Greenhouse and CFM

[SlipperFan]

We are planning for a conservatory that will be attached to our home. One of the things we haven't figured out yet is how to exhaust the air from the area. The greenhouse won't have the typical area at the ridge where a fan could be placed: the front portion is slanted and the rear is attached to the roof of the house.

But the question here is about CFMs. Everything I read says you have to have a fan that will exchange the air once every minute. We'll have awning windows all around, so incoming air is not a problem. There will also be ridge vents. The fan is needed for the hottest days.

So the question is, will I really need a 3000 cfm fan (that's what is calculated for length [22'] x width [16'] x wall height [9']) -- or is this overkill?

 

[John D.]

From the design standpoint you can try to follow the recommendations, from a usage standpoint you can put a speed controler (manual or VFD - variable frequency drive) to be able to slow it down or hook to a temperature sensor for auto. Lower speed is lower energy usage. Rough numbers are at 5 cents per Kilowatt hour operating 1 horsepower for a year (full time) costs $400
so a $100 dollar speed control can pay for itself.

 

[SlipperFan]

I'd like to rephrase my question:

How many of you who have greenhouses follow the guidelines that one is supposed to completely exchange the air in the greenhouse once every minute? http://www.littlegreenhouse.com/fan-calc.shtml

 

[cnycharles]

one of my favorite answers is - "it depends..."

you pointed out that you have some side vents and some ridge vents that would work for air intake, but that at the upper-most edge there wasn't a vent? one thing that will happen if you have more than one vent, is that there will be cross- and passive ventilation. air will move through your greenhouse, so there will be somewhat less need for an exhaust fan to move all of the air. if one vent is higher than another, you will have convective movement. so, lower air will move around and be cooler than air above the vents. if there is a breeze along one wall or over the top, air will be pulled out, or if directly at a vent, then out the other ones. because of this you wouldn't need as much exhausting power.

now, if you had one intake vent on one side and another vent with an exhaust fan on the other side, then you would have much more need of using that formula and include all of the air mass contained in your greenhouse. because of your vents you would need a fair amount less exchange for lower air and maybe use the formula for upper air, but that is only if your plants are exposed to the upper air that was too hot. it would depend on how high above your plants the hotter air was, and how much cross ventilation you can achieve from the vents you have already. if you have a very tall greenhouse with a lot of air mass a ways above, and your vents are at the same level as and/or a little above the plants, then you may not need to vent that air at all. now when it gets to be wintertime if you feel that the sun could get your greenhouse with it's closed vents hot enough so that you would need venting, you would still not likely need a ton of exhausting power because you would already be losing heat through the sides and roof (radiant and through cracks and breezes) and because of the difference in the inside and outside temps you would only want a little cold air to trickle in to mix with your hot air. best in that case would be to just open a ridge or top vent a tiny bit and let the hot bleed out. in the case where you might like to have that heat stay indoors for night-time, it might be better just to have a lot more air movement to keep the leaves fresh so that you can keep it warmer overnight, as it is said that most plants can handle higher than usually like temps if there is a lot of air movement to keep the leaves from getting too hot.

I think in both cases a tiny bit of reflective shading at certain times of year would work better than having a big exhaust fan. it can't hurt to have a small one but in winter you would have to be very careful that you didn't exchange too much too quickly. if you can keep the plant leaves from collecting too much radiant energy (sun on hot leaves in hot greenhouse) with moving air, then you can keep the heat in your greenhouse so that you don't have to heat it as much at night.

summary:
the rating for how much air you would want to exchange would depend heavily on the difference in indoor and outdoor temps. where sun and heat would be seasonal, the needed amount to vent would change drastically as well as the amount of time the exhaust would need to be on, and the effective volume of air that would actually need to be moved/cooled would change alot as well, so you would need some sort of control that you could set automatically that would ramp or adjust how things vent when the potential for being really hot or cold inside/out changes (or just hang out in your greenhouse all day). also to try and answer your direct question myself more directly, very rarely might you need to vent your air by fan like they suggest, and then probably not for very long a period unless the outside temps are at or above the temps you want your greenhouse to be at. if the times of year that your house might get really hot are for only short periods of time, then spritzing a little weak shade compound on the upper southernmost areas of your house may be more effective at preventing the high temperature peaks, and not appreciably change the overall amount of light negatively for your plants. also if there were parts of the roof that were positioned near your house where a lot of sun would shine in and heat up the air/side of house, but very few plants were going to be in those exact areas, those could be roof/wall zones that could have heavy shade compound sprayed on them, with little effect on plants but substantial effect on maximum temps. also using reflective compounds on wall and floor areas where there weren't going to be plants could help bounce some of the light back out instead of letting it be absorbed and release heat inside. so, with a little of this and a little of that, you may not need a big fan, listen to the whirring of fan blades or have to pay for as much electricity

