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#11
01-22-2010, 05:09 PM
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The following is factual and I think anyone would agree. Without assigning any level of risk to any item this is the risk from highest to lowest: 1.Unvented heater 2.Heater vented via natural convection 3.Heater vented via powered blower but without fresh air supplied to the burner 4. Heater vented by powered blower with outside fresh air supplied to the combustion chamber. The opinion comes in when we try to assign just how large the risk is of each opinion. The risk from the case 1 is a function of the status of the heater. How well is it adjusted and maintained? Maybe great, but based on what people do in their houses probably not great. How big is the heater and how much fuel is burned in relation to the volume and the sealing of the GH. Case 1 can be made extremely hazardous or fairly safe depending on these items. Cases 2-4 provide much better protection from the possible flaws that can develop. What's the reward for selecting any of the options. Case 1 is the least expensive, but case 2 is nearly the same cost with improved safety. Case 3 is excellent and costs 5-10% more than 1. Case 4 is the best and costs 20-25% more than 1. Given the cost differences it makes no sense to settle for case 1. Case 2 is the most used situation in homes, but it certainly is dependent on air leaks or occasional door opening to work right. With my GH (1100 sq. ft. with a 16 ft peak) cloudy and cold conditions for days on end (like now) mean that the vent fans are not used for days on end...5 days and counting. There is plenty of opportunity for buildup of CO, CO2, and hydrocarbon fragments under these conditions. When the sun shines it's easy to get the GH 60 degrees above the outside meaning that vent fans are running and there is not a problem with any choice. In Nashville you are more likely to be able to vent and those north of me are less likely and run a higher risk of accumulating undesirable gases. My reason for saying anything here was that I know many people know nothing about this subject and many of those get info from the internet. Given the relative risks and relative costs it makes no sense to use an unvented heater anywhere at any time. I'm sorry you think I exaggerated the risk.That wasn't my intent, but I did intend to get people's attention. 
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#12
01-22-2010, 07:08 PM
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Jim, for more clarification:
1.Unvented heater 2.Heater vented via natural convection 3.Heater vented via powered blower but without fresh air supplied to the burner 4. Heater vented by powered blower with outside fresh air supplied to the combustion chamber. Case #1 - $175.00 (30,000 btu) 99.9% efficient Case #2 - $749.00 (45,000 btu) 80% efficient (5x the cost of #1 not including shipping) Case #3 - $1899.00 (120,000 btu smallest) 80% efficient (10x the cost of #1 not including shipping) Case #4 - $2329.00 (135,000 btu smallest) 82% efficient (12x the cost of #1 not including shipping) http://www.younits.com/gas-unit-heat...html?brand=107 http://www.farmtek.com/farm/supplies...e_heaters.html 1 Therm of propane (1 million btu) is currently priced at $36+. For every 1 Therm of propane burned in a vented greenhouse heater it is allowing $7.20 to go up the chimney. A 1000 square foot greenhouse during very cold winter weather will easily burn 1 Therm of propane every 8 hours. That means that $21 per day goes up the chimney, completely wasted. My reason for posting my information is because I have been running vented greenhouse heaters for fifteen years. I was always told that non-vented heaters in the greenhouse were a bad idea...it would cause bud drop. Then I visted friend after friend that defied the common wisdom and used non-vented gas heaters in their greenhouses...successfully. These included Southern Burner non-vented units made specifically for using in greenhouses, and the "household-type" non-vented heaters. I was flabbergasted and dubious. I have battled with the idea of using a made-for-greenhouse unvented heater for about six years. When my Southern Burner vented units gave me too much trouble this past Fall, I bought an unvented unit on a frigid night, "hoping for the best"...that it wouldn't kill my plants. It worked great. I installed a second heater. It worked great. This occurred during the coldest, longest winter weather we have had here in about eight years. There is no smell from the heaters, no bud drop, no flower wilt. Nada. My recent experience mirrors my friend's decades of experiences. The venting concern is a real one. As I explained in my earlier posts these unvented heaters should not be used in a tightly sealed greenhouse. I provide a constant source of fresh air into the greenhouse, near each heater. This prevents the production of carbon monoxide. Circulating fans also bring a steady source of oxygen to the burners. Using motorized vents attached to the big greenhouse fans to vent-off exhaust is not practical, you must have a small non-motorized opening or have a greenhouse that is not too tightly sealed. I have several areas near the top of my greenhouse that are not sealed well that permit air leakage out. I suppose a small blower fan could be used to duct air out of the greenhouse when ever the heater is on. Even so, the exhaust does not have to remove as much air as a unit used inside a home. As I mentioned earlier the primary products of combustion are water and CO2. Many commercial greenhouses purposely burn propane in their greenhouses during all seasons specifically to add CO2 to the atmosphere because it increases plant growth. These non-vented heaters are rated at 99.9% efficiency. That doesn't mean that they allow 0.1% of the fuel go to unburned, it only means that it sounds ridiculous to say they are 99.999999% efficient. After saying all of this, I agree that anyone who is unable to follow the specific directions that come with the heaters should not use them. Last edited by EdM-in-TN; 01-22-2010 at 11:43 PM.
