I’m working on ideas for moving some of the warm air created in my 1970s-era attached sunroom/sunspace into my home, and I’d appreciate some feedback. A recent picture of the sunroom is posted at https://dl.dropboxusercontent.com/u/1894227/Sunroom.JPG. The dimensions are approximately 18’ x 16’ with the south, west and north sides being all 76 x 34 clear tempered insulated glass units with ½” air space in between. The total air volume within the space is approximately 5000 cu ft and I’ve sealed it to be reasonably airtight (although there are 4 sets of patio doors, 2 up, 2 down, that certainly leak some air). The floor is poured concrete approximately 5” thick; I plan on adding a little more thermal mass with a “flooring” layer of 2.5” pavers later this Fall. There is no other thermal mass other than the shared wall with the house which is stick construction/thin layer of stucco.

The sunroom spans two stories of the home: the basement level of the home is the floor level of the sunroom and there is a standard size door between the two. And there is a balcony/walkway at the level of the main floor of the home. There are two sets of sliding glass patio doors between the main level of the house and the sunroom walkway. When we’re home, it’s easy enough to slide those patio doors open and get some heat moving into the house that way (and even get some circulation by cracking open the door between the basement level of the home and the sunroom). But obviously this arrangement only works when there is active heating and when someone is there to close things up when that heating stops.

I’m wanting to install some kind of fan-driven system that would automatically move air from the sunroom into the house when the sunroom temp rose above a certain level, then shut off that air movement when the temp fell below. The basic design I’m playing around with is running a 6” or 8” standard wall/vent pipe vertically along the wall shared by the sunroom and the home (on the sunroom side). Since the roof/ceiling of the sunroom is a simple sloped/shed design, the “intake” end of the pipe would run vertically to about 6” or so of the ceiling to capture the warmest air pooling there. The pipe would extend straight down vertically and then penetrate through the wall and into the house with a “T” or elbow. I’d install a thermostat-controlled inline fan that would blow heated sunroom air into the house above a set temp, but turn off below that temp. I would install some kind of gravity or spring-controlled damper to close the pipe when the fan was not blowing, hopefully preventing (most) air movement in or out.

Finally, I was considering the need for some kind of “return air” capability, otherwise it would seem I’d just be pulling much colder outside air into the sunroom fairly quickly once the warm air had been pulled into the house. So I was thinking of installing a short section of 6” or 8” pipe (or some kind of vent) through the wall at the basement floor level, again with a gravity or spring damper. The idea would be that when the inline fan was blowing, that would create sufficient pressure (?!) to pull that basement-level damper “open”, pulling cooler basement into the sunroom and creating a continuous “heating loop” from basement to main level.

So that’s the outline of a basic plan, but I have more than a few questions:

1. Is the overall design good, or is there another approach to consider that would accomplish the goal?
   
2. 6” versus 8” pipe? The 8” pipe elbow/”T” is pretty darn big and unsightly, and may be more than I need to move the air for this particular sunroom. 6” inline fans, however, seem to be limited to around 250CFM which may not be sufficient to create the pressure to open the basement-level damper/pipe.
3. Would it be better to penetrate the wall near the ceiling level of the main floor of the home, or near the floor level? I suspect the floor level might be better for air movement, but ceiling level would interfere less with furniture placement.
4.  I’m not sure of the best place in the pipe to mount the fan and the damper…I was thinking the damper right near the “end” of the pipe penetrating into the main floor of the house, with the fan mounted just behind it (although people seem to have more luck with gravity-powered vertically-mounted dampers than spring-powered horizontally-mounted ones)

 ‘Any advice/feedback greatly appreciated...thanks for your time!

Nice looking house. You don't say where you are, which is the most important info. Until you know how much insolation you can expect, you can't make good decisions about mass or recirculation. go here for basics about sunspaces. http://www.builditsolar.com/Projects/Sunspace/LTMSSGuide/LTMSSGuide.htm The site also has page of modeling tools that can help you with planning. One of the resources, the Borst site, is managed by an extraordinarily helpful poster here -- sailawayrb.

