Mucka works out heat loss a room at a time, so you can size a boiler and pick a radiator for every room off the same survey. It shows the watts for each wall, window and floor, so the number is one you can defend rather than one you have to trust.
You need the Gas Calculators skill installed. It sits alongside Gas & Oil Certs.
Read this bit first
Mucka sizes boilers and radiators. It is not an MCS heat pump design.
If you are doing a certified heat pump install, MCS's own standard (MIS 3005-D) says the calculation has to be to BS EN 12831-1:2017, and MCS give certified installers a Heat Load Calculator that does exactly that, free. Use theirs. Mucka asks what you are sizing on the very first screen and will stop you there rather than let you survey a whole house and hand in paperwork that gets rejected.
It also does not size underfloor heating output — a floor circuit works on a different curve to a radiator. You can still use the room watts and take those to the manufacturer's pipe spacing tables.
What you need before you start
- Where the property is and when it was built. The year sets the age band, and the age band sets the thermal bridging allowance. England, Wales, Scotland and Northern Ireland have different bands, so Mucka asks which.
- The design outside temperature. This comes from the property's location and altitude — roughly −1.5 °C on the south coast to −5.6 °C around Glasgow, and colder the higher you are. Mucka does not guess it for you, because a couple of degrees here moves the answer by a good chunk and a wrong guess is one you would never notice.
- A U-value for everything the room loses heat through. Mucka does not look these up yet, so bring them — off the EPC, out of your own tables, or the figures you normally work to.
- Your design flow and return, if you want radiator sizes as well as watts.
Doing a room
For each room: what sort of room it is, its floor area and ceiling height. Then list what it loses heat through — a wall, a window, a floor — and for each one, its area, its U-value, and what is on the other side of it.
That last one is the bit worth getting right. Mucka gives every element its own temperature difference:
- Outside uses the design outside temperature.
- The ground uses the figure you gave for under the floor, usually the mean annual outside temperature, about 10 °C.
- An unheated space — a loft, a garage, an unheated hallway — uses whatever you say it sits at.
- Another heated room uses that room's design temperature.
A party wall to a room at 18 °C is a three degree difference, not twenty-four. Treating it as an outside wall is the classic way a hand calculation comes out miles high.
What Mucka fills in for you
Two figures come from a table unless you type over them, and Mucka tells you underneath the room when it has used one:
- The room temperature — 21 °C in a lounge, 22 in a bathroom, 18 in a bedroom, kitchen or hall. In a property from 2007 onwards it is 21 throughout except bathrooms.
- The air changes — half an air change an hour in a habitable room, none in an inner room with no outside wall.
Those air changes are worth a word. The older guidance in a lot of people's heads says a living room is 1.5 air changes and a bathroom 3. Modern figures are far lower, and using the old ones roughly doubles the ventilation loss on an older house. Mucka uses the lower figure and shows it, so if the job genuinely needs more — an open flue in the room, a permanent vent — you can put it up yourself and see exactly what it did.
What you get back
- Every element: its area, its U-value with the thermal bridging added, its temperature difference and its watts.
- Every room: the fabric loss, the ventilation loss and the total.
- The whole house, in kilowatts.
If you gave a flow and return, each room also gets the radiator output it needs at Δt50 — the figure manufacturers quote in their catalogues. Drop your flow temperature and that number goes up, which is the whole point: a rad that does 1,200 W on 75/65 does about half that on 45/40.
Two things it says every time, and means
The whole-house total has no diversity applied. It is every room at its worst, all at once. Size each radiator on its own room figure. For the boiler, treat the total as the design maximum before you add anything for hot water.
Thermal bridging is added to outside and ground elements only. Party walls and internal walls read very slightly low as a result. It is a small effect and it is stated rather than buried.
Saving it
The whole survey saves against the job, inputs and all, so months later you can show how the number was arrived at rather than just what it was.