Passive Solar, Radiant, High Ceilings....

I am in the planning stages of a small(ish) well insulated home build. The house will basically be a modified A-frame, technically it will be an extra tall gambrel on 4 foot knee walls so that I can have counters and furniture up against walls that are 90 degrees. Footprint of 24×36′ Full ICF basement as well as the two long 4′ knee walls and the rear 24′ knee wall. Building will be south facing and the south gable wall will be primarily glass. The first floor deck I believe I am going to do with an ICF decking system so it will be poured concrete with a ton of thermal mass I believe around 14 yards of concrete or roughly 56,000lbs. There will be an open loft in the back with half wall front (south) half will be open cathedral ceiling. Open floor plan, I believe I will do a spiral staircase roughly centered that goes from basement to loft continuous.  Insulation will be r-49 Minimum in walls and (hot) roof.
Ok now that you have the gist of the build a couple questions.

Am I crazy to do radiant heat with 20′ high cathedral ceilings? I will have some ceiling fans of course to circulate heat down but will I be making heat at the floor and losing it to the ceiling or will this work out fine?

I would really like to add hydronic solar for the radiant but house is already going to be passive solar, so on days where the sun is shining in winter (I am in Vermont and there are less of those sunny days than we would like but its always cold) Anyway, on days the sun is out I really should not be heating the slab at all, the house with all the glass should really heat well on its own. Could I way oversize a basement water storage tank to use as additional thermal battery? There will be a poured patio outside so that could become a dump zone if I needed to get rid of excess heat.

If this is a solid plan how would you size the solar array?

I am not an expert... but radiant heating is not about heating the air, it is about heating things. So my gut is that a high ceiling will not matter as much when it comes to radiant heat. Convection heat would be a different story.

My first though is that you have not mentioned passive solar for cooling.

Jeff said, There will be a poured patio outside so that could become a dump zone if I needed to get rid of excess heat.

How does this get rid of excess heat?

Anne Miller wrote:My first though is that you have not mentioned passive solar for cooling.

Jeff said, There will be a poured patio outside so that could become a dump zone if I needed to get rid of excess heat.

How does this get rid of excess heat?

Sorry miscommunication I didn't mean to cool the house but rather to cool the thermal battery. By this I mean, if I oversize a hydronic solar array, on a sunny day I can not overcharge the floor slab as the house will overheat, I want to build an extra large water storage tank as a secondary thermal battery to store the excess heat, in the event of many warm sunny days in a row where the water battery is also "full" ie as hot as I would want to let it get I could "dump" excess heat to the outdoor concrete slab.

So the patio will help cool the concrete slab aka thermal mass?

My house has a small front patio about 50 feet by 5 ft.  I have not noticed it changing the temperature of the slab much.I have an unheated room at the back half of the house and the slab will keep the room at 60 degrees no matter how cold the temps get.

Anne Miller wrote:So the patio will help cool the concrete slab aka thermal mass?

My house has a small front patio about 50 feet by 5 ft.  I have not noticed it changing the temperature of the slab much.I have an unheated room at the back half of the house and the slab will keep the room at 60 degrees no matter how cold the temps get.

Not quite, the slab should be charged up to probably no more than 70 degrees to keep the house 68 or so. So if its a sunny day and the house is already warm and the slab is already at temp the hot water solar panels cant send any more heat to the slab without just overheating the house. The house will likely already be gaining heat due to the solar gain from the south glass without the floor in the equation.

This is where the secondary thermal mass comes in aka a big water tank in the basement how big, I am not sure? 500 gallons maybe? Not really sure how to size that. But for examples sake lets say the tank is 500 gallons. If that tank is already at whatever maximum design temp is lets say 140-160 degrees and the sun is still shining and the hot water solar collectors are still collecting heat this is where you would "dump" heat to the outdoor patio slab just to bleed some off and not overheat the system. Heat may also need to be dumped in the summer when the only load is domestic hot water and there are a lot more sunny days. Probably some of the collectors might be able to be shut down in the summer to avoid this problem some. In the winter the issue I forsee is in sizing the system big enough to take advantage of the minimal sun there may be times where there is too much heat. I believe a dump zone is common in these types of systems.

suggested watching.  Start at 1 hour and 9 minutes in.  Big water tank.



DYI tank.  using "tank" loosely.  Insulated wood box with a pond liner.  Now you will notice from the previous video depth matters for stratification

[youtube] https://www.youtube.com/watch?v=bI0T7ZegAPU[/youtube]

Now I will argue the big tank is desirable because in warm weather you can use say the top 1/3 for hot water and the bottom 2/3 for cooling allowing stratification to keep them separate.  If you have water based solar panels you can likely use them for cooling during the night thru radiant cooling.

Now combine this drain back information with the above to keep costs down and the system completely simple.

[youtube] https://www.youtube.com/watch?v=CW9YVaPW9wU[/youtube]

Now addition cooling.

[youtube] https://www.youtube.com/watch?v=nV7XJXETr0Q[/youtube]

My thinking is to use this as both heating and cooling.    In heating mode it would create a bottom that the house couldn't fall below.  Look at the trenches you have to dig already.  Can you bury lines deep enough at the same time for this to help without moving any more earth or only moving just a bit more?  Thinking around basements, possible the trenches for septic drain field, down electric line trenches etc.  Then a field of heating and cooling lines clustered in layered zones so you can have a hot middle, cooler middle shell and a cooler still out shell.

Then I will point you at my write up on solar air system.   One of the one I am still struggling with here is amount of air that needs circulated.  The reason is if you are going to heat tons of mass with air then you need to move tons of air past and around it clear to the floor.

solar thermal air heating.

The neat thing about doing active over passive is you can decide how much of the heat you bring in from the active.  I would over design both the active and the passive to bring more heat in than you need.  Then make up the difference by cooling.  Think of it this way.  You run all summer cooling while slowly heating your block of dirt.  You might even cool the block of dirt off every night for the first part of the summer.  In the fall the house will be way to warm early as the passive turns on more but it hasn't cooled enough to matter so you store that heat away too.  So you may use the active or you may just dump its heat outside never bringing it in.  So going into winter you have a block of dirt to pull the heat out of limiting how cold the house can get.  On really sunny runs the house even in winter will get too warm so you are boosting that heat in the block of dirt by basically air conditioning.  By spring the passive has quit working as it shades itself.  Now the active is doing primary heating while the block of dirt is limiting how cold the house can get.  Now you want to start wasting the heat so things are cooled off to begin summer cooling.

As for the radiant being high being a possible problem I am going to answer done correctly I do not think so.  But I would design for a backup plan just in case.  You simply need a way to force the air to mix.  Be it a ceiling fan or a duct with some fairly high speed quiet fans in it.  At least have the place where you can build such a thing if needed.  I am extremely interest in radiant ceiling and it is on the list of things I would like to add here.  More for giving me summer AC than winter but a bit of both.  With sloping ceilings I would design for a drain back design just like an exterior water based collector so I didn't need antifreeze in the system.

Now given how much I fight with my basement floor I would never do a floor without radiant tubing in it.  Between convection and boundary layer affects it is almost impossible to heat the concrete without putting the heat in it.  I have seriously wondered if I actually want 2 layers there?  One in the concrete and one say 2 feet in the dirt.  So I can charge a bigger thermal battery inside the house.