Update on Timeshifting to Minimise Heating Costs.
When we first decided to self-build in 2014, Jan and I visited quite a few passive house builds and talked to various experts; we soon decided that a low energy approach was broadly the way to go for our build. One of these experts, a passive-house evangelist called Seamus O'Loughlin, emphasised that a conventional heating approach (where boiler demand is based on some central thermostat set point) doesn't work well in a passive house, because the time constants of a high-thermal capacity low energy house are a couple of orders of magnitude longer than those anticipated by conventional CH control systems.
At the time this seemed a controversial assertion, but because I have done some mathematical modelling professionally, I was able to and decided to do some time-dependent heat-flow modelling and control strategy simulation of how our designed house would behave and this very much supported this assertion. I have already covered a lot of detail of my CH approach in previous posts and discussions, but it’s probably worth summarising some key headlines to set the context for my changes to our heating strategy:
- We were cash-flow limited during the build phase, so had to make various cost-benefit trade-offs on our build, like most members here. I based these on a general net 10-15 year payback, and it was clear that we wouldn’t be able to achieve a true zero-input passive house largely because of design compromises owing to planning restrictions and our plot size and orientation.
- However, we would be able to build a low-energy house that would need generally low levels of supplemental heating for maybe 6 months a year, with overall heat losses an order of magnitude less than a conventional build, and the thermal capacity of the heated fabric be many factors more.
- We decided to go all electric in the house with wet UFH embedded in the ground floor slab only. Cost benefit trade-offs didn’t even support installing an ASHP, though I did future proof the installation to simply the later addition of one if the cost numbers changed.
- I decided to adopt a simple but unconventional strategy for heating the house: calculate the total heating requirement for the coming day daily at midnight; this is based on actual averages for energy use, average house temperature and forecast average external temperature for the coming 24 hrs. This allows me to dump as much of this heat into the house fabric as practical at the cheapest electricity rate, and for us this is in the 7 hour overnight off-peak window on our E7 tariff.
- We used to get some spill-over into peak rate top-up in the coldest months, but a year ago I added an oil-filled electric radiator on my 1st floor landing, and one in my son’s 2nd floor bedsit controlled by my Home Automation System, with these scheduled to come on in the overnight E7 window to dump extra heat in the upper floors. This simple addition reduced the thermal layering from ground to second floor, and almost eliminated the need for daytime slab top-up.
- In practice we have roughly a 1°C daily ripple on overall winter house temperature. Because using a daily forecast computation does have some intrinsic prediction error, this can add typically less than 0.2°C day-to-day ripple on top, but any longer term drift can be corrected by the daily feedback.
- I have RPi3B running NodeRED attached to some digital thermometers and 4 GPIO controlled solid-state relays (SSRs) to control the time of the UFH pump and Willis heater, plus the 2 × SunAmps for DHW. This was very cheap to implement, and basically has no monthly or annual maintenance.
With the current Electricity price hikes, we have decided:
- To trim our house temperature set-point back from 22.3°C down to 21°C
- To hard limit automatic heating of the slab to the cheaper 7-hour off-peak window. (We can still do peak by request in one hour chunks if we want to.)
- To use electric oil-filled radiators overnight to do any additional top-up. I can automate this through my Home Assistant (HA) that runs on a separate RPi4 and do this using MQTT via WiFi connected powered/metered sockets.
This strategy currently limits heat into the house to:
- ~21 kWh through the slab and
- ~7 kWh through the two radiators.
28 kWh is enough to maintain overall house temperatures so long as the external temperature is at ~7 °C or higher, and it clearly isn’t the case at the time of posting. The house needs about 2½ kWh/K, so with the average daily external temperature at zero today this is 17½ kWh too little to maintain house temperatures. The long term Dec / Jan average where we live is about 4°C, so to maintain temperatures in this case we would need an extra 7½ kWh/day. (This last year, we had 26 days where the average external temperature was 4°C or below and only 2 where temperature was below zero or below.)
So what happens when we underheat our house? Simple: it slowly cools down, and very slowly. For example, in the last 5 days of cold-spell, capping the heating has dropped the average house temperature from 22.3 down to 21.3°C, and given an average of -1°C for today, it will be down to our new target of 21°C by tomorrow . At this point I will need to add more heat or to accept that the house temperature will fall further. I will definitely need to add another 7kWh or so extra radiative capacity for overnight topup. We will play it by ear over the next week or so. I can either accept that I will be paying £0.38/kWh for extra peak period top-up during these really cold spells, or let the average temperature fall a little further if we find it comfortable enough (wear a thicker jumper, etc.)
This approach works well for us because our house is so insulated and it has a huge amount of thermal capacity within the heated envelope. If we accept a small heating ripple then it really doesn’t matter that much when we heat within the day and so we can time-shift our demand to make use of the best tariff rates: currently over 85% of our electricity use is at the off-peak cheap-rate price. This latest exercise of clamping the heat output to 28 kWh when the maintain level is closer to 40 kWh underlines that the heat budget for and given day can be off by 30% or so and the net temperature drift is still on 0.1 °C or so; the time constants of the system are of the order of a week rather than days or hours.
By way of a contrast my daughter lives in a pretty large but conventional 1990s house. When her heating goes off in the evening, the living room temperature drops maybe 4-5°C within an hour.
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