Wherever it's not too expensive, and even in my tiny little greenhouse, I always get two smaller capacity things in preference to one big thing, be it a heater, a fan, or a vent. That way, if one of them dies for some reason, there will still be an underpowered backup item that can function until the other is replaced.

I say go for two heaters if it's not prohibitively expensive.

Our largest gh (32x72) isn't as large as your's but we heat it with one heater with no problem and we're in a COLD, COLD area of VA. As long as you get a heater that will throw the heat sufficiently, there should not be any reason to use 2, especially if you have enough fans to distribute the heat fairly evenly (all gh's have hot and cold spots).

Regardless of whichever route you take, make sure you have overkill on the btu's. All our heaters are in the 200k range in houses from the above mentioned to a small one that is 16x64.

We have three kinds of heaters: Modine, Sterling (I think], and Reznor. Modine is the best, IMHO only because it seems to throw the heat farther.

Regards,

Tom

I agree on the subject of installing a heater rated higher than the calculated need, because for one; the rated BTU's on any given heater may or may not be accurate and what may be sufficient in one GH may not in another and mostly- if you experience an unusually cold spell, you may need extra BTU's in any case.

I may be wrong in my belief, but do know that you can always turn a thermostat down to produce less heat, but can never get more out of the heater than it's designed to produce.

I have 2 greenhouses 30 x 60 with two heaters in each, positioned in opposite corners. Another greenhouse 30 x 48 with a single heater. I like having 2 heaters in the bigger houses, I feel that I have a little better control and I sleep a little better at night knowing there is a back up (not so much now as one of the heaters is not working in one house and the second heater can't quite keep up-brought in portable heat for really cold nights). I have them set to fire at different low temperatures-about 10-15 degrees offset from each other. On relatively mild nights only one heater is used as it can keep up with the heat loss. On really cold nights, the second heater kicks in to keep the house at the set temperature.

Also, be sure to look at the BTU output rating for the heater when choosing one, not the input. Heaters are sold by BTU input. A heater rated at 300,000 only has an output of 240,000 at 80% efficiency. Therefore a 300,000 may not be enough for your needs. The more the BTU's generally means a more powerful fan too.

A little more BTU's than you calculate is good. Too much BTU's however means the heater heats up the house too quickly and will cycle on and off more frequently and therefore potentially wear out quicker.
HAF fans will help prevent cold spots and carry the heat a little further.

Our largest house has a big plastic ventilation tube running at the top and going the whole length of the gh. It is attached to a large fan and the tube is probably 3 feet in diameter and has holes punched on two sides to help distribute the heat and circulate air. For additional air flow we set down or attach to the purlins 20 inch Lakewood portable fans. They aren't box fans but ones with a moveable "foot" so the angle can be adjusted. They have 3 speeds and are really durable, made with metal. We must have at least 10 of them and get them at Sam's club for ~ $35 each. They are similar to the one on the Amazon link below.

As for peace of mind knowing the heat is on, we have Thermalarms in all the greenhouses. You set them to a min and max temp and they'll sound an alarm in the house if the temps go outside the set range. They work very well. Here is a link for them:

http://www.thermalarm.com/

The wiring, battery, and alarm are not included.

Regards,

Tom

Here is a link that might be useful: Lakewood Fan at Amazon

I have a thermalarm wired to all my greenhouses. That does make it easier to sleep, but I still like the security of a second heater. Less likely that I will have to go out and move all my plants into another greenhouse at 3 AM. I also have wireless thermometers in them so I can monitor the temperature from the house. At least with the thermalarm I won't go out to a greenhouse full of dead frozen plants though. The thermalarm is nearly as expensive as a unit heater these days. I bought mine when they were in the low $300 range. Last I checked with my supplier they were well over $400. Still worth it though-I have a second one of them just in case the first one needs replacing.

I'm not sure if we're talking about the same Thermalarm system---the ones we use cost less than $50. See the link below for the one we use. And, we have a generator that can easily run all four greenhouses and, for emergency heat if a unit heater fails we can hook up several propane heaters that sit on the small tanks. Won't "heat" the house, but will keep things from freezing.

Usually, if a heater goes out, its the pilot light or the heat sensor on top. OR!--the power has gone out.

I do have several spare Modine heaters I bought and would like to sell a few of them.

Regards,

Tom

Here is a link that might be useful: Thermalarm III

I actually have a Sensaphone (the expensive part of the system). It is hooked up to the phone line and will call up to 3 numbers to alert me to low/high temperature, high sound level, and with a separate sensor, to flooding.

I use a Thermalarm but it needs to be hard wired to an alarm and have a power source for the alarm itself. Mine just completes the circuit when a high or low temperature set point is reached. It is connected to the Sensaphone. It is too far to run wire from the greenhouse to the house nor could I hear an alarm coming from inside the greenhouse (but my neighbors might)

I also have an automatic back up generator with transfer switch and portable propane heaters.
I have had heaters not start up due to a clogged orifice, faulty electronics, clogged power vent (bird nest).
I'm not sure why the most recent failure has occurred. For some reason the fan doesn't come on and it shuts the gas off. I need to do some more investigating. If I can't fix it I will need a new one.

Tom- where are you in VA and what size heaters do you have?

Our Thermalarms are hard wired and run off a 12volt battery to a small, 85db buzzer in the house. We ran underground phone wire and have the phones on 2 wires and the alarms on the other 2. The guys we bought our first greenhouse from had a red outdoor light wired to a thermastat for another type of alarm. We didn't put that up.

Our generator isn't automatic since we use it elsewhere around the farm but I can plug it into the hot water heater system in the big greenhouse and backfeed it to all 4. We then turn on ONE lightbulb so we can look outside at night and see that the generator is running. We live ~ 100 yards from the furtherest gh.

I take our heaters apart and clean the burners before each house is turned on. I'm lucky to have a class A propane man that lives 1/2 mile away--he does not carry parts but knows his stuff. Our repair man does carry everything but he is expensive, but GOOD. I bought a meter that allows me to check all the pressures on both sides of the gas valve.

We're close to where I-77 and I-81 intersect. I 99% sure the heaters are in the 200,000 btu range; I'd have to go back and check to make certain. We got them from a a warehouse in Charlotte, NC when they upgraded. The heater tags say they were set up for propane but I cannot guarantee that---they might have been converted. We've got them stored in a barn.

Regards,

Tom