ᐅ Operate radiators with a supply temperature of 35 °C

Hello,
I have received a quote for a radiator that is supposed to heat a 15 m² (160 ft²) room in a fully insulated new building. Since we also have underfloor heating in the house, the installer said it should be operated with a supply temperature of 35°C (95°F). I am unsure if this will work. The manufacturer’s performance data only goes down to 45/35/20. Using their performance calculator (an Excel sheet), which allows you to enter any values, it shows an output of 182 watts, which seems rather low. I will probably have to insist on getting the heating load calculation to verify this.
On one of the manufacturer’s social media accounts, it is also mentioned that: "If your system is to operate with a supply temperature below 40°C (104°F), compact radiators are generally unsuitable because below 40°C natural convection collapses (this applies to all manufacturers)."

Does anyone here have experience operating radiators with such a low supply temperature? Oh, and please no discussions about why a radiator is preferred over underfloor heating; that is not the topic here.

Yes, that is possible. I am currently running at just under 37 degrees Celsius (99°F) flow temperature. The problem is that at -12 degrees Celsius (10°F) outdoor temperature, you end up with a 35 degrees Celsius (95°F) flow temperature.

Hausbaer schrieb:

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

At least in my current renovation, I have adapted an apartment completely to 40/35°C (104/95°F). With 33 cm (13 inches) radiators, you can achieve really high outputs. A radiator with only 182 watts is very small.

However, I consider a flow temperature of 35°C (95°F) too high in combination with underfloor heating in a new build. In that case, it is better to invest a bit more money and get a proper radiator designed for low flow temperatures. There are special radiators available specifically for this purpose, for example from Viessmann or Jaga.

Joedreck schrieb:

Yes, that works. I’m currently running a supply temperature just under 37 degrees Celsius (99°F).
The problem is that at -12 degrees Celsius (10°F) outdoor temperature, you end up with a supply temperature of 35 degrees Celsius (95°F).

May I ask what your outdoor temperature is when you have a supply temperature of 37 degrees Celsius (99°F)? Also, do your rooms heat up fairly quickly with your radiators? What size are they?

nordanney schrieb:

At least with my current renovation, I have the apartment set for a 40/35 system. With 33 cm (13 inches) radiators, you can achieve really high outputs. A radiator rated at only 182 watts is quite small.

However, I consider a 35°C (95°F) supply temperature too high when it comes to underfloor heating in a new build. It’s better to invest a bit more and get radiators designed specifically for low supply temperatures. There are special radiators made for this, for example by Viessmann/Jaga.

Well, whether 182 watts are sufficient should actually be indicated by the heat load calculation, if I understand that correctly. From everything I’ve read online so far (30-60 watts per sqm in new builds), this doesn’t seem enough. The question always comes back to the scenario considered (which outdoor temperatures you assume). Speculating doesn’t help here, so I need to follow up with the installer.

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

Hausbaer schrieb:

Well, whether 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 comparison value from my current room-by-room heating load calculation (KFW 55, decentralized controlled residential ventilation) for a 16sqm (172 sq ft) room. In my case, it is 377 watts for a room temperature of 22 degrees Celsius (Norm outdoor temperature -8 degrees Celsius).
You can achieve that with a Buderus type 33 radiator 1200x600mm (47x24 inches), for example at 35/30/22 (supply/return/room temperature), and a smaller one at 20 degrees Celsius room temperature, or with a higher temperature difference. Alternatively, a smaller Viessmann low-temperature (heat pump) radiator type 16 (dimension 600x500mm / 24x20 inches) designed specifically for 35/30/20 performs at 413 watts in comfort mode.

If the room really only requires 182 watts, you are close to Passive House standards ;-)

Hausbaer schrieb:

The question, of course, is for which scenario they are not sufficient (meaning which outdoor temperatures you assume). Speculating won’t help, and I have to check with the installer.

Correct.

Hausbaer schrieb:

I agree with you about the supply temperature – but the question remains 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’re building with a general contractor? Then it’s quite simple. Whoever pays, gets their wishes fulfilled. It’s like buying a car and then saying, “Let’s see what equipment I get.”
You choose and order the service.

Hausbaer schrieb:

May I ask what your temperature difference (ΔT) is at a flow temperature of 37°C (99°F)? Also, do your rooms warm up quickly with the radiators? What size are they?

Well, whether the 182 watts are sufficient should actually be indicated by the heat load calculation, if I understood that correctly. From everything I've read online so far (30-60 watts per square meter in new builds), that doesn’t seem enough. The question, of course, is for which scenario they are insufficient (meaning which outside temperatures you assume). Speculating doesn’t help, so I need to follow up with the installer.

I agree with you about the flow temperature—but again, the question is how I can influence it. We are building with a general contractor, and I have no data about the heating system design.

It must have been around ~2°C (3.6°F). I have a gas boiler with a self-conducted hydraulic balancing.
I don’t know if it heats up quickly. The heating runs continuously 24/7. Very comfortable.