The Mechanical Ventilation Heat Recovery system.......
We had looked at all manner of 'Eco' developments for the house, and spoke at length to a number of unbiased energy specialists. It became clear to us though, that such things as Ground and Air Source Heat Pumps, made no economic sense at all for a property in the town with a piped supply of natural gas.
However, every house has to have ventilation, which would normally consist of 'trickle vents' in windows, passive stack (relying on convection) and mechanical extraction fans in kitchens and bathrooms, the latter initiated automatically when the lights are turned on. One of the downsides of these methods, is the loss of heat that occurs, and often the resulting condensation that almost certainly occurs because of reluctance to open windows in winter months. Even in a reasonably well ventilated house, mould will occur behind cupboards and in cold corners, so this is plainly stone age stuff, and we wanted to improve on it.
We were aware that there were systems on the market that could supply fresh air 24/7, but retain the heat from the exhausted air. We were totally convinced by the principle of it, and so we did some searching. There are several MVHR system available on the market, with various methods of recovering the heat, from flat plate heat exchangers, to rotating wheels. I looked online and found a few installation businesses that supply and fit systems from the manufacturers, such as Vent Axia, Xpelair, Airflow, and we received comparative quotes. Our Self-Build Rep Greg Blee, had some experience with another company, Villavent, and asked them to provide a quotation. It was on the higher side, but it appeared to me to be a high quality system. The high capacity Swedish drive unit with rotating wheel exchanger was produced by Villavents parent company, Systemair. The quotation arrived with schematic plans for the routing of ductwork around the building. In all cases the heat exchanger and fan unit was quite a large box, especially so with Villavents VR400 , which we initially thought could be fitted in the utility room. The more we looked at it, the more it became clear that this would not work, and considered where we might be able to position it elsewhere. Thankfully, the stairwell had a mass of room to fit in an easily accessible loft space, with an outside wall, where the inlet and outlet vents could be housed. As we were in the final design stages for the frame I asked our Architect to make the amendment, and the deck and wall were added to the timber frame plan.
The Sales Director at Villavent, Neil Christer, was absolutely passionate about the system, and when half way through the timber frame build I had reservations about whether we could actually fit in the ducting, he said he would come down to the site to advise us. The main issue of concern was the fact that there was not a path through the wall from upstairs to downstairs, because there was a solid 9" ring beam at floor level. Thankfully, Neil found a solution for us, we would increase the width of two internal stud walls to 150mm, which would allow them to fit rectangular plastic ducting between floors with no reduction in airflow...shimples! A revised schedule and drawings arrived within a couple of days, and Clinton got to work on the walls. The total supplied and fitted price would be £4.7k including filters and guarantee for five years.
On February 1st 2013, all the system components were delivered to site. On Wednesday 6th, their Installer 'Ben' arrived, and by mid afternoon, the first fix was completed.The fan unit fitted really well into the loft we had provided. Although it doesn't seem like it in the picture below, there is plenty of room for access and maintenance when required. It is of course now enclosed behind a wall, but all components are modular and there is nothing too big to remove through the loft hatch. All the ducting in the roof space is insulated to stop condensation.
We would not need Ben again until the plastering and painting was complete, he returned to fit all the vent louvres and commission the system on 13th May.
Living with the system
At the time of writing this, the system has been in use for 3 years and 8 months, and it does definitely deliver a comfortable environment. Although it runs 24/7 (they say you should never turn it off) the wall controller allows you to adjust set periods of fan speeds (slower at night due to lower activity), and the level of heat recovery. In the summer, when doors and windows are unavoidably open on a hot day, you can also set it to low speed, to reduce use of energy. We have had one failure, that being the small motor that drives the rotary heat exchanger. It stopped working, and was replaced under guarantee. There is an electric heater in the unit, which you can set to automatically heat the incoming air if it falls below the set temperature (which is also adjustable). We have turned this off because I don't like the thought of using electricity in this way. As a result, when it is very cold outside, you can sometimes feel the fresh air is a bit 'cool'. We are replacing the filters each year as recommended, one set for each, inlet and outlet, and were initially supplied with three sets as part of the package. Having use them all, I have just bought a set in readiness, and they cost £93.40 delivered from Fans & Spares (a Systemair company). The unit is very quiet in use, you can just about hear the air flowing during the night time silence, but the fans in the unit are pretty imperceptible. You rarely get something for nothing, and so it is in this case, it takes electricity to continually run the fans and motor, and our costs are higher as a result but have to countered against lower heating costs.
Our total Elecricity use is pretty stable month to month, and at or around 4150 Kwh consumed per year. Gas is used for our heating and hot water, and occasionally in the gas fire, which is only used for effect really. Monthly use varies greatly as you would expect with heating not required for 6 months of the year, but annual use of gas has been steady at around 9300 Kwh.
