Tamara-- post to FAQs all these questions? AND, another Q.
kirkhall
8 years ago
Featured Answer
Sort by:Oldest
Comments (21)
kirkhall
8 years agofunkycamper
8 years agoRelated Discussions
Questions to be placed into the FAQ
Comments (36)Well, OD, you didn't answer. May I post a polished version anyway? First, restate the subject as a question: How do I calculate the costs of running a greenhouse? Answer: The question is best answered by using an example, but first some formulas: Watts=amps*volts 1 KWH = 3,413 BTU 1500W electric heater produces 5120BTU Propane is 91,600BTU/gallon Q=deltaT*A/R Q=BTU heat requirement per hour delta T is the difference between outside temp and desired inside temp A= the surface are of the greenhouse is square feet, not including the floor; ie, the roof, side walls, and end walls R= the R value of the glazing Two important points about this last formula...some heating calculators use "heat loss value" (HLV) instead of R-value. Don't get confused, the HLV is just 1/R. The formula using heat loss value is then Q=deltaT*A*HLV. The other important point is that you use a different temperature when calculating your average energy costs over a month than you do when you you calculate what size heater you need. Heater calculations are used to determine how much heat you will need on the coldest night of the year. For this, you use your coldest winter temps, for example the colder end of your zone. For calculating heating costs you need the average temperature for the month. Here are some typical R-values for common glazings or coverings: 4 mil polyethylene 0.83; 4 mm (5/32") twinwall polycarbonate 1.43; 6 mil polyethylene 0.87; 6 mm (1/4") twinwall polycarbonate 1.54; 6 mil poly double layer (inflated) 1.43; 11 mil woven polyethylene 0.95; 3 mm (1/8") glass (single layer) 0.88; 16 mm (5/8") triplewall polycarbonate 2.5; Polycarbonate/fiberglass (single layer) 0.83 OK, let's say we are contemplating an ACF 10 x 12 Cottage Greenhouse. We first need to calculate the surface area of the greenhouse. Using the diminsions found on the website and some simple geometry, we calculate the surface area of each sidewall to be 55 sq ft, each roof side to be 64.6 sq ft, and each end wall to be 75 sq ft. Therefore the total area (A) is sq ft is (55*2)+(64.6*2)+(75*2)=389.2 sq ft. The website tells us that this GH comes in 6mm twinwall polycarbonate, so the R-value is 1.54. A quick search shows that in this particular zone 7a, the average January temperature to be 28.4 degrees. We want to keep our greenhouse at 50 degrees. So the "delta T" is 50-28.4=21.6 degrees. Now we can calculate the BTU per hour necessary to heat the greenhouse in January. Q=deltaT*A/R. Substituting the numbers, Q=389.2*21.6/1.54=5459 BTU. To figure daily use, multiply by 24 hours to get 131016 BTU/day. To figure monthly use, multiply again by 30 (days in the month) to get 3,930,480 BTU/month. Now, lets say we heat with propane. Knowing the BTU in a gallon of propane, we can take our BTU/month and divide by the number of BTU in a gallon of propane. 3,930,480/91,600BTU/gal propane to get 42.9 gallons of propane we would expect to burn in January. We just bought propane at $1.69/gal, so this would mean my January cost would be 42.6*$1.69=$72.52/month. Let's say we heat with electric. Take the same BTU value and instead of dividing by the BTU/gallon of propane, divide by BTU/KWH. 3,930,480/3413BTU per KWH=1151KWH. If my energy cost is 7.169cents/KWH, then heating electric should cost about $82.52 for the month of January. OK, well, our hypothetical DW says that the winter costs are too high and she cannot be convinced to go ahead with the greenhouse. Let's look at what we can do to lower the winter costs. Let's insulate the north wall...no sun comes through this wall, and if we insulate it with foil-backed polyisocyanurate 1" board, which has an R-value of 7.04, we raise the R-value of the north wall to 1.54+7.04=8.58. We can recalculate Q using