Greenhouse Heat problem
 

I built a greenhouse this fall (I bought a kit from the greenhouse megastore). Its 8ft long, 6ft wide and 7 feet tall. I have a 1500 watt electric heater inside the greenhouse hooked up to a thermistat that I normally keep set around 58 degrees. Until today the electric heater alone has been enough to keep it above 55 at night. Today is the coldest weather we have had this winter so far. Its 17 degrees now and the wind was up to 40 mpr today, low of 15 tonight here in St louis.

Today when I got home from work I noticed the temp droping inside the greenhouse to 51 degrees. then I remembered that I bought a kerosene heater to use in case of a power outage. So I decided to see if it would help supplement the electric heater that apparently cant keep the temp consistant. This was my first time ever using a kerosene heater. When I lit the heater and put it in the greenhouse a few minutes later it was very smokey inside. Then I read about how to use the heater, problem was I had the wick too high. After I lowered the wick the smoke was gone and now its 58 degrees inside. So the problem is solved. On nights that drop below about 25 degrees or so I will use the kerosene heater in addition to the electric heater.

Has anyone else had experience with heating a greenhouse when the outdoor temp drops below 20 degrees?

Re: Greenhouse Heat problem
 

I have a 12X7X7 portable greenhouse and it usually gets below 20F in the winter, but hasn't yet. All I use is about 130 gallons of water for a heat sink and foil on the north wall. So far temperatures have been held above freezing despite lows already in the mid 20s. I think it costs too much to heat my greenhouse.

If it gets cold enough to threaten my plants, I might get one of those candles in the glass containers and put it inside a fireproof metal container, but I doubt I need to in order to provide a few degrees of warmth.

Re: Greenhouse Heat problem
 

I've got a little bit of experience with it....lol... Crappy weather we've been having the last few days, huh? I use LP gas in my big production greenhouses. In my glass lean-to up against the house, (10x20)we use a vent-free blue flame type heater. I do have to keep the basement window open to the greenhouse though with a fan running to provide fresh air or the heater goes out because the construction is too tight(it runs out of air).

Re: Greenhouse Heat problem
 

Last night it got down to 9 degrees here in St Louis, with the small kerosene heater and the 1500 watt heater I was able to kept the temp above 50. I need to put some weather stripping on the bottom of the door, that should help as well. Seems like I only need the kerosene heater when the temp drops below about 25 or so.

I have palms, cacti, succulents and some banana plant pups in my greenhouse. I suppose it could get down to about 40 without damaging the plants but I really dont want it to go that low.

Re: Greenhouse Heat problem
 

40's are no problems for palms and most tropicals, tho alot of plants really slow down their growth below 50 degrees.

Re: Greenhouse Heat problem
 

I found an online calculator to calculate the BTUH heat loss (British Thermal Units per hour) for a greenhouse. I have a 6x8 greenhouse so if I want to keep it 57 degrees inside and the outside temp is 20F then I need a heater that generates at least 5120 BTUH. My 1500 watt electric heater generates 5120 BTUH so I only need my kerosene heater when the outdoor temp drops below 20F. Based on our 10 forecast after tonight it will not be below 20F for the next 10 days. With the addition of my kerosene heater I can keep my greenhouse at 57 down to about 7F, below that the heaters would not keep up with the heat loss. Good thing is that it rarely gets that cold here in St Louis. Seems like we normally get about 3 or 4 arctic blasts every winter. I actually did some research on the low temp for each year back 10 years and it has not dropped below zero in the last 10 years here. That would mean we are in USDA zone 7 not USDA zone 6.

TIP: If you want to buy kerosene dont buy it at a home center or hardware store, you will pay $10/gal. Some gas stations sell kerosene. Today it was selling for about $3.60/gal and I bought 5 gallons so I should be set for the winter.

Re: Greenhouse Heat problem
 

I added some more insulation to the inside of my greenhouse and it really helped. I used clear bubble wrap (secured with double sided tape) on the inside panels. I insulated around the inside of the door and used bubble wrap to close off the circular vent. I will take off the bubble wrap in the spring when I put up a shade cloth.

It was only 30 degrees in St Louis today, at noon it was 75 in my greenhouse and the roof vent was open (I have a solar powered roof vent opener). Its 20 out now and its still holding steady at 58 degrees in the greenhouse.

Re: Greenhouse Heat problem
 

Another way to apply bubble wrap is to spray the glass and then put the wrap on it - no need for tape.

Mike

Re: Greenhouse Heat problem
 

What do I spray with? My panels are polycarbonate not glass. I have bubble wrap on the inside walls but not the roof, should I also use on the roof?

Re: Greenhouse Heat problem
 

Yes. Warm air rises and heat goes to cold. The more insulation the better, especially the roof.

Re: Greenhouse Heat problem
 

I have a little pump-up sprayer, I put water in it and use that. As noted, the roof is the most important part.

Mike

PS - Make sure the polycarb is clean!

Re: Greenhouse Heat problem
 

Thanks, I ordered some more bubble wrap and I am going to insulate the inside of the roof when I get it. Hopefully that will help my electric bill.

Re: Greenhouse Heat problem
 

I put up the bubble wrap on the inside of the rood on my greenhouse yesterday. I had to use some tape to keep it up. Had to use a whole role of paper towels to dru off the roof. Hopefully this will help. The test will be if my heater can keep, its supposed to get down to 14 tonight. Brrr!!!!!!!!!!!!!

If anyone is looking for bubble wrap buy it on ebay. If you want to buy locally you will pay twice a much or more. I used the large bubble type and bought a role 24 wide X 70feet long for $18 including shipping.

Nice to see the sun again today, its 80 degrees in my greenhouse now even though its only 38 outside. And thats with the roof vent open.