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#13
01-23-2010, 05:31 AM
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Straight from the current FarmTek catalog, power vented Modine heaters including the vent blower, vent piping, and electronic ignition: (Case 3)
Output in BTU/hr 120,000 $699.00 160,000 Btu/hr $899 The heater in my GH is a Modine power vented,electronic ignition, propane model (Case 3) from Griffin Greenhouse Supply in Knoxville with 140,000 BTU/hr output. It is listed at $829 in the FarmTek catalog and that's exactly what I paid Griffin for the heater including mechanical installation. The electrical installation is not done by Griffin so you need an electrician if you can't attach the wires yourself. I received a verbal estimate from Griffin on a case 4 model for an additional $700 so you're right I understated that case (4). That's what I'll do next time because it's the best case and my life is worth a lot to me and to my dogs. My wife might have other feelings. Notice that FarmTek does not offer an unvented model for sale (except an obviously portable,floor standing propane heater listed as "temporary" heat). That's no oversight. The catalog lists Modine Hot Dawg power vented, electronic ignition models in smaller sizes: 24,000 BTU/hr $495 36,000 BTU/hr $535 The sizes you compared are vastly different from each other so the price comparisons are not valid. The cheapest alternative is not necessarily the best one....in fact it seldom is. Last edited by Kentucky; 01-23-2010 at 05:40 AM. Reason: addition
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#14
01-23-2010, 10:03 AM
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When I purchsed my most-recent heater - a Reznor V3 UDAS that looks a lot like the Modine "Hot Dawg", but with separated combustion (Case #4) - It cost me $710 more than the case #3 version, but it reduced my propane consumption by 40% compared to the equivalent-BTU standard (vertical heat exchanger) Modine it replaced.
Got that $710 back the first winter.
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#15
01-23-2010, 02:24 PM
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Quote:
The household-type ventless heaters are not cheaper because they are less safe, they are cheaper because of supply and demand. The demand for household-ventless heaters is high, and supply is high, and competition drives the costs down. In contrast, vented greenhouse heaters are in low demand and the production is therefore low, and competition is low which drives the costs up. All greenhouse equipment sells at a premium. The power vented greenhouse heaters are relatively no more sophisticated than the ventless ones. What I did not mention earlier is that up until 2005, eight States prohibited the use of ventless gas heaters in homes. But because of newer, safer heaters, seven of those States dropped their objections in 2008. California is still uncertain. There is no State in America that prohibits the use of ventless gas heaters in a greenhouse. If lawyers and scientists can't prove the case, then what is your complaint of safety concerns? Remember, it's routine for large greenhouse companies to burn propane in a greenhouse for the specific purpose of generating more CO2 in the atmosphere for the plants. If this were dangerous why then are they not sued over safety concerns? I will also mention that neither I nor my friends have experienced bud-blast or flower wilt with the use of these new heaters. Orchids are the "canary" of the flower world and will drop buds and flowers with the slightest amount of propane or ethylene in the greenhouse atmosphere. There apparently is none with these new heaters. Not only are these heaters proven safe even in homes, and don't seem to harm the plants or flowers, but they are also more "green" than the vented unit heaters. These new ventless units use 99.9% of their fuel converted to heating the greenhouse versus only 80% by the vented heaters which send 20% of their heat directly to the outdoors unused. The ventless heaters use less fuel and therefore release less CO2 into the atmosphere which is blamed for causing so-called "global warming". There are thousands of the Southern Burner ventless heaters being used in greenhouses all over America. I spoke with the owner of the company, Glenn, and they have never had a report of a death and no report of injury on a properly maintained heater. There has never been a question of the safety of the Southern Burner heaters. And as I said previously, I'm familiar with these Southern Burner heaters and these new ventless heaters are even more sophisticated and safer. I would agree with you that there has always been a "professional" way to heat a greenhouse and that hobbyists are told to use the same heaters that the large greenhouses do. That advice might be changing. We live in a world where we are being asked to make our "carbon footprint" smaller. If we can do that safely, I'm all for it. Last edited by EdM-in-TN; 01-23-2010 at 11:50 PM.
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#16
01-24-2010, 05:19 AM
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Thanks for the input to this discussion. I think we have touched on the alternatives and issues. It should be helpful to someone investigating their choices to know what questions they need to be considering in their quest for heating a GH.
Ray, I'm going to install a second heater in my GH to insure against any failures and I'm going with a fresh air combustion model. I hope I get the savings you have realized. At the moment I'm doing my part for global warming with a propane bill of about $4000 a year at $2 per gallon. JL
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#17
01-24-2010, 10:01 PM
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Lucky you to get propane for $2.00 per gallon, here its $2.85.
Ten years ago, the price of propane was comparable to natural gas. Propane burns much hotter than natural gas, so a slightly higher price for it back then "evened things out". Not any more. There have been large new natural gas fields opened in the US, while demand for propane increases. Currently, per Therm (1 million btu) of heat, propane is selling for an average of $34, while a therm of natural gas is $8.50. If you have a choice, use natural gas.
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#18
01-25-2010, 05:48 AM
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Ed, I found that propane costs vary wildly between competing supplies in the same market. Two years ago I paid as much as $3.25 a gallon. Threatening to leave the supplier got an immediate reduction to $2.80 (unfortunately not retroactive).
Last year I changed to my current supplier at $2.25 a gallon and this year's contract price is $2.09. My house is on natural gas that comes from the Columbia Interstate pipeline that's just 100 yards from the house. It requires a two stage meter to reduce the pressure to residential use. Unfortunately Columbia does not want these high pressure taps any longer. The state won't allow them to disconnect me, but increasing my usage substantially is legitimate reason for disconnect so I had to go to propane for the GH. Doesn't surprise me that the Nashville area is high. Growing places seem to be that way. Try Ferrellgas if they operate there and if possible contact a location as rural as possible for your area. My original supplier (high price) of propane tended towards being a supplier to the local residential market.....suburban houses outside the natural gas distribution system, trailer parks (though none are allowed in this county, they surround it), things like that....in fact their name is Suburban Propane. By going to a supplier in a neighboring county that is more into agricultural, manufacturing, and cheaper rural, eastern Kentucky residential accounts I got the substantial price reduction plus much improved friendliness and service. Propane prices did decline over that same period to be fair.
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