Hi, a few things to mention---in no particular order:

as well as clones, there are counterfeit copies of the Univent on Amazon. I had email discussions with the manufacturer in Denmark in an attempt to find a preferred US vendor. They seemed genuinely surprised when they saw all the look-alikes on Amazon, they thanked me profusely and said they were taking immediate steps to remedy the situation. Other than that, English was a second language and I never did get a preferred vendor list. Instead, I paid a little more and bought it from Johnny's. The peace of mind is priceless

since I put the seedlings out around March 25, I only have a couple of weeks, typically, to keep an eye out for extreme cold. The temps should moderate as we get into April. We did have a few nights---maybe three, where the temp hit 20, 21 and 23. The hot bed/ cold frame (not sure what to call it) bottomed out around 54---which is about what my cellar was every night. All of the thirty-something nights, it has maintained a nice 63-65 degrees. The heater has a lot of reserve capacity but would require more playing with the settings. When the heater was too high, it kept the Univent open---despite it being 35 outside, it was 77 inside. I dropped it down little by little until a 30 degree night was mid-sixties inside and then the following 50 degree overcast day, it stayed below 75---which is where the Univent is set to open. The Univent opening temp can be adjusted up or down but I was trying to find the spot where the heater could maintain 60 at night and 70 during the day. There is an optional thermostat for keeping an even tighter control on the heater but I didn't spring for it.

putting a double layer of polyethylene on the cover would have bought me a few degrees of further cold temp protection, allegedly at the expense of some light transmission. I opted for the extra light.

yes, I do have a generator and it feeds the entire house as well as the circuit in the garden. Should a worse case scenario arise---frigid cold in the single digits or the generator runs out of propane, I guess I could always throw a blanket over it for the night. Between the warm ground and the 5 gallon pail and 1" thick foam insulation, a blanket over the top would probably get me through??? I will never forget the April 1 snowstorm many years ago but by a couple of days later, most of it was melted by the seasonable weather that returned. By putting the plants out on March 25, my window of vulnerability is not that large and I would still have several weeks to start again should the need arise

I placed the heater at the rear in the center of the cold frame. I didn't want to put it directly under the Univent where it might affect it. I placed an old computer fan facing it to disperse the heat, prevent cold spots and strengthen the plants. This worked fine until I tried to rearrange the inside a bit. I moved the 5 gal pail almost into the corner and faced the fan at it, thinking the air would uniformly wend its way around the sides of the square structure. It was the weirdest thing. The temperature probe on the trays with the pots in them said it was 63 degrees---but the vent was now open several inches??? WT? It turned out to be a weird way the heated air bounced around and it then hit the Univent, causing it to open---yet several inches below the vent, where the temperature probe was, it was 63. I put everything back the way it was and will try to remember to never do that again

the cold frame is roughly 40 cubic feet. The heater for a plastic pail was supposed to be capable of maintaining the pail at 60+ degrees on a 35 degree night---with nothing to spare. I opted for the larger 550 watt heater. There was a website somewhere that let you plug in the space the pail/barrel will be in, the ambient temperature and the liquid that will be in the pail and tells you how many watts you need to maintain a certain temperature. I used 40 cu. ft., water at 60 degrees with 35 as the ambient temperature. There are weather sites that will give historic temperatures---highs, lows and averages. Should you opt for a metal pail, they come with several different interior coatings. Basically, the ones that come bare with a spritz of rustproofing are only intended to prevent rust while in the warehouse---not real rustproofing as one might surmise. The different epoxy coatings for the interior are determined by the material that will be in the pail. In my case, it was water which called for the "red" epoxy coating

the cold frame structure was originally a fence for a bed for bush beans. The rabbits loved them so I built a light weight fence that popped in as 4 separate sides and popped out again when you were through. I used black 1/4" hardware cloth (screening) to keep the rabbits out. I decided to keep this setup and just put foam board inside and make a hinged top. The back of the foam board was bright white with blue lettering (which would have been facing out) so I sandwiched in some black landscape cloth for looks. The cold frame doesn't have a sloped roof but it seems to be just fine for my intended purpose and the time of year. I put an ever so slight pitch to the bed (pried up one end with a shovel), hoping it would facilitate rain runoff but it wasn't sufficient. I have decided that I would rather have the small puddles (the vent opens the lid and it drains off quickly) than to look at something I did that was cock-eyed

