Passive/Active Solar Home Heat Loss Calcs.

Constantin

... the only thing I'll add is that you may or may not want to get into the capabilities of the thermal flywheel effects that a house can benefit from. I remember reading about a structure in VT that was on a mountain and still maintained 40°F+ all winter w/o the benefits of heating.

This feature had been designed into the structure to enable a fuel-less winter layup w/o the need for blowing out the water, etc. lines. You'd get into some pretty hard-core calculations, such as the thermal uptake of the home in different weather conditions, NOAA data on said weather conditions, then a 95th-percentile on the worst-case scenario (i.e. a week of overcast+rain, for example), to calc out how much benefit the structure safely derives from the sun.

At other times, as Brad said, the structure will have even less need of heat. But if the aim is to "under-size" the heating plant by taking advantage of variable weather conditions and thermal mass, you have your homework cut out for you.

John Madden

Passive/Active Solar Home Heat Loss Calcs.

Can anyone steer me in the right direction to do accurate heat loss calculations on a passive/active solar house design? I've had 30+ years of experience with conventional building heat loss calcs, but never run across a solar design before. Any suggestions?

Brad White_9

Same method

for any building.

Solar design entails (as you well know) capturing what you can as we pass by the sun. It is just a bonus to offset the losses occuring concurently and over the time that your building mass is being put to work re-emitting. Solar design goes to the supply side of things, not the loss side.

Heat loss is best sized for the coldest times and that is usually the hours just before dawn when there is no sunlight and at times when the house is being asked to possibly recover from a setback.

The subtle difference may be the empirical data collected that says your building may not set back as far at night due to mass. But the emissivity of the envelope is what it is regardless.

Hope this helps.

Brad

Brad White_9

Thanks, Constantin

You bring up an excellent point regarding how the "supply side" aspects of solar gain may affect the system size ultimately.

Modulating input is such a blessing these days and the first solar building I did had the next best thing at the time (early 1980's): modular boilers. There were times when all modules were necessary, a week or more of overcast weather and annoying bitter cold in Vt. On average the solar aspects made a huge difference in oil and wood consumption, no doubt.

But I for one do not have the onions to dictate that the sun will perform to my schedule. I am just not that good!

Still, your points are very well taken. Thanks for the sanity/reality check!

Brad

hr

With passive design

you really need to be able to control the solar gain. by far the biggest mistake I have seen with passive design is lack of controls to handle OVERHEATING.

You need to design in a method to control the amount of gain. Proper siteing and roof overhangs need to be looked into.

I think I would include some sort of motorized window shading. Possibly automatic controls to control the gain if the home is un occupied.

I'm seeing more and more windows with the shading built into the air space between the panes. Now that makes a lot of sense to me.

hot rod

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Brad White_9

Excellent point to bring up...

Overheating and the ability of control systems (on the heating supply side at least) to anticipate and manage heat production. Yes, too many solar buildings at least in their early versions tended to be way to warm in my opinion.

Downside of shading devices is, they block what you want to harvest for later use...

Here we are on our little planet circling the sun and this is what we think about....

Thanks Hot Rod

Brad

GMcD

Depends how detailed you need

You could do rudimentary hand calculations using the solar gain calculations and sun azimuth/angle tables that ASHRAE provides, but there is very little in terms of accurate solar absorption calculations for the actual heat gains by the interior surfaces. There are some high end computer modeling tools like TAS, TRNSYS and IES-VE that will do a reasonably accurate model, if you've got the $$ and time for the learning curve. The key with passive design is to model the "free run temperature" of the spaces to accurately account for the thermal mass and building thermodynamics. Anything less is just a ballpark estimate.

Constantin

Very good point also...

... controlling insolation is just as important as controlling the heating plant. To the extent possible, I would try to make it as passive as possible...

... for a graphic illustration of the possible pitfalls, I invite you to visit the Institute du Monde Arabe in Paris, where thousands upon thousands of irisis open and close to regulate the insolation (for light protection). While well-built, a number of the irisis fail every year...

Back in high school, we had a simple lattice and angled slat system that hung horizontally over the windows outside our classrooms. In the summertime, the slats were too angled to let the summer light in, in the winter the sun could navigate past the slats. Seems workable.