You need to move a lot of air to move small heat differentials. So I'd use larger (perhaps 18") openings and two larger, slower fans in a push/pull configuration. Plus insulated and mechanically controlled automatic dampers.

I'd keep thermal mass out of the sunroom - store heat inside the house where it won't be mostly wasted at night.

+1 on the larger sized openings and balanced flow.

-1 on the keeping the thermal mass out of the sun room. (How do you expect them to do that- dig out the 40 year old slab? :-) ) Having the thermal mass in the sun room moderates how quickly the temperatures fall in the sunroom, lowering the net heat loss through the insulated wall. It also moderates how quickly it heats up, but so be it. It's not worth changing.

The thermal mass of air by volume is about 0.018 BTU per cubic foot per degree-F. A flow of 250 cfm is 15,000 cubic feet per hour. A temperature difference of 10F then yields about 0.018 x 15,000 x 10F= 2700 BTU/hr, which is bit more than you get out of a 750 watt electric space heater- it's something, but not huge. It has to be be truly torrid in the sunroom to be delivering substantial heat at those kind of flows. A typical gas fired hot air furnace is delivering on the order 1000 cfm at a delta-T of about 50-70F. You won't usually need that much heat in the middle of a sunny day, but then you won't get temperatures that high either. But (without doing the math, just a WAG) something on the order of 500 cfm at a 15F delta-T for several hours may be reasonable to expect on a super-sunny winter day out of a sunroom that size, but to do that without consuming a lot of blower power needs a much bigger bore than 6-8".

Wow...great thoughts and suggestions.  That Build It Solar low thermal mass sunspaces link/page is excellent.  Even though that wasn't the original intent of this space--basically from what it looks like it is a 1970s/80s-era enclosed patio--that is what I'd like to use it for.

The location is western Nebraska...about 42deg N Lat and 104deg W Long and around 4500 ft elevation.  We're similar to Colorado in terms of weather/insolation, although we get a little more cloud cover on average.  The lower sun angle during the winter and the nearby trees will cut into the heating of course.  I should have mentioned that my house exposure is not squarely north-south....my "southern" exposure is oriented to about 125deg...southeast....the "west" wall of the sunspace is approx. 225deg

It sounds like 6" or 8" pipe/vent is way too small for what I need to do, and you make good points about the fan size/noise.  The only reason I had thought about a section of pipe/duct was to pull air from the "peak" of the sloped ceiling, but I wonder if that is absolutely necessary?  If I had a large through-the-wall vent that was near ceiling level of the main floor of the house, that would be about 3' below the ceiling peak of the sunspace.  A vent near the floor level of the basement would be about 16' below that.

That webpage recommends 3 CFM of fan for every sq ft of glazing, which would mean about 500CFM if I were just focusing on my south(east) glazing.  Adding in the larger (south)west wall would add another 650cfm needed.

Since we're talking about cutting big holes in my wall I'd like to start cautiously and then maybe expand the size if needed.  If I were to start out with a simple "two vent" system--one at the main floor ceiling level and one at the basement floor level--what would be a conservative minimum size for those vents?  Would a single fan pushing basement air out through that basement floor-level vent be enough to push hot air through the main floor-level vent above, including opening up an inline damper?  I'm thinking that would be much quieter than a fan on the main level, particularly since most of our time during the day is spent on the main level rather than the basement...But maybe that isn't going to be sufficient to move the heat in.

I appreciate all the help-
Colin

If there are doors on both floors, a floor fan is more cautious than wielding a sawzall, tho you'd be cutting a register into the upstairs floor if the areas are not otherwise connected. Use an inline 120V thermostat set initially for 85-90 degrees. (basically an SPST extension cord.) Blows in upstairs. Returns downstairs. Use the fan speed to guesstimate cfm. Should be enough insolation to be neutral with doors open and fan off. It will also give you an opportunity to contemplate distribution through the house, which is your next challenge.

dunno your tolerance for jerry rigging or the interior layout of your house, but two of these fans or similar mounted in a storm door/window is a $100 solution. Downstairs blowing out; upstairs blowing in. http://www.target.com/p/holmes-window-fan-with-digital-thermostat/-/A-10299424

BTW Dana is also extraordinarily helpful.

