CIBSE vs MCS — choosing a calculation method
Heatworx calculates heat loss two ways: the CIBSE Domestic Heating Design Guide (2026) method, or the MCS heat pump method (MIS 3005-D / BS EN 12831-1). You pick one per survey and can switch at any time.
This page explains what each method does, why the same house gives different numbers under each, and which one to use for a given job.
Same survey, two methods
Both methods run on the same measured survey. Same rooms, same dimensions, same construction choices, same U-values, same emitters. Switching method does not change or discard anything you captured — it changes how the numbers are computed from it.
That means you survey once. If a job turns from a boiler replacement into a heat pump quote, you switch the method and the results are recomputed from the survey you already have.
The MCS method (MIS 3005-D)
MIS 3005-D is the calculation method MCS requires for accredited heat pump installations. It sizes for steady-state conditions using BS EN 12831-1: fabric heat loss plus a whole-dwelling ventilation model, with no reheat allowance from heating schedules. Under MIS 3005-D, when the 99.6% design temperature is used, no reheat uplift is required.
The outside design temperature is mandated, not chosen. MIS 3005-D Table 2 lists reference sites with fixed design temperatures; the calculation must use the value for the nearest site, adjusted for the property's altitude. When you switch a survey to MCS, Heatworx applies this temperature automatically. If a manual design temperature diverges from the mandated value, the app warns you.
The CIBSE method (DHDG 2026)
The CIBSE Domestic Heating Design Guide method is schedule-aware. Your heating schedule and setback temperatures drive an intermittent heating allowance — extra capacity to bring rooms back up to temperature after the heating has been off. That allowance raises both emitter sizing and heat generator sizing. A house heated in two short bursts a day needs more from its radiators than one heated continuously; CIBSE prices that in.
The outside design temperature comes from CIBSE weather data for the property's location and altitude, and you can override it manually where a project specifies one.
What changes in practice
The same house can show different Heat Load and Generator Peak figures under each method. This is not an error — the two methods make different, documented choices:
- Reheat. CIBSE adds an intermittent heating allowance from your schedule and setback. MCS adds none — it sizes steady-state. Your zone schedules are kept either way; the annual cost estimate uses them under both methods.
- Design temperature. CIBSE uses location-based weather data with a manual override. MCS mandates the MIS 3005-D Table 2 value for the nearest reference site, altitude-adjusted.
- Ventilation. CIBSE builds room airflows from survey evidence (leakage, vents, flues). MCS uses the BS EN 12831-1 zone model: each room takes its worst-case share of the dwelling's air change.
- Thermal bridging. CIBSE applies a y-value across the envelope. MCS applies an additive U-value uplift to each external element by construction age band.
Neither figure is "the right one" in the abstract. Each is right for the paperwork that asks for it.
How to switch in Heatworx
On the survey page, tap the method chip (CIBSE / MCS) and pick the other method. The switch is per survey profile, takes effect immediately, and recalculates every result — room heat losses, emitter comparisons, generator sizing. It is fully reversible: switch back and the previous results return.
Under MCS, the mandated design temperature applies automatically, and every result states which method and data pack produced it. Reports carry the same attribution, so anyone reading them can see which method was used.
When to use which
Doing an MCS heat pump installation? Use MCS. MIS 3005-D is the method an MCS-accredited installation must file, and the mandated design temperature is part of that requirement.
Everything else? Either method works — pick the one your paperwork needs. CIBSE suits general heating design and schedule-sensitive sizing, such as a boiler replacement where the house is heated intermittently and reheat capacity matters.
Because both methods run on the same survey, it costs nothing to look at both. Switching takes seconds and changes nothing you measured.
Heatworx implements these calculation methods. It is not certified or approved by MCS or CIBSE, and using it does not by itself make an installation compliant. Compliance belongs to the installer and their accreditation scheme; Heatworx provides the calculation.
Frequently asked questions
Is Heatworx MCS certified?
No — MCS certifies installers and products, not calculation apps. Heatworx calculates to the MIS 3005-D method that MCS installations require. The certification, and the responsibility for the submission, remain with the accredited installer.
Why did my numbers change when I switched?
Because the methods compute differently: reheat allowance, design temperature, ventilation model and thermal bridging all differ (see "What changes in practice" above). Your survey data did not change. Switch back and the previous numbers return.
Do I lose my heating schedules under MCS?
No. Schedules and setbacks stay on the survey. MCS sizing does not use them — MIS 3005-D sizes steady-state — but the annual running cost estimate still does, and they are used again the moment you switch back to CIBSE.
Can I set my own design temperature under MCS?
MIS 3005-D mandates the design temperature — the nearest Table 2 reference site, altitude-adjusted — so Heatworx applies it automatically when you switch. If a manual design temperature diverges from the mandated value, the app shows a warning, because a submission using a different value would not meet the method.