The Reactive House

Building BMR

Richard Rodgers -

Can the reactive house be built? Rather, can it be built today (with current technology), at reasonable cost? Most analyses would suggest the answer is a resounding no: the off-grid requirement and reliance on only intermittent (renewable) energy sources would seem to argue that energy storage needs alone would vastly exceed what falls into the affordable range.

Consider a late December day in a heating climate, with little daylight (so minimal PV generation), and frigid temperatures (so great heating demand): without a warehouse full of batteries, how would we survive one night, let alone a week? But before we dismiss the case outright, let's try to unpack all the assumptions that might be at work here, and try to construct a more detailed model of generation and consumption, and resources that could contribute.

Rather than just assuming that the average (US, or regional, etc) energy consumption must be supplied, let's start from zero and examine each need, and what it would entail for energy production. We can liken the problem to a well-known animal physiological measure: the basal metabolic rate. What does a house need in energy terms to be habitable at all, and maintain its integrity? Human BMR is defined as the number of calories required to keep your body functioning 'at rest'; for buildings, it is the amount of energy (kWh) required to keep a house in a habitable state 'at rest'. We do not yet have a precise sense of 'at rest' for buildings, but roughly it means 'independent of specific occupant behavior'. But what is 'habitable'? Of course, this will vary from season to season, from climate to climate, and occupancy, etc. But we can establish a baseline, and adjust for these factors. How low/small can we make building 'BMR' in the 'worst' case (during winter in a heating climate)? Here's a starter list of building 'metabolic functions' and their energy requirements (the total of which equals the building BMR) - expressed in watts per hour:

  • Respiration Our house will be 'passiv' (tight building envelope), so mechanical ventilation will almost certainly be required. But the house will also be small (say 1,200 - 1,400 sqf), so requirements could be met by a HRV/ERV of modest capacity. A Zehnder ComfoAir Q350 HRV (an efficient unit) claims 0.34 W/cfm in energy consumption. Assuming a daily average of 40 cfm (using AHSRAE 62.2-2016 1400 sqf, 2 occupants), this translates to a 13.6 watt per hour contribution to the BMR. Lets make it 20 to allow bursting for showers, etc.
  • Fat Storage We will need to refrigerate food. Modern fridges have become quite efficient, so a 35 watt contribution will suffice for a small model.
  • Nervous System Our home can't be reactive without a nervous system - allow 10 watts for a WiFi router to transmit nerve signals.
  • Circulation Let's make our heating system as lean as possible - zoned, low temperature hydronic. Using Thermostatic Radiator Valves at the emitters, we might only need 35 watts to power a single circulation pump, if we have enough hot water available.
  • Vision Our house relies on natural light sources wherever possible, but in the winter scenario we are describing, daylight is limited. Assuming 6 hours of usable daylight, and 8 hours of sleeping time, we will need power to provide 10 hours of illumination per day. Modern LED-based lighting is very efficient - approaching 10 times better than traditional incandescent fixtures. So to replace 3 60 watt incandescents for those 10 hours (in 3 areas, e.g.) we would need only 18 watts of LED lighting - 180 watt hours for the day, or around 8 watts added to our BMR.
  • Lymphatic System How about water? Rules of thumb for the US - 80+ gallons per person per day - should not apply for a number of reasons in the reactive house (one: no flush toilets). Rather, we will assume the following: one 10 minute shower using a low-flow (1.5 gpm) head, and 5 more gallons per day for all other uses (drinking, hand-washing, brushing teeth, etc) yielding 20 gallons per person per day. For occupancy of 2, that is 40 gallons, which an efficient well pump (like Grundfos SQFlex 11 SQF-2) can produce for 60 watt hours, or 2.5 make it 3 watts added.

So far, our BMR is a mere 111 watts per hour. In winter, when we have only 6 or so hours of PV-producing daylight, we would need 18 * 111 = 2 kWh to 'cover' the non-power producing hours in a day, or 2.7 kWh for all 24 hours in the worst case that there was a storm, etc. While not negligible, we still fall comfortably within the abilities of residential battery systems on the market today. A Generac PWRcell M6 battery claims a usable capacity of 18 kWh: which could supply BMR-level energy for about a week. So why do conventional houses use so much more energy? There are several other services/needs one might claim fall in the BMR (cooking, dish washing, clothes washing & drying, etc) but we would argue that they are deferrable in a way the others are not, so should not be included. However, the single largest energy requirement - hot water for hydronic heating and domestic use - must still be addressed.