The BTU numbers on heating equipment is maximum BTU per HOUR, though the "per hour" is often assumed, not explicit.

A BTU is a unit of energy (the amount of energy it takes to raise 1lb of water 1 degree F), and is not time dependent.

The calculated or measured heat load is also a rate, a rate that varies with the temperature difference between the interior & exterior temperature. A fixed output heating element designed to deliver 5000 BTU per hour if running continuously will have to run at a duty-cycle to not overheat the space when the room or zone isn't losing heat at 5000 BTU per hour. A heating element that can only deliver 2500 BTU/hour would have to run twice the duty cycle as the 5000 BTU/hr element to achieve the same output.

For example, if the room loss of the room at -5C is 1000 BTU/hr, delivering that heat with a 5000 BTU/hr radiation would be at a 20% duty cycle for the heat input to balance with the heat being lost, whereas a 2500 BTU/hr heater would have to run a 40% duty cycle to balance. Either can heat the space, and it's the same total amount of power going in (or energy used over time). What we're trying to figure out is the heat rate necessary to heat the room at -5C, and extrapolate that to -25C (or whatever your outdoor design temp is.)

If your bathroom is just one room on a zone that is controlled by a thermostat elsewhere, if the bathroom is running cool it's a room-to-room balance issue, not a total capacity issue. The system may be able to heat the bathroom just fine with the existing radiation if the zone thermostat is located in the bathroom, but that means it would probably overheat the other rooms on that zone. If we know the total amount of heat it takes, and calculate the heat that the floor radiation is putting out relative to the radiation output & loads on other rooms, we have a better shot at figuring out just how much radiation needs to be added for the room-to-room temperatures to be about the same under any load condition.

Dana1, I just wanted to say I really appreciate the time you take to explain things and the method and manner at which you do. I have learned an awful lot just reading yours and some others posts on this site.

Dana1, Thanks for clarifying how the labels work. After your last post I figured that the heater would just need to pickup the pace a bit as it got colder.

Yes, the ensuite is just one room in a 4 area zone. Because the ensuite is the most critical area that is where the thermostat is. The other 'room' in the zone is the laundry room and is not so critical. The last two areas are the front foyer and the back foyer (mud room). Because these are pretty much open to the house they are considered supplemental heat to the main floor air heat system.

Got an actual quote back from the distributor on the ECO 24-24, $247CAD. I think I will look at a slightly larger unit and also wait to see what kinda quotes I get on the Express Radiant heaters. One again, Thanks.

So you should put a bypass valve on the new radiator so you can balance the heat output. Otherwise you risk under radiant heating the other rooms in the same zone. Perhaps not critical, but not optimal.

Not sure if I'm understanding you correctly but the 5 port manifold controlling that zone has balancing flow valves for each port. I can control the flow and therefore the BTUs to each area of the zone including the new radiator. I think this is what you are referring to?

Since you're only needing to make up the difference for that room vs. the others, you could just try throttling back flow on the other rooms on the zone (particularly the room with the thermostat), and opening the flow to the cold bathroom wide open.

The 24" Biasi can probably heat the whole bathroom with 104F EWT, even without the floor radiation, there's no real need to go bigger.

Thanks for the suggestion Dana. I may have caused some confusion by using two terms; There is only one bathroom, that's the Ensuite and that's where the thermostat is. Because I upgraded to 1/2" Pex I am able to cut back on my flow a bit in a couple of areas(Mud room/Laundry). The front foyer is 5/16 Pex, the same as the Ensuite, and although not critical I will benefit from as much flow as possible. I will take another look and maybe that 24-24 will do. I'd just hate to go to all the time and expense and not have some 'wiggle room' with the BTUs. Once again, Thanks.

I see - yes, with a separate valve for the ensuite/bathroom you are all set to balance properly even if that circuit gains excess capacity.