There is a bypass pipe from the supply to the return on this basement radiator. Apparently the water is whistling by the supply. Are there "Mono-flow" fittings on the supply and return to this register? A monoflow tee fitting has a sorts scoop that catches the water and forces it down to your radiator. You need one on both the supply and return. They have to be oriented correctly on both the supply and return. Water, like electricity, will take the path of least resistance.

Converting baseboard to radiators should not be difficult especially if the house orginally had radiators. Old cast iron radiators can be put back in service even if they have been sitting idle for a long time. I agree radiant hot water heating is the most comfortable. However variable speed furnaces have come a long way in increasing comfort. Since you are already going through the expense of adding duct work for the AC, and you dislike the ugly baseboard, then consider integrating a furnace. You can get an electronic air cleaner if you have allergy problems. There are thermostats which will turn on the furnace blower to circulate and filter the air at very low speeds.

I would see if your city has an architectural salvage store. Since most new hot water systems would use baseboard-style radiators, these types of covers are not very commonly sold new, at least in my region. You could also check with a local sheet metal shop and find out about having something custom made to your needs.

My understanding is that boiler, pump and radiator reps often do a lot or all of the engineering calculations for boiler and radiator installations. I must be missing something because I don't understand this at all" Runtals use 1/4 of the water of Myson, so this justifies their higher price - you get it back in energy savings. A btu is a btu. The boiler makes it, the structure looses it. How it gets to the room from the boiler has little effect on efficiency provided the return water is the same temperature. Even if you have to put 2 or three times the electricity into pumping with an inappropriate piping/pump and maybe radiator system, it won't make that much of an impact. I can't say much about CI rad design with respect to internal water volume. Water carries the heat so flow has to be sufficient and transfer from water to metal and metal to room has to come out right. It really boils down to the rated heat output of the rad at a given temperature and the rate needs to maintain that given temperature. Cast iron baseboard need not be any more of a dust collector than the higher mounted Runtal rads and has the advantage of putting the heat low where you need it most and heat transfer will be maximized. Are you sure about that 40k calculation? Right now I am looking at heat loss calcs for a home about that size. It is 50 years old and, although the insulation and air sealing is excellent for a home of that vintage, it is only good overall. The calculation comes in at below 50k at a design temp of of 0F. My fuel use calculations, however, come up below 30k. I put more faith in the latter since there is guesswork involved in the energy rater's estimate.. The walls have insulation blown in, The ceiling is filled to over the joists and there are double pane windows. Exterior doors are good.