Hydronic Boiler Sizing: How to Match Your Radiators for Peak Efficiency

Walk the house and log every radiator (fast checklist)

Start by making a quick radiator map. For each radiator, jot down:

• Type (column/tube, cast-iron panel, etc.)

• Height (inches or cm)

• Number of sections

• Columns/tubes per section

You can do this room-by-room with a clipboard or your phone. Why it matters: boilers don’t heat air directly here; they feed emitters, and emitters are sized in EDR. If we miscount emitters, the boiler won’t match the load.

Snap a photo of the end view of each radiator; that angle shows columns/tubes clearly. If you want a second set of eyes, send those pics through our Quote by Photo page and we’ll help confirm what you’ve got at no charge.

Turn measurements into EDR using tables (the simple lookup)

With measurements in hand, use EDR tables to find output per section. Example: if your notes say “5-tube, 25-inch high,” the table might list ~5 sq ft EDR per section (exact values depend on the table). Then compute:

Radiator EDR = (Sections) × (EDR per section) Do this for each radiator. You’re not guessing heat loss yet just rating the emitters. This is crucial for steam and still very helpful for hot water, because it shows what the radiators can deliver at a given temperature.

Keep a separate column for “Notes” (painted radiators, covers, partial obstructions). These can slightly impact real-world output. If a room has a decorative cover, flag it so we verify later that the boiler and temps can still satisfy the space.

Add it all up your Total System EDR (the backbone number)

Add every radiator’s EDR to get Total EDR. This single number is the emitting surface your boiler must serve. It’s the backbone for steam sizing and a solid reference for hot-water systems.

• Steam systems: Total EDR is king.

• Hot-water systems: Total EDR helps, but we’ll also weigh Manual J heat loss and Delta-T later.

Why this matters: radiators don’t care what boiler you buy; they'll only release so much heat per square foot at a given water/steam temperature. If the boiler is too small, rooms lag. Too big, and you’ll short-cycle and waste fuel.

Keep your EDR sheet. When you later change windows or insulate, you’ll already have the “emitter side” documented for future tweaks.

Steam systems: size by connected radiation (EDR × 240 BTU/hr)

For steam, we keep it straightforward: Boiler Output ≈ Total EDR × 240 BTU/hr. That 240 figure is a common rule used to match steam production to the connected radiation. Size strictly to the total connected EDR so the boiler can feed all radiators and piping without starving the system.

What about home upgrades? Even if you added insulation or new windows, steam sizing still follows connected radiation. Undersizing a steam boiler because “the house is tighter now” often backfires,radiators won’t fill properly, and comfort suffers.

When comparing boilers, look at the manufacturer’s NET steam rating (sometimes shown as “sq ft steam”). Match or slightly exceed your Total EDR. This avoids confusion with gross vs net ratings and keeps you out of trouble.

Hot-water systems: use EDR, Manual J, and NET I=B=R (with piping allowance)

Hot-water (hydronic) gets one more step because water temps can vary and insulation upgrades matter. Use EDR to understand radiator capacity, then run or reference a Manual J heat loss. Good news: if you’ve improved insulation, air sealing, or windows, you can often downsize.

Rules of thumb that help:

• Hot-water output ≈ EDR × ~170 BTU/hr per sq ft (at typical conditions).

• Choose boiler capacity based on NET I=B=R ratings, which include ~15% allowance for distribution losses.

Workflow: verify your room-by-room load with Manual J, confirm that your radiator EDR and chosen water temps can meet that load, then pick a boiler whose NET rating covers it without big overshoot.

If rooms are overheating in mild weather, a smaller modulating boiler plus outdoor reset can tame cycling and save fuel.

Delta-T: match water temperature to room temperature (avoid surprises)

Delta-T = Average radiator water temperature − Room temperature. Example: 75°C (167°F) water average − 20°C (68°F) room = 55°C (99°F) Delta-T. Radiator outputs are rated at specific Delta-T values commonly ΔT 50 or 60. If you plan to run cooler water (to save energy), those same radiators will emit less heat unless they have the EDR to compensate.

Why it matters: Boiler sizing and emitter sizing must agree on the design Delta-T. If you size the boiler for high-temp water but run low-temp, rooms lag. If you size for low-temp but need high-temp on design day, efficiency drops.

Note the supply and return temps you want to run. Check your radiator EDR tables at that ΔT. This is where hydronic systems earn their efficiency right-sized temps for the actual load.

Crunching the BTUs: two quick examples you can copy

Let’s say your Total EDR = 300 sq ft.

Steam example:

• Required output ≈ 300 × 240 = 72,000 BTU/hr.

