Introduction

If you’ve ever swapped a boiler, added a heat pump, or changed a stack of radiators only to hear “it’s still cold in that room,” you know the pain. The fix starts with a proper room heat‑loss survey. What is it? A structured way to measure the building fabric and air leakage so your emitter sizing and flow temperature choices are right the first time. Why bother? Because accurate heat loss cuts callbacks, improves comfort, and reduces running costs. How? Follow a consistent survey process, use sensible assumptions, size to flow temperature (not folklore BTU lists), and record everything clearly so clients buy in.

Table of Contents

• Key Takeaways
• Plan the Survey Like a Job
• Get the Fabric Numbers Right
• Nail Infiltration and Ventilation
• Size for Flow Temperature, Not Just BTU
• Allow for Intermittency, Setbacks, and Warm‑Up
• Document Assumptions, Options, and Next Steps
• Frequently Asked Questions
• Conclusion

Key Takeaways

• In general, UK practice uses 21 °C for living rooms and 18 °C for bedrooms; design around those targets and state them clearly.
• Commonly, infiltration can account for 20–30% of total heat loss in older, leaky homes; don’t ignore air changes per hour (ACH).
• A radiator rated at ΔT50 typically delivers around a third less output at ΔT30; size emitters for the flow temperature you’ll actually run.
• Many installers report that lowering flow temperature by 5–10 °C can reduce cycling and improve comfort on modern systems.

Plan the Survey Like a Job

The Problem

Rushed surveys lead to guesswork. Guesswork leads to cold rooms and expensive revisits. Many contractors find that when they “wing it,” they miss details like voids, bay projections, or uninsulated floors.

The Solution

Treat the survey as a scoped task with a kit and a route:

• Tools: laser measure, hygrometer/thermometer, simple IR thermometer, notepad (or voice notes), and a phone for photos. A blower door is great but not essential.
• Route: start at the loft and work down, or by zone. Capture floor, wall, window, and door areas for each room.
• Record: design temperatures, window types, insulation clues (loft depth, cavity notices, retrofit signs), trickle vents, and draught hotspots.
• Time: allow 30–45 minutes for a small flat and 60–90 minutes for a typical house. It’s common for this investment to save 2–3 hours of rework later.

Tip: Dictate findings room by room and snap photos as you go. With Donizo, those voice, text, and photo notes become a clean, branded proposal the same day, with e‑signature ready when the client says “go”.

Real‑World Example

Two‑bed terrace, street‑facing bay, suspended timber floor: laser the room, estimate wall areas excluding internal partitions, note single‑skin bay, single vs double glazing, and obvious air gaps. Capture all via voice to keep your hands free and your head in the details.

Get the Fabric Numbers Right

The Problem

Heat‑loss calcs go sideways when U‑values are guessed poorly or areas are wrong. Many contractors struggle with older stock where drawings don’t exist and materials vary room to room.

The Solution

Use sensible, stated U‑value assumptions and measure areas properly:

• Walls: In general, uninsulated solid brick is around 2.0 W/m²K; older cavity walls without fill are commonly 1.5–1.8 W/m²K; insulated walls can be 0.3–0.6 W/m²K.
• Windows: Commonly, older double glazing lands around 1.6–2.0 W/m²K; new high‑performance units can be near 1.2 W/m²K or better.
• Roof/loft: With 270 mm quilt, it’s common to assume 0.16–0.2 W/m²K; thin or patchy insulation drives that higher.
• Floors: Suspended timber over ventilated voids are often leaky and “colder than expected”; solid floors vary widely depending on insulation.

In general, windows might be only 10–25% of the envelope area but can contribute 30–50% of losses in homes with poor glazing and draughty frames. State this in the proposal so clients understand why emitter upgrades alone may not fix a freezing bay window.

Real‑World Example

1930s semi with a north‑facing bay: your survey shows the bay’s single‑skin cheeks and old DG units. You flag two options—upgrade glazing/U‑values, or oversize the radiator for that bay and accept higher running costs. You document both paths and let the client choose.

Nail Infiltration and Ventilation

The Problem

Ignoring air leakage is the classic under‑sizing trap. That “one cold room” is often the leakiest.

The Solution

Use realistic ACH assumptions and adjust for exposure:

• In general, older, leaky homes can sit around 1.0–1.5 ACH at typical conditions; tighter retrofits might be 0.5–0.7 ACH; very airtight builds can be less than 0.5 ACH.
• Wind‑exposed sites and corner plots often feel “colder for the same numbers.” Add a small allowance.
• Note intended ventilation: trickle vents, extract fans, or MVHR. You don’t want to double‑count.

Commonly, infiltration makes up 20–30% of the heating load in pre‑retrofit homes. Write the ACH you used into the proposal so you can defend the numbers later.

