Bringing back an oldie! I think that makes 3. Here's my update.
I ended up replacing the valve on that radiator and it worked.
Then it froze because I'm a fool (long story) and cracked, so I replaced it with a towel warming rad that's much much smaller (and more importantly, lighter - no cast iron, just brass I think. 1,500 btu's for a 60 sqft bathroom). I lived with it the past winter and the bathroom was a little chilly. Not that bad.
I figured I could get by with this smaller one because I planned on remodeling and hyperinsulating the exterior wall. I have done this, 3" total width of rigid foam and a triple pane window.
However I'd like the marble floor to be warm, and I know there are 2 risks in connecting pex loop floor heat to my boiler but I think I've got them addressed.
1 - Floor too hot, ouch! Shouldn't be an issue if I don't use heat transfer plates. I don't see how PEX would be able to transfer heat above 125 through a plywood subfloor into a marble floor to make it too hot.
2 - Water too cold returning to boiler! Again, if I don't use heat transfer plates the plywood subfloor shouldn't pull away too much heat to the point where it's too low upon return. I'm planning on insulating with a 1" air gap to avoid striping.
Naturally, it's probably going to be a very mellow heat which is ok. I just want it not to be cold and I want to supplement the towel warming rad a bit in case I've overestimated the impact of the hyperinsulating.
Any thoughts from the team?
Last fall I removed a radiator from the bedroom that we were turning into a nursery and replaced it with PEX under the sub floor and it works great! The sub floor is 5/8" or 3/4" OSB (not Advantech) and the floor covering is a medium pile carpet with padding below it. It seems to heat the room sufficiently and I haven't noticed a change in performance with the rest of the system.
It is the first room on the whole house loop so the radiator in that room got super hot and I was worried about having a little one around it. I did some research about adding a PEX loop to an otherwise radiator system and found a lot of conflicting information so I decided to just go for it. Since the water will take the path of least resistance I figured if I just plumbed the loop into the supply and return that was previously to the radiator, there would be too much resistance for the water to actually flow through the loop on its own and it would just flow through the main "through" line. To combat that I installed a shutoff valve in the "through" line so I could force the water to go through the loop. In fear of adding a significant amount of resistance to the whole system (since resistance adds up through the whole system) I added an additional "T" on each side of the shutoff valve and ended up making two shorter loops instead of one long one and also used 3/4" PEX in lieu of the standard 1/2". Having two loops running alongside one another (not consecutively) meant the water would only have to travel half the distance and navigate half the number of bends as well as providing additional volume capacity for the water to flow through since the main line is 1" and the water has no choice but to flow through one of those loops so necking it down to a single long loop of 3/4" would be adding resistance from several angles (added travel distance, bends, reducing from 1" to 3/4", etc.).
Of course, I overthought my approach and instead of splitting the room in half and servicing one half with one loop and the other half with the other loop and pulling the PEX down and back within the same joist cavity, I started at one end and snaked a single run of tubing down each cavity and through a drilled hole and back the next one and so on. Then with the other loop I started at the other end of the room and did the same thing working my way back to where the first loop started, giving me one piece of PEX from each loop in each cavity (2 per cavity). Again, I overthought my approach (like I do on most projects) and thought that starting at one end of the room with one loop and the other end with the other loop, the water temperature loss from the beginning of the loop to the end should even out since each cavity would have on piece of PEX from closer to the beginning of one loop and closer to the end of the other. To be honest, I don't think you'd be able to tell the difference in water temp from the beginning of the loop to the end but I didn't realize that until I had everything hooked up and it didn't matter anymore (at least not for that room). I wouldn't have even bothered to explain all of this but it turned out to be a huge pain to pull around 100' of PEX through the first hole, then 90' through the second and so-on. And I didn't want to start in the middle and pull half through one way and half through the other way because I didn't know exactly how much I was going to need to cut off the roll and with my luck I would have ended up being a few feet short. Drilling holes at one end of the joists and pulling it straight through and then pulling a loop of slack between each joist long enough to run the length of that cavity and back would have been so much easier.
I have my (coal) boiler set to maintain between 140 degrees and 180 degrees. Its warm enough to make it through the carpet and heat the room fairly well so far has not proven to be too hot on the feet where the loop runs under the corner of the bathroom which has a tile floor, however, the PEX is only secured up in the corner where the joist and sub floor meet with J-Hooks every few feet in that area so there isn't continuous contact with the sub floor. With this setup the tile does get fairly warm and the warmth can be felt a few tiles away form where the loop is so it stands to reason that it could get too hot to comfortably stand on if there were continuous contact with the sub floor. Also, I believe there is only one run of PEX in the joist cavity below that tile (since its the outer edge of the loop) so two runs of PEX per cavity could also contribute to too much heat for the tile. I think your idea of holding the PEX down an inch or so from the sub floor might be the way to go and if it ends up being too hot you could always lower it a bit more or open the valve a bit on the through line so the loop doesn't see quite as much hot water.
When I first installed everything i had all of the PEX held up with the J-Hooks but wasn't 100 percent pleased with all the more heat I was getting through the carpet so I had the guys in the sheet metal shop at work fab me up a bunch of little pieces of aluminum a couple feet long that were sort of a "U" shape with a nailing flange coming off one side that I installed in place of the J-Hooks in most places (picture attached). These pieces of aluminum served two purposes, the held the PEX in contact with the sub floor more consistently and also acted like a heat sync to help transfer more heat from the PEX to the floor. I put a couple pieces up and after a little while I took a picture from the top side with a thermal camera that I borrowed from work and was amazed to see the difference the pieces of aluminum made (picture attached). The 4 bright areas in the middle are where I installed the aluminum pieces, the rest were just fastened as previously stated.
Some other things I learned along the way worth noting:
-Be sure to use the PEX with an oxygen barrier coating on it. Apparently regular PEX is oxygen permeable and when oxygen is introduced into the system you begin to get corrosion on the inside of your copper lines and cast radiators.
-Wear gloves when pulling the PEX through the joist cavities. The tips of the screws and nails used to secure walls above and to secure cement board for tile stick through the sub floor just enough to rip your hands open but barely enough to see.
-Get or build an uncoiler if you can. Fighting all of the twists in the PEX is just another thing that makes the job harder than it needs to be. I built my own out of some scraps that I had laying around (picture attached). The only thing I had to buy was a few PVC fittings to make the axle. A second person to spin the roll would work in place of an uncoiler too but my wife was 8 months pregnant when I was doing this so she was in no mindset to be helping with this sort of project.
-Heating the PEX up makes it so much easier to work with. I had a small electric heater blowing on the roll for a little while before I started pulling it through the joists and it made life a lot easier ...until it cooled down... then it was a struggle. Having it easier to work with for a little while is better than nothing though!
I plan to add more loops to my system in the near future and eliminate radiators along the way. My first experience with this stuff was a bit frustrating at times but I'm confident that it will go smoother next time. Ultimately I'd like to replace all of my radiators with PEX, at which time it would make sense to put a manifold in but I don't know if I will go the manifold route since everything will already be configured to work (hopefully) without one. Additionally, it would make sense to lower the water temp to the more proper level at that point to maybe save on heating costs, but again, everything will have been installed with 170 degree +/- water in mind so the number of runs per joist cavity and mounting distance from the sub floor might rule that option out.
I'm sure I am forgetting some stuff so if you have any questions let me know. It sounds like we have pretty similar applications so I think you will be fine. It might take a little tweaking but I think you'll be able to make it work!