Nearly Passive in Somerset

On the way back from the Homebuilding & Renovating show, I pop into a development of near Passive Houses in Chewton Mendip. There I meet the project manager Arthur Bland who shows me around the terrace of three houses, which have been commissioned by the Waldegrave estate and are going to be rented out on completion.

It turns out that this project has been driven by Arthur and his interest in low energy housing and Arthur is a big thermal mass fan. He chose to work with Logix, one of our more innovative ICF businesses, and he assembled a crew to work with him. He particularly liked Logix because their formwork is manufactured in the South-west and therefore if he needed something extra he wouldn’t have to wait six weeks for it to arrive from overseas. “I went on their training course, I went and visited two sites and then I just got going with it,” he said. “I picked up local builders to work with me; some were brilliant at it, others couldn’t get their heads around it at all.”

There were problems with the site. For a start, building control had identified a radon issue, which had to be managed. Arthur chose to use Eco Slab which, he said, was a brilliant solution, very easy to use and no mechanical handling required and, because it gives a ventilated sub-slab, all it needed was a gas barrier installed above to meet the requirements. Arthur also went for a poured Quad Lock intermediate floor, specifically to increase thermal mass — there is a hell of a lot of concrete in these homes. All around, the fabric is designed to have U values equivalent to Passivhaus standards, with extra insulation added where the proprietary systems didn’t quite get there.

I had been told that these were another example of no heat homes, but it transpired that this wasn’t really true. What they did each have in their airing cupboards was £6,000 worth of Genvex Combi, a bit of kit I hadn’t come across before. This machine is a hybrid between a mechanical ventilation unit with heat recovery (MVHR), and an air source heat pump (ASHP), which also supplies 185litres of domestic hot water. It’s a beast, but it’s a quiet beast. One of the oft-heard criticisms of ASHP is that it is noisy, but these units ran with a barely discernable hum at the standard setting. Even on full blast setting, it made less noise than a gas boiler.

It’s a neat idea. On the other hand, it’s electric heating, which doesn’t sound too great an idea. But to be fair, this is also pretty close to the standard set up in a German Passivhaus, where they usually have something called a post-heater added to the MVHR system. Arthur said that without the unit on, the houses were settling at an internal temperature of around 12°C (it’s been fairly chilly recently), but when I visited they were holding a temperature of 17°C. None were yet occupied.

What I don’t yet know is how much energy these machines are using, though Arthur promises to let me know when he has some figures.