Heat Loss

Introduction

The first step in designing an efficient space heating system is understanding the specific needs of the property. This begins with a thorough assessment of heat loss, which determines the required size and capabilities of the system. The goal is to supply each room with just enough heat to maintain the target temperature—no more, no less.

Ideally, this assessment would only be conducted after all possible insulation and air-tightness improvements have been made. However, with winter approaching, I had to bring forward the installation of the new heating system.

Heat Loss

The foundation of the entire heating system design is the heat loss calculation. All subsequent decisions, from system size to component selection, are based on this crucial step. If the calculated heat loss is too high, the system will be oversized, leading to unnecessarily expensive components and reduced efficiency due to frequent cycling (the system turning off and on trying to maintain temperature). On the other hand, if the heat loss calculation is too low, the system may struggle to provide sufficient heat, working all day at 100%, and leaving the property uncomfortable.

Estimating Heat Loss Using Typical Values

When calculating heat loss, an assessor will usually rely on typical values based on the insulation type, roof construction, window quality, and other building factors. These values help establish a baseline for expected heat loss in homes with standard construction. For example, the U-values (a measure of heat transfer) of various materials, such as insulation, roof types, and window glazing, are factored in to estimate how much heat is lost through walls, floors, ceilings, and windows.

However, this method can lead to inaccurate results if the building construction is misunderstood, or if the building materials are sub-standard/poorly fitted. For instance, the assumed U-value could be far too high if an entire section of insulation is missing entirely, window seals are no longer airtight, or a ventilation hole isn't sealed properly.

Measuring Heat Loss

Instead of relying solely on theoretical values, I conducted a practical test to measure the actual heat loss in each room. This involved using a 1.5kW electric heater to raise the room temperature to a target, measured by a digital thermometer, then maintaining that temperature for one hour. With the outside temperature known, the amount of electricity consumed during this period, measured via a digital power meter, provides a direct measure of how much heat was required to offset losses in that specific room.

By repeating this test for each room, factors such as room size, insulation quality, window performance, and air leakage are automatically accounted for. This real-world data would help identify any areas with higher-than-expected heat loss, which might indicate poor insulation, drafts, or construction flaws that wouldn't be apparent from standard U-value estimates alone.

Results

The total measured heat loss of all rooms was slightly under the estimated value, which I still consider a little high due to surrounding rooms being cooler than the target temperatures during test.

This approach provides a more accurate heat loss assessment, allowing for a heating system design based on the actual performance of the building rather than idealized assumptions.

Conclusion

From the data, the heating requirements for the hall, utility room, en-suite, and bathroom are negligible. These rooms will not be heated directly (with the exception of the bathroom) but still contribute to the total heat loss calculation. This is because the heat they leech from the adjacent rooms must be accounted for in the overall system design.

Provided the building envelope is well insulated from the outside, these rooms will eventually reach the same temperature as the adjacent spaces once thermal equilibrium is achieved. This process causes the temperature in these rooms to lag behind the adjacent ones. However, since the system is designed to run continuously without cycling, the temperature lag is only relevant during the initial startup phase.

The exception to this is the bathroom, which is a thermally-isolated extension located outside the bounds of the main apartment and into the shared area of the building.