Just to be clear, when you say "there is one manifold" and all three rooms are on the "same single loop", do you mean the radiators are in series? That is, does the out of radiator #1 feeds the in of radiator #2 and the out of radiator #2 feeds the in of radiator #3? OR, are you saying each room is independently fed from the manifold at the boiler?

Yes, the radiators are in a series. 1 goes into 2 goes into 3.

Every radiator has a BTU rating (how much heat it will put out, at a given water temp). Absent having an insulation or air infiltration problem in this room that others don't have, your problem is that the radiator isn't putting out enough heat for the room relative to the radiators in the other rooms.

While the material and configuration of a radiator affect the output of any given unit, a 5000 BTU output at a given temp is a 5000 BTU output, no matter what material the radiator is made of. I believe what you need is a radiator with a greater output than what's there now.

Three things to consider.

First, as snidely says, the question is what the BTU output of the radiators is in each room. Try to find the specs for what you have by googling online. As an example, page 3 of the following is for Weil McLain cast iron baseboard:
http://www.weil-mclain.com/en/assets/pdf/en-snug-072011.pdf . The BTU per hour ratings are given for various water temps, one or two flow rates and per linear foot. For the above example, a 1 foot section of baseboard with 180 degree average water temp will get you 570 BTUs per hour with a 500 pound per hour flow rate. (500 lbs per hour is about 1 gallon per minute (GPM)). You then multiply by the total linear feet or use the table.

Page 4 of the following gives similar info for Weil McLain's fin baseboard.
http://www.weil-mclain.com/en/assets/pdf/en-hightrim-072011.pdf
You'll note that at the same average water temp, the output is 720 BTU/hour for 1 GPM flow rate.

For cast iron radiators, it will take a little more tracking down or just use something close. Here's a page with similar info for this brand:
http://www.ocsind.com/cast-iron-radiators.php
These are given in BTUs per square foot per hour. First you have to calculate the number of square feet of radiator you have by determining the specific model (height) which gives sq. ft. per section, multiply by the number of sections and then multiply by the BTUs per square foot. A 'section' is one vertical rib. It can either be an end section (with feet) or the intermediate sections (in the middle without feet). The value doesn't matter for end or middle section.

Given the above, you should be able to determine how many BTUs each room is getting. If the baby's room is similar (size, exposure, etc.) to another room that is warm, you can do a comparison.

Second, if the baby's room is the last radiator in the series before the return to the boiler, I'd check what the water temperature coming into the room is. As you can see by the tables, the output is close tied to the water temp. The best way to do that would be an infrared thermometer aimed at the pipe going into and out of the radiator/baseboard. (See link below.) I'd measure the water temp in and out for each room and compare. Use that sample to do your calculations above a second time. As an example of a problem, a relative of mine had all fin-type baseboard. One series loop in their system was the living room and a bedroom. The living room only had baseboard under the picture window. They did a remodel and added baseboard along the other outside living room wall AND added baseboard to the basement, all on the same loop. As a result, the bedroom was much cooler the next winter because of the large temperature drop of the water by the time it got to that room (the bedroom was the last on the loop).

Third, the old wives tale (or conventional wisdom) is to not mix cast iron with fin/steel baseboard. The ratings given in the specs are for water flowing given the average water temp. When the boiler/pump shuts off, all bets are off. Cast iron baseboard and radiators have more thermal mass with the iron and with water volume (although they take longer to heat up). The fin baseboard cools off faster and basically just has the pipe running the length full of water. So, there is some basis to your concern.

If you find that there is a significant temp drop of the water coming into baby's room, you might see if the plumber can run a separate branch loop to the room from the boiler off the same source manifold (depending on how your zoning is controlled). He could also add a balancing valve to the branch so you could throttle down the flow if needed in case it's too hot in the room. But before changing anything, do the calculations.

If there's not enough baseboard based on BTU calculations, you could add more or replace with more cast iron baseboard. It's been a while, but it used to run something like $40 to $50 a foot, plus installation for cast iron baseboard.

Hope this gives you a few ideas.

Here is a link that might be useful: Infrared thermometers

This post was edited by DreamingoftheUP on Sat, Feb 22, 14 at 15:23

I'd use a ceramic space heater, they can really warm up a small room, try one for a month before deciding if it's too expensive.

It would be easy and quick enough and most likely the cheapest way to go. I use them all the time and have several for our 5000 sq. ft house, I keep the thermostat set very low, 58, and warm up the rooms I am in- although we have gas logs for the main room for when we are there. ( it doesn't effect the bedrooms much at all, however).

Lots of good advice given on this forum. I guess the cast radiators will yield narrower temp swings between cycles, but the average temp will be the same with both types and you might not notice the difference. I just can't get over what poor advice you got from the locals. That was all bull snot. You were 0 for 3!

I would suggest modifying the system from a simple series configuration to a manifold configuration so that each room is separate, piped in parallel and each receiving the same water temperature. I would change the slant fin rad to a properly sized cast iron radiator. Install mechanical thermostatic radiator valves on each rad for individual, zoned temperature control in each room and install a pressure regulated variable speed circulator that would efficiently regulate the flow of water as required.

I’m sure that you can find a contractor in Boston that is well versed in modern, efficient hydronic systems.

SR