Greenhouse heater size? calculation vs. table
 

Now heating: Modine Hot Dawg seems to be solid. I calculated BTU as glass area x delta T/R-value and get ~ 18,000 BTU requirements
2000 sqft x 30 (from 30 to 60F)/3.4 for low E double pane glass.
On greenhouse megastore, HD30 with 24K output BTU is indicated for GH up to 250 sqft, while up to 550 sqft suggests a 48K output BTU. Should I tust my calculations, or manufacturer's indications?

---------- Post added at 04:10 PM ---------- Previous post was at 04:04 PM ----------

Second: Relying upon a single heater is not ideal. But the one that is of appropriate size is already the smallest one.

One option may be to plug electric heaters in swamp-cooler outlets and set them as emergency back-up in the cold period. I have about 16A available for that. A space heater running at 1500W (so around 16A) produces about 5K BTU. May be sufficient to prevent a catastrophe. Also cheap at ~$60.

If you put large water storage containers under your benches, they will absorb enough heat during the day you will not need as much heat at night.

Thanks for the water storage pointer. I'm aware of it. Not sure how serious I am about implementation. I currently plan for a 100 gal RO storage tank under a bench. I've been toying with rainwater storage, but not sure how far I want to take this.

I agree with the 24k BTU calculation, but that is the output needed, so the heater input, which is how they are specified, will need to be about 25% greater (check the stated efficiency, and recalculate, to be sure - I arbitrarily used 80%).

A little safety factor is important. It 30F the likely lowest temp experienced, or is that 10 degrees lower than that?

If a greenhouse heater fails, I have found that a gas grill can be the saving backup.

Hi Ray, on the HD30 output is 24K input is 30K, alleged efficiency 80%, which numerically adds up.

Re lowest temp, freezing in Santa Barbara,CA, is extremely rare, and 60F is rather generous as target temp in GH. And required 18K with a 24K BTU heater has a safety factor of 33% = 10F, or delta T of 40F.

In an area with a reasonably stable climate the idea of putting the whole greenhouse or a nice piece of it on top of a 2'-3' deep pond is very attractive from the standpoint of stabilizing the minor swings you're talking about. Keeping it passive and having the water protected from sun exposure to avoid heating is the ticket for your situation. I've been contemplating this for a while now. I plan to use a solar hot water collector to supplement the heat in the winter, but I'm dealing with below freezing temps for extended periods of time.

Big water body or having GH lowered into ground are certainly theoretical options. For underground pond or storage tank you would also need some sort of circulation system. Passive radiation is most likely too slow. Haven't seen any calculations for that.

FWIW, when I lived in SC, I used 55-gallon steel drums with bread flats on top as my benches. They were filled with water and antifreeze (for the corrosion inhibitors).

There were 10 in a 14'x14', double-poly structure, and they absorbed solar energy well enough that my electric heater rarely came on, even when the temp dipped into the upper 20's.

Consider a Solar Assisted Radiant Heating System with a 30% gas fired and 70% solar collector, it can give you a nice economical heating system.

I echo what Ray says about water as a heat sink. I will show how it helped me. This will be long because I want to give you enough data that you can do calculations and compare it to your proposed structure.

I have a sunroom, approximately 11.5 x 31 feet / 3.5 x 9.5 meters. It is a converted southeast facing patio, tucked into one concave corner of my T-shaped house. The interior short side has one double French door to the house, and the long side has two. The doors have cheap single-pane glass.

The sunroom has a solid roof contiguous with my house. It is polyurethane foam with an elastomeric rubber coating, and an extremely high R value for a roof.

I enclosed the two open sides years ago, with 7 foot / 2.13 meter tall sliding glass doors all the way around, and windows averaging 2.5 feet / 76cm above. There is a 6" / 15.25cm of wood all around between the doors and windows. I used standard, non-efficient sliding glass doors and window glass. The structure is not as tight as it could be; I did not seal the cracks where electrical conduit enters the structure from outside, and the sliding glass door weatherstripping is old.

I left the concrete floor the original 4" / 10cm below the house slab and left it plain concrete, because I knew it would be wet frequently.

I built it to overwinter cacti and succulents, which mostly tolerate winter night temperatures just above freezing. Before enclosure, the patio was far too hot to be usable, for almost any purpose, in summer. For several years before I enclosed it, I grew vigorous annual summer vines up trellises to shade the patio completely, but it was still too hot. I did get a lot of nice yard-long beans, luffa sponges and moon flower morning glories. Enclosing the patio did not change the summer heat problem.

In winter, for 4 hours or so each morning, the sun hits the exposed masonry southeast wall of my house under the overhang. This warms the sunroom to over 80 F / 27C almost every winter day. In summer the sun reaches under the overhang only for an hour or so in the morning. Due to the cheap glass I used, unmodified summer temperatures in the sunroom are above ambient while the sun shines into it, then ambient for the rest of the day and night - far too hot for most plants. I now use a portable evaporative cooler during spring, summer and fall.

It has two electrical circuits: one electric socket, shared with indoor lighting for the adjacent family room, and one overhead light circuit shared with exterior lights. Total amperage is low, since the house was built in 1960.

My winter nights can be colder than yours. I have 0-10 nights down to the upper 20s F / -5C or lower. Most nights are in the 40s F / 3C. My winter days are almost always much warmer than this, so solar gain is substantial.

When I began using the structure, unheated, to overwinter cacti and succulents, I occasionally lost plants close to windows on the coldest nights. They froze solid. Many nights temperatures were somewhat above freezing, warmer near the masonry wall.

When I began growing orchids needing warmer nights, I used an electric space heater and a fan in winter. At close to the maximum amperage for the circuits, I could keep the room at 48-50 F / 9-10C on the coldest nights, but the heater ran all night. Although the room is not tight, I did not want to use combustion to heat it.

When I decided to collect rain I realized it would also serve as a heat sink. I now have 10 x 32 gallon / 121 liter covered (mosquitos!) plastic trash barrels filled with rain in my sunroom, lined up against the house wall, away from the windows. These are the cheapest covered plastic barrels Home Depot sells, $15 each. The plastic is treated with UV stabilizer. Mine are in the shade so this doesn't matter, but they could be used as passive solar collectors.

The sun does not reach my barrels, because shelves and plants along the windows are in the way. Nevertheless, the water is noticeably warm to the touch; they absorb heat passively, even through the plastic. The space heater now only runs briefly during the coldest parts of the coldest nights.

I started with three barrels, and every time I ran out of water before the next rain, I bought two more. That is how I settled on ten. But if I had more barrels in there, I would probably never have to heat the room.

Your proposed structure probably has more surface to lose heat than my sunroom, but you will use more energy-efficient glass. I suspect you could cut your heating bill very low by using interior water storage. If you had an outdoor passive solar collecting system to warm your water storage, I suspect you might not need any supplemental heating on nights following sunny days. I have looked into modifying a solar hot water heater collector to warm my storage tanks, but I really don't need it.