I’ve been invited to attend a MCS working group meeting next Monday, discussing among other things heat loss calculation, so this post alongside looking through the new MCS heat demand tool is part of my preparation for the meeting.
1. New MCS heat load calculator
I’ve been given access to the new MCS heat load calculator Heat Load Calculator - MCS. This was released December 2024. I noticed in the discussion at the time that this new version included the option to use an air tightness test result, a welcome step forward.
This new tool also implements a ventilation heat loss calculation that does follow the EN 12831 standard as opposed to the simpler calculation that originated from the CIBSE domestic heating design guide.
2. My earlier analysis of EN 12831
I wrote up my own analysis of the ventilation calculation in EN 12831 back in April 2024 in my post here Measuring air change rates with CO2 sensors for heat pump heat loss calculation - #84 by TrystanLea and then in more detail in the documentation page here: Air change rate calculation methods — OpenEnergyMonitor 0.0.1 documentation.
The key thing to note is that the total ventilation heat loss for the building is not the sum of the ventilation heat loss for each room. The building ventilation heat loss is half the sum of the rooms, at least in naturally ventilated buildings.
As I mention in the documentation post, the exact reasons for these factors are not clearly articulated in the standard. A logical explanation could be that under wind load, rooms facing the wind might have cold air pushed into them. This air would then move, pre-warmed, to adjoining rooms on the other side of the building, resulting in higher heating requirements for wind-facing rooms than those on the leeward side. The latter halving likely reflects an averaging out of these effects across the entire building.
3. MCS EN 12831 uses pre-2000 air change rates which then halve for whole building..
Interestingly rather than using 0.5 ACH for n.min.i as I had read from the EN 12831 standard the MCS calculator uses the pre-2000 air change rate values from the CIBSE domestic heating design guide (for my pre-2000 house), BUT the halving factor in the calculation halves the impact of using these air change rate values from an average of about ~1.7 ACH for my house to ~0.8 ACH.
Is there an evidence basis to use the pre-2000 air change rates from the domestic heating guide in the EN 12831 calculation?
4. Are other domestic heat loss tools adopting EN 12831?
It’s interesting that the MCS calculator is now using the latest EN 12831 calculation method but Im not aware of it being used yet in popular heat loss tools used by most domestic installers? These tools still use the simpler calculation that’s originates from the domestic heating design guide. Are there other domestic heat loss tools adopting EN 12831?
5. Radiator design and system temperatures
There a bit of a peculiar practical radiator system design & control point that results from the EN 12831 calculation in that you need to size the emitter system to be able to output more heat to each room individually than the total heat actually required by the sum of all of the rooms.
Taking the logical explanation (mine, not stated in the standard): that under wind load, rooms facing the wind might have cold air pushed into them. This air would then move, pre-warmed, to adjoining rooms on the other side of the building, resulting in higher heating requirements for wind-facing rooms than those on the leeward side. The latter halving likely reflects an averaging out of these effects across the entire building.
This would require higher flow temperatures in the wind-ward facing rooms and lower flow temperatures on the leeward side.. I guess it assumes that when it’s windy you increase the system flow temperature and then that TRV’s in the rooms on the leeward side partially close as they need less heat..
From a heat loss and system design perspective it seems like an over-complication, why not just design to an even whole house air change rate, heat loss and flow temperature but then just check that the heat pump is capable of delivering the higher temperatures for a scenario where you do balance the same heat output differently across the different rooms & emitter system?..
I’m looking forward to learning how the MCS working group process works and what the thinking is of members on the above. Comments and reflections on this welcome!
that’s the result of the calculation! Make sure your reading the 2017 version of the standard as it all changed.