These are good thoughts, but I'm really focusing on a system that would be working while I'm away during the day, and would be able to close off the openings if the sun wasn't shining or later in the day, etc. That's why I was considering standard pipes that I could use with inline fans and dampers.

As far as heat transfer within the house, it's a classic 1970s open floor plan with an open stairase (6' x 8') that connects the 3 floors. This equalizes the temps between the floors somewhat, but also creates the issue of a "chimney" where heat on the main floor, for example from our wood stove, travels along the low 7.5' ceiling and heads upstairs via the staircase rather than spreading around the main floor--where we spend most of our time. 'Not sure the best way around that issue, but for now I'm just tackling how to get some sunspace heat into the house

Dana1 is the most knowledgeable and smartest person on this board, but at my peril I'm going to disagree with him on one point: although the traditional wisdom has been to maximize thermal mass in a sun room and that has real advantages, the opposite, minimal thermal mass, can be good too and is sometimes better.  Given the slab in this case, I agree that it might not be worth changing, but it is possible--you could put a 1" layer of foamboard on top of the slab, followed a tongue-and-groove plywood subfloor above that and your choice of flooring material on top.  Or if you don't plan to use the space for purposes other than as a solar collector, just the foam board.

If you just have a greenhouse not attached to another building, and you don't actively heat or cool, the average temperature inside will be about the same with or without thermal mass.  The difference will be that without it, the space will be miserably hot during the day and miserably cold at night.  Having the space cool at night means less heat loss during that time, but having it warm during the day means more heat loss then, and those two cancel out leaving your with the same average temperature.  However, if you have people or plants or anything of value in there during the day and you don't want to bake them or it, you're going to need to open the windows during the day to throw away some heat.  Then at night you'll wish you had that heat.  So in that case, the average temperature becomes colder than with the thermal mass.

If you attach the greenhouse to a regular house, and you can control some fans, the equation changes.  If you have thermal mass in the sun space, it works similarly to before.  During the day you gain more through the window than you lose, and night you lose more than you gain, and you hope the equation is balanced in favor of the gain, and if that's the case you have some heat you can blow into the house.  With a low thermal mass sunspace, in the morning, it heats up fast.  You have to turn on the fan and deliver heat to the house, or else the sunspace will overheat.  But if you have the fan working, you can keep the sunspace temperature similar to what you would have with the thermal mass.  You gain more than you lose, same as with the thermal mass, during the day.  But at night, the temperature in the sunspace quickly drops.  That's a good thing, because for the whole long winter night, you are losing much less heat through the window than you would in the high thermal mass case.  In the extreme of low thermal mass, the temperature drops almost all the way to the outdoor temperature, and the amount of heat you lose at night is exactly as much as was stored in the thermal mass.  The less thermal mass you have, the less loss you have.  And the faster it warms up in the morning.

With the low thermal mass system, it's probably necessary to close dampers, rather than just turning off the fans.  And really the less thermal mass the better.  Many of the designs for this discussed on the builditsolar site approach being flat plate collectors more than being rooms. 

The only downside of low thermal mass is that you have the high temperature for a shorter time, so you might want a higher fan speed to deliver the heat during that short time.  If you use a lower fan speed, the temperature in the sunspace goes up, which then means your heat delivered per CFM also goes up, so to some extent it's self correcting, but if it gets hotter in the sunspace you'd lose more heat through the U-value of the windows, and you might even damage some materials in there.  Still, the total daily heat loss would be less than with the thermal mass:  Suppose we start with Dana's 15F rise during the day, to 85F and say the high thermal mass keeps it up to 75 F at night.  And say outside is 40 F during the day and 20F at night.  Through a 1 sq foot U-1 window, we'd lose 45 BTU/hr during the day and 55 BTU/hr at night (more if the thermal mass was higher).  Since it's winter, I'll say day is 10 hours and night is 14 hours.  That's 1220 BTU/sq ft/day loss with the thermal mass.  (The heat loss is actually a little worse than that because of the phase shift between the indoor temperature swing and the outdoor temperature swing, but that's not worth fussing about--I'm going for the general concept here)