(well, I guess my summary wasn't really short enough to be called that... )
hope this helps

 

[SlipperFan]

http://floriangreenhouse.com/monarch.html If you look at the photo of the conservatory attached to the house with the chimney beside it (upper left corner), you will see basically the configuration of the structure we will be getting. It will have temperature-controlled motorized ridge vents, and will have awning windows all around the base-wall. There is no place on this structure to have an additional vent up high, because of the slanted panels up to the ridge vent.

However, where the structure will attach to our home, we have to build a little space so that new structure will fit into the existing roof-line of our home. It is in that section we are thinking of putting in some kind of exhaust fan, but it's architecturally difficult if not impossible to put in a 3000 CFM fan there. The passive air-flow from the awning windows up through the ridge vents should take care of any heat build-up problem except during our hottest days when the temperature goes over 90ºF outside.

My question is I'm just wondering if anyone used that formula for determining what size exhaust fan they use. Simple question, really.

 

[SlipperKing]

I used it BUT I didn't have the varables you have added with low vents and roof vents.
Friends of mine have this same conservator, picture #3(cabana). I don't think they have a exhust fan BUT they do have a great little evaporator cooler on the far end from the house. Works great, cool air on the ground heat out the vents.

 

[cnycharles]

a standard formula for ventilating only (not venting using a fan-pad cooler or trying to determine replacement air in winter when venting humid air) is length x width x ((top peak height - ridge height)/2). this translates to the volume of the box below times the volume of the triangle on top . using this formula, there still will be some rise in temps gradually (according to online help pages), so the formula you posted which is about the same except for adding on another .2 to the figure, may be realistic for not allowing a slow rise in temps. the formula is a standard fairly close to that used by greenhouse industry for ventilation for hoop houses with fans on one end and vents on the other. you would need even more ventilation exhaust if you were going to use a fan-pad cooler.

now, I have never built a greenhouse or used one in my yard, but have worked in many different sized ones of all makes and sizes already formed. it would help you more probably to have someone to step up who has built their own little one (our put up a pre-made one) and say yes this has worked or no not enough to make you feel comfortable.... but I have a feeling most people probably used whatever the greenhouse vendor said would be good. but again, the formula your greenhouse vendor used is a little more than the industry standard, but isn't unrealistic. better to have a little more and have a speed controller or timer than not have enough.

looking at the greenhouse design, I was thinking more of a lean-to (much smaller) type right up against the side of the house. now, you do have places you can use an exhaust fan, just you would be replacing the clean glass in one section probably on the 'near end' in the pictue, probably with doubly wall pvc and cutting out the shape to fit whichever exhaust fan you want. I have seen these exhaust fans pretty much supported by the double poly, maybe along with one side or other of the fan unit edged onto the frame between the glass sections. it does also look like that unit does have peak vents, if I'm looking at the right one. also if there is a small space near your house, not big enough for the rated fan, you could have one or two smaller ones on each side (opposite walls) and stage them; temperature reaches 90 and one fan or sets turns on, it reaches 95 then another one or sets turns on. we have this setup on one set of ranges at work; three different stages and more fans for each one as the temp above nominal gets higher. in these houses there is only exhaust fan venting, no passive and the fans cover most of the exhaust end, with the whole opposite intake end being an intake vent which opens and closes

 

[SlipperFan]

it does also look like that unit does have peak vents, if I'm looking at the right one.

Yes, it will have ridge vents, as I described above.

also if there is a small space near your house, not big enough for the rated fan, you could have one or two smaller ones on each side (opposite walls) and stage them; temperature reaches 90 and one fan or sets turns on, it reaches 95 then another one or sets turns on. we have this setup on one set of ranges at work;

That is one of the things we are considering. But how strong the fans should be -- well, that's why I asked my question.

Rick, that is one of the things we are considering as an option. I think we'll go through a season first to see if something like that is needed.

 

[gonewild]

If you have ridge vents there is no real need to have exhaust fans. Ridge vents are very effective to vent out the heat. You would not want to use exahus fans with the ridge vents open because that would tend to reverse the natural drafted air flow.

