Operate radiators with a 35 °C flow temperature

Hello,
I have received an offer for a radiator that is supposed to heat a 15 sqm room (fully insulated new building). Since we also have underfloor heating in the house, the installer said it should also be operated with a 35 °C flow temperature. However, I am unsure if that can work. The performance data from the manufacturer only goes up to 45/35/20. Using the performance calculator (an Excel sheet) where you can enter any values, you get 182 watts output, which seems relatively low. I will probably need to insist on receiving the heat load calculation to verify this.
On a social media account of the manufacturer, it is also mentioned that: "If your system is to operate with a flow temperature below 40 °C, compact radiators are generally unsuitable because natural convection breaks down below 40 °C (applies to all manufacturers)."

Is there any experience in this forum with operating radiators with such a low flow temperature? Oh, and please no discussion about why a radiator is desired and not underfloor heating, that is not the topic here.

Yes, that works. I am currently at just under 37 degrees supply temperature. The problem is that at -12 degrees outside temperature, you end up at 35 degrees supply temperature.

Is there any experience in this forum with operating radiators at such a low supply temperature?

At least with my current renovation, I have had an apartment completely set for 40/35. With 33 radiators, you can achieve really high outputs. Only 182 watts is a mini radiator.

Although I consider a supply temperature of 35° in connection with underfloor heating in a new building to be too high. Then better spend a few extra euros and buy a proper radiator for low supply temperatures. There are special radiators made for this. For example Viessmann/Jaga.

Yes, that works. I am currently at just under 37 degrees flow temperature. The problem is that at -12 degrees outdoor temperature you end up at 35 degrees flow temperature.

May I then ask what your outdoor temperature is when you are at 37 degrees flow temperature? Furthermore: Do your rooms warm up fairly quickly with the radiators? What size are they?

At least with my current renovation, I have had an apartment designed completely for 40/35. With 33 cm radiators, you can get really high output. Only 182 watts is a mini radiator. Although I consider a flow temperature of 35° in connection with underfloor heating in a new building to be too high. Then you might as well spend a few extra euros and buy a proper radiator for low flow temperatures. There are special radiators for that. For example Viessmann/Jaga.

Well, whether the 182 watts is sufficient should actually be told to me by the heat load calculation, if I have understood that correctly. From everything I have read so far on the internet (30-60 watts per sqm in new buildings), it is not enough. The question here, of course, is again for which scenario it is not enough (i.e. from which outdoor temperatures you start). Speculating probably does not help, and I have to follow up with the installer. I also agree with you about the flow temperature - but here too the question is how I can influence it. We are building with a general contractor and I have no data regarding the design of the heating system.

Well, whether the 182 watts are sufficient should actually be indicated by the heating load calculation, if I understood that correctly.

For example, I can give you a reference value from my current room-specific heating load calculation (KFW 55, decentralized controlled residential ventilation) for a 16 sqm room. In my case, it is 377 watts for a room temperature of 22 degrees (standard outside temperature -8 degrees).
You can achieve that with, for example, a type 33 radiator Buderus 1200x600mm at 35/30/22 (at 20 degrees room temperature also smaller, as well as with a higher temperature difference). Alternatively, a small Viessmann low-temperature (heat pump) radiator type 16 (dimensions 600x500) designed specifically for 35/30/20. It delivers 413 watts in comfort mode.
If the room really only needs 182 watts, you are close to the passive house standard ;-)

The question, of course, is again for which scenario they are not enough (i.e., which outside temperatures are assumed). Speculating won’t help here, and I have to follow up with the installer.

Correct.

I also agree with you on the flow temperature – but here too, the question is how I can influence that. We are building with a general contractor and I have no data regarding the design of the heating system.

You are building with a general contractor? Then it's actually simple. Whoever pays gets their wishes fulfilled. You don’t buy a car and say “Let’s see what features I get.”
You select and order a performance.

May I then ask what your delta T is when you are at 37 degrees flow temperature? Furthermore: Do your rooms warm up quite quickly with the radiators? What size are they? Well, whether the 182 watts are sufficient should actually be told to me by the heat load calculation, if I understood that correctly. From everything I have read on the internet so far (30-60 watts per sqm in new buildings), they are not enough. The question here is of course again for which scenario they are not sufficient (so from which outside temperatures one assumes). Speculating probably does not help, and I have to follow up with the installer. I agree with you about the flow temperature – but here too the question is how I can influence that. We are building with a general contractor and I have no data regarding the design of the heating system.

It must have been about ~2 degrees. I am running a gas boiler with a self-performed hydraulic balancing. I don’t know if it warms up quickly. The heating runs continuously 24/7. Very comfortable.