I’ve been testing whether it matters how our DS18B20 temperature sensors are affixed to the radiators – it’s a classic round peg in a square hole problem. We want to know what temperature the water is inside the radiators, and all we can do is tape a sensor to the surface metal. The question is really, how fiddly should we be about that process, and what do the readings actually mean? I had the thought that shielding the side of the sensor that’s away from the radiator might make the reading approach the truth better, and so I set up that test. A saucepan stands in the for radiator; a sensor in the water gives the actual temperature, and two sensors strapped to the side give the insulated and uninsulated results. The insulation is just 6 layers blackout curtain lining I had lying around; it’s suppose to have reasonably good thermal properties.
I thought I’d try some of what we’ll be doing at St Columba’s, but from the relative comfort of my own home – and potentially answering some questions at the same time. Here’s the set-up.
That was the problem back in 2012 when I looked up these Christ Church Morningside drainage records, anyway. One of the access points is right next to their two boilers. The ignition on one is still a bit touchy – I guess they didn’t like what came up any better than I did! At the time we wanted to know whether there was any connection to the drains out the back. The answer appears to be “no”.
Now I do just about remember how radiator heat outputs are specified. It’s all about Δt, the difference between the average temperature of the radiator surfaces and the temperature of the surrounding air…
St. Columba’s by the Castle – where we ran an Innovative Learning Week event last year – has an interesting follow-up question for us. How much will it help the space warm up when they remove that wood paneling around the radiators?
Whenever I look at them, I think “this is a place that takes the name ‘radiator’ seriously” – after all, there will be radiation off that nice exposed surface. Unfortunately, radiators are misnamed. They really heat spaces almost entirely by convection. These are designed to take cold air in at the bottom, heating it so it rises and comes out the top. You can’t see from the picture, but there is actually a grill in the shelf above the radiator. Not so underneath. The air intake is seriously obstructed, whereas the air outflow is just slowed down by extra turbulence, dumping heat right there instead of further into the space.
Answering their question starting from first principles is tricky. There are data sheets that will tell us how much heat output this radiator would give if unobstructed, but of course, not what happens with all of the interesting things that happen to them in practice. And I know from talking to Dimitri about building a test rig for the hot wire anemometry that turbulence is tricky stuff! They’re really looking for “how much more heat output will we get, and will it be enough or do we need to make more infrastructure changes”. It’s a bit hard to mount equipment on the radiator for the “before” picture. These are my thoughts about how we might answer the question.
Those hall control requirements are a bit complex to fill… and it’s become clear with just how fast the hall now heats up, we have to think about user controls that keep it from overheating, whereas before that simply wasn’t often a problem! I’m not quite sure what we’ll do this time, but I’m working on it.
At least, there is a top cut-out and control within a reasonable temperature band – just not the one that’s set on the thermostat, which I think they said is at 17.5 now! Lots of models come with a behind the scenes display adjustment (“add or subtract this amount”) but not this one; since some users feel the cold with their eyes, best to sort something.
And it’s a pretty mild day – let’s look at that warm up, blown up.
So, we’re snagging a church hall, and so far, we’ve got plenty of heat input and output, and a boiler running at 90C under ineffective control.
Here’s some of the views. First up, we have the answer to the question: how quickly does it warm up?