Hydronic Radiators

Radiators are useful in a retrofit situation or where there will be no concrete slab in which to bury the pipes.

The water temperature needs to be hotter than with in-slab heating. In-slab heating needs to be at 35-40 degrees or so, while the heat output of radiators will be half of their “rated” performance at this temperature. So, when used with heat sources that are typically at a lower temperature than a traditional gas boiler, it can be useful to oversize the radiators – the larger the radiator the more heat it will give off. Note that this also applies to condensing boilers, which run much more efficiently at temperatures below 60°C

In any case, it is a good idea to run with a lower water temperature to avoid the risk of burns.

Factors to apply for difference between room temperature and mean water temperature other than 60°C.

35°C – 0.496
40°C – 0.590
45°C – 0.688
50°C – 0.789
55°C – 0.893
60°C – 1.000

So from the above figures, if the design temperature of the water is 45°C, the radiators need to be about 30% bigger for the same heat output.

Stainless steel might look good but the shiny surface has low emissivity, which means they will not warm up the room as well.