working with the foam board was challenging. In order to cut it cleanly, with no jagged edges or little white pebbles falling out, I used a straightedge and a 3-4" paint scraper. At first, using a belt grinder, I put a sharp edge on the paint scraper and with several successive passes against the straightedge, I got fairly clean cuts. Then I tried heating the paint scraper with a propane torch until it was quite hot. One pass along the straightedge and the results were perfect. Any place I had to make a cutout---say for the mounting bracket for the Univent---I didn't want square inside corners, which break. I wanted rounded inside corners. I used a pair of Channel Locks and a copper 3/4 pipe coupling and heated that with the torch. When pressed in to the corner, it melted a perfect 3/4" radius. Word of warning---the fumes are beyond nasty so this should be done outdoors!

in order to hold the lid open, I opted for simple boards on the outside of the cold frame. Initially, I wanted them inside---out of sight---but decided, sooner or later, something would go wrong and the lid would come crashing down and the lid stay would poke a hole in the polyethylene.

I made all of the joints in the wood and the insulation as tight as I could. Then, after several days of rain, the wood started to swell up and move around. The joint between the lid and the sides now had a slight gap. I stapled on some felt weatherstripping to the top of the sides. Then the wood shrank and the lid now couldn't close as tightly as before because the weatherstripping near the hinges prevented a clean seal on the front of the cold frame. I transferred the weatherstripping to the lid stays and it seemed to solve all of the wood movement/leak problems.

I would be happy to post some pictures. A cursory search here as well as a Google search left me in the dark. I don't have any cloud storage that I use. Directions to the how-to page?

A very inexpensive addition could be reflective mylar (emergency space blankets they were once called, about $2 each about 20 years ago) on the side walls (and floor?), and if you ever find it necessary to cover it at night with a blanket, a space blanket under the blanket.

Cutworm wrote: ↑Sun Apr 03, 2022 8:40 pmI would be happy to post some pictures. A cursory search here as well as a Google search left me in the dark. I don't have any cloud storage that I use. Directions to the how-to page?

This is the easiest forum to post pictures to--I thought there was a thread with some instructions but I can't find it. There might be other ways to do this, but go to the quick reply box at the bottom of this page. Click on the "Full Editor and Preview" button. Then go underneath the text box and click the "attachments" tab, and there will be an "add files" button. Click that and you can browse to the picture(s) you want. Just double click the picture and it will insert it into the thread. At least that's how to do it on a regular computer...might be a little different on your phone.

Thanks for the picture posting directions---it really was the easiest thing I've done with unfamiliar technology in a looong time

Well---maybe not. I got "invalid extension", kicked out and signed off

I tried adding the pictures as files and got the warning, the screen flashed and it kicked me out of my computer (Chromebox). I had to log back in to the computer, where I then saw duplicate posts???

Plan B was to try dragging the pictures over. That failed as well

I don't know if Chrome just doesn't play well with this forum but I'd be happy to email the pictures if anyone was interested

Cutworm wrote: ↑Mon Apr 04, 2022 10:46 am Thanks for the picture posting directions---it really was the easiest thing I've done with unfamiliar technology in a looong time

Well, you're certainly familiar with building stuff, I was particularly impressed with the hot copper tube corner rounding on the foam.

Cutworm wrote: ↑Mon Apr 04, 2022 10:47 am Well---maybe not. I got "invalid extension", kicked out and signed off

Were the pictures jpegs?

@Cutworm I deleted your duplicate post, although you could have done it yourself, using the 'x' at the top right corner of the post. And if you would like, you could PM or email me the pictures and I can put them in your post to share with the forum.

Yes, they were.

