Over the past couple of weeks I’ve been trying to measure the heat input to the house from the heating system, and gain a better understanding of the way the house responds to changes in temperature. I already knew that the air source heat pump (ASHP) only seemed to come on for a couple of hours or so each morning, and also knew (from power measurements on the supply to the ASHP) that it rarely seemed to use more than around 800 W to 900 W, settling down after about 10 minutes or so of running to around 400 W to 500 W. What I wanted to find out was how much heat was actually being supplied to the house from the underfloor heating (UFH).
To do this, I used the data that the built-in house measurement and data collection system uses, together with a portable data logger than I adapted to measure the surface temperature of the heated floor. After a couple of weeks there is a fair bit of data, as the house data logger records the outside air temperature, inside air temperature, inside relative humidity, inside CO2, floor slab internal temperature, buffer tank temperature, ASHP flow temperature and the hot water preheat plate heat exchanger temperature. The portable data logger records the temperature of a remote temperature probe, relative humidity and CO2 concentration normally, but I adapted the temperature probe so that I could stick it to the heated floor surface, near the centre of the house, to measure floor surface temperature.
There’s a formula for calculating the amount of heat power, in Watts/ square metre (W/m²) that an UFH heating system is delivering:
P (W/m²) = 8.92 x (floor surface temperature – room temperature) ^1.1
The two data loggers both take and store ten measurements per hour, and so I ended up with over 4000 sets of data to process after a couple of weeks or so. The data is challenging to present in a form that’s readable in this blog, but some things are quite clear. The inside temperature is pretty stable, fluctuating by a bit over 1°C in total, around the room thermostat set point of 20.5°C, no matter what the outside temperature.
This plot shows the hourly changes in outside, inside and floor surface temperature over just a part of the measurement period, where the outside temperature varied the most, to give an idea of the temperature stability:
Unfortunately, we didn’t get any really cold weather, but even so, with a change of outside temperature of nearly 10°C the house air temperature barely changes, never getting lower than 20°C. There is a little bit of room temperature over-shoot when the heating comes on, as the room thermostat turns the heating off at 20.6°C, but the heat stored in the floor slab continues to heat the house up by nearly 1°C above the turn-off point, without the UFH running.
I may be able to fine tune this, as it is directly related to the UFH flow temperature. I have this set for around 24°C to 25°C at the moment, as if it is any higher the room temperature over-shoot increases. The problem is finding a thermostatic mixing valve for the UFH that will reliably hold a relatively low flow temperature; most are designed for heating systems in houses with a far greater heating requirement, so don’t regulate well below around 30°C.
From all this data, I was able to calculate the arithmetic mean daily heat input power, as well as the actual daily heat energy input, together with the arithmetic mean daily temperature:
As we’re not yet living in the house, this is very much a worst case heating requirement, as each person in the house will add around 80 W to 100 W of heat, plus there will be additional incidental heat gains from cooking, running showers, appliances etc. I suspect that the actual heating requirement would be non-existent for many of the days in the chart above, once we’re living in the house.
I also tried to estimate what the heating cost would be per day, using an assumed coefficient of performance (COP) for the ASHP of 3. This is probably slightly pessimistic, as for much of the time the ASHP COP is likely to be well over 3, but nevertheless it gives an idea of heating cost. It looks as if the average daily heating cost, during the winter, is unlikely to be higher than around 25p, and may well be a fair bit less than this once we’ve moved in. Even in the winter months we usually generate more electricity than this from the photovoltaic (PV) panels, so, as hoped, it looks like we won’t have any energy bills at all.