How to size a built-in radiator

holograham

Looking at having a built in radiator for a room renovation we are doing. 115 sq foot room with 2 exterior walls, 2 wooden windows, and insulated all over (walls, ceiling, floor - R-20 mineral wool).

I used the castrads website calculator and it got me ~16 EDR. But given the size of room it's likely going to need to be larger if it's enclosed (with a grate for airflow).

How do I estimate this?

Jamie Hall

How enclosed is enclosed? Somewhere on this site there is a discussion about loss of capacity from enclosures; hopefully someone with a better memory than mine can locate it.

As a completely wild guess, I'd upsize it about 30%.

Steamhead

https://www.heatinghelp.com/systems-help-center/radiator-covers/

Jamie Hall

Thank you, @Steamhead

dabrakeman

I don't understand the Castrads calculator. It doesn't seem to care where you live. What kind of temperature rise is it assuming?

EdTheHeaterMan

https://forum.heatinghelp.com/discussion/comment/1888153#Comment_1888153

Assumptions Built Into the Calculation

1. Standard Test Conditions (ΔT50)

Radiator heat outputs used in sizing tools are based on the BS-EN 442 standard, which is a legal requirement and MARC-supported for all hydronic radiators.
That standard defines a Delta T (ΔT) of 50°C as the benchmark for published output figures.

• Flow water temperature: ~75 °C (~170°F)
• Return water temperature: ~65 °C (~150°F)
• Room (ambient) temperature: ~20 °C (~68°F)
  This produces an average radiator water temperature of 70 °C (160°F) and a ΔT of 50 (122°F).

Because outputs are tested at these conditions, Castrads (and MARC) size based on that standard output figure.

2. Radiator Outputs Are Declared at ΔT50 (and ΔT30 for low-temp systems)

Under BS-EN442/MARC compliance, radiators must have independent testing and a Declaration of Performance (DoP) showing:

• Output at ΔT50
• Output at ΔT30 (for low-temperature systems like heat pumps)

The calculator will use ΔT50 by default unless otherwise specified — which explains why it doesn’t ask for outdoor design temperature or target indoor temperature directly. Those are implicitly folded into the ΔT50 test assumption.

3. Implicit Room Target Temperature

Rather than asking you to enter a design indoor temperature, the calculation assumes a target indoor temperature of about 20 °C (68°F) — the common baseline used in the EN442 standard. That room temperature is built into how ΔT50 is defined.

4. Heat Loss and System Conditions Are Idealized

Actual heat loss depends on:

• insulation levels
• window area
• air leakage
• building orientation
• outdoor design temperature
  …but the basic calculator simplifies it by using typical assumptions tied to standard conditions rather than requiring all of those detailed inputs. Real engineers often add corrections or separate heat-loss calculations for precise work.

In Summary

A MARC-approved calculation (in UK practice) means three key assumptions are baked into the output figures and radiator sizing:

• Radiator outputs are based on EN-442 standard test conditions (ΔT50/ΔT30) — essentially a flow/return and room temp assumption.
• Target indoor temperature (~20 °C) and a defined water temperature differential are assumed, so you don’t enter them explicitly.
• Heat outputs are certified and independently tested values so that radiator sizing comparisons are fair between products.

Because of these standard assumptions, the Castrads calculator doesn’t ask about design cold or room temp — it already assumes the standard conditions used in industry and regulations. And pretty much the entire UK is in a miserable winter climate zone all the time.

dabrakeman

I'll admit I don't fully understand it. I see how EN442 works to standardize radiator output determination but missing the direct connection to room heat loss calculation. If you have a heat loss the standards can be used to determine radiator size. Bottom line which probably goes back to another recent thread on calculators and the wildly different results they generate, how confident should the OP be in the 16 EDR or 3840btu he is starting out with (I think he has steam)?

mattmia2

What kind of system are you connecting this to? Is it something where you can adjust the output or is it something like 1 pipe steam or a series loop where you need to get the size right?

mattmia2

When one of my coworkers said they were going to the UK in the winter I asked if that was a good idea and another coworker that is from the UK said that it doesn't get cold there the way it does here so maybe you might want to do a manual J.