• Pick a steam boiler whose NET steam rating meets or slightly exceeds 72,000 BTU/hr (or the equivalent “sq ft steam”).

Hot-water example:

• Estimated radiator capacity ≈ 300 × 170 = 51,000 BTU/hr (at typical ΔT).

• Confirm your Manual J is ≤ this number at design conditions.

• Choose a boiler with NET I=B=R output that covers the calculated load (remember ~15% distribution allowance is baked into NET).

If your Manual J is lower than the EDR-based estimate, that’s a green light for modulating operation and possibly lower water temps more comfort, less cycling.

Common pitfalls (and how we dodge them)

• Mixing rules: Using hot-water math on steam (or vice-versa) will skew the size.

• Ignoring piping losses: For hot water, rely on NET I=B=R to capture ~15% distribution loss.

• Delta-T mismatch: Radiators rated at ΔT 60 won’t deliver the same heat at ΔT 40.

• Bad counts: Misreading columns/tubes or sections. Photograph and re-check end views.

• Covers and clutter: Decorative covers or furniture in front of radiators reduce airflow and output.

• Assuming upgrades don’t matter (hydronic): If you tightened the house, Manual J may justify a smaller boiler.

• Assuming upgrades do matter (steam): Steam still sizes by connected EDR stick to it.

When in doubt, ask. Our Help Center can sanity-check your sheet before you buy.

The field (the little things that save big time)

• Label radiators with painter’s tape while counting. Mistakes drop fast.

• Measure height carefully EDR tables are height-sensitive.

• Check vents and traps (steam) so the new boiler isn’t blamed for distribution issues.

• Purge points and valves (hot water): Make sure the system can be bled quickly after install.

• Outdoor reset (hot water): Pairing your boiler with reset control lets water temp float with weather steady comfort.

• Keep the old EDR sheet: Future remodel? You’ll know exactly what your emitters can do.

Want a sanity check? 

Browse our HVAC Tips blog or send your sheet through Quote by Photo.

Choosing equipment: matching your numbers to a boiler spec

When you shop, compare apples to apples: focus on NET ratings for hydronic and NET steam for steam. Check the fine print:

1. Fuel and venting that fit your home.

2. Output range (modulating if hydronic) that brackets your load.

3. Control capability (outdoor reset for hydronic).

4. Warranty and parts availability.

If you’re also adding spot cooling or a shoulder-season solution,or install odds-and-ends, see Accessories. If financing helps, we’ve got options here: HVAC Financing.

Verifying radiator output at your design Delta-T (final check)

Before you hit “buy,” re-verify radiator output at your chosen water temps:

• Pick your design indoor temp (commonly 68–70°F / 20–21°C).

• Pick design water temps (e.g., 180°F supply/160°F return for classic hydronic; lower if you’ve got the EDR to support it).

• Confirm the EDR tables match your ΔT. If not, adjust expected output.

• Compare the result to your Manual J room loads.

If the numbers are close and the boiler can modulate, you’re in the sweet spot. If they’re far apart, you may need to raise design water temp or consider adding an emitter area in the coldest rooms.

Keep rooms balanced with working valves and good circulation; sizing won’t fix a stuck valve.

When to right-size vs. downsize (practical scenarios)

• Old, leaky house, steam: Size to connected EDR. Don’t downsize because “we’re insulating next year.”

• Upgraded hydronic home: If Manual J shows a lower load and your radiators can meet it at reasonable ΔT, choose a smaller, modulating boiler.

• Mixed radiators: If one area is perennially cold, verify its local EDR and flow before blaming boiler size.

• Future remodel: If you plan to finish a basement or add a sunroom, note that in your selection slight extra capacity (with modulation) can be smart.

Need a gut check on your plan? Our Design Center can align loads, emitters, and boiler options so you buy once.

Tools, worksheets, and next steps (keep it organized)

Here’s a simple flow you can follow today:

1. Map and measure every radiator.

2. Look up EDR per radiator; total the system.

3. For steam: EDR × 240 BTU/hr → match NET steam rating.

4. For hot water: run Manual J, check EDR × ~170 BTU/hr, select by NET I=B=R (includes ~15% piping loss).

5. Confirm ΔT and water temps; re-check outputs.

6. Pick equipment and plan controls.

Grab quick references in our Sizing Guide and Help Center. We'll review your sheet and point you to the right gear from The Furnace Outlet.

Ready to size your boiler? Let’s finish the plan together

You’ve got the roadmap: measure EDR → total it → apply the right BTU rules → verify at your ΔT. From there, selecting a boiler is straightforward. If you want a quick double-check, send your radiator list and any photos via Quote by Photo. 

If you’re also planning cooling for summer, compare ductless mini-splits or compact through-the-wall units while you’re here.