It was tedious at times trying to find answers and/or solutions but I totally enjoyed the learning experience

Re: the copper coupling---at first I filed the side of a solder tip on a Weller solder gun to a knife edge. It seemed like a plausible idea at the time for cutting the foam panels down to size. But, before trying it, I saw a video where a guy used the side edge of a paint scraper to make multiple passes on the foam to finally cut through. Then I had the light bulb moment---what if I heated the paint scraper? It worked great! When it came time for the corners, I figured I was on a roll. I selected the size copper coupling for the radius and "voila" perfect inside corners. I laid it out on the foam with a Sharpie (washes off with isopropyl) and did the corners. Then, I used the paint scraper to cut from the radius to the edge of the foam. I edge banded all the foam with Gorilla duct tape for aesthetics as well as a little ding protection.

Thanks---I just tried to PM you the JPEGs, thinking possibly that might work---but I got the same error?

pondgardner wrote: ↑Mon Apr 04, 2022 2:03 pm @Cutworm I deleted your duplicate post, although you could have done it yourself, using the 'x' at the top right corner of the post.

I think there's a time limit on that, looks like it's probably an hour after the original post.

@Setec Astronomy Probably so...just wanted to mention it for the benefit of any new members. And I should have added that if someone has replied to the post, you cannot delete it.

Cutworm wrote: ↑Mon Apr 04, 2022 2:11 pm Thanks---I just tried to PM you the JPEGs, thinking possibly that might work---but I got the same error?

How about email?

OK---how does that work? Do you PM me your email?

If you click on my user name, it should take you to my profile, where you can either PM or email me...

I tend to be a wee bit obtuse with this stuff---but all I saw for options was private message

I tried opening the image in my Gmail account and then dragging the image over from there but that didn't work either

@Cutworm I sent you an email that you can possibly attach any jpeg's to in reply.

Cutworm wrote: ↑Mon Apr 04, 2022 2:45 pm I tend to be a wee bit obtuse with this stuff---but all I saw for options was private message

I tried opening the image in my Gmail account and then dragging the image over from there but that didn't work either

And I was incorrect about the email option. That is used by the moderators here only.

@Cutworm I added the pictures and I will send you a follow up email. Nice work!

Well I'm seeing the pics this morning and admiring your work! Very nice build indeed.

Thanks for that. As I said in earlier post, it would have been easier to have started from scratch---but I had it in my head to just utilize the existing rabbit fence sections, add some poly and make a top---what could be easier After having been outdoors for several years, the pressure treated 1x4s I had used to frame the sides were all cupped and bowed. The fence would still keep rabbits out but was not suitable to keep drafts out. Something would need to be done--or just start over. I figured I had all winter to kill, so why not try to straighten the boards.

I found an old clothes steamer gathering dust and between that and some clamps, I managed to get the bowed sides 90% straight again---or straight enough. When it came to the cupped boards, as evidenced by the two vertical stiles on the front, I chose discretion over valor. I had the feeling that any pressure on those would split the board and then I'd have more work myself. Perfection being the enemy of progress and all. I will say this, though, the cupped boards don't look anywhere near as bad in person as they do in the picture---not sure why?

Tormato had asked what difficulties I encountered and what I'd do different. I think I've covered most of the bases on the difficulties and as far as what I'd do different---I would suggest just starting from scratch rather than the retrofit. This was built to come apart and go into storage when finished. I spent a great deal of time trying to devise tool-free ways to to this and still be weather-tight. Ultimately, I had to settle for 6 screws in the front and six in the back and unscrew half of the hinges. The sides are very light for removal in my old age. We don't need the bed so it may stay up after all till next year

Speaking of hinges. I used galvanized strap hinges for the lid. The way they are mounted, I needed them to swing 270 degrees. I had some old ones which did this but they were really too big. I went to every big box store and hardware store around. Every last one of them had 6" strap hings that would only open 260 ish degrees---and then bind and stop. It is just poor manufacturing---and poor oversight. I had several old ones that worked just the way they were supposed to. So---I went on eBay and found some old brand new galvanized strap hinges made by Stanley. Due to their age, I hoped they were made properly. Now I had to pay for shipping, but what the heck. They worked perfectly. How can everyone screw up the engineering on a strap hinge that's been around for eons??

Starting from scratch would have let me choose the volume of the space to be heated and the build process would have been quicker.

Thanks again to everyone for the help along the way.