Installing a Vent-Axia HR25H Single Room Heat Recovery Unit


Our downstairs bathroom is in a solid walled, solid floored, single story, 'lean-to' on the north end of the offshot. There is non-functioning? air brick above the window and the outside wall gets wet when the gutter is blocked by wind-blown garbage from the neighbour's roof.

The bathroom is used for baths and showers, but also as the drying room when it's too wet to dry outside: so the window is often opened to clear the condensation, losing what heat there is.

There has been some mildew on the inside of the wall beside and below the toilet cistern, presumably encouraged by the cold (damp) wall and general humidity.

What Happened

So inspired by (link-rot: was "") Jack at Peckham Power and using his research I've installed my own humidity controlled extractor fan.

With 243mm of solid wall the drilling was hard going:
The (diamond-tipped) core bit has to grind its way through the wall, which is does — slowly. The bit has to be kept straight to stop it catching on the surface: the clutch prevents the drill spinning out of your hands, but it slows the cut so should be minimised. Holding a front-heavy drill and bit weighing ~4kg almost completely still at head height and keeping the trigger pressed while wearing padded gloves, dust mask, googles and ear-protectors wasn't much fun!

But I broke through eventually (after ~2h30, with lots of rests):

The wall wasn't actually that solid: I hope there's enough brick there to avoid draughts:
Loose bits of brick or mortar slowed the drilling by moving around: this is after all the loose pieces were pulled out.

And there was a lot of cleaning up to do :-(

But I got there in the end (mains connection and PSU installed inverted for wiring convenience):


Too soon to tell!

The documentation says it uses 1.9W in trickle and 25.1W in boost modes (20m3/h and 55m3/h respectively). I'm not sure whether we'll notice either the increase in electricity usage (under 0.5 kWh/week) or reduction in gas usage. I hope it will make the back bathroom (and utility and kitchen) more comfortable: warmer in winter and with fresher air all year. The volume of the bathroom is 8m3 and bathroom, utility and kitchen together about 60m3, so trickle should give a change of air every 3 hours.

In trickle mode it's just audible when standing in front of it. In boost mode I can hear it from the kitchen with both the bathroom and utility doors closed. Air is being sucked out though the left-hand side, blown back in on the right.

After 24 hours, I've found that

is enough to get RH over 70 and activate boost mode. I'm surprised how insensitive I am to humidity!

After a week, I've adjusted the sensitivity to come on at about 75% RH (as reported by a portable thermometer/hygrometer): the fan was on boost mode too often and too long otherwise. The manual says the default is 70% at 25C. But this room is usually 16-18C, I've yet to see 20C since installing the ventilator. I wonder if the humidistat is reacting to absolute- rather than relative-humidity?

2010-09-05 first noticable change: normally if I put a loaf on overnight the smell reaches the bedroom and gets me up in time to take it out. Not anymore! This is not an advantage, but shows that something's happening.

Other Thoughts

Our kitchen is also poorly ventilated: we have to open the outside door when doing a lot of boiling. Putting a proper extractor fan over the hob is difficult: it really needs to wait until the kitchen ceiling comes down. So I'm wondering whether this fan will help in the kitchen or whether to fit another one of these in the kitchen. I would need to find a way to avoid making any dust inside: it was feasible to empty a 3.2m2 bathroom and clean up afterwards (and we have a spare!), it would not be possible in the 18.7m2 kitchen.

Chris Benson