Measuring the temperature of flat panel rads Help and information on all topics relating to your central heating, air conditioning and ventilation issues.

[nehoc]

My house is cold - must be the time of year ...

2 years ago I got a condensing boiler, but it was still not warm enough so last year I had some rads replaced with bigger ones. My old ones were cast aluminium with flat front panels, but the plumber said he could not get ones like it, so I have pressed steel rads with a flat front panel.

What he did not say was the the flat panel is just welded on to a normal rad, and there is an air gap behind it. I soon found that the front temperature was lower than the back, but he told me it did not matter as the heating was by convection rather than radiation [I'd have thought the hotter the surface, the more convection, but never mind].

Any way I want to measure the temperature and I wonder if the is a good way to do it. The best I can do in hold a jam-making [!] thermometer with a brass housing against it. This is easier on the flat front than the back: the temperatures back and front are obviously too low as the contact is poor, but even so there is a big percentage difference. Two sets of readings show back/front respectively as 36°/28° and 42°/32°: clearly if this ratio was maintained when the hotter of the two is increased to whatever temperature the rad is really achieving [?70°C] the difference represents a huge amount of lost heat.

If this is the case, why are double-skinned rads like this made, and how can I get accurate measurements?

Cheers, Colin

[DEEARR2]

There is a possibiliy that the radiators you have, by description, could be low surface temp rads (LST). These are installed in areas like old folks homes, nurseries etc where the front surface area is protected from the variable heating system flow temperatures. This in turn protects patrons from heat related injury. The actual heat output is from the protected rear section of the radiator.

[htg engineer]

"rad is really achieving [?70°C] the difference represents a huge amount of lost heat"

WHAT ????

You need the temperature of the flow and return, you need a 12oC temperature difference.

As you plumber said convection is what heats the room, so the air gap - is there to heat the air then circulate. If the radiator is the right size for the room and you have a temp difference between the flow and return (as above) it'll heat the room - surface temp is irrelevant there has to be heat loss to heat the room.

htg

[nehoc]

Thank you deearr2 and htg engineer - sorry for the delay as I was on a different computer without my log-in.

Yup, it seems they are LST, but I’m not ready for the old folks home yet! As I said my earlier rads were cast ali, the front being flat, but the same temp as far as i can tell as the back or middle [in the case of double panels] and my plumber just said he could not find them tho I have seen them in other places since.

I know from reading about the subject before I posted that I need a 12° difference between flow and return, but I don’t have a suitable thermometer, I would swear that the installer never measured it, and it is not what I was asking about.

The room was measured, so in theory the rads are the right size. But there are heavy curtains [I have no double glazing] which were there before the rads were changed and they effectively block convection at the back [the hottest bit] and the middle.

I was guessing at 70° as the temp at the back as I could not measure it properly and the temperature differences I mentioned were the best I could get from back and front with a brass mounted thermometer which was not in perfect contact and I assume that if the temperature back and front was actually higher than I got the difference is also greater.

While, despite their name, radiators primarily heat by convection if the front has a low surface temperature there has to be less convection, and clearly there is some heating by radiation that must be lower too.

What I am looking for is an idea of what the efficiency loss in convection and radiation is with LSTs [at best they can only have the equivalent of about 3.5 convecting surfaces] - presumably there is a formula used in the trade to calculate how much bigger an LST has to be than a conventional rad.

Thanks, C

[DEEARR2]

When the KW requirement for each area to be heated has been calculated, then reference to the LST manufacturers radiator tables would show the necessery rad sizes. They also show the flow temperature, to the rads, required to give the rated output. Perhaps a re-check on calculated heat requirement for effected areas and a rad size check against rated outputs .

[nehoc]

Thank you again deearr2.

As a cold end-user I think I am getting there. Of course I never asked for LST rads, the clue being in the Low bit, just flat panel like before which being cast were the same temp all over.

I don't think I was even told the make, but can someone give me an idea of how much bigger typical LSTs need to be to give the same heat output? Then I'll have a better idea of how significant a factor it is.

Cheers, C