of floor area equivalent in south glazing, overhangs engineered for the latitude and window height, and direct-gain thermal mass in the correct ratio to the solar glazing and of the correct thickness will maintain uniform temperatures AND receive, store, and release free solar heat on a diurnal cycle.

It is only indirect-gain thermal mass that has to be heated by overheated air, which is why indirect mass is so much less effective than direct-gain mass. Direct-gain mass does NOT require overheated air in order to absorb solar heat - it is directly heated by radiant transfer from the sun to the mass. It has to be in the sun's path (and not shaded by furniture or rugs), of low specular reflectivity, of medium-to-dark hue (reds, browns, greens are best) for good solar absorptivity, with good thermal diffusivity and effusivity, and ideally 4" thick for a floor or 8" for a wall for an effective diurnal heat capacity.

With a properly-engineered and designed passive solar house, there is no inherent incompatibility between a solar thermal mass floor and radiant high-mass in-floor heat. In fact, I think they make excellent partners, since (as I described earlier) the radiant tubing effectively redistributes solar direct-gain heat to the non-solar parts of the floor."

Answered by Robert Riversong

In an R50 very-tight house in Sask the slab would never hit 75F in space heating mode (not even at -40C/F outdoor temps), and if it ever hits 80F the occupants would be throwing open the windows. Overglazing for excess solar gain would be a hazard in this house, because most days when the sun shines it'll be well above -40. I'd revisit the glazing area and orientation (even simulate it) before building this thing. Your bigger heat loads will be at dawn you might consider some amount of east glazing and designing in some controllable exterior shade for those windows to keep from cooking yourself in the summer.

If you put too MUCH thermal mass in the slab and heat it hydronically it won't drop in temp quickly enough in the hours after daybreak, delivering too much heat to the house. (This would be less of an issue at R100 than at R50.) The solution might be to run it cooler at night, even though it might leave you slightly chilly in the AM.

jonr's panel-radiator solution still makes more sense, letting the thermal mass be just thermal mass for the whole house, not a radiator. If you would prefer radiant, go radiant-ceiling, which has the ability to respond quickly and will work well year-round. No matter how much mass you put in, a few days of sunny weather in not super cold conditions could still roast you out if you go overboard on the south-facing glazing without designing in the means to shed heat. Using lower-gain more highly insulated windows plus overhangs helps.

If you're not going to simulate it to a higher accuracy, standard rules of thumb of south glazing for to floor area ratios still apply at R50, but err to the smaller side in an R50 house. The old-school rule of thumb on lesser-R buildings isfor the glazing no more than 7-10% of heated floor area. At R50 cut that by at least 1/3- make 7% an absolute upper bound or you'll be heat-dumping most of the year. IIRC Orr's 1970s "Saskatchewan House" superinsulation experiment was too severe a heat-trap, requiring higher ventilation rates than anticipated to keep it cool even in winter, and the active solar thermal system designed into it for space heating was never
needed.

It might be useful to use the PassiveHouse tools to fine-tune the envelope design, even if not going to a full PassiveHouse level of heat loss.

If you still feel you want to use the slab as a primary radiator use a non-sunny sections of the slab, and only PART of it, not the entire thermal mass. That way you can have a warm spot for that cushy feel without running into response-time & overheating issues. Do the math on the design day heat load, and how much slab it would take to meet that heat load at 23-25C surface temps, and do it all on the north half of the house. It'll be more responsive, and fewer overheating situations will arise. Even if you have to add some radiant ceiling to meet the peak heat loads (you probably won't), don't get sucked into slab-heating in the heavily solarized rooms.

Direct-gain mass does NOT require overheated air in order to absorb solar heat

It doesn't require it, it creates it. You are going to apply lots of solar radiant heat to an exposed surface and somehow this heat is only going to go downwards and sideways?

Gotta agree with Dana1. Modeling is a must if expect passive solar to provide most of your heat. I like UCLA's HEED software because it is easy and free. http://www.energy-design-tools.aud.ucla.edu/heed/ Google Sketchup in conjunction with Google Earth can show you how the sun will hit your floor at any given hour during the year. http://sketchup.google.com/ EnergyPlus is available as a sketchup plugin but the modeling at this point is no longer easy. http://apps1.eere.energy.gov/buildings/energyplus/openstudio.cfm

Gotta disagree with Dana1 about the prospects of roasting. You should expect days like April 7, 2010, when it hit 93 degrees in Pa., and at a time when my overhangs would not have completely shielded my windows, You should have the appropriate window treatments ready. (Shoji screen shutters in my case that incorporate radiant barrier film and bubble wrap insulation. No blackout conditions. Decor friendly.)

Play with the modeling for a while and you will see what kind of tools you need to live with passive solar in your part of the world. (The models load local weather data.) Active and passive solar makes little sense in Pa for example. A long string of overcast days, which is the risk here, would disappoint you twice. Wood heat is the natural complement of passive solar. I'm not sure I'd mess with a radiant slab without it.

I'd definitely put emphasis on the "...properly-engineered and designed..." portion of Riversong's response- it's a real design problem to but done by the numbers, not just hacked into place 'cuz the PEX is cheap and a "oh, what the hell" approach. The size & distribution of the mass and the slab temps required to support the heat all have to be carefully configured to the anticipated solar gains, etc. Crayon on napkin or "design by web-forum" it ain't (but jonr's solution could be.) The do-over should you get the coefficients wrong could get ugly.