Inside temperature fell during a power cut - but central heating wasn't on

[NY]

Yesterday we had a power cut for a little over 2 hours from about 11:00 to 13:10. The inside temperature fell during the time of the power cut and rose again as soon as the power came back.

The central heating had not been on, so the lack of it during a power cut is not an issue. That part of the house gets some sun from south-facing windows, and a little bit of heat from the gas Aga which remains on during a power cut, but that is in a kitchen about 50 feet (two rooms) away.

I don't remember the sun going behind the clouds during that time. The weather station is positioned so, as much as possible, it is shaded from the direct heat of the sun all day.

And yet the inside temperature (green line on graph) recorded by my weather station fell gradually by about 4 deg C between the start of the power cut (about 11:00) and the time when the power came back (13:10). The time when the power went off is a bit vague: from memory it was around 11:00. The time it came back is easy to determine because the line after the power was restored and my Raspberry Pi running weather station software began logging every minute becomes more jagged due to readings every minute, whereas all those readings that were recovered later from the weather station's internal memory are at 10-minute intervals, smoothing out minute-by-minute variations

It is possible that the battery which keeps the weather station going during a power cut was not supplying as much power, affecting the interior temperature sensor, though if that was the case I'd have expected a sudden drop, rather than a gradual fall.

If it *wasn't* a spurious result due to the station losing mains power, can anyone think why a room that was not being heated by central heating would cool when the mains went off, and start to warm up as soon as the power came back.

The only thing that would not be on during a power cut is a couple of freezers, but they are two rooms away from the one where the weather station is. Would freezers really give out enough heat to warm a room about 50 feet away by 4 deg C?

[Theo]

26C is pretty warm. Does the room feel that warm to you in normal times?

If it's say 16C outside there must be a source of heat to keep the room at that temperature. It doesn't look like solar gain (why is the room still warm at midnight and cooling during the morning?).

So either the room is fairly leaky and there's a constant source of heat which cut out when the power went out, or the measurement is suspect.

The freezers could be a source of that heat, but it would have to be a small room for that amount of heat to build up - certainly not 50 feet away. (I've been in a corner shop where it happened - that was half chillers and the whole shop was the size of a single garage)

Ground truth the temperature by what you feel. Is the room warm enough that it would be more comfortable to wear shorts? (yes I know personal ideas of temperature vary a lot)

Theo

[Chris Hogg]

Someone shut the door to the room where the interior sensor was located, allowing the room to cool (although 4°C does seem a lot), and then someone opened it soon after power was restored.

As Theo commented, 26° does seem rather warm. Is your interior sensor calibrated correctly? Is it picking up a spurious signal from somewhere that stopped when the power went off? Does the sensor cable run alongside a mains cable?

If the inside temperature fell that quickly, it suggests you're losing a lot of heat somewhere, and it must be costing you a fortune.

Just suggestions.

[Animal]

No, a freezer isn't going to be much more than 100w intermittent. Possible factors: cloud rain high wind loss of electric power consumption, pretty much all of which becomes indoor heat.

[Tim+]

Check your CH timer programming. Ours had been set to “off” for months but following brief power glitch the other day it seemed to have reset itself to the previous (winter) settings.

Tim

[Jeff Layman]

I don't understand your high figures for inside temperature, or the extreme fall during the power cut.

I also have a weather station, and rather oddly we also had a short power cut on Wednesday evening from about 1900 to 2000. The lounge room temperature has usually been around 20°C during the day in this cool August, and so it was on Wednesday when the power cut started. It was

20.2°C when the power cut started, falling to 19.5°C after an hour when I was able to get the next reading (they aren't stored but are uploaded, which couldn't occur during the power cut). Within 30 minutes it had returned to 20.0°C. Our lounge is triple-aspect, and without heating in summer. The only things which provide heat in the lounge are a few watts from wall-warts, etc, and about 60W from the TV set. I assume that the slight drop in temperature was due to the TV going off, and me going out to check our neighbours to see if they'd lost power too, so letting cooler air from outside in.

What system are you using, and where is the inside temperature sensor?

[Chris Hogg]

PS: have you tried simply turning off the power at the mains isolator or simply pulling the fuse and seeing if the sudden cooling/re-warming repeats itself?

[Brian Gaff]

What wall warts are in use in that room and was the telly on standby?Although not huge a couple of old fashioned wallwarts that do tend to get warm, and a TV which also runs warm, could be the reason if you are well insulated. I'd not discount fridges, as although they are not in the same room they may make a difference.

Brian

[Brian Gaff]

/this is where a simple low tech thermometer would be handy as a checkpoint. Don't get the dial variety they are not very accurate, get a normal column of Mercury one assuming we are allowed to own them these days oof course. Brian

[Jeff Layman]

To follow up on my post, there are three wall warts; one for the weather station console, one for the router, and one for the dect phone - probably less than 10W in total. I also forgot there is a power brick for the laptop (and it *is* a brick as it's rated at 180W! Of course, most of the time it's just using a few tens of watts to power the laptop). So, overall, I reckon around 100W in average use.

[The Natural Philosopher]

Enough to heat a small insulated room at least a couple of degrees. I worked on 50W/sq m on the UFH calculations to get a 19°C rise.

[Paul]

"A fluorescent lamp, or fluorescent tube, is a low-pressure mercury-vapor gas-discharge lamp"

As a society, we still use plenty of mercury, and are as careless in its disposal, as you would imagine.

The best mercury thermometers, work over a 5C range and are +/-0.01 C. The reservoir on the bottom of those is quite large, compared to a department store thermometer. the advantage of liquid based thermometers, is filtering out temperature transients. If you break one of those, the instructor will surely be angry :-) They are quite expensive.

Some of the digital techniques can measure a huge temperature spike within a one second period (diode based temperature), and on mercury, you would not see the spike, but only the time-filtered value.

Paul

[NY]

It's a Hive controller - actually with two different "zones" fpr different parts of the house (controlled by motorised valves). The Hive app, which talks to the Hive boiler controller, shows in its temperature/boiler-on-time readings that the CH hasn't come on for either zone for a few months apart from brief occasions when the temp has got cold at night due to the cold summer weather we're having.

I agree that 25-26 deg C is unusually warm: I'm inclined to wonder whether it's mis-calibrated. The room feels comfortably warm, but not too hot - apart from days when the sun is shining in and the ambient temp outside is 25-30 deg C. But I will check it against another thermometer in a similar place.

[NY]

Although it is a simple software problem with a digital thermometer to do a rolling average over 1 second, 5 seconds, 1 minute etc according to what is needed. The disadvantage is that there will be delay of the averaging time before the first reading is displayed - unless the sensor is permanently reading and logging temperatures and it is only the display that it turned on/off. A mercury or dial thermometer is "always on" ;-)

[John J]

Any chance you've got something running in a concealed space? It could be a secondary hot water circulation pump.

[GB]

That was my guess, too. We have a circulating hot water system, and some of the pipes are not properly lagged.