Without the thermal mass, in the extreme case, we'd have zero heat loss at might, but let's say the interior temperature only drops to 40 F, so we have 20 BTU/(sq ft hr) loss, or 280 BTU/sq ft overnight.  That means that for the same heat loss over the whole day, we could have 940 BTU/sq ft lost during the day, or 94 BTU/(sq ft hr), corresponding to an average temperature in the sunspace of 40+94=134 F.   That's 64 F above the house temperature, so our heat delivered per CFM is more than 4X higher than in the thermal mass case.  We could therefore use a lower fan speed than in the thermal mass case, and despite the higher daytime temperature in the sunspace, we'd have less heat loss through the glass.  And we could run the fan fewer hours, saving electricity as well as improving the heat yield from the space.

It's pretty clear that low thermal mass provides better heat collection ability for the house.  If you want to keep the sunspace comfortable during the day, you can either use a higher power fan or use thermal mass.  But if you don't care about keeping it comfortable during the day, you can use the same fan you would with the thermal mass, and still get better heat collection.  If you want the sunspace to stay comfortable at night, thermal mass would be the best way to do that.

As far as fan choice, if you are going to be running this most of the day, most of the winter, you want efficient fans, well matched to the job.

Most HVAC fans are made to move air through long ducts that have significant resistance. Most of them also use low efficiency single-phase induction motors. If you are basically pushing air through a hole in the wall, you need much lower static pressure capability for the fan. You can solve both of those problems by buying fans made for computer cooling. For example a Conair Rotron brand Carvel DC model fan is rated for 550 CFM with no restriction on the flow, at 30 W. At 0.04 in water back pressure, it provides 500 CFM. In Fantech's highest efficiency EC series, model FG 10 EC has about the same flow at that same backpressure, but uses 87 W to do it. It can drive a higher backpressure than the Conair can (2" vs. 0.3"), but if you are not using that capability, it's just wasting electricity.The regular FG series uses 134 W at that same operating point. And best of all, the Conair is not only cheaper than the Fantech EC model--it's less than half the cost of the regular induction-motor-based FG 10. http://www.digikey.com/product-detail/en/19031767A/CR303-ND/

You could also gain some fan efficiency by adjusting the speed based on the temperature in the sunspace, so you run it at a lower speed when the temperature difference is moderate and only go up to full space when the temperature is high. But it's probably easier to set up with simple on-off control with a threshold. You could have automatic fan control and then manually close dampers or hatches at night and open them in the morning, if the thermal mass is low enough that the sunspace temperature drops significantly below the house temperature at night.

You are correct chrs. You need the appropriate amount of thermal mass given the goal of each specific project.

For a passive solar room/building, you need the appropriate amount of thermal mass to adequately regulate the inside temperature by quickly absorbing and then slowly releasing the daily irradiance solar heat gain. The appropriate amount of thermal mass in this case depends on the maximum daily clear sky solar heat gain and on the absorptivity, density, heat capacity, emissivity and volume (thickness times area) of the thermal mass being used. Thickness is typically limited to only a couple of inches given the associated heat transfer time lag associated with material that is typically used for thermal mass in building construction.

For a solar collector where the intent is to maximize the collector temperature and quickly move the collected heat, you need low thermal mass and high thermal conductivity. So if this sun room will now be used as a solar collector, increasing performance will result with decreasing thermal mass. However, Dana is correct that it would likely not be easy or practical to reduce the thermal mass of this sun room now. If the sun room thermal mass is reduced, the inside temperature of the sun room would also have to be carefully regulated via the ventilation control system. This is frequently done successfully as you clearly illustrated with your greenhouse example. In this case, the excess sun room heat gain would be vented into the house in lieu of vented outdoors. Build It Solar also has many great examples of diverse solar collector methodology being successfully used to significantly augment building heating and water heating systems.

