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Air Changes per Hour - ACH and the MCS requirement

 
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Air Source Heat Pumps (ASHPs)
Last Post by Batpred 3 months ago
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 robl
(@robl)
Reputable Member Member
Joined: 4 years ago
Posts: 214
29/01/2026 3:58 pm  

A blower door test typically pressurises the home to +/-50Pa relative to outside, and measures the airflow required to achieve this in m^3/hour.  Using the total building surface area and total internal volume you can get the whole house air changes per hour @50Pa, and also the airflow per m^2 of building fabric - the two numbers usually come out quite similar when metres are used - hence often it's not clear which one is used.  Normally there isn't 50Pa difference between inside and outside, so the actual airflow through the building is a lot lower - by default I think a factor 20 is applied.  This magic factor of 20 is an approximation - in reality it depends on house shape (taller houses should have a lower factor applied) and how exposed the location is (more exposed will give a lower factor) and where the leaks are(encouraging stack ventillation again gives a lower ratio).

Building regs presently ask for 8m^3/hr/m^2 @50Pa or better(lower).  I'm not sure why 0.5 is picked for ventillation loss rather than the 8/20=0.4 that my maths gives, my guess is still thats where the value comes from.



   
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trebor12345
 trebor12345
(@trebor12345)
Estimable Member Member
Joined: 2 years ago
Posts: 147
29/01/2026 5:43 pm  

Posted by: @robl
↑

A blower door test typically pressurises the home to +/-50Pa relative to outside, and measures the airflow required to achieve this in m^3/hour.  Using the total building surface area and total internal volume you can get the whole house air changes per hour @50Pa, and also the airflow per m^2 of building fabric - the two numbers usually come out quite similar when metres are used - hence often it's not clear which one is used.  Normally there isn't 50Pa difference between inside and outside, so the actual airflow through the building is a lot lower - by default I think a factor 20 is applied.  This magic factor of 20 is an approximation - in reality it depends on house shape (taller houses should have a lower factor applied) and how exposed the location is (more exposed will give a lower factor) and where the leaks are(encouraging stack ventillation again gives a lower ratio).

Building regs presently ask for 8m^3/hr/m^2 @50Pa or better(lower).  I'm not sure why 0.5 is picked for ventillation loss rather than the 8/20=0.4 that my maths gives, my guess is still thats where the value comes from.

My Air Permeability is : 2.35 m³/(h·m²) @ 50 Pa.  So if I use your magic factor: 2.35/20=0.1175.  According to my SAP report the overall Infiltration Rate for my building is 0.099.  So not far out.

FromAi:

Screenshot 2026 01 29 at 16.50.19

This post was modified 3 months ago by trebor12345

Hitachi Yutaki SCombi Heat Pump
(Indoor Unit ) RWD-3.0RW1E-220S-K
(Outdoor Unit) RAS-3WHVRP1

2024 build bungalow, Southern england, 179 m2, 14w/m2
Underfloor heating all fully open
7KW heat pump
50 litre buffer tank (4 port)
3.6KW solar panels


   
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 Judith
(@judith)
Prominent Member Member
Joined: 2 years ago
Posts: 519
30/01/2026 4:22 pm  

Mick Wall has a very thorough measurement set of air changes per hour for his house, on his web site https://energy-stats.uk/air-changes-per-hour-heat-loss/

 


2kW + Growatt & 4kW +Sunnyboy PV on south-facing roof Solar thermal. 9.5kWh Givenergy battery with AC3. MVHR. Vaillant 7kW ASHP (very pleased with SCOP 4.7) open system operating on WC


   
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 Judith
(@judith)
Prominent Member Member
Joined: 2 years ago
Posts: 519
30/01/2026 4:33 pm  

The generally accepted ratio from the 50Pa test to natural usage is 20. But I wanted to understand why and can’t now re-find the references I used to verify a factor of ~20 for us.

https://building-performance.org/bpa-journal/ach50-achnat/

This American web site gives a wider range of weather exposure conditions than the UK but that’s useful in itself because of how broad the factor is, from 9 to 30.


2kW + Growatt & 4kW +Sunnyboy PV on south-facing roof Solar thermal. 9.5kWh Givenergy battery with AC3. MVHR. Vaillant 7kW ASHP (very pleased with SCOP 4.7) open system operating on WC


   
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Batpred
 Batpred
(@batpred)
Prominent Member Member
Joined: 1 year ago
Posts: 801
30/01/2026 5:46 pm  

When evaluating SmartHome temperature sensors, I noticed many would also include hygrometers.

So that could provide a simple way to have targeted ventilation. In tropical locations, aircon operation is typically driven by a combination of temperature and dampness. To be clear, the sensors work in our range as well..


8kW Solis S6-EH1P8K-L-PLUS hybrid inverter; G99: 8kw export; 16kWh Seplos Fogstar battery; Ohme Home Pro EV charger; 100Amp head, HA lab on mini PC


   
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