I’ve spent a bit of effort draughtproofing my house, which has been great for keeping the place warm. The downside is that a certain level of ventilation is necessary to prevent humidity building up. This means condensation, mould and other nastiness. This winter has been quite bad, with black mould growing on cold spots around the place. That’s unacceptable to me with wee kids in the house, so something needed to be done.
The problem has two solutions:
- Get the humidity down by increasing ventilation
- Eliminate cold spots where humid air would condense
The main sources of damp in a house will be the bathroom and kitchen. Using a cheap humidity monitor I’ve been tracking our humidity levels and found them consistently above the 70% danger level where problems arise. Clearly we needed to avoid pumping any more moisture into the house, so extract ventilation in the bathroom seems like the top priority.
I’ll be tackling the cold spots in due course too, but for now it’s time for some mechanical ventilation with heat recovery (MVHR).
What is MVHR?
I’m allergic to the idea of simply blowing air we’ve heated with fossil fuels straight out the side of the house, so instead of an extractor fan I went shopping for a through-wall mechanical ventilation heat recovery unit. Unlike a regular extractor fan these also draw in fresh air and pass it over a heat exchanger that recovers some of the heat from the outgoing air. As well as reducing draughts by supplying air to replace that extracted, it will reduce the cooling effect of sucking out the warm moist air.
Is it worth fitting MVHR?
New tightly sealed houses use whole-house MVHR systems, but these are a different kettle of fish. Indeed, fitting one is only worth it if your air tightness is very good. Given that I had a humidity problem I took my air tightness to be somewhat higher than it should be, and coupled to the carbon-saving and comfort boosting properties it seemed the extra expense was worth it.
The Vent-Axia HR25H
I chose the HR25H over several competing products (Envirovent Retrovent and it’s replacement the cringingly-named Heat Sava, Vent-Axia HR25 Solo and Tempra) due to the combination of good performance, low price, and generally positive reviews from owners. The unit is not balanced, it extracts slightly more than it inputs, but this is a plus point in my books as it will draw air from the rest of the house into the bathroom so that other rooms get the benefit too.
Essentially the HR25H is a plastic tube with a divider down the middle. This penetrates the wall, and the fans are mounted on the exterior end, with the filter and electronics on the inner end. A cartridge type plastic heat exchanger sits in the middle of the tube and the whole lot is powered by a switched-mode power supply unit located up to 5m away. Like all MVHR it’s designed to run constantly on trickle and speed up when needed. Like most people I opted for the humidistat controlled version so that it would automatically boost whenever it was needed. Sensitivity is controllable, although getting it right can take a bit of fiddling.
Just a smidge of drilling required to make this usable…
To install this MVHR you’ll need a 100mm or 152mm hole through your wall. I already had a large airbrick through the wall, which had been bodged about with during the time of previous owners and currently had a nasty plastic grille. This was a good thing, as core drilling all the way through 300mm of wall is not my idea of fun.
I had hoped to avoid drilling altogether, and simply bash enough of the airbrick out, but the vent narrowed inside the wall too much (difficult to see in the pics). You can hire core drilling kits for around £50, or hire someone else to do it for about £40-90.
It’s important to make sure you drill with a slight downwards slant (which is why the hole isn’t going through the top of the vent in the picture). Sitting a spirit level on the chuck of your drill and tilting until the bubble moves is good enough. This will ensure that any condensation that forms inside the heat exchanger drains outwards. This is important, you will get condensation inside it, and if it can’t escape it will fill up the heat exchanger and the unit will conk out.
Note the tile-over-tile job from previous owners. I constantly find nasty things like this whenever I do any DIY on this house. Luckily they left some spare tiles for me so I can patch up the hideousness when I’m done.
I fitted a plastic wall sleeve to the hole and filled the rest of the void with expanding foam. I’m sure the latter is awful for the environment, but it’s supremely useful stuff for sealing weird shaped holes and penetrating little gaps. Then it’s simply a matter of sliding the HR25H into the sleeve where it seals very tightly due to the rubber seals around it. The kit includes 5m of cable to run to the power supply unit. I went straight up through the ceiling, across the top of the insulation in the roof and down to the wall outside the bathroom where an existing junction box was close. After wiring the MVHR to the low voltage side of the power supply box and having the mains connected to the high voltage side I hit the switch and it all powered up nicely. The power supply even includes a fuse and a switch, so there’s no requirement for any additional hardware to spur off your existing wiring.
The unit is quiet in trickle mode. You can hear it nearby, but only if you listen for it. After all my hard work I needed a shower and was pleased when boost came on almost immediately.
Boost mode is much noisier, but so is any extractor fan. If I listen up the stairs I can hear it running in the bathroom if the door is open. If you shut the bathroom door it’s barely audible in the bedrooms adjacent to the bathroom, so I won’t be waking anybody up when I’ve got an early start.
Some action snaps:
Immediately pre-shower. Temperature is reading a little high as I inadvertently left the monitor over a radiator for a few minutes just prior. So much for a controlled experiment…
Peak humidity. The monitor does lag a bit, I assume it’s averaging readings from the last few minutes.
Seems to be working…
…yep, definitely working!
Note the drop in temperature. Some of this is due to the accidentally high reading pre-shower, then the effect of running the hot shower, but some drop while ventilating is inevitable. External temperature was pretty chilly, probably no more than 10ºC. No heat exchanger is 100% efficient, and just think how much colder it would have got if we weren’t recovering some of the exhaust heat.
The humidistat is adjustable between 60-90% relative humidity. While the adjustment is just a case of twiddling a knob, you do have to remove the face of the MVHR to do this, which means it’s a screwdriver job. To be honest this is a pain , but once you get it right there should be no need to tinker. I found the best way was to set mine to be relatively insensitive so that it shut off too early. Then I tweaked it downwards until it went into boost again, and kept repeating this until I was happy with the RH level when it shut off.
Power usage is 2W in trickle and 22W in boost. I estimate mine will run in boost for about 2h a day, putting annual consumption at under 16kWh (assuming it’s switched off for half the year). That’s around £2.25 for me, so worth every penny to prevent nasty mould spores getting at my kids.
Besides a regular clean of the filters little is required. Replacement heat exchangers are available and simple to fit. Some users have reported prematurely dead fans, but they have a five-year warranty on them, so it should only be nuisance value if they do conk out. The electronics are all easily accessible, and you can pull the whole unit out from inside, so there shouldn’t ever be any need to go up a ladder to attend to the outside.
I’m happy with the HR25H. At around £275 delivered it’s substantially cheaper than some of the competition, and performs well. My humidity readings are now in the low 60’s or high 50’s (max has been 67%), and it’s done so without wasting heat. Fitting it was pretty easy, especially since I could use an existing airbrick. Besides an occasional clean out it should do it’s thing without any effort from us, which is just how I like it.