Registered: 1359070732 Posts: 2,240
Reply with quote #21
FWIW, how about using a common or garden central heating radiator, like this...
Painted BLACK and suspended horizontally in the apex of the GH... Additional finning could if necessary be force-fitted into the gaps between the radiator elements. This model is steel and 6 ft long by 2 ft wide, and it has a power of 1200 watts. That supposes a water capacity of around 15 liters... An aluminum radiator would heat up faster, but would be somewhat more expensive... The radiator is connected (by PEX etc.) to a water holding tank in the bottom of the greenhouse, with a circulator of sufficient power. Air can also be blown over the radiator by a fan. Best results would be achieved by ducting the radiator, but care would be required to ensure that (at least the) top of duct were transparent, to allow solar heat absorption also... The price of this radiator is 189 Euros = about 250 USD... Cost of system could be kept minimal by using (say) a converted 40-gallon oil drum as a tank, and a salvaged automobile radiator cooling fan for the air source. System is basically same as Gary's existing car-radiator model, but somewhat scaled up. A smaller CH radiator could obviously be used - for example, this is an aluminum model, dimensions 60 cms x 96 cms x 10 cms thick, 1770 watts, Price 115 E (= 150 USD) G_H __________________
(1) "Heat goes from hot to cold, there is no directional bias" (2) It's wrote, "voilà" unless talking musical instruments...
Registered: 1396734277 Posts: 3
Reply with quote #22
Sorry I'm late getting in on this discussion. Let me start with my reasoning on the setup I decided on for the greenhouse. I decided to mount the fan near the peak of the greenhouse for a couple of reasons. First, that's where most of the heat is. Simply by circulating the air from the peak to the lower portion of the greenhouse, I lower the heat at the point where the greatest loss is, and raise the temperature where the mass is (brick floor, barrel of water, bottles of water, etc.). This would be similar to a ceiling fan in your home blowing up in the winter. It pushes the warm air at the ceiling to the lower part of the room. The temperature difference from peak to floor in a greenhouse can be more than 10 degF. By putting a heat exchanger in between, I can store part of that heat. The fan also doubles as a circulation fan. I can set it to automatic or manual. When the heat pump is not needed, I can turn the fan on for circulation. As I explained to Mattie, if I had it to do again, I would have placed the tank under the floor of the greenhouse. The heat I'm loosing to the atmosphere, as much as 3 deg. overnight on cold nights, would radiate back into the greenhouse. The rest of the system would not change. The tank would be well insulated to allow for maximum control over the heat released into the greenhouse. To answer another of Mattie's questions, the optimal flow rate through the heat exchanger varies depending on the difference in air and water temperatures. If the difference is large, the flow rate should be higher. If the difference is small, the rate should be lower or none at all. The 006-VTF4 Taco pump I mentioned in the article does exactly this. My pump does not, so I settled for a mean 3 to 4 GPM. The Taco 005-F2 I used is only 1/35th HP and uses a little over half a watt. It will pump anywhere between 0 and 18 GPM with a head range of 9.5 feet. I think Garage_Hermit did a great job of answering most of the questions presented. I fall into a combination of his classifications. I didn't do all of the calculations that should have been done, but I guess you can say I made intelligent guesses. I did a lot of research to back my decisions. Putting it simply, the important factors are rate of water flow and air flow. The size of the heat exchanger is not important, as long as it is big enough. The amount of water and air flowing through it will determine the exchange rate. You want the flow rate slow enough to maintain good stratification in the tank. I could have used a larger fan. The best way to go with the fan is with a variable speed fan. The system GaryBIS is talking about is where I got my original idea for my system. One thought there. The drain back system would be ideal, but if you are not going to use the greenhouse for a period of time in the winter, you could simply blow the system out with air. That's what I do each year with my sprinkler system. Also, the heat exchanger you're using is probably like mine. the tubes loop through the exchanger a couple of times, trapping the water. A car radiator's tubes do not, so it would drain back completely. On cooling the greenhouse in the summer, I think it will be simple. Actually I'm cooling it during the day now, and storing the heat I remove from it. With an exterior heat exchanger, I can reverse the process. I haven't gotten into the design yet, but I'm thinking the cooling portion would not change. Instead of heating at night, by diverting the flow through an exterior exchanger, the water in the tank would be cooled at night. I think a couple of manual valves in the right place could do this. I would also have to put a thermostat at the tank. Any ideas on this would be welcomed. This system will perform well, as long as I can get the tank temperature down to about 60 deg. At this temperature, the system will handle 100% of the cooling during the day. Russell
Registered: 1352940256 Posts: 1,052
Reply with quote #23
Russell thanks for signing up and answering questions. I was wondering if you were planning on adding a drainback hydronic panel to the roof of tank shed for extra heat in the winter?
