Original URL: http://www.theregister.co.uk/2012/02/04/eco_home_part_3/
Tame the gas monster with sensors, suckers and a spiffy new fan
More performance-tuning for your home and office
Part 3 I'm pleased to say that what with the relatively warm 2011 and our conservation efforts we had the lowest consumption of electricity and gas at home of any year yet, a bit over 1,500kWh ('units') of electricity and under 4,000kWh of gas. (A typical UK household is nearer 3,300kWh 'leccy and 18,000kWh gas.) With our solar PV exports we were just carbon-negative for power by my calculations.
Even allowing for the warmer year with Heating Degree Days (about 820 in 2011 for us compared to a more typical 1,100, and a whopping 1,481 in 2010), I could see a small further improvement in heating efficiency last year. Some will have come from the aerogel insulation in my boy's room, but one more gadget had not yet had time to contribute by this year end...
MHRV: Warm story
To save energy at home , you need not just better insulation, but also better air-tightness to avoid leaking warm air to the outside, especially in an icy gale.
Somehow at the same time it is important to maintain decent ventilation to avoid condensation, stuffiness and frankly a build up of pong (my youngest's nappies were all the evidence you needed).
But this ventilation cannot just be 'unplanned' or 'accidental' via leaky walls or constantly open windows in the winter, so efficient buildings often use mechanical heat-recovery ventilation (MHRV) which forces stale air out, pulls fresh air in, and transfers the heat from the outgoing to the incoming air to get the best of both worlds. The same principle of countercurrent heat exchange allows penguins to stand on the ice and yet not have blood return to their hearts icy cold!
Because our house will probably always be too leaky for a whole-house MHRV system like the big boys have, and because I'm kinda cheap, I have installed a single-room MHRV (a Vent-Axia HR25H ) in our bathroom. It does seem to keep the air upstairs fresh without opening windows, and air is brought in at remarkably close to room temperature, but it is not helping as much as I'd hoped to keep humidity and condensation down overnight with all of us snoring away (up to 80 per cent relative humidity at night: recommended levels are 30%RH--60%RH).
Partly I'm tackling the humidity coming from the kitchen by putting on a portable dehumidifier in there (for example when laundry is drying) which is a lot more energy-efficient than the tumble dryier and also provides heat somewhere between electric and gas heating in terms of CO2 per kWh to the room.
It may also be time very soon to replace all or nearly all of our ageing double-glazing with triple glazing to take away the cold surfaces for the condesation to form, plus continuing to superinsulate a bit at a time to make sure damp does not form on (or in) the exterior walls.
In summer we'll just turn the MHRV off and have windows open as usual!
I continue to monitor the temperature with iButton sensors (read via OWFS  on Linux on my SheevaPlug) in and around the boy's room to keep an eye on insulation performance. Late January's graph:
shows the heating down when we were away a few days over Christmas. Also, after Christmas, it shows a higher daily minimum temperature and more stability since we are no longer throwing open windows in the morning to vent the fog now that we have the MHRV purring away 24x7.
Solar thermal, pumping heat
Thanks to continuing government flapping on both solar PV (electricity) and solar thermal (for hot water) in the domestic FiT and RHI schemes, plotting some combined solar PV/T for the space left on our roof has been more ... um ... interesting than really necessary.
However, the nice man from Isoenergy  came to visit and confirmed that we could probably do a ground-source heat-pump (GSHP) combi (to replace our ageing and not-massively-efficient gas combi) using a ~100m borehole (we don't have the lawn space for the normal pipe-runs), but for a kewl £17k or thereabouts. And that a good air-source heat-pump (ASHP) would not be much cheaper but would, as expected, be less likely to cut much footprint compared to gas. Ouch. I am keen to stop being committed to burning fossil fuel at home but there's keen and there's more than twice the cash per tonne of CO2 avoided as even relatively-pricey solar PV.
However, if the heat-pump could be combined with solar PV/T to take away any need to import energy at all for hot water in the summer for example, and generate a trickle more 'leccy to spill to the grid, the numbers start to look at least competitive with solar PV, and might eat away another 500kg or more of CO2 per year from our footprint, talking us firmly carbon negative for energy. (Plus we could disconnect gas entirely, which would save us £100 per year on standing charges alone!)
And like with my solar PV and MHRV I want a system that I can just forget about and let get on with the job, and that the rest of the family don't need a training course to manage. I can choose to take lots of meter readings if I want, but the basic kit must be reliable and almost entirely automatic. Just like in IT, being lazy and letting machines do the work can be a virtue; false heroics are not smart!
At the start of 2011 I massively upgraded the batteries for the off-grid power system for my SheevaPlug Internet server  and it coasted through December and January off-grid without a glitch.
December/January battery levels; the days without spikes over 14V were days without any sun to speak of (click to enlarge). 
But that is 2kWh of effective storage or four biggish car-batteries in size for a device that uses about 0.1kWh per day; given that a typical UK household used 9kWh/day imagine what would be needed to take just one house off the grid and thus why grid-scale storage is so hard at the moment!
I hope to experiment with the Raspberry Pi and a network of temperature and humidity and maybe even CO2 sensors this year, amongst other things, and may do some of the same for my kids' school... ®