However for ridge vents to work they must be opened and closed. Are your vents going to be motorized with climate control?

Even with ridge vents you may still want an exhaust fan for those days you want to keep the roof closed.

I don't think you need to worry about the CFM formulas. There are so many variables that the recommended formulas can only be a guideline. Just remember hen you are thinking about ventilation with a fan there is no such thing as "to much".

If I were you I would put in what ever size fan fits in your space.

The only reason to need a huge air turnover is to cool the air. You can acheive this with internal fans and fog nozzles without an exahust fan.

 

[SlipperFan]

If you have ridge vents there is no real need to have exhaust fans. Ridge vents are very effective to vent out the heat. You would not want to use exahus fans with the ridge vents open because that would tend to reverse the natural drafted air flow.

However for ridge vents to work they must be opened and closed. Are your vents going to be motorized with climate control?

As I posted above, yes.

Even with ridge vents you may still want an exhaust fan for those days you want to keep the roof closed.

I don't think you need to worry about the CFM formulas. There are so many variables that the recommended formulas can only be a guideline. Just remember hen you are thinking about ventilation with a fan there is no such thing as "to much".

If I were you I would put in what ever size fan fits in your space.

The only reason to need a huge air turnover is to cool the air. You can acheive this with internal fans and fog nozzles without an exahust fan.

This is in the direction of my thought. It's nice to have confirmation.

 

[Rick]

When its > 90 outside simply moving the air in an out of your greenhouse will do nothing for cooling, and will also dry it out.

So you either need a swamp cooler (wet pad wall) or foggers to get some kind of evaporative cooling to control the temps.

I've gone through iterations of mister/fogger/wetpad, and found the most efficient use of water and cooling is through wet pad (swamp cooler). Followed by fogger, with misting systems in last place.

My system uses a combination of wet pad to handle about 60-70 of maximum temp/humidity control and fogger to handle the rest. The wet pad runs all the time and the fogger comes on when the wet pad cant keep up.

The efficiency of the wet pad is based on how much water trickles down through the pads, and how much air is pulled or pushed through the pads. The one turnover/min may be a good rule of thumb for sizing the wet pad and fans.

I have designed my wet pad system to run on recirculated interior air to keep humidity up when temps aren't high enough to need lots of external air exchange too.

 

[Jim Toomey]

I have two two stage exhaust fans, these are hooked up to a single thermostat. Wet pads are at the other end...
If I had to do the wetpad over again I would design it like what Rick says above and recirculate the air to humidify it more.

At stage one (I usually have it set somewhere around about 86-90) it kicks on the low speed which is pretty fast, if the temp climbs any higher, stage two kicks in and they really speed up.

If I had to do it over again, I would do the same thing, I only wish I had used 220 instead of 110, it would have saved some electric cost.

I have recently added a fogger made by hydrofogger.com it really does an excellent job of keeping the humidity level consistent.

Jim

 

[cnycharles]

i've also heard about the using 220 instead of 110, that for muscle work it's better to have the 220. at work we also thought about having a simple microfogger system set up where our intake vents were, when we were venting during warm weather to help keep the plugs and cuttings cooler. unless you live in an area where it is normally very dry, (unlike upstate ny or michigan which have very similar environments) when it gets cooler and cloudy the humidity often doesn't need much raising, but you wish you could vent to get rid of the indoor fog. at that point, you have to start venting and then running the heat to get it drier indoors (or botrytis kicks in and other lovely things).

your problem is, there are many ways to skin your cat! I think I would do more preventing the heat buildup as usually in our necks of the woods it isn't often expected to get over 90 degrees, and if you have some of this weather you can just circulate the air a lot more. one thing I forgot to add before, is a few years ago when the temps for a month were over 90 every day, the greenhouses that are cooled through full exhaust ventilation were reaching over 110 degrees, and that was with reflective curtains over the crop. there was constant air movement, so no plants died but at that heat there wasn't a point letting sun through because photosynthesis was suffering mightily. also because there wasn't radiant heat on the leaves of these plants (and us in these greenhouses) it was more bearable. now when you would go into the houses that had upper vents or roofs that opened up directly, it was much cooler. having shade (on the glass) of some sort for the hottest times really would be of the most benefit if you had strong passive venting, and if you had all or mostly exhaust/venting you would really need shading of some sort. reflective curtains work well except for where you have passive upper venting because it interferes with the air movement

 

[SlipperFan]

Thanks Rick, Jim and Charles. Interesting solutions -- I appreciate the input.

