Earth Notes: MHRV (Mechanical Heat-Recovery Ventilation)

Why MHRV is a good idea, and our own installation experience...
Vent Axia HR25H MHRV in bathroom


To improve heating efficiency, houses must become more air-tight so as to lose less heat through "unplanned ventilation". But that then brings increased stuffiness (elevated CO2 levels) and moisture/humidity from people inside breathing. Unless dealt with this is unpleasant and unhealthy and can cause mould to grow for example.

MHRV can also help keep the heat out in summer, when the building is cooler than outside. Keep the ventilation and your cool!

(Note that there are various similar names/terms in use for the technology, eg: MVHR or Mechanical Ventilation with Heat Recovery is another common one, and SRHRV ie Single Room Heat Recovery Ventilation for this style.)


Heating/energy efficiency and ventilation seem at odds with one another, but by imposing a heat-exchanger between incoming and outgoing air (similar to the counter-current exchanger that lets penguins stand on ice without freezing their hearts) enough fresh air can be provided while retaining most of the heat.

Very energy-efficient and air-tight buildings such as Passivhaus don't even need a conventional heating system; a whole-house mechanical (ie with fans!) heat-recovery ventilation system, (typically drawing air out from kitchen, bathroom and toilet and feeding fresh air in elsewhere), with a little in-line electric heating as backup is enough.

Most UK housing stock will not be air-tight or otherwise efficient enough for Passivhaus-style whole-building MHVR, but any reasonably air-tight building that's starting to see condensation can benefit. Just think of MHVR as an extractor fan that dumps the pongs and moisture while retaining most of the heat.

We installed a nominally single-room MHRV in our bathroom in December 2011 with a humdistat (automatically goes into turbo mode when the air is very humid) to help reduce condensation in the bathroom itself, but also in all the upstairs bedrooms.

Without the MHVR every (cold) morning we've had to shut all the bedroom doors, turn the radiators TRVs down to zero, open the windows and wipe them down.

After installing the MHRV the aim is not to have to open the windows at all in cold weather (which should reduce heating costs) nor mop the windows.

MHRV in our Bathroom

HR25H Review Summary

  • Vent-Axia HR25H
  • Reviewed by: Damon Hart-Davis on 2012/01/07
  • Keeps air fresh, retains heat.
  • Does what it claims on the tin, though has not helped as much with condensation as I'd hoped, and still has some issues that need to be resolved with the install.
  • Rating: 3.5 out of 5
Vent Axia HR25H MHRV seen working in IR

As of 2011/12/21 a single-room MHRV fan with humidistat-driven boost mode (a Vent-Axia HR25H) was installed in our bathroom (photos).

From Vent-Axia's page:

Performance: m^3/h l/s
Max ventilation rate 54.8 15.2
Normal Supply Rate 7.8 2.1
Normal Extract Rate 15.7 4.3
Boost Supply Rate 35.7 9.9
Boost Extract Rate 54.8 15.2
Number of speed settings 2

Efficiency: The unit should retain up to 84% of the temperature differential of out going air.

Heat exchanger: Should be of a multi plate counter flow type constructed out of a polymeric plastic with ultra sonic welded joints.

Motor: Should be 24V DC with sleeve bearings, greased for life. The motor shall operate up to an ambient temperature of 40°C and be fitted with a thermal overload protective device.

Fan: Impeller should be a polymeric centrifugal backward curved type. Configured as a split single wheel.

Controls: The unit should be operated via integral pull cord switch, PIR or humidistat.

Filter: Should be a reticulated foam type coarse filter.

Condensation: The outlet should be via drain holes in the lower part of the external grille.

Construction: The unit outer case should be terracotta coloured extruded ABS wall tube with black EPDM wall seals and terminating in a weather cowl. The internal grille should be white ABS.

Sound Levels: dB(A) @ 3m
Trickle: 16.8
Boost: 49.2

Power consumption is 1.9W in normal/trickle mode and 25.1W in boost.


As usual we find out something about the house's construction every time we work on it. In this case, going in the small gap above the bathroom window from the outside the construction was:

  • Tiles hung on horizontal wooden battens
  • Bituminous felt (~2mm, quite stiff), maybe like Onduline 'crinkly bitumen board'
  • 0.5"/12.5mm of wood fibre board
  • 2 lots of 8"x2" with 9mm ply between
  • 0.5"/12.5mm plasterboard
  • ceramic tiles on inner face of wall

The rubber sealing rings on the outside of the MHRV through-the-wall tube are not going to make a very good seal against the wood hole, so some squirty foam is going to be deployed to make everything airtight.

The factory setting of the humidity sensor was having the fan run in 'high' setting continuously (which is quite loud) given the humid overcast day, so I adjusted that until the fan just went off and then checked that breathing on the sensor fired the fan up to full blast mode again. Low speed is quiet. My humdity meter is claiming about 75%RH (at ~12:30), having had all the windows open all morning.