Real‑World Example

Coastal terrace, end‑plot, timber floors: you select 1.2 ACH due to obvious draughts and exposure. You offer to revisit the calcs after basic air‑sealing—client understands why draught‑proofing plus emitter sizing beats emitter sizing alone.

Size for Flow Temperature, Not Just BTU

The Problem

Old BTU charts assume higher flow temperatures. Then you commission the system at lower flow (to condense better or suit a heat pump) and rooms lag.

The Solution

Design to the flow temperature you’ll run:

• Targets: Many UK gas systems are moving toward 60/40 or 55/45 to improve condensing behaviour. Heat pumps commonly run 45/35 to 55/45.
• Outputs: A radiator rated at ΔT50 will commonly deliver around a third less output at ΔT30. If you plan to run cool, you must size up.
• Comfort: Lower flow temperatures reduce cycling and can stabilise room temps. Many installers report that dropping flow 5–10 °C improves comfort at part‑load.

Real‑World Example

Victorian bedroom sized at 1.2 kW using ΔT50 ratings. Client wants a future‑proof low‑temp system. You specify a 1.7 kW emitter at ΔT30 (larger unit or fan‑assist panel) and explain the why in the proposal. Fewer questions later.

Allow for Intermittency, Setbacks, and Warm‑Up

The Problem

Clients rarely heat 24/7 at a fixed setpoint. Morning warm‑up and evening boosts need headroom.

The Solution

• In general, many contractors add a small margin (often 10–15%) for intermittency, unknowns, and local cold spots.
• Note floor type and time constant: solid floors warm slowly; timber floors and UFH zones behave differently.
• Clarify the control strategy (modulating stats, TRVs, weather compensation) because it changes how much “extra” you need.

Real‑World Example

Bungalow with mixed UFH (kitchen) and rads (bedrooms). You size the bedrooms with a modest intermittency margin and recommend weather compensation to smooth warm‑up. You document both the margin and the controls.

Document Assumptions, Options, and Next Steps

The Problem

Disputes start where assumptions end. If the client doesn’t see the “why,” every cold snap becomes your fault.

The Solution

Package your survey so it’s decision‑ready:

• List per‑room design temps, U‑values used, and ACH assumption.
• Show emitter options (standard radiator vs larger vs fan‑assist vs UFH) tied to the chosen flow temperature.
• Note fabric improvements (glazing, loft top‑up, draught‑proofing) and their impact.
• Include a simple “what changes if” note: if they drop bedroom setpoints to 17 °C, or add insulation, what happens to emitter size?

Many contractors find that clear documentation halves back‑and‑forth. With Donizo, you can speak your room notes, attach photos, and generate a branded PDF proposal on the same day. Clients can e‑sign it, and once accepted you can convert it to an invoice in one click—no re‑typing.

Real‑World Example

You include two pathways: “Keep existing rads at 60/40, accept slightly higher fuel use” versus “Swap three undersized rads to run 50/40 now and stay heat‑pump‑ready.” The client signs the low‑temp path in the Donizo portal—no printer, no delay.

Frequently Asked Questions

Do I really need room‑by‑room, or can I size by rule of thumb?

Room‑by‑room wins. In general, older UK homes vary room to room in U‑values and leakage. A quick survey (30–90 minutes) pays back by avoiding that single cold room that triggers a callback.

What design temperatures should I use?

Common UK practice is 21 °C for living rooms and 18 °C for bedrooms. If the client likes it warmer or cooler, document the change and its impact on emitter size.

How do I account for future heat pumps?

Size emitters for lower flow temperatures (for example 50/40 or 45/35) where practical. Note that a radiator at ΔT30 gives around a third less output than at ΔT50, so you may need larger panels or fan‑assisted options.

What if the house is very draughty?

Use a higher ACH assumption (for example 1.0–1.5 for leaky stock) and recommend basic air‑sealing. In general, infiltration can be 20–30% of losses in such homes, so sealing plus right‑sized emitters is the winning combo.

How can I present findings fast without hours at a desk?

Capture notes by voice on site, take photos, and turn them into a professional proposal with Donizo. You can send a branded PDF, get a legally binding e‑signature, and convert the accepted proposal to an invoice without re‑entering data.

Conclusion

Reliable heat‑loss surveys aren’t about fancy kits—they’re about a consistent process, realistic assumptions, and clear documentation. Measure areas, choose sensible U‑values, include infiltration, size to the flow temperature you’ll run, and state your margins. Do that and you’ll cut callbacks, improve comfort, and build trust. If you want to reduce admin time, use Donizo: talk through the survey on site, generate and send a branded proposal, collect a digital signature, and turn the “yes” into an invoice in one click. Less typing, more time on the tools.