I'm wondering, though, in Michigan, when the temp gets over 90º, it's also very humid. I don't think drying in the greenhouse will be a problem.

I've also read that swamp coolers work less effectively in humid conditions. Have any of you found that to be true?

 

[Ray]

When its > 90 outside simply moving the air in an out of your greenhouse will do nothing for cooling, and will also dry it out.

Rick - isn't that contradictory?

If the air movement is sufficient to dry out the greenhouse, wouldn't that constitute some level of evaporative cooling?

My GH relies on internal air circulation plus vents in the front wall that open at 70°, and a VERY large exhaust fan (rated for a complete air exchange every 30 seconds) that kicks in at 85°. It dries the greenhouse sufficiently - even in 90+% RH - that a fogger kicks in periodically.
________________________________________________________
Concerning the swamp cooler and humidity, it's physics, Dot.

It takes energy to heat water and turn it into a vapor. With evaporative cooling, that energy comes out of the air, resulting in cooler, moister air.

The evaporation rate, however, is also controlled by the ambient humidity. The lower the RH, the faster the evaporation rate, hence the more "heat" taken out of the air.

That said, I have seen evaporative coolers at work in greenhouses in Houston, where the heat and humidity are hard to top. They may not be overly effective, but they do work to some extent.

 

[gonewild]

It takes energy to heat water and turn it into a vapor. With evaporative cooling, that energy comes out of the air, resulting in cooler, moister air.

The evaporation rate, however, is also controlled by the ambient humidity. The lower the RH, the faster the evaporation rate, hence the more "heat" taken out of the air.

An important factor to remember......when it comes to healthy plant growth it is not air temperature that is most important. The most important aspect is leaf temperature. Leaf tissues have a tendency to absorb and collect heat from the sun and air so the leaf temperatures can and do get higher than air temperatures. If you have a film of water on the leaf surface as that water evaporates it will pull stored heat from the leaf tissue thus lowering the leaf temperature. This is why frequent misting of the foliage is very effective.

 

[SlipperFan]

This is helpful. Thanks Gentlemen!

Keep the suggestions coming...

 

[Rick]

Rick - isn't that contradictory?

If the air movement is sufficient to dry out the greenhouse, wouldn't that constitute some level of evaporative cooling?

I have found that the amount of moisture available for evaporative cooling from plants in even the most packed GH is insufficient to keep up with high volumes of hot (and relatively dry) air. Generally you end up with generally hotter conditions inside the GH (because of solar input), and more moisture getting sucked out of the plants (because hot air holds less moisture than cold air. As long as there is some sunlight entering, then evaporitive cooling at the leaf surfaces is often canceled out by solar input to the leaf surface itself.

As you point out this is a matter of physics. On an energy basis the actions happening at the leaf surface are very small compared to the amount of energy required to offset large volumes of heat being pulled through the GH by ventilation.

My GH is small, but well filled with plants. I don't exchange lots of air in the summer, but the GH is fairly leaky anyway. Before I added automated filling to the wet pad sump, I used to need to replace about 10 gallons a day to evaporative losses on the system (peak summer), and the fogger uses another few gallons a day too. This is a lot compared to what I mist on the plants every morning, and whatever selected pot watering happens on a daily basis.

So at least in my GH, the amount of evaporative cooling performed by the plants in the GH is negligable compared to what is occuring in the wet pad and foggers.

 

[Rick]

Thanks Rick, Jim and Charles. Interesting solutions -- I appreciate the input.

I'm wondering, though, in Michigan, when the temp gets over 90º, it's also very humid. I don't think drying in the greenhouse will be a problem.

I've also read that swamp coolers work less effectively in humid conditions. Have any of you found that to be true?

Living in TN it gets hotter and more humid than Michigan, and swamp coolers are the mainstay of all good GH operations here.

You would be surprised on the drying effects of heat behind glass. Before the swamp coolers and foggers, a hot summer day in TN with 50% shade cloth and fans blowing into the open doors of the GH would produce 100+ temps with 30% humidity (ambient humidity around 50 to 60% except just after a rain shower). Definitely plan on some type of humidity control for your GH.

 

[SlipperFan]

Thanks, Rick. We do have a plan for humidity control, but I think we'll have to live with it before we'll know if it work adequately or if supplemental will be necessary. From what I've been reading here, it sounds like every time a new greenhouse is built, it's rather an experiment that needs tweaking as time goes on.