The mains supply has been installed in the toilet next door (to keep it away from bathroom humidity) with a mains 'off' switch to turn the unit off entirely in summer.

The complete install took about 4 hours.

At about 5pm, with six people in the house (including two guests and two small children running around) and some cooking, the fan cut in, so I adjusted it up a little further so as not to come on until ~85%RH. (Viking House on the Green Building Forum was suggesting that we should aim for closer to 60%RH.)

8pm: the fan has gone very quiet or has cut out after just a few hours' service! Will have to investigate more in the morning...

2011/12/22 9am: the fuse was fine when checked, and opening the wiring pattress and wiggling wires looking for loose connections revealed nothing but the fan operates when power is re-applied, so the sparks will come and check it out.

2011/12/23 9am: only a little condensation on bedroom windows this morning, though it was ~11°C outside so not a real test. The fan went into boost mode automatically during a shower (>90%RH), which is good.

Purely in a spirit of scientific enquiry I took a long hot bath early afternoon and for about an hour the MHRV managed to keep the place from steaming up without getting cold as it automatically went to 'boost' mode after minutes. At the end of that time the bathroom was >95%RH (the meter steamed up!) and ~20°C (so ~2°C warmer than the room next door for example, and ~9°C warmer than outside). After about 20 more minutes the fan stopped entirely, I suspect from overheat protection at the fan or the (warm) 24V power supply...

6pm: turned HR25H off at mains for a couple of minutes and back on and it restarted, in 'boost' mode, then dropped back to slow after a couple of minutes. It is no use if it needs babysitting like this, but VA's tech support line was shut mid-afternoon when I tried to call...

2011/12/31: haven't had to open any bedroom windows at all since MHRV went in, and condensation on bedroom windows seems to be reduced, though we haven't yet had a very tough test (sub-0°C night) yet. Also, cut-out hasn't happened again, and fan seems to be switching between low and boost modes appropriately. I hope to see reduced fluctuations in bedroom temperatures and reduced heat demand from not needing to open windows to disperse moisture and pong...

2012/01/01: fan cut out again after about 90 minutes in 'boost' mode (somewhat after another long hot bath), though it had survived quicker/cooler showers and baths before that, and come on briefly to expell moist air escaping from the kitchen from time to time, as might be hoped.

2012/01/04: Vent-Axia's (helpful) technical support line suggested that the cutting out I observered my have been the power supply box not having enough clearance to the ceiling and thus overheating. I'll try to get it moved. Separately, it seems that after cold nights (<5°C) we may well still have to wipe down windows (though maybe less than before), but we should not have to open them subsequently for adequate fresh air or to complete drying.

2012/01/05: it looks like the fan isn't sloping slightly down to the outside (≥1°) as indicated in the fitting instructions, so condensation has run back through the fan and into the bathroom tonight after a bath.

2012/01/13: the builder is back today fixing the slope on our HR25H to drain condensation to the outside: the heat exchanger was full of water. We have tested pouring water in at the room end and made sure that it flows out freely. The power-supply box has also been moved a few inches down from the ceiling (it is in the toilet next to the bathroom) to improve cooling. The next few days are forecast cold, around/below freezing min and ~5°C max, which should test the MHRV efficacy. Also hope to start running the portable dehumidifier in the kitchen when laundry is drying to avoid that moisture finding its way into the rest of the house.

2012/01/14 8am: -2°C overnight and though the fan is running (on low) I can't detect much if any airflow through it, and it seems that the unit has been re-assembled wrongly, so I've turned it off until I can get a grown-up back to look at it.

2012/01/16: -1°C overnight and lots of condensation with the fan off. Builder turned up at 8am to readjust so that flow is now correct in and out. I've given the fan an extended test on boost (by running/having a hot bath, life is tough) and it ran for over 2 hours on boost before switch back to low mode having brought the humidity down to ~80%RH as per the control. And in the evening, partly with the assistance of the portable dehumidifier in the kitchen while laundry has been drying, humidity levels are around 70%RH (kitchen 63%, living room 64%, front bedroom 72%, bathroom 76%).

2012/01/17: -1°C overnight and a little less condensation with the fan on. Still needed the windows wiping down, but noticeably less. Quite possibly time for better (3G/triple-glazed) windows!

2012/01/20: attempted to slightly reduce humidity threshold for 'boost', now about 75%RH, but should still be high enough not to come on at night. I am now aiming to keep below ~70%RH inside, mainly with the MHRV upstairs and the portable dehumidifier in the kitchen downstairs as necessary. One site recommends an indoor RH of 30% to 50%; another suggests a similar range of 55% down to 30%. (Had to adjust threshold upwards several times during the evening to avoid 'boost' mode which is a bit noisy for night-time.)

