Mechanical ventilation with heat recovery (MVHR) is often put forward as part of a low-energy design strategy for homes. The principle is that you build your house air-tight to minimise air infiltration and then mechanically supply fresh air via a heat exchanger. This heat exchanger captures the heat energy in the warm outgoing air and transfers it to the fresh incoming air. Some MVHR units can reclaim as much as 90% of outgoing heat, potentially making drastic cuts to your heating load while maintaining high internal air quality (that’s the theory). All of this is part of the build tight, ventilate right strategy that you hear so much about.
But how air-tight do you have to build before MVHR makes sense? I took a look using SAP 2005, and here are the results:
Pretty disappointing. For this house, MVHR only narrowly outperforms natural ventilation when you get down to extremely low air tightness values (about 2 m3 of air per m2 of external fabric area per hour at 50 Pascals pressure difference between inside and outside – also called the q50 value). Maximum q50 for building regs is 10 – and many UK builders have trouble just hitting that target. A q50 of 2, while not up to German Passivhaus standards, is still very very tight.
I did the same exercise for a 120m2 detached house, 70m2 detached, 70m2 terraced, and 70m2 flat. The curve is nearly identical in all cases.
This is assuming a specific fan power (SFP) of 2W/l/s, the default value in SAP. But best practice for MVHR as defined by the EST requires an SFP of less than 1W/l/s. Changing the SFP makes a significant difference to our 120m2 terraced house:
Suddenly MVHR breaks even at a q50 of 10 and at very low air tightness reduces the overall emissions by almost 10%. If you look at homes with less exposed fabric area (like a mid-level flat) the MVHR curve is even steeper. This is because the q50 figure translates into fewer air changes per hour. In other words, less exposed fabric equals less overall infiltration even if you have a crappy builder. However, the maximum savings in emissions is still only around 10%.
So MVHR looks pretty good after all, but you’ve got to be careful when specifying the unit and pay close attention to the SFP. A next step would be to compare £/kgCO2 saved for MVHR and alternatives, such as solar thermal panels.
I’m not going to include all the information about the houses, like insulation values and so on, just because I just wanted to give the general results. Let me know in a comment if you want to know any of the assumptions I made.
One interesting point is that SAP allows you to put in your own q50 value. But in the calculations it treats this number as air changes per hour at 50 Pa rather than m3/m2 of building fabric as it says in the SAP documentation. As a result, SAP can arrive at much higher infiltration rates than the authors may have intended, particularly for homes with little exposed fabric area.