I can always renovate/modify the sunspace in the future if need be.  For now let's just assume I want to leave the sunspace "as is" as I've described previously (= no changes in thermal mass, installation of new flooring, etc.).  What I'm trying to decide on is the best first attempt at a basic ventilation control system to move warmed air into my house.  Obviously I'm free to revise/improve that system later, but I want to make my best educated guess on a basic simple to try that I won't regret later in terms of basic design decisions.

The Build It Solar examples are great, but only one of them that I could find (Mike's Colorado home) had an attached sunspace layout fairly similar to mine.  However, his sunspace volume is considerably smaller, and his sunspace/home was only one story.

So in terms of keeping it simple at this point, let me ask some pointed questions to get some responses to help me make an educated guess on a good first design, that could be improved upon later if needed :)

1. Is the basic approach/design I've described...a "warm air in" vent on the main floor level and a "cooler basement air out" vent a sensible first design attempt to try before something more involved/elaborate?
2. If not, what would be?
3. If so, what about some details of this design:
1. In terms of locating the two vents, I assume there would be no reason to locate the basement level vent anywhere other than near the basement floor (same level as the sunspace floor)?
2. What about the main floor vent?  Should that vent be close the main floor ceiling (which would place it about 3' below the peak of the sunspace and about 16' above the sunspace floor/basement vent), or down near the main floor floor (add/subtract about 7.5' ceiling height to those earlier figures)?
3. If the main floor vent is located at ceiling level...only 3' below sunspace ceiling peak...would it still make sense to run a 2.5' or so section of duct/pipe up closer to that peak to pull in the warmest air?  Or is 3' close enough?
   
4. Fan model selection/vent duct opening size.  chrs I appreciate that link to that Conair computer fan.  That fan is only about 3.5" wide if I'm reading the specs right...what would that mean in terms of the size of my vents/wall penetrations?  I assume you mean installing it into a space that is larger than 3.5" wide right?  I can always make a small hole larger so "starting small" and increasing it later if I don't like the results makes sense, but I'd like to make a decent guess to begin with before I buy a fan that I won't end up using because it is undersized.  So what size "hole"/vent penetration would be that first educated guess to try with a fan like that?
   
5. Two fans or one?  Would the first thing to try be mounting a single fan in the main floor "intake" and seeing how well that not only blows the warm air into the home, but how well it pulls that cooler basement air in?  Or vice versa (with the benefit of less fan noise since it is in the basement)?
6. Dampers.  Any creative ideas on dampers?  Or should I just stick with some inline dampers like this or this?

Thanks everyone for your help.

The Conair fan is actually 10" diameter.

An advantage of using two fans, one in and one out, is that you'd maintain balanced pressure and wouldn't drive infiltration. And you would get double the pressure capability, and about double the power consumption. With no ducts and 10 inch holes, I would guess you wouldn't need the pressure capability and the pressure imbalance would be small. If you run a length of duct it is probably worth checking the pressure drop with some duct calculator or formula.

I think your proposed locations are good. The 2.5 foot duct stub doesn't seem like it would make a big difference, but might help. I think I'd install it without that and measure temperatures, and add it if it seems like it would be worthwhile based on the temperatures.

Another idea would be to run a duct down to the main house floor level, to keep more of that hot air on the first floor vs upstairs. How much that matters depends on the layout and envelope quality. (/matters less with a good envelope.

My thoughts:

1. Yes

2. N/A

3.a. Yes, heat rises so locate all exhaust vents near floor as is done for typical hot air heating system.

3.b. Near floor would be better as discussed in 3.a. response. The sunroom inlet vent should be as high as practical to capture hottest air.

3.c. Pulling the hottest air which will be at the peak is clearly best, but trading a couple of feet of height for duct installation ease might be okay.