the same formula, but adding together the BTU lost through the north wall and the BTU lost through the rest of the greenhouse. The formula would be Q=(deltaT*A/R)+(deltaT*A/R). Our greenhouse is oriented north-south, so the north wall is an end wall with a square footage of 55 sq ft. Our BTU formula changes: Q = (334.2*21.6/1.54) + (55*21.6/8.58) = 4826 BTU/hour This equals a monthly use of 3,474,720 BTU, equal to about $72.99/month heating electric. What else could we do? Well, we could add some thermal mass. Let's put 8 black 55 gallon drums in the greenhouse. One BTU is defined as the amount of heat required to raise one pound of water one degree F. The water will warm during the day and release this heat at night. One chap we know has measured the temperatures of his water at dawn and at dusk. The temp is 74F at dusk and 66F in the morning, a drop of 8F. Eight 55 gallon drums hold 440 gallons of water which is 3520 pounds of water. A 1F drop in the water temp releases 3520 BTU. An 8F drop releases 28120 BTU overnight. Over the course of a month, this is 844,800 BTU. Therefore, the total monthly BTU could hypothetically drop further to 3,474,720-844,800=2,629,920 BTU. This is 770.55KWH heating electric. At $0.07169/KWH, this is $55.24 for the month of January. Our calculations assume 100% efficiency. Some heaters, particularly vented heaters, are not 100% efficient. If you know the efficiency of your heater, you can adjust the calculations accordingly! Now on to summer costs. The greenhouse in the example will need an exhaust fan and HAF fans. First the exhaust fan. The greenhouse is 900 cubic feet in volume. Therefore, we need an exhaust fan that is rated at least 900 cfm. A 16" exhaust fan at Charley's is rated at 1095 cfm. The website tells me it draws 1.8 amps. Remember that watts=amps*volts. So when it is on, the fan uses 1.8*120=216 watts. If this fan runs 16 hours per day in the summer, it will use 216*16=3.456KWH/day. This is 103.68KWH/month, which, at the same price of energy we have been using above, amounts to $7.43/month for the exhaust fan. Let's say we use two 8" long life HAF fans. This may be overkill for a greenhouse of this size, but you will get the idea. Each fan draws 0.6 amps, and at 120V, this is 72 watts. Running 24 hours a day, each fan uses 1.728KWH/day, or 51.84KWH/month. Total use with the two fans is again 103.68KWH/month or $7.43/month. Know the usage pattern and amperage draw of each electrical appliance in the greenhouse allows us to calculate the cost of running each item. OD, I cleaned it up a little and also reinsterted the line breaks in the formulas. Thanks. SB...See MoreFAQ Version 3.0
Comments (7)Jodik says: The coir might look dry on top, but down in the center of the pot, it could be quite moisture-laden! I have found it helpful to insert little wooden skewers carefully into the soil to about root level, and leave them there... I take them out and press them against my cheek to test for dampness... if they feel at all damp, I wait to water... but if the skewer comes out dry, it's time to water. My coir is 4" deep in a closed pot. I put the skewer to the bottom of the pot. When I take it out how many inches of bone dry top "soil" should there be before it's time to water again? Is it 1", 2" or 4"? I just went 10 days without water and only 1" of the top was dry so am I supposed to water this direct sun pot once every two weeks or so to avoid over watering, really? And how much water for my 6" closed pot? I had been watering 1 1/2 cup or so once a week and my plant was fine but I'm now worried that was too much so I'm re-evaluating. I know what conventional wisdom says (water deep, not shallow) but that advice doesn't seem to apply to my situation. Also how long should it be before I see new leaf growth in the summer time (first season outside)? My 18 month old Red Lion so far has only grown new leaves once a year. At potting and at dormancy end....See MoreHow to get a FAQ posted?
