Q&A with Jason: explain heating load vs demand The first in a series of actual client questions, answered

Q. What is the difference between heating demand and heating load and how is each measured? How do these numbers impact on Passive House certification?

A. Both these numbers are important. Passive House certification sets an upper limit for both these metrics: breach both and you’ll miss out on certification. However, the point of measuring these two things is to make sure you’re comfortable in your new home and that it delivers what it promised, ie radical energy efficiency.

A building can be certified to the Passive House by meeting either the heating demand or the heating load, in addition to other quality metrics.

First, heating demand. This is the amount of heat required to keep the home within an acceptable, comfortable temperature range. This will vary a little from project to project depending on client preference; let’s assume 20 degrees. Heating demand is expressed as the amount of kilowatt hours per square metre per year (kWh/m²/a). The Passive House standard sets an upper limit; to gain certification, a building cannot use more than 15kWh/m²/a. This is approximately 5-30% of the amount of energy needed by a modern house built only to the Building Code minimum.

The heating demand is calculated as part of the energy modelling that happened at design stage. It’s a computer simulation of how that individual building will perform, taking all specifics into account. So it’s theoretical but many studies have validated its accuracy. If you put a meter on your electric heater or heat pump over winter, the amount of power it outputs should, when annualised, match the heating demand prediction from your home’s energy model.*

Heating load is the power used by a heater of sufficient size to keep the temperature comfortable on the coldest days of the year. A 300m² Passive House can typically rely on a single plug-in, roll-around 3kW heater. In a home half that size, a 1.2kW heater would be sufficient to maintain the temperature over 20C but I do recommend one with a larger capacity. This will be useful if the house were unoccupied for a time and the temperature had dropped a little more. The Passive House standard upper limit is 10W/m².

The heating load can be measured directly via a co-heating test. When these tests were run on certified Passive Houses, the result was real world performance closely matched the performance predicted by the design stage energy models.

Note that the numbers for either heating demand or load are not zero. Even a certified Passive House will need some heating to remain at 20C through the winter. How much depends on many factors. Climate is a big one but how many and how people live in the house has a significant impact too … the length of hot showers, the use of ovens or slow cookers, lots of electronics etc. All of these generate heat. Doors left open for pets or by children in winter has the opposite impact.