Nice room/house. You don't give enough information. Your contractor should know how to do this. They should take into consideration the construction (windows, insulation), boiler type (water temperature), and outdoor design temperature.

Thanks. Yes, our contractor is very good and I don't doubt that they will figure all of this out. I mainly ask out of curiosity because we are trying to learn as much as possible as we go along. Maybe there is too much technical knowledge for us to pick up on this issue, and we will have to rely on the contractor. If so, that's fine.

There are lots of large nice, attractive cast iron rads that will go nicely in place of the aluminum convectors. Note that typically cast iron are run at lower temperatures than aluminum fin convectors. As the names imply, the heat delivery mechanism is different. Cast iron relies more on radiative heat transfer. That is the kind of heat that you feel on your skin in front of a fire or in front of a radiative electric or gas heater (with reflector, not a fan).

You seem to have at least 15 feet of radiator run along that wall, I wonder why that's not enough? Maybe because of the enclosures? Are they low BTU units or you live somewhere cold or there's not enough water flowing through? Is this a converted garage, even if not maybe the floor isn't well insulated?

Another observation (on a topic you haven't asked about), that tree is too close to the house. Anytime is a good time to take it down, and I would lower the planter area along the wall to return it to ground level.

Good luck.

"You seem to have at least 15 feet ... low BTU units "

Low output from fin-tube convectors might be low because the water temp is low. Water temp is generally set lower for CI radiators.

I'll bet the existing install would work much better with a cutout panel on the top of the enclosures, the same as on the front. I've seen a lot of enclosures like that, with openings on the bottom or side and then on the top too, which allows air to flow past the units and rise into the room.

One addition question: We cannot find a single long radiator that throws off enough BTU for the space. Is there any issue with putting multiple, separate radiators along the wall and linking them together?

Sort of like this but with the radiators maybe closer together and a couple more radiators? We probably would need 20,000 BTU, which requires several radiators.

Central London townhouse - work in progress · More Info

Air heated by the radiator (or, more accurately 'convector') will tend to rise, drawing in cooler air from below to be heated in turn. If air is blocked from entering or leaving, the convector cannot do its job.

You can put as many radiators as you want, anywhere you want to put them. If the room's air sealing and insulation can be improved, you might better do that first.

We really don't know what is in those boxes or what is going on in them. If they are open on the bottom, there might be plenty of air flow.

Where are you located? Are you estimating you need 20K BTUs for that wall or 20k BTUs for the 400 sq ft room? My house is about 613 sq ft per floor with 8'4" ceiling height and we have 4 cast iron radiators for each floor. Each radiator is about 4 feet long and two columns deep and a water temp of about 150 degrees max. This works fine in Northern VA outside of DC. I calculated the BTUs once, but don't remember the details.

We don't have a wall with a large number of windows, but do have 6 windows and 2 doors on the 1st floor and 8 windows on the 2nd floor. All but 2 windows are 1940 vintage wooden double hung with add-on aluminum storm windows. The house is brick and fairly tight, but there is no insulation in the exterior walls, and other upgrades have been minimal. Eight radiators seems to heat very well and it is relatively economical with a .84 efficiency Buderus gas boiler.

Bruce

Normally, hot water radiators is good choice and permits the use of "zone heating". However, for hot water radiators to do their job, they must have unrestricted air flow from bottom to top of the unit. I can't tell for sure from the posted picture, but it appears that your radiators are completely boxed in and can not work. The woodwork over the radiators is pretty.