3.d. Consider using standard size residential heating ducts.

3.e. One blower at the inlet and adjustable grills at the outlets as is done for typical hot air heating systems. Keep it simple and minimal cost. Target between 500-1000 CFM. Perhaps an adjustable speed blower to allow some optimizing.

3.f. You might want to consider an insulated control damper. Perhaps checkout the Greenheck products that are thermally broken to reduce condensation. When your sunroom is cold and your house is warm, you don’t want to draft any warm house air into the sunroom. You also don’t want condensation running down the duct into the house either. These are likely your biggest risks and they could easily offset the benefit if not considered/addressed.

Efficient 30 W, 48 V power supplies are pretty readily available. That said, there's a 24 V version of the same fan (http://www.digikey.com/product-detail/en/19031573A/CR302-ND), and 24 V power supplies are even more readily available, e.g this one:
http://www.digikey.com/product-detail/en/VEH40US24/1470-2320-ND/
Actually rated 40 W.
You could then switch the 120 V input to the power supply with your control system. Switching 24 V is also quite feasible, and safer to work with in a DIY setup. And the power supply has low standby power consumption so it's OK to leave it on all the time.

The backdraft damper, however, might significantly limit the flow of the poor gentle fan I'm recommending, if the spring is too strong and it doesn't open all the way. The best low pressure backdraft dampers I know of are the Tamarack Cape units, http://www.tamtech.com/8-Cape-Backdraft-Damper_p_83.html . Their biggest is 8", whereas the airflow path of the fan is 9.120" diameter. 8" might be close enough, but it might be better to use the concept to make your own bigger one. For the return, you could use two 8" holes with those backdraft dampers in them, and have plenty low enough impedance for the fan. Or maybe if you just use backdraft dampers on the returns, and use a manual or motorized damper on the one with the fan?

On running a duct down, I meant from the top of the sunspace to the floor level of the main floor of the house, rather than to the top of the main level of the house ... but maybe I misunderstood--maybe the short duct is all that is needed to do that? The only point of that is that if the hot air goes into the main level near the ceiling, you might worry that it would stay there and never mix with air closer to the floor.

Thanks chrs, this really helps. I think I have a pretty good idea of a gameplan to move forward with. Once I get things installed, I'll report back some results Colin

PS interesting design on that "cape" low pressure damper.  The online reviews on this design seem to be mixed; it probably depends on lot on your specific application.  Have you actually used one or know of people who have had success with them?  They are fairly inexpensive, so probably worth a gamble either way.  As you say it might not be difficult to build your own...key would be choosing the correct fabric/material that is "fluid" enough to collapse in a way to block most air leakage, and light enough to easily "blow open"...

It seems like with a little tweaking you could build a damper similar to "Mike's" in Colorado (page 3 of this PDF) with some foamboard pivoting around a thin metal rod, slightly weighted at the bottom to return it to a "closed" position.  Getting a reasonably snug fit at the edges without risking an edge "catching" and getting stuck in place would be the challenge...

You got me thinking about backdraft damper designs. With a rigid flap design like Mike's, it would seem that you'd ideally want it to require a small, but significant, pressure to start to move, but then not much more force to swing all the way open. A flap that is closed when vertical with a weight at the bottom is almost the opposite. The torque created by the weight would be a maximum when it's fully open, and would go to zero when it's all the way closed. That's almost opposite what I'd think you'd want. I'd want to add an arm, rigidly attached to the flap, but sticking out at almost 90 degrees from the flap. That weight being pulled down would hold it closed when it was closed, but would apply less and less torque as the flap swung up, meaning very little torque would be needed to hold it all the way open. That would help it swing all the way out even with a low flow.

I'm probably making it more complicated than it needs to be, as the airflow you'd get by natural convection when the sun space is cold would be in the reverse direction, and that's naturally stopped with no weight needed, but I thought it was interesting to think about.

Is there any advantage of putting a heat pump in the sunspace to relocate the heat?