Ask the slab.
We have ten thermometers in the slab, recording and logging the temperature every few minutes. There are five buried at the bottom of the foundation, one near each corner and one in the middle, and another five in the screed, hopefully a safe distance from the underfloor heating pipes so that they are measuring the temperature of the floor and not just the pipes. They should give us an idea of when the heating went on though.
The best one to look at is probably the thermometer in the middle of the floor. Before the heating went on, it was nicely cycling day to day with the temperature at its lowest mid-morning, then rising from 10 or 11 am, as the heat of the sun found its way into the slab. The heat either does this directly by hitting the floor and conducting through the tiles and concrete, or indirectly by heating the air in the house, and the air heating the floor. My instinct is that solar radiation is going to have more effect heating the floor than ambient air, probably because I've been indoctrinated into the mantra of hot air rising, and the consequence that not much heat will be going down from the air into the floor.
The data is a bit grainy, since we only have precision to a tenth of a degree while James Joule reckoned he could measure the temperature of his beer vats to 1/200 of a degree Farenheit. Making the best of our 21st century tools, the floor seems to stop warming around 3.30, which is about when the sun stops reaching it directly. The room temperature is still a couple of degrees warmer at this point, but can be five or six degrees warmer around noon. Hopefully you can see this in the first chart, where the green line is the room temperature, the red line the temperature just under the floor, and the blue line the temperature at the bottom of the slab. Of course a bigger temperature difference means more heat would be conducted from air to floor, and in fact the ambient thermometer is half way up the wall on the south side of the house, so it's possible that the air temperature at the floor is only above the floor temperature until 3:30. Anyway, this is not strictly relevant to my question.
On 5th December, we must have switched the heating on from around 5:30 am because the slab started to heat up then. From then, depending on the weather, there's a double peak effect when the morning injection of heat starts wearing off and the solar gain hasn't kicked in yet.
On some days there was obviously no solar gain, and the temperature just falls after the morning boost. This happened on December 8th, 9th and 10th when there was a fair bit of snow.
Then from 12 December we get a triple peak effect when the heating was also on from 7:30 or 8 pm for half an hour. This only lasted for a couple of days, perhaps until my Yorkshire genes got the better of me.
The triple peak starts up again from 19th December, this time the heating going on from 10 pm for half an hour.
We went away from 22nd December for 2 weeks, leaving the heating off. The screed went back to the diurnal cycle, with the temperature at the bottom of the foundation plumetting to an all-time low of 19.2 degrees centigrade. The lowest trough in the screed was 18.7 degrees.
The heating went back on again in the mornings from 3rd January, and I can't tell exactly when it went on in the evenings. There is a rise in screed temperature around 8pm on 7th, 24th to 26th and 30th Janary, and 2nd, 3rd and 8th February. On other days there is a slight plateauing of temperature around that time, before the fall over the night, so I guess night time heating was on at least until 16th February. There are a couple of days when the heat went on around 10:30 pm.
We turned the morning heating off on 8th March, and just turned it on again 12th December. Instinct once again tugs at my coat tails, urging me to switch on the heating so the house doesn't lose too much heat making it more difficult later. Knowledge of thermodynamics suggests that making the house warmer is just going to mean losing more heat, so if we can survive the temperature, we should be OK. Experience also shows that it's not going to get that cold. It was still above 18 degrees in the middle of the slab with no heating on for two weeks at the end of December. Also, experience of the underfloor heating is that the response is not so bad, and while it doesn't give the instant blast of hot air you get from a fan heater or air conditioner, it feels warm within ten or fifteen minutes of switching it on. And if things get really desparate we can switch on the air conditioner, or do something really drastic like put socks on.
Whether to turn it on in the mornings or evenings is another question for another day.