2012/01/26: had to turn off fan overnight which went into boost mode, and had been a bit oversensitive for a few days before, so turned up threshold in the morning. (Trying to find suitable tools and then adjust the humidistat threshold in the pitch dark, balancing on a stool in the bathroom, without cursing, became an unwanted new skill.)

2012/01/30: in an exchange with 'djh' on GBF it turns out that tickle mode is giving us ~0.1ACH (air change per hour, for the whole house rather than just upstairs) and the (noisy) boost mode gets closer to the ≥0.3ACH suggested as 'adequate'.

(UK regs may require 0.44ACH and PHPP says that DIN 1946 Part 6 requires a min of 30 m^3/h/person, min 0.3ACH, or about 8 of the the HR25Hs on trickle for this house, but there was also a suggestion that ~0.6ACH was the maximum even with MHRV that avoids unacceptable heat losses.)

This is against the backdrop of a presumably-still-leaky house, but as djh puts it clearly too humid and "in mite & mold territory". So I have to work out where to go from here to improve ventilation without ruining heating performance.

Maybe for the remaining month or two of heating season we could leave a bedroom window open a crack at night to take advantage of the excess extract volume and draw the humid air out that way? RH rises at night towards 80% and falls slowly during the day with fewer people in.

2012/02/01: at around freezing outside and with some wind, and the master bedroom open a crack, we had ~60%RH overnight and *much* less condensation than usual. House temperature dropped a few degrees (C) overnight, but not too bad.

2012/02/03: seems to have survived -5°C outside overnight, though doesn't seem to be bringing a lot of air in!

2012/02/11: turned the fan off in the early hours of this morning with the external temperature well below freezing (possibly -7°C) due to a slight grating sound which I took to be a build-up of ice. Tried turning on again a few times in the morning, but turned off after ~15 minutes each time when rattling restarted, presumably to ice reforming. Coming up to noon with external temperatures at about -3°C, the fan seemed to stay rattle-free.

2012/02/12: again circa -several°C induced a rattle in the night, so MHRV turned off until the morning.

2012/02/20: although apparently -3°C by this morning, with only one of us (and thus less humidity) in the house, there was no rattle and the unit ran fine all night.

2012/03/23: a week or so with enough sun to have windows open (and MHRV off) during the day and typical RH in the kitchen is now nearer 50%.

2012/09/02: just bought Milton Sterilising Tablets to clean the filter and heat exchanger for the HR25H before nights get chilly and we want to start using the unit again. A few nights ago hit ~8°C. 28 Milton tablets (of which I used 2 this time) cost £1.25. Cleaning instructions are in the leaflets that come with the unit, and consist of the following:

  • Isolate the mains power supply.
  • Unclip the removable grille from the unit and remove filter.
  • Loosen the two securing screws and remove the grille surround.
  • Remove the divider board and slide out the heat exchanger.
  • Wash the grille, surround, filter and heat exchanger in warm water using a mild detergent and dry thoroughly. NOTE: Keep water away from all electrical components and wiring within the unit. If it is not possible to fully clean the filter, it must be replaced.
  • Reassemble in reverse order ensuring the divider board seals against the heat exchanger. NOTE: Follow the Inside/Outside instruction labels on the heat exchanger. The recess in the heat exchanger outer wall must be aligned with the wiring tunnel on the unit to ensure the divider board slot is vertical.
  • Switch the power supply on and check the operation of the unit.

(I also gave the central heating a quick test even though sunny and warm!)

2012/09/22: had the MHRV on last night with just the master bedroom window open a crack, and similarly on a couple of previous nights with external air temperatures around the ~7°C mark. Condensation has been seen on the outside of the new triple glazing; none inside yet.

2012/09/23: MHRV making interesting noises with 14mph easterly rain straight into it!

2013/01/17: with overnight temperatures down to -4°C the HR25H started rattling badly (presumably from ice on the fan) but the Tempra was fine, possibly because the fan is on the warm interior-side of the unit.

2013/03/21: Neil W wrote to me to dispute the claimed energy recovery.



As of October 2012 we'd refitted part of the kitchen and were thinking about doing the rest, including replacing the existing gas stove and replacing the old passive (air-grille) ventilation with MHRV such as an HR-25 (but with manual boost only).

Read more about our eventual choice and the installation.

2012/12/08: between the two fans, and especially trying to avoid high RH building up in, and escaping from, the kitchen, and with cold weather (and thus low absolute humidity outside), RH has been hovering around and dropping below 70%, which is good.

7°C/70%RH Rule of Thumb

My observations in the last quarter of 2012 suggest a simple rule-of-thumb that additionally supports the notiona that 70%RH should be an upper limit as far as practicable.

We only see bad condensation inside our (triple-glazed) windows when internal relative humidity is above 70% and external tmperatures are at or below 7°C outside, eg overnight.