Comments (12)Dave, I guess I'm not sure why you are against having a simple text file ON THIS SITE with a list of "Frequently Asked Questions" and their answers. I'm not opposed to anything of the sort Rob! You have misunderstood what is being discussed here I'm afraid. Isn't that what this is all about, sharing information? Passing on information from the 'experts' to the neophytes? Yes, exactly. In fact a list of FAQ and answers already exists here and has for years. There is a master list as well as a list specific for each forum ON EACH forum. But apparently none of us can add to it or modify it any longer as we used to be able to do before the site was sold. Only the iVillage admin can do that now that they own Garden Web. Are the admins here at GW afraid of the extra load it might cause on their servers/network? I would think they would be all for it since in increases their advertising exposure. I have no idea. You'd have to ask them that. However as they already have an extensive advertising and market section on the site and since the FAQ are not usually advertising oriented, perhaps they don't feel it is necessary. Just a guess on my part. You'll have to take that issue up with them. Considering you only joined in March, you sure seem to know a LOT about how this site works, and all of the people involved. Yes, I re-joined in March true but was a regular here for many years. Spike (the previous owner) set up GW as a free gardening message board back in the early 1990's but in 2003 or 2004 it changed to a subscription service and I left then. All my "way things used to be" posts are just that - USED to be info that is no longer applicable - and only referenced because both Carolyn and Trudi were here then also. My original post was just an inquiry directed at current members (and at Admin if they care to read it) about how FAQ modifications and adding new FAQ to the existing list are done here NOW that iVillage owns the site. Just that - nothing more - no agenda. Dave...See Morebathroom tile FAQ's
Comments (64)I changed the entrance of my shower tub. The panel board was moldy; water was getting in between the top tiles and the panel board. the joint was not well sealed. I put new everything for the entrance, even a membrane between the plywood and the tiles. However, it seems to leak from the wall now. Water is coming out of the wall outside the shower tub. Would it be the grout, tiles...? Tiles and grout are probably 20+ yrs old ( turquoise tiles). I tried to put some ready grout ( in a tube), but it still leaks! Calking is well done. Should I remove the old grout and put new one everywhere? Thanks for your help!...See Moreschicksal
8 years agofunkycamper
8 years agoschicksal
8 years agofunkycamper
8 years agokirkhall
8 years agoBuehl
8 years agohvtech42
8 years agokirkhall
8 years agokirkhall
8 years agokirkhall
8 years agoBuehl
8 years agofunkycamper
8 years agohvtech42
8 years agotea4all
8 years agoTamara
8 years agotea4all
8 years agokirkhall
8 years agomudhouse
8 years ago
Related Stories

MOST POPULAR8 Questions to Ask Yourself Before Meeting With Your Designer
Thinking in advance about how you use your space will get your first design consultation off to its best start
Full Story
REMODELING GUIDESSurvive Your Home Remodel: 11 Must-Ask Questions
Plan ahead to keep minor hassles from turning into major headaches during an extensive renovation
Full Story
FEEL-GOOD HOMEThe Question That Can Make You Love Your Home More
Change your relationship with your house for the better by focusing on the answer to something designers often ask
Full Story
GREEN BUILDINGWhat's LEED All About, Anyway?
If you're looking for a sustainable, energy-efficient home, look into LEED certification. Learn about the program and its rating system here
Full Story
HOUSEKEEPINGHow to Clean Grout — Stains and All
If your grout is grossing you out, this deep-cleaning method will help it look new again
Full Story
KITCHEN DESIGNWhat to Know When Choosing a Range Hood
Find out the types of kitchen range hoods available and the options for customized units
Full Story
BEDROOMS15 Nightstands That Are Not Your Average Bedside Tables
Take another look at that unusual antique or flea market find. It could have a new life next to the bed
Full Story
MOST POPULARHow High Should You Mount Your TV?
Today we look at an important question to consider when locating your television: How high should you set it?
Full Story
LIFESurprising Ways to Pare Down at Home
All those household items you take for granted? You might not need them after all. These lists can help you decide
Full Story
KITCHEN DESIGNHow to Design a Kitchen Island
Size, seating height, all those appliance and storage options ... here's how to clear up the kitchen island confusion
Full Story
Tamara