Best Of
Re: Smoke smell in house with oil boiler
Have a chimney sweep inspect your chimney. It may be partially obstructed with dead birds or critters. If you don't have a chimney cap to keep the birds out, you might want to have one installed.

Re: New Return Pipe Noise During Renovation
Here is an example of an older boiler where the water line (Red arrow) is much higher than your present boiler. I suspect the boiler that was originally in your house when the wet returns were installed had a higher water line. The excessive pressure was not helping any with the annoying noises.
Re: Hi could somebody tell me what the difference is in these hago nozzles
Age.
The red top nozzle vial has a nozzle that was made within the last 10 years. the clear top vial was from an older batch of nozzles. There is no actual functional difference. They are the same.
I can tell you this about those two nozzles, now that they are out of the factory container, if you have touched the cindered filter with your fingers, no matter how clean your hands are, you can throw both those nozzles away. Proper nozzle handling recommends that nothing touches the cindered filter except the oil that it is to filter once it is installed in the oil burner's nozzle assembly. Especially the nozzles below 1.00 GPH, the orifice for that nozzle spray along with the tangental slots inside the nozzle are very small and can get clogged easily
Re: Air to Water Heat Pumps
I'd rather put the buffer on the return side so there's no delay in conditioned water hitting the emitters.
Re: Used baseboard questions
Yes the fins need to be open top to bottom or no convection is possible. Judging by the plastic sliders they were not installed correctly.

Re: Possible Tekmar replacement or tweaks?
I've also used the 279 for about seven years. Because of the unknown algorithm regarding the room sensors (which Tekmar refuses to divulge), I decided NOT to use them. I tweaked the heating curve, and the radiator venting over the course of one year and got it to the point where MOST apartments (there are six on three floors) maintain a comfortable temp throughout the year. A couple get up to 74° or so on occasion…………..not a concern for me. I run it on a 45 minute cycle which keeps the insulated mains quite warm and gets steam to the end of the mains in less than two minutes.
As mentioned, the WWSD is your best friend. This one is set to 62° due to plenty of residual heat in the building from people and cooking.
If you are dead set on keeping the sensors, you could use the second sensor in a different room and the 279 will average the two. I understand the difficulty in wiring it…………..!
The probe for the steam temp is notorious for failing. I've already gone through three of them. You need the capability to access it.

Re: Proposed Piping Layout
Thanks @Big Ed_4 , the purpose of the mixing valve is boiler return water protection, it keeps the return above 130 degrees. When it is just Zone 1 running, since the piping and radiation was designed for another era, it would take over 45 minutes to get the return temperature over 120-125
The smaller zones, 2-4, never had that issue since the mass is much smaller, but I think I read that Dan's book suggests having the boiler protection in place.
Re: What I think
You made the right decision then, no one would invest in a 37 year old system.
What's your design temp there?

Re: Possible Tekmar replacement or tweaks?
Tek mar is the best. The other option is a Heat Timer which is big $$$ and does not work as well. Balance the system with venting and run the pressure as low as possible.