Mike __________________ Both temperature rise and airflow are integral to comparing hot air collectors
Registered: 1396734277 Posts: 3
Reply with quote #24
My original plans were to put a panel on the roof of the shed, until I realized the efficiency of the heat pump. Since the middle of February, I've used the backup heater only one time, and I didn't really need it then.
The heat pump system and a drain back system, for example, operate at different temperatures requiring two storage systems. The heat pump storage could be used to preheat the hydronic system, but that's probably not necessary. Therefore, the two systems would have to operate independently of each other. In a colder climate this would probably be feasible, but I can't see the benifit for such a small gain in this climate.
If I were designing a system and knew the efficiency of each, I think that climate would dictate which one I chose, but not both. If I chose the hydronic system, I think I would still implement the circulation portion of the heat pump system. I didn't analyze that portion of the system in my article, but I have noticed a significant difference in how well the greenhouse holds heat on cool nights when the storage system is depleted. In other words, the system is running, but the store doesn't have enough heat to make a difference.
Registered: 1383482035 Posts: 199
Reply with quote #25
Welcome to the board Russel.
Registered: 1383913384 Posts: 283
Reply with quote #26
Welcome to the forum Russell
No worries in arriving late,in fairness I should have let you know there was a discussion taking place here on your design. Regards Mattie
Registered: 1383913384 Posts: 283
Reply with quote #27
Thanks for the detailed reply earlier made for some good reading(1000th post! thats a lot of typing,thanks for taking the time to do so) I like the radiator idea its a simple means to get the job done.When you mention ducting does this mean the radiator is enclosed ?You also mention leaving one side transparent does this mean it would sit at the peak under glazing? For me i would position the radiator tight to the 30 degree side of the roof , to allow the maximum amount of light into the greenhouse.Although Im not sure how much the angle would effect the flow. The reason i mentioned piping (aluminum being a possible choice here), is that it would allow a streamlined footprint near the peak that could run the lenght of the roof and may possibly be lighter ,allthough a coil and water will certainly add plenty of kg. Thanks again Russell for the detailed replies, i have a few questions that i will add later if you have time to answer them. Regards Mattie
Registered: 1353115870 Posts: 243
Reply with quote #28
Originally Posted by
Russell Mike; My original plans were to put a panel on the roof of the shed, until I realized the efficiency of the heat pump. Since the middle of February, I've used the backup heater only one time, and I didn't really need it then. The heat pump system and a drain back system, for example, operate at different temperatures requiring two storage systems. The heat pump storage could be used to preheat the hydronic system, but that's probably not necessary. Therefore, the two systems would have to operate independently of each other. In a colder climate this would probably be feasible, but I can't see the benifit for such a small gain in this climate. If I were designing a system and knew the efficiency of each, I think that climate would dictate which one I chose, but not both. If I chose the hydronic system, I think I would still implement the circulation portion of the heat pump system. I didn't analyze that portion of the system in my article, but I have noticed a significant difference in how well the greenhouse holds heat on cool nights when the storage system is depleted. In other words, the system is running, but the store doesn't have enough heat to make a difference. Russell Hi Russell, Thanks for coming by! I'm wondering why the drain back collector would want to operate at a different temperature? It seems like it could just take water from the tank at whatever temperature the tank was at and warm it up and bit and return it to the tank? The drain back collector would actually operate more efficiently if the tank temperature was low. But, I can see that if you are already getting enough heat from the current setup why the drainback collector would just be an unnecessary addition. I'm thinking that it would probably be a good addition for colder climates. Gary
Registered: 1396734277 Posts: 3
Reply with quote #29
I agree with you on the addition of a drain back system for colder climates. You may not have enough excess heat in the greenhouse for the heat pump system to operate properly. The drain back system could add the needed heat even with a small collector. My reasoning is coming from the little experience I have with solar applications. Last winter I was considering adding small solar collectors to the top portion of the greenhouse near the glazing, on the inside, to heat a storage tank inside the greenhouse. I did an experiment with one collector which had about five square feet of surface area and a barrel of water. I used a fountain pump submerged in the water of the barrel to circulate water through the collector. It worked pretty well. The water temperature would get up as high as 110 degrees. The reason I wouldn't want to combine the two systems is the heat pump system seems to be the most efficient when operating within a temperature range from about 70 to 80 degrees. At that range the system will cool during the day, preventing the exhaust fan from coming on, and heat at night. With a collector hooked up, the tendency would be to get the tank too hot to be able to cool during the day. Therefore, the exhaust fan would have to be used to cool the greenhouse. If you did combine the two systems, a second tank for the collector could supplement the heat pump store during periods of cold and cloudy days and still allow the heat pump to operate efficiently. In that case, a third valve would probably be needed to switch the backup tank on. At a hotter temperature, the backup tank could probably be smaller. Russell