Additional Ventilation in conjunction with Passive Solar
We are constructing a passive solar house (In Charlotte, NC) – the main windows for the winter solar heat gain are second floor windows open to a two story family room. The heat sink wall is midway up the two story wall. The architect thinks most of the hot air released from the heat sink wall will be “wasted” up in the second story section of the living room (most of the living area is on the first floor) and wants us to install some sort of ventilation to bring the hot air to the first floor although he did not give us any specifics. I was thinking of installing a Panasonic (or equal) continuous running exhaust fan in the ceiling of the second floor and having it exhaust on the first floor near the HVAC return. Probably put it on a swicth so it can be turned on during the winter and off for the summer. Anyone have any better ideas? As a side note, we will be installing an ERV as well if that should somehow be worked into this – have not decided if it will be part of the HVAC system or a stand alone ERV – hearing conflicting opinions on that. Thanks.
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Your architect is right, so the question remains: why did he design a passive solar house that needs remediation in order to function? A solar house that needs constantly running fans to operate is no longer a passive solar house. The beauty of good passive solar design is that it needs no additional power to function well.
Here's an idea that you probably won't like: design 8' ceilings to keep the heat (whether solar or fossil fueled) where you live, and place the thermal mass where the sun will shine on it - preferably in the floor because people prefer warm feet and cool heads.
If you put form before function, you'll get exactly what you wished for: a poorly functioning house.
Robert - point taken but I guess I only gave you half the story - there is a heat sink floor as well that runs the length of the family room and has several large exterior doors and a few windows to allow for the majority of the solar heat gain in this area. I am not a designer so cannot really say for sure what his intent was but I think the additional windows and heat sink was probably trying to meet some sort of design criteria of square footage of windows and heat sink as they relate to the SF of floor area. He has been doing passive solar design since the early 70s so suspect that he halfway knows what he is doing. I think the home will function properly without the fan - just trying to see if there may be some added benefit from adding it.
With a 2-storey room and lots of solar glazing, you will have significant temperature stratification, and most of the indirect solar heat (in the air) will be unavailable to the thermal mass. The only way to undo the stratification is, as the architect suggested, by mechanically moving the warm air back down.
Air temperature stratification also means more positive pressure at the ceiling which will increase the air and vapor drive, making exfiltration and moisture problems more likely.
Danny,
Robert's points are well taken. In addition, I'll add this: in any home where you are worried about temperature stratification, the situation will be improved by an impeccable air barrier and thick insulation.
Homes with cold floors and hot ceilings are usually homes with air leakage problems.
We are building to the National Green Building Standard so will be a pretty tight, well insulated home - shooting for a HERS around 50. 2x6 walls with blown cellulose with 1" rigid foam, closed crawl with an R-12 exterior wall and a foam roof deck for a semi-conditioned attic. Will be seling all joints and following proper air sealing guidelines. The first 6' along the south facing exterior wall is a raised concrete floor with tile on it for our heat sink floor - the remainder of the floor will be either oak hardwoods or bamboo (will be a homeowner selection). Thanks for the replies.
I’ve done several large home designs like yours in the South and Southwest; however, I design a dedicated return air duct (or 2) from the ceiling of the 2-story room DIRECTLY to the plenum or return air system and install an IAQ thermostat or an air-cycler that will run the air handler 15-20 min. every half hour continuously. All you want to do is to keep moving that hot-humid air from the ceiling and bring it down as often as you can. I do insulate under/over the roof sheathing so my returned duct system in the semi-conditioned attic; otherwise I would design some kind of Coffer ceiling to keep the return ducts under the ceiling.
Thanks Armando - that was our original idea but though that was defeating the purpose of the passive solar - we are trying to prevent the heat from running - that was why we were thinking of the small Panasonic fan. Also - We are planning on having a seperate system for the second floor so a few minor concerns were - getting the return ductto the closed crawl where the first floor system will be and also wondering how the overall design of the system would be affected if we had a return so far away from the system - all other runs would be very short. Neither of these was a huge concern but small details that were pushing us towards the small fan again. Thanks for the reply and thoughts.
Danny,
Be careful of any design for a "passive" solar house requiring extensive use of a furnace fan. Many furnace fans draw 700 or 800 watts -- ouch! You'll be heating your home, all right -- with waste heat from your furnace fan motor.
Of course, if you choose a furnace with an efficient ECM blower, you won't waste as much electricity.
Martin is correct on the ECMs. I should have mentioned that we always use high efficient HVAC systems with ECMs. I guess I "assume" everyone uses them in green homes.
Thanks guys - yes - will be on the upper end whereever we end up - they are still considering a geothermal option - if they do not do that they will have a top of the line system for the first floor. They are retired and second floor will not be used except when their children are in town which is why they want a seperate system for that floor - they plan on leaving it off (or set very low) most of the year. That is a totally seperate topic I am investigating - want to give them good advice on not running a system and weather that would overwork the main system, etc - working thorugh all of that with our HVAC contractor once they make up theri mind. Thanks for the tips - good ideas to keep in mind.